A role for both Ets and C/EBP transcription factors and mRNA stabilization in the MAPK-dependent increase in p21 (Cip-1/WAF1/mda6) protein levels in primary hepatocytes

Mitomycin C and its analog trigger cytotoxicity in MCF-7 and K562 cancer cells through the regulation of RAS and MAPK/ERK pathways

Mitomycin C (MC) is an anti-cancer drug which functions by forming interstrand crosslinks (ICLs) between opposing DNA strands. MC analog, 10-decarbamoyl mitomycin C (DMC), unlike MC, has stronger cytotoxic effects on cancer cells with TP53 mutation. We previously demonstrated that MC/DMC could activate p21WAF1/CIP1 in MCF-7 (TP53-proficient) and K562 (TP53 deficient) cells in a TP53-independent mode. We also found that MC/DMC regulate AKT activation in a TP53-dependent manner and that AKT deactivation is not associated with the activation of p21WAF1/CIP1 in response to MC/DMC treatment. RAS proteins are known players in the upstream mediated signaling of p21WAF1/CIP1 activation that leads to control of cell proliferation and cell death. Thus, this prompted us to investigate the effect of both drugs on the expression of RAS proteins and regulation of the MAPK/ERK signaling pathways in MCF-7 and K562 cancer cells. To accomplish this goal, we performed comparative label free proteomics profiling coupled to bioinformatics/complementary phosphoprotein arrays and Western blot validations of key signaling molecules. The MAPK/ERK pathway exhibited an overall downregulation upon MC/DMC treatment in MCF-7 cells but only DMC exhibited a mild downregulation of that same pathway in TP53 mutant K562 cells. Furthermore, treatment of MCF-7 and K562 cell lines with oligonucleotides containing the interstrand crosslinks (ICLs) formed by MC or DMC shows that both ICLs had a stronger effect on the downregulation of RAS protein expression in mutant TP53 K562 cells. We discuss the implication of this regulation of the MAPK/ERK pathway in relation to cellular TP53 status.

Role of ERK signaling in bladder urothelium in response to cyclophosphamide injury

Mice with inducible urothelial deletion of fibroblast growth factor receptor 2 (ShhCreERT2;Fgfr2Fl/Fl ) injured with cyclophosphamide had aberrant basal cell endoreplication and poor regeneration. The endoreplication correlated with an absence of phosphorylated (activated) ERK expression in urothelium. We assessed whether inhibiting ERK activity phenocopied the urothelial defects in injured Fgfr2 mutant mice. We co-administered cyclophosphamide and an ERK inhibitor (ERKi) systemically in mice and assessed general histology and immunofluorescence for various markers post injury. Since AKT also signals downstream of FGFR2, we assessed effects of an AKT inhibitor (AKTi) on cyclophosphamide injury. ERK knockdown did not affect urothelial injury or proliferation 24 h after cyclophosphamide. Conversely, ERK inhibition led to larger basal cell nuclei, more submucosal hemorrhage and attenuated uroplakin staining 3 days after injury versus vehicle-treated mice. Compared to vehicle-treated mice, ERKi-treated mice had a trend for more Ki67+ urothelial cells and had statistically fewer phospho-Histone H3+ cells normalized to Ki67 and higher basal cell DNA content, consistent with endoreplication 3 days after injury. Ten days after injury, ERKi-treated mice still had signs of poor urothelial regeneration with absent or aberrant expression of differentiation markers and ectopic lumenal expression of keratin 14 (basal progenitor marker). Co-administration of the AKTi led to no apparent urothelial defects 3 days after cyclophosphamide. Thus, ERK knockdown (but not AKT knockdown) leads to urothelial regenerative responses after cyclophosphamide reminiscent of Fgfr2 mutant mice. Together, it appears that FGFR2 acts through ERK to prevent aberrant urothelial basal cell endoreplication and ensure normal regeneration after cyclophosphamide.

Yorkie drives Ras-induced tumor progression by microRNA-mediated inhibition of cellular senescence

The activation of Ras signaling is a major early event of oncogenesis in many contexts, yet paradoxically, Ras signaling induces cellular senescence, which prevents tumorigenesis. Thus, Ras-activated cells must overcome senescence to develop into cancer. Through a genetic screen in Drosophila melanogaster, we found that the ETS family transcriptional activator Pointed (Pnt) was necessary and sufficient to trigger cellular senescence upon Ras activation and blocked Ras-induced tumor growth in eye-antennal discs. Through analyses of mosaic discs using various genetic tools, we identified a mechanism of tumor progression in which loss of cell polarity, a common driver of epithelial oncogenesis, abrogated Ras-induced cellular senescence through microRNA-mediated inhibition of Pnt. Mechanistically, polarity defects in Ras-activated cells caused activation of the Hippo effector Yorkie (Yki), which induced the expression of the microRNA bantam bantam-mediated repression of the E3 ligase-associated protein Tribbles (Trbl) relieved Ras- and Akt-dependent inhibition of the transcription factor FoxO. The restoration of FoxO activity in Ras-activated cells induced the expression of the microRNAs miR-9c and miR-79, which led to reduced pnt expression, thereby abrogating cellular senescence and promoting tumor progression. Our findings provide a mechanistic explanation for how Ras-activated tumors progress toward malignancy by overcoming cellular senescence.

Loss of Fibroblast Growth Factor Receptor 2 (FGFR2) Leads to Defective Bladder Urothelial Regeneration after Cyclophosphamide Injury

Cyclophosphamide may cause hemorrhagic cystitis and eventually bladder urothelial cancer. Genetic determinants for poor outcomes are unknown. We assessed actions of fibroblast growth factor receptor (FGFR) 2 in urothelium after cyclophosphamide exposure. Conditional urothelial deletion of Fgfr2 (Fgfr2KO) did not affect injury severity or proliferation of keratin 14⁺ (KRT14⁺) basal progenitors or other urothelial cells 1 day after cyclophosphamide exposure. Three days after cyclophosphamide exposure, Fgfr2KO urothelium had defective regeneration, fewer cells, larger basal cell bodies and nuclei, paradoxical increases in proliferation markers, and excessive replication stress versus controls. Fgfr2KO mice had evidence of pathologic basal cell endoreplication associated with absent phosphorylated ERK staining and decreased p53 expression versus controls. Mice with conditional deletion of Fgfr2 in urothelium enriched for KRT14⁺ cells reproduced Fgfr2KO abnormalities after cyclophosphamide exposure. Fgfr2KO urothelium had defects up to 6 months after injury versus controls, including larger basal cells and nuclei, more persistent basal and ectopic lumenal KRT14⁺ cells, and signs of metaplasia (attenuated E-cadherin staining). Mice missing one allele of Fgfr2 also had (less severe) regeneration defects and basal cell endoreplication 3 days after cyclophosphamide exposure versus controls. Thus, reduced FGFR2/ERK signaling apparently leads to abnormal urothelial repair after cyclophosphamide exposure from pathologic basal cell endoreplication. Patients with genetic variants in FGFR2 or its ligands may have increased risks of hemorrhagic cystitis or urothelial cancer from persistent and ectopic KRT14⁺ cells.

Down-regulation of long non-coding RNA MALAT1 inhibits granulosa cell proliferation in endometriosis by up-regulating P21 via activation of the ERK/MAPK pathway

STUDY QUESTION

Is there a specific mechanism underlying the association between lung adenocarcinoma transcript 1 (MALAT1) and endometriosis-related infertility?

SUMMARY ANSWER

The down-regulation of MALAT1 in endometriosis granulosa cells (GCs) may have an adverse effect on the growth and development of oocytes by inhibiting GC proliferation, due to cell cycle-dependent mechanisms that enhance P21 expression through activation of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway.

WHAT IS KNOWN ALREADY

The association between endometriosis and infertility is well supported throughout the literature, and endometriosis per se and its surgical treatment have an adverse effect on the ovarian reserve and on oocyte development. MALAT1, one of the most extensively expressed and evolutionarily conserved transcripts, has been implicated to play a role in human development and many diseases. However, little is known about the role of MALAT1 long non-coding RNA (lncRNA) in endometriosis and its associated infertility.

STUDY DESIGN, SIZE, DURATION

We measured MALAT1 lncRNA expression levels in GCs from 52 endometriosis patients and 52 controls. Also, MALAT1 was knocked down in a human GC tumor-derived cell line, KGN, to investigate the role of MALAT1 and its molecular mechanism in cell proliferation.

PARTICIPANTS/MATERIALS, SETTING, METHODS

GCs were collected from women with or without endometriosis undergoing IVF or ICSI treatment. All endometriosis patients were diagnosed by laparoscopy or laparotomy, and control patients were limited to male factor or tubal disease and had a normal ovarian reserve. Quantitative real-time PCR (qRT-PCR) was used to measure the differential expression levels of MALAT1 lncRNA between endometriosis patients and controls. The receiver operating characteristic (ROC) curve was drawn to evaluate the diagnostic values of MALAT1 in endometriosis. In the KGN cell line, MALAT1 was knocked down with locked nucleic acid GapmeRs. Cell counting kit-8 assays, ethynyl-2-deoxyuridine assays and flow cytometry were used to study the role of MALAT1 in cell proliferation and cell-cycle progression, and western blotting was performed to detect the potential underlying mechanism.

MAIN RESULTS AND THE ROLE OF CHANCE

We first found that MALAT1 lncRNA was significantly down-regulated in endometriosis GCs and was associated with the antral follicle count (R = 0.376, P < 0.001 versus control). In addition, MALAT1 lncRNA levels were significantly lower in the GCs of infertile women with advanced stages of endometriosis (P = 0.01 versus control). The ROC curves illustrated strong separation between all the endometriosis patients and the control group (AUC: 0.705; 95% CI: 0.606-0.804; P < 0.001), Stage I-II and control group (AUC: 0.651; 95% CI: 0.536-0.767; P = 0.016), and Stage III-IV and control group (AUC: 0.827; 95% CI: 0.718-0.936; P < 0.001). MALAT1 lncRNA was primarily localized in the nuclei of GCs. We found a negative correlation between MALAT1 lncRNA and P21 mRNA in the GCs from patients (R = -0.628; P < 0.001). MALAT1 knockdown in KGN cells inhibited cell proliferation and cell-cycle progression. In addition, MALAT1 knockdown induced an increase in both the mRNA and protein levels of P21, and of P53, phosphorylated ERK1/2 (p-ERK1/2) and phosphorylated c-Jun N-terminal protein kinase (p-JNK) protein levels, as well as causing a decrease in cyclin dependent kinase 2 (CDK2), cyclin D1 and p-P38 MAPK protein levels. Furthermore, inhibition of the ERK/MAPK pathway with U0126, the up-regulation of p-ERK1/2, P21 and P53, and the down-regulation of CDK2 and cyclin D1 by the knockdown of MALAT1 were all attenuated by MALAT1 knockdown. Therefore, MALAT1 may regulate GC proliferation through P21/P53-dependent control of the cell cycle, and the ERK/MAPK pathway participates in this process.

LARGE SCALE DATA

None.

LIMITATIONS, REASONS FOR CAUTION

The hormonal treatment used in IVF and surgical removal of endometriotic lesions may have altered MALAT1 expression in GCs. The ovarian granulosa-like tumor cell line, KGN, was used for further functional and mechanistic studies due to the difficulties in obtaining human GCs in sizable amounts and maintaining primary cultures.

WIDER IMPLICATIONS OF THE FINDINGS

Our finding represents the first example of an lncRNA-based mechanism in endometriosis GCs. Women with endometriosis show altered MALAT1 expression levels in GCs that may impair fertility by regulating the function of GCs. Therefore, analysis of MALAT1 and its molecular mechanisms of action provide new insights into the pathogenesis of endometriosis and its associated infertility.

STUDY FUNDING/COMPETING INTEREST(s)

This work was supported by the National Natural Science Foundation of China (grant number: 81671524) and the National key research and development program of China (grant number: 2017YFC1001100). The authors declare there is no conflict of interest.

Resistance to ERK1/2 pathway inhibitors; sweet spots, fitness deficits and drug addiction

MEK1/2 inhibitors are clinically approved for the treatment of BRAF-mutant melanoma, where they are used in combination with BRAF inhibitors, and are undergoing evaluation in other malignancies. Acquired resistance to MEK1/2 inhibitors, including selumetinib (AZD6244/ARRY-142866), can arise through amplification of BRAFV600E or KRASG13D to reinstate ERK1/2 signalling. We have found that BRAFV600E amplification and selumetinib resistance are fully reversible following drug withdrawal. This is because resistant cells with BRAFV600E amplification become addicted to selumetinib to maintain a precise level of ERK1/2 signalling (2%-3% of total ERK1/2 active), that is optimal for cell proliferation and survival. Selumetinib withdrawal drives ERK1/2 activation outside of this critical "sweet spot" (~20%-30% of ERK1/2 active) resulting in a p57KIP2-dependent G1 cell cycle arrest and senescence or expression of NOXA and cell death with features of autophagy; these terminal responses select against cells with amplified BRAFV600E. ERK1/2-dependent p57KIP2 expression is required for loss of BRAFV600E amplification and determines the rate of reversal of selumetinib resistance. Growth of selumetinib-resistant cells with BRAFV600E amplification as tumour xenografts also requires the presence of selumetinib to "clamp" ERK1/2 activity within the sweet spot. Thus, BRAFV600E amplification confers a selective disadvantage or "fitness deficit" during drug withdrawal, providing a rationale for intermittent dosing to forestall resistance. Remarkably, selumetinib resistance driven by KRASG13D amplification/upregulation is not reversible. In these cells ERK1/2 reactivation does not inhibit proliferation but drives a ZEB1-dependent epithelial-to-mesenchymal transition that increases cell motility and promotes resistance to traditional chemotherapy agents. Our results reveal that the emergence of drug-addicted, MEKi-resistant cells, and the opportunity this may afford for intermittent dosing schedules ("drug holidays"), may be determined by the nature of the amplified driving oncogene (BRAFV600E vs. KRASG13D), further exemplifying the difficulties of targeting KRAS mutant tumour cells.

Metabolic stress regulates ERK activity by controlling KSR-RAF heterodimerization

Altered cell metabolism is a hallmark of cancer, and targeting specific metabolic nodes is considered an attractive strategy for cancer therapy. In this study, we evaluate the effects of metabolic stressors on the deregulated ERK pathway in melanoma cells bearing activating mutations of the NRAS or BRAF oncogenes. We report that metabolic stressors promote the dimerization of KSR proteins with CRAF in NRAS-mutant cells, and with oncogenic BRAF in BRAFV600E-mutant cells, thereby enhancing ERK pathway activation. Despite this similarity, the two genomic subtypes react differently when a higher level of metabolic stress is induced. In NRAS-mutant cells, the ERK pathway is even more stimulated, while it is strongly downregulated in BRAFV600E-mutant cells. We demonstrate that this is caused by the dissociation of mutant BRAF from KSR and is mediated by activated AMPK. Both types of ERK regulation nevertheless lead to cell cycle arrest. Besides studying the effects of the metabolic stressors on ERK pathway activity, we also present data suggesting that for efficient therapies of both genomic melanoma subtypes, specific metabolic targeting is necessary.

ST2 gene products critically contribute to cellular transformation caused by an oncogenic Ras mutant

The ST2 gene was originally identified as a primary responsive gene, and the expressions of its gene products are induced by stimulation with growth factors and by oncogenic stresses. In this study, we observed that oncogenic Ras mutant induced the expression of ST2 and ST2L proteins. Interestingly, the enforced expression of ST2 gene products in NIH-3T3 murine fibroblasts remarkably enhanced Ras (G12V)-induced cellular transformation. Furthermore, when the expression of ST2 gene products was silenced by RNA-interference technique, Ras (G12V)-induced cellular transformation was drastically suppressed. According to these observations, it was indicated that the oncogenic Ras-induced expression of ST2 gene products is required for the acceleration of cellular transformation, and this seems to be independent of the stimulation with IL-33, a ligand for ST2/ST2L. Interestingly, knockdown of ST2 gene products caused a reduction in Rb phosphorylation in transformed murine fibroblasts, suggesting the functional involvement of ST2 gene products in cell cycle progression during cellular transformation. Our current study strongly suggests the importance of ST2 gene products in cellular transformation, and the presence of novel mechanism how ST2 gene products affect the cellular transformation and cell proliferation.

Bis(acetylacetonato)-oxidovanadium(IV) and sodium metavanadate inhibit cell proliferation via ROS-induced sustained MAPK/ERK activation but with elevated AKT activity in human pancreatic cancer AsPC-1 cells

In this study, the antiproliferative effect of bis(acetylacetonato)-oxidovanadium(IV) and sodium metavanadate and the underlying mechanisms were investigated in human pancreatic cancer cell line AsPC-1. The results showed that both exhibited an antiproliferative effect through inducing G2/M cell cycle arrest and can also cause elevation of reactive oxygen species (ROS) levels in cells. Moreover, the two vanadium compounds induced the activation of both PI3K/AKT and MAPK/ERK signaling pathways dose- and time-dependently, which could be counteracted with the antioxidant N-acetylcysteine. In the presence of MEK-1 inhibitor, the degradation of Cdc25C, inactivation of Cdc2 and accumulation of p21 were relieved. However, the treatment of AKT inhibitor did not cause any significant effect. Therefore, it demonstrated that the ROS-induced sustained MAPK/ERK activation rather than AKT contributed to vanadium compounds-induced G2/M cell cycle arrest. The current results also exhibited that the two vanadium compounds did not induce a sustained increase of ROS generation, but the level of ROS reached a plateau instead. The results revealed that an intracellular feedback loop may be against the elevated ROS level induced by vanadate or VO(acac)₂, evidenced by the increased GSH content, the unchanged level at the expression of antioxidant enzymes. Therefore, vanadium compounds can be regarded as a novel type of anticancer drugs through the prolonged activation of MAPK/ERK pathway but retained AKT activity. The present results provided a proof-of-concept evidence that vanadium-based compounds may have the potential as both antidiabetic and antipancreatic cancer agents to prevent or treat patients suffering from both diseases.

High alpha-methylacyl-CoA racemase (AMACR) is associated with ERG expression and with adverse clinical outcome in patients with localized prostate cancer

Alpha-methylacyl-CoA racemase (AMACR) is a well-characterized marker extensively utilized in prostate cancer (PCA) diagnosis. However, the prognostic value of AMACR expression and its relation to TMPRSS2-ERG gene rearrangement as one of the most common molecular alterations in PCA is not fully explored. AMACR expression was investigated in a cohort of 218 men with localized PCA treated by radical prostatectomy and correlated with ERG and various clinical and pathological parameters. In vitro studies assessed AMACR changes to ERG knockdown and other related genes. In addition, bioinformatics validated the significance of AMACR/ERG expression and assessed relevant genetic signatures in relation to AMACR/ERG expression. AMACR expression was significantly associated with disease progression and with ERG (p ∼0). Seventeen percent of cancer foci showed negative/weak AMACR expression while being ERG positive. High AMACR expression was significantly associated with positive surgical margins (p = 0.01), specifically in tumors with lower Gleason score <7, with ∼95 % exhibiting positive surgical margin (p = 0.008). High AMACR showed marginal association with PSA biochemical recurrence (BCR) (p = 0.06) which was slightly more pronounced in ERG-positive tumors (p = 0.04). This was validated in other public cohorts. However, in this cohort, the association with BCR was not statistically significant in multivariate analysis (p = 0.09). Using in vitro cellular models, AMACR messenger RNA (mRNA) expression, but not protein levels, showed an association with ERG expression. We report for the first time a significant association between AMACR and ERG with prognostic implication. Patients with high AMACR/ERG-positive PCA may be at higher risk for disease progression, and additional studies in larger cohorts are needed to confirm the above findings. Functional studies investigating the molecular pathways connecting AMACR and ERG may provide an additional insight into PCA progression pathways.

Sp1 regulates Raf/MEK/ERK-induced p21(CIP1) transcription in TP53-mutated cancer cells

We previously reported that the upregulation of mortalin, an Hsp70 family chaperone, is important for B-Raf(V600E) tumor cells to bypass p21(CIP1) expression, which is activated as a tumor-suppressive mechanism in response to aberrant MEK/ERK activation (Wu et al., 2013). Interestingly, mortalin depletion induced p21(CIP1) transcription not only in wild-type TP53 but also in TP53-mutated B-Raf(V600E) cancer cells, suggesting the presence of an additional mechanism for p21(CIP1) regulation. In the present study, using luciferase reporter truncation analysis in a TP53-mutated B-Raf(V600E) cancer cell line, SK-MEL28, we identified a proximal p21(CIP1) promoter region responsive to mortalin depletion. Interestingly, when Sp1-like cis-elements in this promoter region were mutagenized, the p21(CIP1) promoter luciferase reporter was no longer responsive to mortalin depletion. Consistent with this, our ChIP analysis revealed that mortalin knockdown could induce Sp1 binding to p21(CIP1) promoter in a MEK/ERK-dependent manner. Moreover, RNA interference of Sp1 substantially attenuated p21(CIP1) expression induced by mortalin depletion in SK-MEL28 cells. Consistent with this observation in SK-MEL28 cells, Sp1 was necessary for the tamoxifen-regulated ∆Raf-1:ER to induce p21(CIP1) transcription in U251 cells, in which TP53 is mutated. However, in contrast, Sp1 was not necessary for ∆Raf-1:ER to induce p21(CIP1) transcription in LNCaP cells, in which TP53 is wild type. These data suggest that Sp1 may address TP53-independent p21(CIP1) transcription in Raf/MEK/ERK-activated cancer cells and that its requirement in Raf/MEK/ERK-induced p21(CIP1) transcription is subject to TP53 status.

Mig-6 suppresses endometrial cancer associated with Pten deficiency and ERK activation

PTEN mutations are the most common genetic alterations in endometrial cancer. Loss of PTEN and subsequent AKT activation stimulate estrogen receptor α-dependent pathways that play an important role in endometrial tumorigenesis. The major pathologic phenomenon of endometrial cancer is the loss of ovarian steroid hormone control over uterine epithelial cell proliferation and apoptosis. However, the precise mechanism of PTEN/AKT signaling in endometrial cancer remains poorly understood. The progesterone signaling mediator MIG-6 suppresses estrogen signaling and it has been implicated previously as a tumor suppressor in endometrial cancer. In this study, we show that MIG-6 also acts as a tumor suppressor in endometrial cancers associated with PTEN deficiency. Transgenic mice, where Mig-6 was overexpressed in progesterone receptor-expressing cells, exhibited a relative reduction in uterine tumorigenesis caused by Pten deficiency. ERK1/2 was phosphorylated in uterine tumors and administration of an ERK1/2 inhibitor suppressed cancer progression in PR(cre/+)Pten(f/f) mice. In clinical specimens of endometrial cancer, MIG-6 expression correlated inversely with ERK1/2 phosphorylation during progression. Taken together, our findings suggest that Mig-6 regulates ERK1/2 phosphorylation and that it is crucial for progression of PTEN-mutant endometrial cancers, providing a mechanistic rationale for the evaluation of ERK1/2 inhibitors as a therapeutic treatment in human endometrial cancer.

MAPK signaling mediates sinomenine hydrochloride-induced human breast cancer cell death via both reactive oxygen species-dependent and -independent pathways: an in vitro and in vivo study

Sinomenine, the main alkaloid extracted from the medicinal plant Sinomenium acutum, is known for its anti-inflammatory effects. Recent studies have suggested its anti-cancer effect in synovial sarcoma, lung cancer and hepatic cancer. However, the underlying molecular mechanism for its anti-cancer effect still remains unclear. This study investigated the anti-tumor activity of sinomenine hydrochloride (SH), a hydrochloride form of sinomenine, in human breast cancer cells in vitro and in vivo. We found that SH potently inhibited cell viability of a broad panel of breast cancer cell lines. Two representative breast cancer cell lines, namely ER(-)/PR(-) MDA-MB-231 and ER(+)/PR(+) MCF-7, were used for further investigation. The results showed that SH induced G1/S cell cycle arrest, caused apoptosis and induced ATM/Chk2- and ATR/Chk1-mediated DNA-damage response in MDA-MB-231 and MCF-7. The anti-cancer effect of SH was regulated by increased expression levels of p-ERK, p-JNK and p-38 MAPK. Further studies showed that SH resulted in an increase in reactive oxygen species (ROS) and inhibition of ROS by N-acetyl-L-cysteine (NAC) almost blocked SH-induced DNA damage but only mitigated SH-induced MAPK expression changes, suggesting that both ROS-dependent and -independent pathways were involved in MAPK-mediated SH-induced breast cancer cell death. The in vivo study demonstrated that SH effectively inhibited tumor growth without showing significant toxicity. In conclusion, SH induced breast cancer cell death through ROS-dependent and -independent pathways with an upregulation of MAPKs, indicating that SH may be a potential anti-tumor drug for breast cancer treatment.

MEK1/2 overactivation can promote growth arrest by mediating ERK1/2-dependent phosphorylation of p70S6K

The extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein (MAP) kinase pathway has been involved in the positive and negative regulation of cell proliferation. Upon mitogen stimulation, ERK1/ERK2 activation is necessary for G1- to S-phase progression whereas when hyperactived, this pathway could elicit cell cycle arrest. The mechanisms involved are not fully elucidated but a kinase-independent function of ERK1/2 has been evidenced in the MAPK-induced growth arrest. Here, we show that p70S6K, a central regulator of protein biosynthesis, is essential for the cell cycle arrest induced by overactivation of ERK1/2. Indeed, whereas MEK1 silencing inhibits cell cycle progression, we demonstrate that active mutant form of MEK1 or MEK2 triggers a G1 phase arrest by stimulating an activation of p70S6K by ERK1/2 kinases. Silencing of ERK1/2 activity by shRNA efficiently suppresses p70S6K phosphorylation on Thr421/Ser424 and S6 phosphorylation on Ser240/244 as well as p21 expression, but these effects can be partially reversed by the expression of kinase-dead mutant form of ERK1 or ERK2. In addition, we demonstrate that the kinase p70S6K modulates neither the p21 gene transcription nor the stability of the protein but enhances the translation of the p21 mRNA. In conclusion, our data emphasizes the importance of the translational regulation of p21 by the MEK1/2-ERK1/2-p70S6K pathway to negatively control the cell cycle progression.

An integrated approach for the identification of USF1-centered transcriptional regulatory networks during liver regeneration

Liver regeneration after partial hepatectomy (PH) is a synchronized process that is precisely controlled by system-wide transcriptional regulatory networks. To clarify the transcriptional changes and regulatory networks that involve transcription factors (TFs) and their target genes during the priming phase, an advanced mouse oligonucleotide array-based transcription factor assay (MOUSE OATFA), mRNA microarray analysis, bioinformatic analysis and ChIP-on-chip experiments were used. A total of 774 genes were upregulated or downregulated in PH liver samples compared with the sham operation (SH) group. Seventeen TFs showed significant changes in activity in the regenerating livers, some of which have not been extensively studied in previous reports, including upstream stimulatory transcription factor 1 (USF1). The TF signatures from MOUSE OATFA were combined with mRNA expression profiles and ChIP-on-chip analyses to construct experimental transcriptional regulatory networks in regenerating livers. USF1-centered regulatory networks were further confirmed by ChIP assays, revealing some of its target genes and novel coregulatory networks. The combination of MOUSE OATFA with transcriptome profiling and bioinformatic analysis represents a novel paradigm for the comprehensive prediction of transcriptional coregulatory networks during the early phase of liver regeneration.

A role for interleukin-1 alpha in the 1,25 dihydroxyvitamin D3 response in mammary epithelial cells

Breast cancer is the most common non-cutaneous malignancy in American women, and better preventative strategies are needed. Epidemiological and laboratory studies point to vitamin D3 as a promising chemopreventative agent for breast cancer. Vitamin D3 metabolites induce anti-proliferative effects in breast cancer cells in vitro and in vivo, but few studies have investigated their effects in normal mammary epithelial cells. We hypothesized that 1,25(OH)2D3, the metabolically active form of vitamin D3, is growth suppressive in normal mouse mammary epithelial cells. In addition, we have previously established a role for the cytokine interleukin-1 alpha (IL1α) in the anti-proliferative effects of 1,25(OH)2D3 in normal prostate cells, and so we hypothesized that IL1α is involved in the 1,25(OH)2D3 response in mammary cells. Evaluation of cell viability, clonogenicity, senescence, and induction of cell cycle regulators p21 and p27 supported an anti-proliferative role for 1,25(OH)2D3 in mammary epithelial cells. Furthermore, 1,25(OH)2D3 increased the intracellular expression of IL1α, which was necessary for the anti-proliferative effects of 1,25(OH)2D3 in mammary cells. Together, these findings support the chemopreventative potential of vitamin D3 in the mammary gland and present a role for IL1α in regulation of mammary cell proliferation by 1,25(OH)2D3.

Induction of p53-dependent p21 limits proliferative activity of rat hepatocytes in the presence of hepatocyte growth factor

Background

Hepatocyte growth factor (HGF), a potent mitogen for hepatocytes, enhances hepatocyte function without stimulating proliferation, depending on the physiological conditions. p53, a transcription factor, suppresses the cell proliferation by expressing p21^WAF1/CIP1 in various tissues.

Aim

To investigate the mechanism through which the hepatocytes maintain mitotically quiescent even in the presence of HGF.

Methods

We studied the relationship between p53 and p21 expression and the effect of p53-p21 axis on hepatocyte proliferation in primary cultured rat hepatocytes stimulated by HGF. Hepatic p21 levels are determined serially after partial hepatectomy or sham operation in rats.

Results

DNA synthesis was markedly increased by HGF addition in rat hepatocytes cultured at low density but not at high density. Cellular p53 levels increased in the hepatocytes cultured at both the densities. p21 levels were increased and correlated with cellular p53 levels in hepatocytes cultured at high density but not at low density. When the activity of p53 was suppressed by a chemical inhibitor for p53, cellular p21 levels were reduced, and DNA synthesis was increased. Similarly, p21 antisense oligonucleotide increased the DNA synthesis. In rats after partial hepatectomy, transient elevation of hepatic p21 levels was observed. In contrast, in sham-operated rats, hepatic p21 levels were increased on sustained time scales.

Conclusion

p53-related induction of p21 may suppress hepatocyte proliferation in the presence of HGF in the setting that mitogenic activity of HGF is not elicitable.

Beta-naphthoflavone (DB06732) mediates estrogen receptor-positive breast cancer cell cycle arrest through AhR-dependent regulation of PI3K/AKT and MAPK/ERK signaling

Beta-naphthoflavone (BNF, DB06732) is an agonist of aryl hydrocarbon receptor (AhR) and a putative chemotherapeutic agent that has antitumor activity against mammary carcinomas in vivo. However, the mechanism by which BNF exerts this antitumor effect remains unclear. Thus, we explored mechanisms of BNF's antitumor effects in human breast cancer cells. This study showed that BNF suppressed cell proliferation and induced cell cycle arrest in the G0/G1 phase with downregulation of cyclin D1/D3 and CDK4 and upregulation of p21(Cip1/Waf1), leading to a senescence-like phenotype in estrogen receptor (ER)-positive MCF-7 cells, but not in ER-negative MDA-MB-231 cells. In addition, BNF inhibited PI3K/AKT signaling, and the PI3K inhibitor, LY294,002, exhibited the same inhibitory effects on cyclinD1/D3, CDK4 and the cell cycle as BNF. Interestingly, BNF activated mitogen-activated protein kinase-extracellular signal-regulated kinase (MAPK-ERK) signaling, and more notably, MEK inhibitor PD98059 significantly blocked the BNF-induced cell cycle arrest and upregulation of p21(Cip1/Waf1). Furthermore, specific ERα and AhR siRNA studies indicate that ERα is required in BNF-induced p21(Cip1/Waf1) expression, and BNF-mediated cell cycle arrest and modulation of AKT and ERK signaling is AhR-dependent. Taken together, AhR-dependent inhibition of the PI3K/AKT pathway, activation of MAPK/ERK and modulation of ERα is a novel mechanism underlying BNF-mediated antitumor effects in breast cancer, which may represent a promising strategy to be exploited in future clinical trials.

Macrophage-specific transgenic expression of cholesteryl ester hydrolase attenuates hepatic lipid accumulation and also improves glucose tolerance in ob/ob mice

Cellular cholesterol homeostasis is increasingly being recognized as an important determinant of the inflammatory status of macrophages, and a decrease in cellular cholesterol levels polarizes macrophages toward an anti-inflammatory or M2 phenotype. Cholesteryl ester hydrolase (CEH) catalyzes the hydrolysis of stored intracellular cholesteryl esters (CE) and thereby enhances free cholesterol efflux and reduces cellular CE content. We have reported earlier reduced atherosclerosis as well as lesion necrosis and improved insulin sensitivity (due to decreased adipose tissue inflammation) in macrophage-specific CEH transgenic (CEHTg) mice in the LDLR(-/-) background. In the present study, we examined the effects of reduced intracellular accumulation of CE in CEHTg macrophages in an established diabetic mouse model, namely the leptin-deficient ob/ob mouse. Macrophage-specific transgenic expression of CEH improved glucose tolerance in ob/ob-CEHTg mice significantly compared with ob/ob nontransgenic littermates, but with no apparent change in macrophage infiltration into the adipose tissue. However, there was a significant decrease in hepatic lipid accumulation in ob/ob-CEHTg mice. Consistently, decreased [(14)C]acetate incorporation into total lipids and triglycerides was noted in precision-cut liver slices from ob/ob-CEHTg mice. In the primary hepatocyte-macrophage coculture system, macrophages from CEHTg mice significantly reduced the incorporation of [(14)C]acetate into triglycerides in hepatocytes, indicating a direct effect of macrophages on hepatocyte triglyceride biosynthesis. Kupffer cells isolated from ob/ob-CEHTg mice were polarized toward an anti-inflammatory M2 (Ly6C(lo)) phenotype. Taken together, these studies demonstrate that transgenic overexpression of CEH in macrophages polarizes hepatic macrophages (Kupffer cells) to an anti-inflammatory M2 phenotype that attenuates hepatic lipid synthesis and accumulation.

[Biological functions of the duplicated GGAA-motifs in various human promoter regions]

Transcription is one of the most fundamental cellular functions and is an enzyme-complex mediated reaction that converts DNA sequences into mRNA. TATA-box is known to be an important motif for transcription. However, there are majority of promoters that have no TATA-box. They are called as TATA-less promoters and possess other elements that determine the transcription start site (TSS) of the genes. Multiple protein factors including ETS family proteins are known to recognize and bind to the GGAA containing sequences. In addition, it has been reported that the ETS binding motifs play important roles in regulation of various promoters. Here, we propose that the duplication and multiplication of the GGAA motifs are responsible for the initiation of transcription from TATA-less promoters.

LIP expression is regulated by IGF-1R signaling and participates in suppression of anoikis

Background

The transcription factor, CCAAT enhancer binding protein-β (C/EBPβ), is expressed as several distinct protein isoforms (LAP1, LAP2 and LIP) that have opposing actions in cellular proliferation and differentiation. Increases in the ratio of LIP/LAP are associated with aggressive, metastatic breast cancer; however, little is known regarding the molecular mechanisms that regulate LIP expression or the biological actions of an increase in the LIP/LAP ratio. Metastasis is highly dependent upon the suppression of anoikis and the role of C/EBPβ and LIP in this anchorage-independent, survival process is currently not known in mammary epithelial cells. IGF-1R signaling is important for the survival of breast cancer cells and crosstalk between IGF-1R and EGFR signaling pathways have been implicated in the development of more aggressive disease. We therefore evaluated in mammary epithelial cells whether IGF-1R signaling regulates the LIP/LAP ratio, analyzed the potential interplay between EGFR and IGF-1R signaling and addressed the biological significance of increased LIP expression in cellular survival and suppression of anoikis.

Results

Our data provide the first evidence that IGF-1R signaling regulates LIP expression in an EGFR independent manner to increase the LIP/LAP ratio in mammary epithelial cells. Although crosstalk between IGF-1R signaling and EGFR signaling is detectable in MCF10A cells, this crosstalk is not required for the IGF-1 mediated regulation of LIP expression. Rather, the critical regulator of IGF-1 induced LIP expression appears to be EGFR-independent, Akt activity. Our data also demonstrate that increases in LIP expression promote cell survival via suppression of anoikis. Likewise, knockdown of total C/EBPβ leads to increased cell death and suggest that C/EBPβ expression is important for survival and resistance to anoikis. IGF-1 treatment can partially rescue vector control cells from anoikis; however, cells with reduced C/EBPβ expression do not survive anoikis.

Conclusions

Taken together, our data demonstrate that IGF-1R signaling regulates LIP expression in an EGFR independent manner to increase the LIP/LAP ratio in mammary epithelial cells. C/EBPβ expression and elevations in LIP play an important role in regulating cellular survival via suppression of anoikis, in an IGF-1R mediated context or in a manner independent of IGF-1R signaling.

Hierarchies of transcriptional regulation during liver regeneration

The remarkable capacity of the liver to regenerate after severe injury or disease has excited interest for centuries. The goal of harnessing this process in treatment of liver disease, and the appreciation of the parallels between regeneration and tumor development in the liver, remain a major driver for research in this area. Studies of liver regeneration as a model system offer a view of intricate and precisely timed regulatory pathways that drive the process toward completion. Successful regeneration of the liver mass demands a hierarchal and well-controlled balance between proliferative and metabolic functions, which is orchestrated by signaling and regulation of transcription factors. Control and regulation of these cascades of transcriptional activities, necessary for induction, renewal, and cessation of liver growth, are the focus of this chapter.

The ETS family transcription factor ELK-1 regulates induction of the cell cycle-regulatory gene p21(Waf1/Cip1) and the BAX gene in sodium arsenite-exposed human keratinocyte HaCaT cells

Cyclin-dependent kinase inhibitor (CDKN1A), often referred to as p21(Waf1/Cip1) (p21), is induced by a variety of environmental stresses. Transcription factor ELK-1 is a member of the ETS oncogene superfamily. Here, we show that ELK-1 directly trans-activates the p21 gene, independently of p53 and EGR-1, in sodium arsenite (NaASO(2))-exposed HaCaT cells. Promoter deletion analysis and site-directed mutagenesis identified the presence of an ELK-1-binding core motif between -190 and -170 bp of the p21 promoter that confers inducibility by NaASO(2). Chromatin immunoprecipitation and electrophoretic mobility shift analyses confirmed the specific binding of ELK-1 to its putative binding sequence within the p21 promoter. In addition, NaASO(2)-induced p21 promoter activity was enhanced by exogenous expression of ELK-1 and reduced by expression of siRNA targeted to ELK-1 mRNA. The importance of ELK-1 in response to NaASO(2) was further confirmed by the observation that stable expression of ELK-1 siRNA in HaCaT cells resulted in the attenuation of NaASO(2)-induced p21 expression. Although ELK-1 was activated by ERK, JNK, and p38 MAPK in response to NaASO(2), ELK-1-mediated activation of the p21 promoter was largely dependent on ERK. In addition, EGR-1 induced by ELK-1 seemed to be involved in NaASO(2)-induced expression of BAX. This supports the view that the ERK/ELK-1 cascade is involved in p53-independent induction of p21 and BAX gene expression.

The possible functions of duplicated ets (GGAA) motifs located near transcription start sites of various human genes

Transcription is one of the most fundamental nuclear functions and is an enzyme complex-mediated reaction that converts DNA sequences into mRNA. Analyzing DNA sequences of 5′-flanking regions of several human genes that respond to 12-O-tetradecanoyl-phorbol-13-acetate (TPA) in HL-60 cells, we have identified that the ets (GGAA) motifs are duplicated, overlapped, or clustered within a 500-bp distance from the most 5′-upstream region of the cDNA. Multiple protein factors including Ets family proteins are known to recognize and bind to the GGAA containing sequences. In addition, it has been reported that the ets motifs play important roles in regulation of various promoters. Here, we propose a molecular mechanism, defined by the presence of duplication and multiplication of the GGAA motifs, that is responsible for the initiation of transcription of several genes and for the recruitment of binding proteins to the transcription start site (TSS) of TATA-less promoters.

Sorafenib activates CD95 and promotes autophagy and cell death via Src family kinases in gastrointestinal tumor cells

Sorafenib and vorinostat interact in a synergistic fashion to kill carcinoma cells by activating CD95; the present studies have determined how sorafenib and vorinostat individually contribute to CD95 activation. Sorafenib (3-6 micromol/L) promoted a dose-dependent increase in Src Y416, ERBB1 Y845 and CD95 Y232/Y291 phosphorylation, and Src Y527 dephosphorylation. Low levels of sorafenib-induced (3 micromol/L) CD95 tyrosine phosphorylation did not promote surface localization whereas sorafenib (6 micromol/L), or sorafenib (3 micromol/L) and vorinostat (500 nmol/L) treatment promoted higher levels of CD95 phosphorylation which correlated with DISC formation, receptor surface localization, and autophagy. CD95 (Y232F, Y291F) was not tyrosine phosphorylated and was unable to localize plasma membrane or induce autophagy. Knockdown/knockout of Src family kinases abolished sorafenib-induced CD95 tyrosine phosphorylation, DISC formation, and the induction of cell death and autophagy. Knockdown of platelet-ived growth factor receptor-beta enhanced Src Y416 and CD95 tyrosine phosphorylation, which correlated with elevated CD95 plasma membrane levels and autophagy, and with a reduced ability of sorafenib to promote CD95 membrane localization. Vorinostat increased reactive oxygen species levels, and in a delayed NF kappa B-dependent fashion, those of FAS ligand and CD95. Neutralization of FAS-L did not alter the initial rapid drug-induced activation of CD95; however, neutralization of FAS-L reduced sorafenib + vorinostat toxicity by approximately 50%. Thus, sorafenib contributes to CD95 activation by promoting receptor tyrosine phosphorylation, whereas vorinostat contributes to CD95 activation via the initial facilitation of reactive oxygen species generation and subsequently of FAS-L expression.

Tumour angiogenesis is reduced in the Tc1 mouse model of Down's syndrome

Down's syndrome (DS) is a genetic disorder caused by full or partial trisomy of human chromosome 21 and presents with many clinical phenotypes including a reduced incidence of solid tumours. Recent work with the Ts65Dn model of DS, which has orthologues of about 50% of the genes on chromosome 21 (Hsa21), has indicated that three copies of the ETS2 (ref. 3) or DS candidate region 1 (DSCR1) genes (a previously known suppressor of angiogenesis) is sufficient to inhibit tumour growth. Here we use the Tc1 transchromosomic mouse model of DS to dissect the contribution of extra copies of genes on Hsa21 to tumour angiogenesis. This mouse expresses roughly 81% of Hsa21 genes but not the human DSCR1 region. We transplanted B16F0 and Lewis lung carcinoma tumour cells into Tc1 mice and showed that growth of these tumours was substantially reduced compared with wild-type littermate controls. Furthermore, tumour angiogenesis was significantly repressed in Tc1 mice. In particular, in vitro and in vivo angiogenic responses to vascular endothelial growth factor (VEGF) were inhibited. Examination of the genes on the segment of Hsa21 in Tc1 mice identified putative anti-angiogenic genes (ADAMTS1and ERG) and novel endothelial cell-specific genes, never previously shown to be involved in angiogenesis (JAM-B and PTTG1IP), that, when overexpressed, are responsible for inhibiting angiogenic responses to VEGF. Three copies of these genes within the stromal compartment reduced tumour angiogenesis, explaining the reduced tumour growth in DS. Furthermore, we expect that, in addition to the candidate genes that we show to be involved in the repression of angiogenesis, the Tc1 mouse model of DS will permit the identification of other endothelium-specific anti-angiogenic targets relevant to a broad spectrum of cancer patients.

Intracellular chloride regulates cell proliferation through the activation of stress-activated protein kinases in MKN28 human gastric cancer cells

Recently, we reported that reduction of intracellular Cl(-) concentration ([Cl(-)](i)) inhibited proliferation of MKN28 gastric cancer cells by diminishing the transition rate from G(1) to S cell-cycle phase through upregulation of p21, cyclin-dependent kinase inhibitor, in a p53-independent manner. However, it is still unknown how intracellular Cl(-) regulates p21 expression level. In this study, we demonstrate that mitogen-activated protein kinases (MAPKs) are involved in the p21 upregulation and cell-cycle arrest induced by reduction of [Cl(-)](i). Culture of MKN28 cells in a low Cl(-) medium significantly induced phosphorylation (activation) of MAPKs (ERK, p38, and JNK) and G(1)/S cell-cycle arrest. To clarify the involvement of MAPKs in p21 upregulation and cell growth inhibition in the low Cl(-) medium, we studied effects of specific MAPKs inhibitors on p21 upregulation and G(1)/S cell-cycle arrest in MKN28 cells. Treatment with an inhibitor of p38 or JNK significantly suppressed p21 upregulation caused by culture in a low Cl(-) medium and rescued MKN28 cells from the low Cl(-)-induced G(1) cell-cycle arrest, whereas treatment with an ERK inhibitor had no significant effect on p21 expression or the growth of MKN28 cells in the low Cl(-) medium. These results strongly suggest that the intracellular Cl(-) affects the cell proliferation via activation of p38 and/or JNK cascades through upregulation of the cyclin-dependent kinase inhibitor (p21) in a p53-independent manner in MKN28 cells.

Cascades of transcription regulation during liver regeneration

An increasing demand for new strategies in cancer prevention and regenerative medicine requires a better understanding of molecular mechanisms that control cell proliferation in tissue-specific manner. Regenerating liver is a unique model allowing use of biochemical, genetic, and engineering tools to uncover molecular mechanisms and improve treatment of hepatic cancers, liver failure, and fibrotic disease. Molecular mechanisms of liver regeneration involve extra- and intracellular factors to activate transcription of genes normally silenced in quiescent liver. While many upstream signaling pathways of the regenerating liver have been extensively studied, our knowledge of the downstream effectors, transcription factors (TFs), remains limited. This review describes consecutive engagement of pre-existing and de novo synthesized TFs, as cascades that regulate expression of growth-related and metabolic genes during liver regeneration after partial hepatectomy in mice. Several previously recognized regulators of regenerating liver are described in the light of recently identified co-activator and co-repressor complexes that interact with primary DNA-binding TFs. Published results of gene expression and chromatin immunoprecipitation analyses, as well as studies of transgenic mouse models, are used to emphasize new potential regulators of transcription during liver regeneration. Finally, a more detailed description of newly identified transcriptional regulators of liver regeneration illustrates the tightly regulated balance of proliferative and metabolic responses to partial hepatectomy.

PDEF is a negative regulator of colon cancer cell growth and migration

ETS is a family of transcriptional regulators with functions in most biological processes. Dysregulated ETS factor function leads to altered expression of multiple genes that play critical roles in many of the processes required for cancer progression. While the Ets family gene, prostate-derived ETS factor (PDEF), is expressed in epithelial tissues including prostate, breast, and colon, PDEF protein expression has been found to be reduced or lost during prostate and breast cancer progression. The goal of this study was to examine the expression and biologic impact of altered PDEF expression in colon cancer. PDEF mRNA and protein are not detectable in several colon-cancer-derived cell lines. Re-expression of PDEF in colon cancer cells inhibits growth and migration. Growth affects are due to altered cellular proliferation, indicated by increased altered cell population in G(1) and S phases of the cell cycle, as well as increased apoptosis. Relevant to its modulation of growth and migration phenotypes, PDEF expression resulted in altered expression of genes with established roles in cell cycle, motility, and invasion. Furthermore, chromatin immunoprecipitation studies show that p21 and urokinase plasminogen activator (uPA) are direct PDEF transcriptional targets. While non-tumor colon epithelium expresses PDEF mRNA and protein, the majority of tumors showed decreased mRNA and/or protein expression. In human tumor tissue samples, PDEF expression was inversely correlated with the expression levels of uPA. Collectively, the data support the model that PDEF is a negative regulator of tumor progression by modulating the expression of growth and migration promoting genes.

Control of adipogenesis by the SUMO-specific protease SENP2

Here, we demonstrate that SENP2, a desumoylating enzyme, plays a critical role in the control of adipogenesis. SENP2 expression was markedly increased upon the induction of adipocyte differentiation, and this increase was dependent on protein kinase A activation. Remarkably, knockdown of SENP2 led to a dramatic attenuation of adipogenesis with a marked decrease in PPARgamma and C/EBPalpha mRNA levels. Knockdown of SENP2 also caused a marked reduction in the level of C/EBPbeta protein but not in that of C/EBPbeta mRNA. Interestingly, sumoylation of C/EBPbeta dramatically increased its ubiquitination and destabilization, and this increase could be reversed by SENP2. In addition, overexpression of C/EBPbeta could overcome the inhibitory effect of SENP2 knockdown on adipogenesis. Furthermore, SENP2 was absolutely required for adipogenesis of preadipocytes implanted into mice. These results establish a critical role for SENP2 in the regulation of adipogenesis by desumoylation and stabilization of C/EBPbeta and in turn by promoting the expression of its downstream effectors, such as PPARgamma and C/EBPalpha.

Multiple division cycles and long-term survival of hepatocytes are distinctly regulated by extracellular signal-regulated kinases ERK1 and ERK2

We investigated the specific role of the mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase 1 (ERK1)/ERK2 pathway in the regulation of multiple cell cycles and long-term survival of normal hepatocytes. An early and sustained epidermal growth factor (EGF)-dependent MAPK activation greatly improved the potential of cell proliferation. In this condition, almost 100% of the hepatocytes proliferated, and targeting ERK1 or ERK2 via RNA interference revealed the specific involvement of ERK2 in this regulation. However, once their first cell cycle was performed, hepatocytes failed to undergo a second round of replication and stayed blocked in G1 phase. We demonstrated that sustained EGF-dependent activation of the MAPK/ERK kinase (MEK)/ERK pathway was involved in this blockage as specific transient inhibition of the cascade repotentiated hepatocytes to perform a new wave of replication and multiple cell cycles. We identified this mechanism by showing that this blockage was in part supported by ERK2-dependent p21 expression. Moreover, continuous MEK inhibition was associated with a lower apoptotic engagement, leading to an improvement of survival up to 3 weeks. Using RNA interference and ERK1 knockout mice, we extended these results by showing that this improved survival was due to the specific inhibition of ERK1 expression/phosphorylation and did not involve ERK2.

CONCLUSION

Our results emphasize that transient MAPK inhibition allows multiple cell cycles in primary cultures of hepatocytes and that ERK2 has a key role in the regulation of S phase entry. Moreover, we revealed a major and distinct role of ERK1 in the regulation of hepatocyte survival. Taken together, our results represent an important advance in understanding long-term survival and cell cycle regulation of hepatocytes.

Artemisinin selectively decreases functional levels of estrogen receptor-alpha and ablates estrogen-induced proliferation in human breast cancer cells

MCF7 cells are an estrogen-responsive human breast cancer cell line that expresses both estrogen receptor (ER) alpha and ERbeta. Treatment of MCF7 cells with artemisinin, an antimalarial phytochemical from the sweet wormwood plant, effectively blocked estrogen-stimulated cell cycle progression induced by either 17beta-estradiol (E(2)), an agonist for both ERs, or by propyl pyrazole triol (PPT), a selective ERalpha agonist. Artemisinin strongly downregulated ERalpha protein and transcripts without altering expression or activity of ERbeta. Transfection of MCF7 cells with ERalpha promoter-linked luciferase reporter plasmids revealed that the artemisinin downregulation of ERalpha promoter activity accounted for the loss of ERalpha expression. Artemisinin treatment ablated the estrogenic induction of endogenous progesterone receptor (PR) transcripts by either E(2) or PPT and inhibited the estrogenic stimulation of a luciferase reporter plasmid driven by consensus estrogen response elements (EREs). Chromatin immunoprecipitation assays revealed that artemisinin significantly downregulated the level of endogeneous ERalpha bound to the PR promoter, whereas the level of bound endogeneous ERbeta was not altered. Treatment of MCF7 cells with artemisinin and the pure antiestrogen fulvestrant resulted in a cooperative reduction of ERalpha protein levels and enhanced G(1) cell cycle arrest compared with the effects of either compound alone. Our results show that artemisinin switches proliferative human breast cancer cells from expressing a high ERalpha:ERbeta ratio to a condition in which ERbeta predominates, which parallels the physiological state linked to antiproliferative events in normal mammary epithelium.

Nuclear factor-1 and metal transcription factor-1 synergistically activate the mouse metallothionein-1 gene in response to metal ions

Metal activation of metallothionein (MT) gene transcription is dependent on the presence of metal regulatory elements (MREs), which are present in five non-identical copies (MREa through MREe) in the promoter of the mouse MT-1 gene and on the capacity of metal transcription factor-1 (MTF-1) to bind to the MREs in the presence of zinc. We detected a protein, distinct from MTF-1, specifically binding to the MREc region. DNA binding competition experiments using synthetic oligonucleotides and specific anti-NF1 antibodies showed that this protein binds to an NF1 site overlapping the MREc element as well as to a second site upstream of the Sp1a site and corresponds to NF1 or a related protein. Transfection experiments showed that loss of the two NF1 sites decreased metal-induced MT promoter activity by 55-70% in transiently transfected cells and almost completely abrogated metal and tert-butylhydroquinone (tBHQ) induction in stably transfected cells. Similarly, expression of an inactive NF1 protein strongly inhibited MT-1 promoter activity. Using synthetic promoters containing NF1 and MRE sites fused to a minimal MT promoter, we showed that these NF1 sites did not confer metal induction but enhanced metal-induced promoter activity. Chromatin immunoprecipitation assays confirmed that NF1 binds to the mouse MT-1 promoter in vivo and showed that NF1 binding is zinc-inducible. In addition, zinc-induced NF1 DNA binding was MTF-1-dependent. Taken together, these studies show that NF1 acts synergistically with MTF-1 to activate the mouse MT-1 promoter in response to metal ions and tert-butylhydroquinone and contributes to maximal activation of the gene.

Delayed and persistent ERK1/2 activation is required for 4-hydroxytamoxifen-induced cell death

Tamoxifen (Tam), and its active metabolite, 4-hydroxytamoxifen (OHT), compete with estrogens for binding to the estrogen receptor (ER). Tam and OHT can also induce ER-dependent apoptosis of cancer cells. 10-100nM OHT induces ER-dependent apoptosis in approximately 3 days. Using HeLaER6 cells, we examined the role of OHT activation of signal transduction pathways in OHT-ER-mediated apoptosis. OHT-ER activated the p38, JNK and ERK1/2 pathways. Inhibition of p38 activation with SB203580, or RNAi-knockdown of p38alpha, moderately reduced OHT-ER mediated cell death. A JNK inhibitor partly reduced cell death. Surprisingly, the MEK1/2 inhibitor, PD98059, completely blocked OHT-ER induced apoptosis. EGF, an ERK1/2 activator, enhanced OHT-induced apoptosis. OHT induced a delayed and persistent phosphorylation of ERK1/2 that persisted for >80h. Addition of PD98059 as late as 24h after OHT largely blocked OHT-ER mediated apoptosis. The antagonist, ICI 182,780, blocked both the long-term OHT-mediated phosphorylation of ERK1/2 and OHT-induced apoptosis. Our data suggests that the p38 and JNK pathways, which often play a central role in apoptosis, have only a limited role in OHT-ER-mediated cell death. Although rapid activation of the ERK1/2 pathway is often associated with cell growth, persistent activation of the ERK1/2 pathway is essential for OHT-ER induced cell death.

p21 and p27 induction by silibinin is essential for its cell cycle arrest effect in prostate carcinoma cells

Recent studies have shown that silibinin induces p21/Cip1 and p27/Kip1 and G1 arrest in different prostate cancer cells irrespective of p53 status; however, biological significance and mechanism of such induction have not been studied. Here, using two different prostate cancer cell lines DU145 and 22Rv1, representing androgen-independent and androgen-dependent stages of malignancy, first we investigated the importance of p21 and p27 induction in silibinin-mediated G1 arrest. Silencing p21 and p27 individually by RNA interference showed marked reversal in G1 arrest; however, their simultaneous ablation showed additional reversal of G1 arrest in 22Rv1 but not DU145 cells. These results suggest that whereas relative importance of these molecules might be cell line specific, their induction by silibinin is essential for its G1 arrest effect. Next, studies were done to examine mechanisms of their induction where cycloheximide-chase experiments showed that silibinin increases p21 and p27 protein half-life. This effect was accompanied by strong reduction in Skp2 level and its binding with p21 and p27 together with strong decrease in phosphorylated Thr(187) p27 without considerable change in proteasomal activity, suggesting a posttranslational mechanism. Skp2 role was further elucidated using Skp2-small interfering RNA-transfected cells, where decreased G1 arrest and attenuated Cip/Kip induction were observed with silibinin treatment. Further, silibinin caused a marked increase in p21 and p27 mRNA levels together with an increase in their promoter activity, also indicating a transcriptional mechanism. Together, our results for the first time identify a central role of p21 and p27 induction and their regulatory mechanism in silibinin-mediated cell cycle arrest.

ERK implication in cell cycle regulation

The Ras/Raf/MEK/ERK signaling cascade that integrates an extreme variety of extracellular stimuli into key biological responses controlling cell proliferation, differentiation or death is one of the most studied intracellular pathways. Here we present some evidences that have been accumulated over the last 15 years proving the requirement of ERK in the control of cell proliferation. In this review we focus (i) on the spatio-temporal control of ERK signaling, (ii) on the key cellular components linking extracellular signals to the induction and activation of cell cycle events controlling G1 to S-phase transition and (iii) on the role of ERK in the growth factor-independent G2/M phase of the cell cycle. As ERK pathway is often co-activated with the PI3 kinase signaling, we highlight some of the key points of convergence leading to a full activation of mTOR via ERK and AKT synergies. Finally, ERK and AKT targets being constitutively activated in so many human cancers, we briefly touched the cure issue of using more specific drugs in rationally selected cancer patients.

ERK2 but not ERK1 plays a key role in hepatocyte replication: an RNAi-mediated ERK2 knockdown approach in wild-type and ERK1 null hepatocytes

The mitogen-activated protein kinases (MAPKs) ERK1 and ERK2 have been implicated in various physiological events, and specific targeting of these MAPKs could affect cell proliferation in many cell types. First, to evaluate the potential specific roles of these two MAPKs, we analyzed the mitogenic response in regenerating liver after partial hepatectomy (PH) and in primary culture of hepatocytes isolated from ERK1-deficient mice. We show that ERK1 knockout and wild-type (wt) cells replicate with the same kinetics after PH in liver, in vivo, and in primary cultures of hepatocytes, in vitro. Indeed, Cyclin D1 and Cdk1 appear to be expressed concomitantly in knockout and wt cells, highlighting that hepatocytes progress in the cell cycle independently of the presence of ERK1. Second, we specifically abolished ERK2 expression by RNA interference in mouse and rat hepatocytes. We investigated whether small interfering RNA (siRNA) targeting ERK2 could specifically inhibit its expression and interfere with the process of replication. In ERK1-deficient hepatocytes, silencing ERK2 expression by RNA interference and ERK2 activation by U0126 clearly demonstrate that DNA replication is regulated by an ERK2-dependent mechanism. Furthermore, in rat wt hepatocytes, whereas ERK2 targeting inhibits late G(1) and S phase progression, ERK1 silencing is devoid of any effect on cell proliferation, indicating that ERK1 cannot rescue ERK2 deficiency.

CONCLUSION

Our results emphasize the importance of the MAPK cascade in hepatocyte replication and allow us to conclude that ERK2 is the key form involved in this regulation, in vivo and in vitro.

Regulation of Smooth Muscle Cell Proliferation by β-Catenin/T-Cell Factor Signaling Involves Modulation of Cyclin D1 and p21 Expression

We previously observed that stimulation of vascular smooth muscle cell (VSMC) proliferation with growth factors is associated with dismantling of cadherin junctions and nuclear translocation of β-catenin. In this study we demonstrate directly that growth factors stimulate β-catenin/T-cell factor (TCF) signaling in primary VSMCs. To determine whether β-catenin/TCF signaling regulates VSMC proliferation via modulation of the β-catenin/TCF responsive cell cycle genes, cyclin D1 and p21, we inhibited β-catenin/TCF signaling by adenoviral-mediated over-expression of N-Cadherin, ICAT (an endogenous inhibitor of β-catenin/TCF signaling), or a dominant negative (dn) mutant of TCF-4. N-cadherin, ICAT or dnTCF-4 over-expression significantly reduced proliferation of isolated human VSMCs by approximately 55%, 80%, and 45% respectively. Similar effects were observed in human saphenous vein medial segments where proliferation was reduced by approximately 55%. Transfection of dnTCF-4 in the ISS10 human VSMC line significantly lowered TCF and cyclin D1 reporter activity but significantly elevated p21 reporter activity, indicating regulation of these genes by β-catenin/TCF signaling. In support of this, over-expression of N-cadherin, ICAT or dnTCF-4 in isolated human VSMCs significantly lowered levels of cyclin D1 mRNA and protein levels. In contrast, over-expression of N-Cadherin, ICAT or dnTCF4 significantly elevated p21 mRNA and protein levels. In summary, we have demonstrated that increasing N-cadherin and inhibiting β-catenin/TCF signaling reduces VSMC proliferation, decreases the expression of cyclin D1 and increases levels of the cell cycle inhibitor, p21. We therefore suggest that the N-cadherin and β-catenin/TCF signaling pathway is a key modulator of VSMC proliferation via regulation of these 2 β-catenin/TCF responsive genes.

Mechanisms of induction of cell cycle arrest and cell death by cryptolepine in human lung adenocarcinoma a549 cells

We investigated p53-dependent and -independent molecular events associated with cell cycle alteration and cell death in human lung adenocarcinoma A549 cells using cryptolepine, a DNA-damaging agent. After a 24-h treatment, cryptolepine caused an accumulation of p53 at concentrations of 1.25-10 microM and induction of p21(Cip1/WAF1) but only at concentrations up to 5muM. p21(Cip1/WAF1) was also strongly induced by cryptolepine (2.5-5 microM) in cells with p53 largely ablated via small interfering RNA-mediated gene silencing. Cryptolepine induced G1-phase block at 1.25-2.5 microM, S-phase and G2/M-phase block at 2.5-5 microM, and cell death at 10 microM. The dead cells displayed condensed and fragmented nuclei, features of apoptosis. Wortmannin, an inhibitor of ataxia telangiectasia-mutated and DNA-dependent protein kinase (DNA-PK), caused cell cycle arrest at G1 phase without inducing p53 and p21(Cip1/WAF1) expression and cell death. The addition of wortmannin partially prevented cryptolepine-induced expression of p53 and p21(Cip1/WAF1) together with the S-phase block and sensitized cells to induction of cell death. NU7026, a DNA-PK-specific inhibitor, showed neither induction of cell cycle arrest and apoptosis nor the expression of p53 and p21(Cip1/WAF1). The presence of NU7026 caused further reduction of cells in G1 phase induced by cryptolepine at 5 microM without affecting the induction of p53 and p21(Cip1/WAF1) and cell death. This study using the A549 cell as a model demonstrated that cryptolepine selects different molecular pathways to cell cycle checkpoint activation in a dose-specific manner and evokes a wortmannin-sensitive antiapoptosis response.

Ethanol reduces p38 kinase activation and cyclin D1 protein expression after partial hepatectomy in rats

BACKGROUND/AIMS

Chronic ethanol consumption inhibits liver regeneration. We examined the effects of chronic ethanol consumption on two mitogen-activated protein kinases in relation to induction of cell cycle proteins after partial hepatectomy (PH).

METHODS

Male Wistar rats were ethanol-fed (EF) or pair-fed (PF) for 16 weeks before PH. Hepatic activation of extracellular signal regulated kinase (ERK)1/2, p38 kinase and expression of cyclinD1, cyclin-dependent kinase-4 (cdk4) and proliferating cell nuclear antigen (PCNA) were studied.

RESULTS

In PF rats, PH-induced p38 activation was evident at 2h and was maximal at 12h. There was a close temporal relationship between p38 activation, cyclin D1 and PCNA expression. Alcohol exposure reduced p38 activation, cyclin D1 and PCNA, each by approximately 50%. ERK1/2 activation occurred during the first 2h post-PH in both EF and PF rats, and there was no later increase in PF rats. In vivo inhibition of p38 suppressed PCNA expression whereas the effect of ERK1/2 inhibition was inconsistent.

CONCLUSIONS

p38 kinase activation is linked temporally with cyclin D1 expression after PH and appears to exert cell cycle control in the adult liver. p38 signaling also appears to be a target for the inhibitory effect of chronic alcohol on liver regeneration.

Lipid synthetic transcription factor SREBP-1a activates p21WAF1/CIP1, a universal cyclin-dependent kinase inhibitor

Sterol regulatory element-binding proteins (SREBPs) are membrane-bound transcription factors that regulate lipid synthetic genes. In contrast to SREBP-2, which regulates cellular cholesterol level in normal cells, SREBP-1a is highly expressed in actively growing cells and activates entire programs of genes involved in lipid synthesis such as cholesterol, fatty acids, triglycerides, and phospholipids. Previously, the physiological relevance of this potent activity of SREBP-1a has been thought to regulate the supply of membrane lipids in response to cell growth. Here we show that nuclear SREBP-1a and SREBP-2 bind directly to a novel SREBP binding site in the promoter of the p21(WAF1/CIP1) gene, the major cyclin-dependent kinase inhibitor, and strongly activate its promoter activity. Only the SREBP-1a isoform consistently causes induction of p21 at both the mRNA and protein levels. Colony formation assays and polyploidy of livers from transgenic mice suggest that activation of p21 by SREBP-1a could inhibit cell growth. Activation of endogenous SREBPs in lipid deprivation conditions was associated with induction of p21 mRNA and protein. Expression of p21 was reduced in SREBP-1 null mice. These data suggest a physiological role of SREBP-1a in p21 regulation. Identification of p21 as a new SREBP target might implicate a new paradigm in the link between lipid synthesis and cell growth.

Ceramide- and ERK-dependent pathway for the activation of CCAAT/enhancer binding protein by interleukin-1beta in hepatocytes

Interleukin-1beta (IL-1beta) is a major inducer of liver acute-phase protein expression in response to infection. Several transcription factors, including CCAAT/enhancer binding protein (C/EBP), are known mediators in this process, although the mechanisms by which they modulate IL-1beta's action are not completely understood. Activation of sphingomyelinase (SMase) and the subsequent generation of ceramide are early steps in the IL-1beta signaling cascade. In this study, we investigate the role of ceramide in the IL-1beta regulation of C/EBP in primary hepatocytes. The C/EBP DNA binding activity was found to increase in a dose-dependent manner after stimulation with IL-1beta and exogenous addition of C2-ceramide or treatment with SMase. These changes were accompanied by an increase in the nuclear content of C/EBPbeta. Both IL-1beta and ceramide led to extracellular signal-regulated kinase 1/2 (ERK1/2) activation as early as 15 min after treatment. Furthermore, the increase of cellular ceramide content resulted in increased phosphorylation of C/EBPbeta at serine 105 at later time points. Concurrently, the cytosolic levels of C/EBPbeta decreased, suggesting that IL-1beta and ceramide induced nuclear translocation of C/EBPbeta. Ceramide-induced C/EBPbeta phosphorylation, translocation, and DNA binding were suppressed by the addition of PD98059, an inhibitor of ERK1/2 phosphorylation. These results suggest that ceramide and ERK mediate a pathway in the IL-1beta signaling cascade, which results in rapid posttranslational activation of C/EBPbeta.

CCAAT/Enhancer binding protein delta (c/EBPdelta) regulation and expression in human mammary epithelial cells: II. Analysis of activating signal transduction pathways, transcriptional, post-transcriptional, and post-translational control

CCAAT/enhancer binding protein delta (C/EBPdelta) plays a key role in mammary epithelial cell G0 growth arrest. C/EBPdelta gene expression is down-regulated in rodent mammary tumorigenesis and in human breast cancer, suggesting that "loss of function" alterations in C/EBPdelta gene expression are common in mammary gland malignancies. The goal of this study was to systematically investigate the mechanisms controlling C/EBPdelta gene expression in MCF-10A and MCF-12A human mammary epithelial cell lines. The results demonstrate that G0 growth arrest conditions (i.e., serum and growth factor withdrawal or Oncostatin M (OSM) treatment) result in the activation of JAK1, JAK2, and Tyk 2, members of the Janus kinase family of non-receptor tyrosine kinases, in MCF-10A and MCF-12A cells. Growth arrest or OSM treatment also specifically increases activated (phosphorylated) signal transduction and activators of transcription 3 (STAT3) levels, demonstrating that STAT3, not STAT1 or STAT5, is the downstream target of the activated Janus kinases in MCF-10A and MCF-12A cells. Whole cell lysates from G0 growth arrested (GA) and OSM-treated MCF-12A cells exhibit increased acute phase response element (APRE) binding compared to lysates from growing (GR) MCF-12A cells. Transient transfection using C/EBPdelta promoter-luciferase constructs demonstrated that the APRE (STAT3) consensus binding site is essential for growth arrest or OSM induction of the C/EBPdelta promoter. Mutation of the C/EBPdelta promoter STAT3 site or expression of a dominant negative STAT3 construct (STAT3delta) reduces C/EBPdelta promoter activity in response to growth arrest conditions. The human C/EBPdelta promoter also contains an Sp1 site at -61 bp (relative to the transcriptional start site) which is required for basal transcriptional activation. Mutation or deletion of the Sp1 site decreases promoter activity in response to growth arrest conditions. Treatment with the transcriptional inhibitor actinomycin D demonstrated that the C/EBPdelta mRNA exhibits a relatively short half-life (approximately 40 min). Similarly, treatment with the translational inhibitor anisomysin demonstrated that the C/EBPdelta protein half-life was also relatively short (approximately 160 min). These results indicate that the human C/EBPdelta gene is controlled at multiple levels, consistent with a role for C/EBPdelta in cell cycle control and/or cell fate determination.

Characterization of Cdk9(55) and differential regulation of two Cdk9 isoforms

Positive transcription elongation factor b (P-TEFb) controls the fraction of initiated RNA polymerase II molecules that make full length transcripts. This important factor is a heterodimer of cyclin-dependent kinase 9 (Cdk9) and one of four cyclin partners, cyclin T1, T2a, T2b or K. There are two isoforms of Cdk9 in mammalian cells, Cdk9(42) and Cdk9(55). Cdk9(55) has a 117 residue amino terminal extension not present in Cdk9(42). An expression vector with a tetracycline-responsive promoter driving FLAG-tagged Cdk9(55) and a HeLa 37 Tet-Off cell line were constructed. FLAG-tagged Cdk9(55) was inducibly expressed and was found to be localized to the nucleus by immunofluorescence. Western analysis of murine tissues showed that the relative abundance of the two forms of Cdk9 varied across different tissues with liver having more Cdk9(55) than Cdk9(42). During adaptation of primary rat hepatocytes to culture the ratio of the two forms of Cdk9 changed. Initially, Cdk9(55) was the predominate form, but as the cells began to enter the cell cycle Cdk9(42) became the major form. During this change, expression of Cdk9(42) was induced, while Cdk9(55) remained relatively constant.

Up-regulation of p21 gene expression by peroxisome proliferator-activated receptor gamma in human lung carcinoma cells

PURPOSE

The peroxisome proliferator-activated receptor gamma (PPARgamma), a ligand-dependent transcription factor belonging to the family of nuclear receptors, has been implicated in the regulation of cell growth and differentiation although the exact mechanism(s) of this activity has not been elucidated. In this study, we explored the role of PPARgamma signaling on the control of gene expression of the cycle-dependent kinase inhibitor p21 in human lung carcinoma cells.

EXPERIMENTAL DESIGN

Using several human lung carcinoma cell lines (small and non-small carcinoma cells), we assayed for cell growth inhibition and apoptosis induction. We also assayed for p21 mRNA and protein expression by reverse transcription-PCR, real-time reverse transcription-PCR, and Western blot analysis. Nuclear protein binding activities to three response elements located in the p21 promoter [nuclear factor (NF)-kappaB, Sp1, and NF-interleukin 6 (IL6) CAAT/enhancer binding protein (C/EBP)] were measured by gel mobility shift assays. We used transient transfection assays with p21 promoter reporter gene constructs to determine the transcriptional regulation by PPARgamma ligands. Finally, by using p21 antisense oligonucleotides, we tested the link between PPARgamma activation and p21 signaling in cell growth inhibition assays and by Western blot analysis.

RESULTS

We showed that the PPARgamma ligands PGJ2 and ciglitazone inhibit the growth and induce the apoptosis of several human lung carcinoma cell lines, whereas the PPARalpha agonist WY14643 has little effect. Treatment of lung carcinoma cells with the PPARgamma ligands PGJ2, ciglitazone, troglizaone, and GW1929 elevated p21 mRNA and protein levels and reduced cyclin D1 mRNA levels. These results were supported by transient transfection assays, which indicated that PPARgamma ligands increased p21 gene promoter activity in human lung carcinoma cells. In addition, p21 antisense oligonucleotides inhibited PPARgamma ligand-induced p21 protein expression and significantly blocked lung carcinoma cell growth inhibition induced by PPARgamma ligands. Finally, electrophoresis mobility shift experiments demonstrated that PPARgamma ligands increased the nuclear binding activities of Sp1 and NF-IL6 (C/EBP), two transcription factors with regulatory elements in the promoter region of the p21 gene.

CONCLUSION

PPARgamma ligands inhibit human lung carcinoma cell growth and induce apoptosis by stimulating the cyclin-dependent kinase inhibitor p21 and by reducing cyclin D1 gene expression. The induction of p21 gene expression by PPARgamma ligands may be mediated through increased Sp1- and NF-IL6 (C/EBP)-dependent transcriptional activation. These observations unveil a mechanism for p21 gene regulation in lung carcinoma that represents a potential target for therapy.

The calcium channel blocker amlodipine exerts its anti¬proliferative action via p21 (Waf1/Cip1) gene activation

Proliferation of vascular smooth muscle cells (VSMC) contributes to the progression of atherosclerotic plaques. Calcium channel blockers have been shown to reduce VSMC proliferation, but the underlying molecular mechanism remains unclear. p21(Waf1/Cip1) is a potent inhibitor of cell cycle progression. Here, we demonstrate that amlodipine (10(-6) to 10(-8) M) activates de novo synthesis of p21(Waf1/Cip1) in vitro. We show that amlodipine-dependent activation of p21(Waf1/Cip1) involves the action of the glucocorticoid receptor (GR) and C/EBP-alpha. The underlying pathway apparently involves the action of mitogen-activated protein kinase or protein kinase C, but not of extracellular signal-related kinase or changes of intracellular calcium. Amlodipine-induced p21(Waf1/Cip1) promoter activity and expression were abrogated by C/EBP-alpha antisense oligonucleotide or by the GR antagonist RU486. Amlodipine-dependent inhibition of cell proliferation was partially reversed by RU486 at 10(-8) M (58%+/-29%), antisense oligonucleotides targeting C/EBP-alpha (91%+/-26%), or antisense mRNAs targeting p21(Waf1/Cip1) (96%+/-32%, n=6); scrambled antisense oligonucleotides or those directed against C/EBP-beta were ineffective. The data suggest that the anti-proliferative action of amlodipine is achieved by induction of the p21 (Waf1/Cip1) gene, which may explain beneficial covert effects of this widely used cardiovascular therapeutic drug beyond a more limited role as a vascular relaxant.

C/EBP beta isoforms LIP and LAP modulate progression of the cell cycle in the regenerating mouse liver

The CCAAT enhancer-binding protein (C/EBP) beta gene can produce several N-terminally truncated isoforms. Liver-enriched activator protein (LAP) is a transcriptional activator in many systems, whereas liver-enriched inhibitory protein (LIP) is regarded as a functional LAP antagonist. In this study, we examined the impact of these two proteins on cell cycle progression in the regenerating liver. Adenoviral overexpression of LAP, in addition to its role as a transactivator of liver-specific genes, led to a delayed S-phase entry of hepatocytes after partial hepatectomy (PH) in vivo. This delay was accompanied by decreased expression of cyclin A and E as well as proliferating cell nuclear antigen and decreased cyclin-dependent kinase 2 activity at the G1/S boundary. This observation is not explained by increased p21(CIP1/Waf1) expression or lack of phosphorylation of external LAP, but LAP overexpression triggered a decreased C/EBP-alpha/C/EBP-alpha-30 ratio and a reduced basal c-jun level in the liver. In contrast, adenoviral overexpression of LIP resulted in a stronger and earlier induction of cyclin A and E after PH, but did not change the timing and extent of cyclin-dependent kinase 2 activity or the amount of hepatocytes that entered S phase in this model. In the LIP expressing group, both C/EBP-alpha isoforms and c-jun were more strongly induced after PH. In conclusion, the LAP/LIP ratio is an important modulator of cell cycle progression during liver regeneration. In the context of previous studies, our results demonstrate that LAP, through a dose-dependent effect, withholds a dual activating and inhibiting role on hepatocyte proliferation in vivo.

ERK1/2 and p38 cooperate to induce a p21CIP1-dependent G1 cell cycle arrest

To study the mechanisms by which mitogen- and stress-activated protein kinases regulate cell cycle re-entry, we have used a panel of conditional kinases that stimulate defined MAPK or SAPK cascades. Activation of DeltaMEKK3:ER* during serum restimulation of quiescent cells causes a strong activation of JNK1 and p38alpha but only a modest potentiation of serum-stimulated ERK1/2 activity. In CCl39 cells this promoted a sustained G1 arrest that correlated with decreased expression of cyclin D1 and Cdc25A, increased expression of p21CIP1 and inhibition of CDK2 activity. In Rat-1 cells, in which p21(CIP1) expression is silenced by methylation, DeltaMEKK3:ER* activation caused only a transient delay in the S phase entry rather than a sustained G1 arrest. Furthermore, p21CIP1-/- 3T3 cells were defective for the DeltaMEKK3:ER*-induced G1 cell cycle arrest compared to their wild-type counterparts. These results suggest that activated DeltaMEKK3:ER* inhibits the G1 --> S progression by two kinetically distinct mechanisms, with expression of p21CIP1 being required to ensure a sustained G1 cell cycle arrest. The ERK1/2 and p38alphabeta pathways cooperated to induce p21CIP1 expression and inhibition of p38alphabeta caused a partial reversal of the cell cycle arrest. In contrast, selective activation of ERK1/2 by DeltaRaf-1:ER* did not inhibit serum stimulated cell cycle re-entry. Finally, selective activation of JNK by DeltaMEKK1:ER* failed to inhibit cell cycle re-entry, even in cells that retained wild-type p53, arguing against a major role for JNK alone in antagonizing the G1 --> S transition.

The mitogen-activated protein kinase cascade promotes myoblast cell survival by stabilizing the cyclin-dependent kinase inhibitor, p21WAF1 protein

During myogenesis, proliferating myoblasts withdraw from the cell cycle and are either eliminated by programmed cell death or differentiate into mature myotubes. Previous studies indicate that mitogen-activated protein kinase (MAPK) activity is significantly induced with the onset of terminal differentiation of C2 myoblasts. We have investigated the part played by the MAPK pathway in the differentiation of C2 myoblasts. Specific activation of MAPK by expression of an active Raf1-estrogen receptor chimera protein reduced significantly the number of myoblasts undergoing programmed cell death in the differentiation medium. Activation of Raf1 prevented the proteolytic activation of the proapoptotic caspase 9-protein during differentiation. The antiapoptotic function of Raf1 correlated with accumulation of the p21WAF1 protein resulting from its increased stability. Antisense expression of p21 was used to determine whether the p21WAF1 protein mediated the antiapoptotic activity of Raf1. Reduction of p21WAF1 protein in muscle cells abolished the antiapoptotic activity of the MAPK pathway. We conclude that MAPK contributes to muscle differentiation by preventing apoptotic cell death of differentiating myoblasts and that this activity is mediated by stabilization of the p21WAF1 protein.