Expression of the E2F1 transcription factor overcomes type beta transforming growth factor-mediated growth suppression

The TFDP1 gene coding for DP1, the heterodimeric partner of the transcription factor E2F, is a target of deregulated E2F

The TFDP1 gene codes for the heterodimeric partner DP1 of the transcription factor E2F. E2F, principal target of the tumor suppressor pRB, plays central roles in cell proliferation by activating a group of growth-related genes. E2F also mediates tumor suppression by activating tumor suppressor genes such as ARF, an upstream activator of the tumor suppressor p53, when deregulated from pRB upon oncogenic changes. Among 8 E2F family members (E2F1∼E2F8), expression of activator E2Fs (E2F1∼E2F3a) is induced at the G1/S boundary of the cell cycle after growth stimulation by E2F itself. However, mechanisms regulating DP1 expression are not known. We show here that over-expression of E2F1 and forced inactivation of pRB, by adenovirus E1a, induced TFDP1 gene expression in human normal fibroblast HFFs, suggesting that the TFDP1 gene is a target of E2F. Serum stimulation of HFFs also induced TFDP1 gene expression, but with different kinetics from that of the CDC6 gene, a typical growth-related E2F target. Both over-expression of E2F1 and serum stimulation activated the TFDP1 promoter. We searched for E2F1-responsive regions by 5' and 3' deletion of the TFDP1 promoter and by introducing point mutations in putative E2F1-responsive elements. Promoter analysis identified several GC-rich elements, mutation of which reduced E2F1-responsiveness but not serum-responsiveness. ChIP assays showed that the GC-rich elements bound deregulated E2F1 but not physiological E2F1 induced by serum stimulation. These results suggest that the TFDP1 gene is a target of deregulated E2F. In addition, knockdown of DP1 expression by shRNA enhanced ARF gene expression, which is specifically induced by deregulated E2F activity, suggesting that activation of the TFDP1 gene by deregulated E2F may function as a failsafe feedback mechanism to suppress deregulated E2F and maintain normal cell growth in the event that DP1 expression is insufficient relative to that of its partner activator E2Fs. a maximum of 6 keywords: E2F, DP1, TFDP1 gene, pRB, gene expression.

TGF-β in the microenvironment induces a physiologically occurring immune-suppressive senescent state

TGF-β induces senescence in embryonic tissues. Whether TGF-β in the hypoxic tumor microenvironment (TME) induces senescence in cancer and how the ensuing senescence-associated secretory phenotype (SASP) remodels the cellular TME to influence immune checkpoint inhibitor (ICI) responses are unknown. We show that TGF-β induces a deeper senescent state under hypoxia than under normoxia; deep senescence correlates with the degree of E2F suppression and is marked by multinucleation, reduced reentry into proliferation, and a distinct 14-gene SASP. Suppressing TGF-β signaling in tumors in an immunocompetent mouse lung cancer model abrogates endogenous senescent cells and suppresses the 14-gene SASP and immune infiltration. Untreated human lung cancers with a high 14-gene SASP display immunosuppressive immune infiltration. In a lung cancer clinical trial of ICIs, elevated 14-gene SASP is associated with increased senescence, TGF-β and hypoxia signaling, and poor progression-free survival. Thus, TME-induced senescence may represent a naturally occurring state in cancer, contributing to an immune-suppressive phenotype associated with immune therapy resistance.

E2F transcription factor 1 (E2F1) promotes the transforming growth factor TGF-β1 induced human cardiac fibroblasts differentiation through promoting the transcription of CCNE2 gene

The differentiation of cardiac fibroblast to myofibroblast is the key process of cardiac fibrosis. In the study, we aimed to determine the function of E2F Transcription Factor 1 (E2F1) in human cardiac fibroblasts (HCFs) differentiation, search for its downstream genes and elucidate the function of them in HCFs differentiation. As a result, we found that E2F1 was up-regulated in TGF-β1-induced HCFs differentiation. Silencing the expression of E2F1 by siRNA in HCFs, we found that the expression of differentiation-related genes (Collagen-1, α-Smooth muscle actin, and Fibronectin-1) was significantly suppressed, combining with proliferation and migration assay, we determined that HCFs differentiation was decreased. Luciferase report assay and immunoprecipitation proved that the oncogene CCNE2 was a direct target gene of E2F1, overexpression of CCNE2 was found in differentiated HCFs, silencing the expression of CCNE2 by siRNA decreased HCFs differentiation. Our research suggested that E2F1 and its downstream target gene CCNE2 play a vital role in TGF-β1-induced HCFs differentiation, thus E2F1 and CCNE2 may be a potential therapeutic target for cardiac fibrosis.

Targeting TGFβ signal transduction for cancer therapy

Transforming growth factor-β (TGFβ) family members are structurally and functionally related cytokines that have diverse effects on the regulation of cell fate during embryonic development and in the maintenance of adult tissue homeostasis. Dysregulation of TGFβ family signaling can lead to a plethora of developmental disorders and diseases, including cancer, immune dysfunction, and fibrosis. In this review, we focus on TGFβ, a well-characterized family member that has a dichotomous role in cancer progression, acting in early stages as a tumor suppressor and in late stages as a tumor promoter. The functions of TGFβ are not limited to the regulation of proliferation, differentiation, apoptosis, epithelial-mesenchymal transition, and metastasis of cancer cells. Recent reports have related TGFβ to effects on cells that are present in the tumor microenvironment through the stimulation of extracellular matrix deposition, promotion of angiogenesis, and suppression of the anti-tumor immune reaction. The pro-oncogenic roles of TGFβ have attracted considerable attention because their intervention provides a therapeutic approach for cancer patients. However, the critical function of TGFβ in maintaining tissue homeostasis makes targeting TGFβ a challenge. Here, we review the pleiotropic functions of TGFβ in cancer initiation and progression, summarize the recent clinical advancements regarding TGFβ signaling interventions for cancer treatment, and discuss the remaining challenges and opportunities related to targeting this pathway. We provide a perspective on synergistic therapies that combine anti-TGFβ therapy with cytotoxic chemotherapy, targeted therapy, radiotherapy, or immunotherapy.

Copy number variation (CNV) in the IGF1R gene across four cattle breeds and its association with economic traits

The insulin-like growth factor 1 receptor (IGF1R) plays a vital role in immunomodulation and muscle and bone growth. The copy number variation (CNV) is believed to the reason for many complex phenotypic variations. In this paper, we statistically analyzed the copy number and the expression profiling in different tissue types of the IGF1R gene using the 422 samples from four Chinese beef cattle breeds, and the mRNA of IGF1R was widely expressed in nine tissue types of adult cattle (heart, liver, kidney, muscle, fat, stomach, spleen, lung and testis). Results of CNV and growth traits indicated that the IGF1R CNV was significantly associated with body weight and body height of Jinnan (JN) cattle and was significantly associated with body height and hucklebone width of Qinchuan (QC) cattle, making IGF1R CNV a promising molecular marker to improve meat production in beef cattle breeding. Bioinformatics predictions show that the CNV region is highly similar to the human genome, and there are a large number of transcription factors, DNase I hypersensitive sites, and high levels of histone acetylation, suggesting that this region may play a role in transcriptional regulation, providing directions for further study of the role of bovine CNV and economic traits.

TGF-β induces growth suppression in multiple myeloma MM.1S cells via E2F1

Transforming growth factor-β (TGF-β) has an important role in multiple target genes and signaling pathways. The E2F family of transcription factors is a group of DNA-binding proteins that are involved in cell-cycle progression, and therefore have a key role in proliferation. The present study demonstrates that inhibition of cell growth by TGF-β occurs in the multiple myeloma cell line MM.1S. However, the growth-suppressive effects of TGF-β may be reversed by small interfering (si)RNA to reduce the expression of E2F1. TGF-β1 and E2F1 siRNA were manipulated in MM.1S cells to investigate the association between these genes. FACScan Flow Cytometer, western blot analysis and other methods were adopted to confirm such interrelation. The present data showed that TGF-β mediated growth suppression in MM.1S cells, while inducing E2F1 protein expression levels rapidly and transiently. The present data support the hypothesis that E2F1 is a central mediator of TGF-β-induced growth suppression in MM.1S cells and control of E2F1 may be a downstream event of TGF-β action, at least in one multiple myeloma cell line.

The phosphatidyl inositol 3 kinase pathway does not suppress activation of the ARF and BIM genes by deregulated E2F1 activity

The transcription factor E2F plays crucial roles in tumor suppression by activating pro-apoptotic genes such as the tumor suppressor ARF. The regulation of the ARF gene is distinct from that of growth-related E2F targets, in that it is specifically activated by deregulated E2F activity, induced by over-expression of E2F or forced inactivation of pRB, but not by physiological E2F activity induced by growth stimulation. The phosphatidyl inositol 3 kinase (PI3K) pathway was reported to suppress expression of some atypical pro-apoptotic genes by over-expressed E2F1. However, the effects of the PI3K pathway on the distinct regulation of typical pro-apoptotic E2F targets have not been elucidated. We examined whether the PI3K pathway suppressed activation of the typical pro-apoptotic E2F targets ARF and BIM. Activation of the PI3K pathway by growth stimulation or introduction of a constitutively active Akt/PKB did not reduce induction of ARF or BIM gene expression or activation of their promoters by over-expressed E2F1. These results suggest that the PI3K pathway does not suppress induction of typical pro-apoptotic genes that are selectively activated by deregulated E2F1.

Non-basic amino acids in the ROMK1 channels via an appropriate distance modulate PIP2 regulated pHi-gating

The ROMK1 (Kir1.1) channel activity is predominantly regulated by intracellular pH (pHi) and phosphatidylinositol 4,5-bisphosphate (PIP2). Although several residues were reported to be involved in the regulation of pHi associated with PIP2 interaction, the detailed molecular mechanism remains unclear. We perform experiments in ROMK1 pHi-gating with electrophysiology combined with mutational and structural analysis. In the present study, non basic residues of C-terminal region (S219, N215, I192, L216 and L220) in ROMK1 channels have been found to mediate channel-PIP2 interaction and pHi gating. Further, our structural results show these residues with an appropriate distance to interact with membrane PIP2. Meanwhile, a cluster of basic residues (R188, R217 and K218), which was previously discovered regarding the interaction with PIP2, exists in this appropriate distance to discriminate the regulation of channel-PIP2 interaction and pHi-gating. This appropriate distance can be observed with high conservation in the Kir channel family. Our results provide insight that an appropriate distance cooperates with the electrostatics interaction of channel-PIP2 to regulate pHi-gating.

E2F1 transcription factor and its impact on growth factor and cytokine signaling

E2F1 is a transcription factor involved in cell cycle regulation and apoptosis. The transactivation capacity of E2F1 is regulated by pRb. In its hypophosphorylated form, pRb binds and inactivates DNA binding and transactivating functions of E2F1. The growth factor stimulation of cells leads to activation of CDKs (cyclin dependent kinases), which in turn phosphorylate Rb and hyperphosphorylated Rb is released from E2F1 or E2F1/DP complex, and free E2F1 can induce transcription of several genes involved in cell cycle entry, induction or inhibition of apoptosis. Thus, growth factors and cytokines generally utilize E2F1 to direct cells to either fate. Furthermore, E2F1 regulates expressions of various cytokines and growth factor receptors, establishing positive or negative feedback mechanisms. This review focuses on the relationship between E2F1 transcription factor and cytokines (IL-1, IL-2, IL-3, IL-6, TGF-beta, G-CSF, LIF), growth factors (EGF, KGF, VEGF, IGF, FGF, PDGF, HGF, NGF), and interferons (IFN-α, IFN-β and IFN-γ).

Mullerian inhibiting substance induces apoptosis of human endometrial stromal cells in endometriosis

CONTEXT

Müllerian inhibiting substance (MIS) is produced in Sertoli cells of fetal testis and causes regression of müllerian ducts in male embryos. MIS also can induce the cell cycle arrest and apoptosis in müllerian duct-derived tumors in vivo and in vitro.

OBJECTIVE

Our objective was to investigate the expression of MIS type II receptor (MISR II) and whether MIS can inhibit the proliferation and induce apoptosis in primary cultures of endometrial stromal cells (ESC) of endometriosis.

DESIGN AND SETTINGS

In vitro experiments were performed in the university research laboratory.

PARTICIPANTS

Tissue samples from 12 patients who had undergone evisceration for ovarian endometrial cysts were included in this study.

INTERVENTIONS AND MAIN OUTCOME MEASURES

The expression of MISR II in ESC was investigated by immunohistochemistry. The cell viability and apoptosis in ESC treated with MIS was measured by methylthiazoletetrazolium assay and annexin V analysis. The expression of regulatory proteins in ESC treated with MIS was shown by Western blotting.

RESULTS

ESC showed specific immunostaining for the MISR II. ESC treated with MIS exhibited 32% growth inhibition (P = 0.0001). The changes in cell cycle distribution after MIS exposure at 72 h demonstrated that S and G(2)M phases were decreased; G(0)G(1) and sub-G(0)G(1) phases were increased. ESC treated with MIS showed 13.72% annexin V-fluorescein isothiocyanate positivity. In the ESCs, which contain defective p16, MIS increased the expression of pocket proteins p107 and p130 and decreased E2F transcription factor 1.

CONCLUSIONS

The results support a central role for MIS in endometriosis. Although the precise mechanism of MIS-mediated inhibition of ESC growth has not been fully defined, these data suggest that MIS has activity against ESC in vitro and may also be an effective targeted therapy for endometriosis.

A genetic reporter system to gauge cell proliferation rate

In higher eukaryotes, E2F transcription factors often drive expression of genes necessary for the cell cycle, notably the G1/S phase transition. With conventional transcriptional reporter systems, expression of a reporter gene from an E2F-responsive promoter would allow one to identify the fraction of cells making this transition. Here, we have engineered an E2F-responsive genetic reporter system that outputs the proliferation rate. The system takes advantage of the long half-lives of fluorescent protein reporters and output signal normalization. By doing so, it converts dynamic pulses of E2F activity into an analog output proportional to the proliferation rate. Such a system should be useful for applications involving high-throughput drug or genetic screens, investigation of cellular environment, and biological engineering.

Tumor stroma-derived TGF-beta limits myc-driven lymphomagenesis via Suv39h1-dependent senescence

Activated RAS/BRAF oncogenes induce cellular senescence as a tumor-suppressive barrier in early cancer development, at least in part, via an oncogene-evoked DNA damage response (DDR). In contrast, Myc activation-although producing a DDR as well-is known to primarily elicit an apoptotic countermeasure. Using the Emu-myc transgenic mouse lymphoma model, we show here in vivo that apoptotic lymphoma cells activate macrophages to secrete transforming growth factor beta (TGF-beta) as a critical non-cell-autonomous inducer of cellular senescence. Accordingly, neutralization of TGF-beta action, like genetic inactivation of the senescence-related histone methyltransferase Suv39h1, significantly accelerates Myc-driven tumor development via cancellation of cellular senescence. These findings, recapitulated in human aggressive B cell lymphomas, demonstrate that tumor-prompted stroma-derived signals may limit tumorigenesis by feedback senescence induction.

Cell-context specific role of the E2F/Rb pathway in development and disease

Development of the central nervous system (CNS) requires the generation of neuronal and glial cell subtypes in appropriate numbers, and this demands the careful coordination of cell-cycle exit, survival, and differentiation. The E2F/Rb pathway is critical for cell-cycle regulation and also modulates survival and differentiation of distinct cell types in the developing and adult CNS. In this review, we first present the specific temporal patterns of expression of the E2F and Rb family members during CNS development and then discuss the genetic ablation of single or multiple members of these two families. Overall, the available data suggest a time-dependent and cell-context specific role of E2F and Rb family members in the developing and adult CNS.

The E2F family and the role of E2F1 in apoptosis

The E2F family of transcription factors plays a pivotal role in the regulation of cellular proliferation and differentiation. Although the deregulation of E2Fs is considered an oncogenic event that predisposes immortalized cells to transformation, paradoxically, E2F1 is also equipped with an ability to induce apoptosis under certain cellular contexts. It has become evident that E2Fs, in particular E2F1, participate in many aspects of the apoptotic process, either by acting alone or in cooperation with other factors, such as p53, to protect organisms from tumor development in the face of oncogenic lesions. Given the frequent inactivation of p53 in human cancers, the E2F1-induced apoptosis pathway is rapidly gaining attention as a key mechanism to compensate the loss of p53 in human tumors. In this review, we will focus on the recent progress in our understanding of E2F1-mediated apoptosis and discuss how these discoveries can be translated into potential therapeutic intervention.

TGF-beta induces growth arrest in Burkitt lymphoma cells via transcriptional repression of E2F-1

Transforming growth factor-beta (TGF-beta) is a potent regulator of tissue homeostasis and can act as both a tumor suppressor and a tumor promoter. The ability to induce cell cycle arrest is a major component of the tumor suppressor function of TGF-beta. Lung, mammary, and skin epithelial cells exhibit a common minimal cytostatic program in response to TGF-beta signaling involving the repression of the growth-promoting factors c-MYC, Id1, Id2, and Id3. Loss of c-MYC expression is a pivotal event in this process, resulting in derepression of the cyclin-dependent kinase inhibitors CDKN1A (p21) and CDKN2B (p15) and ultimately leading to growth arrest. It is not clear, however, which responses are necessary for TGF-beta-mediated growth arrest in other cell types. Here, in human Burkitt lymphoma cells transformed by deregulated c-MYC expression, we demonstrate that efficient TGF-beta-induced cytostasis can occur despite both maintenance of c-MYC levels and a lack of p21 and p15 induction. TGF-beta treatment also results in induction, rather than repression, of Id1 and Id2 expression. In this context, growth arrest correlates with transcriptional repression of E2F-1, and overexpression of E2F-1 in Burkitt lymphoma cells largely overcomes the TGF-beta-mediated G(1) arrest phenotype. These data indicate that deregulation of c-MYC in lymphoma cells does not overcome the tumor suppressor function of TGF-beta and that repression of E2F-1 transcription is sufficient for the efficient induction of cytostasis.

Characterization of TBX20 in human hearts and its regulation by TFAP2

The T-box family of transcription factors has been shown to have major impact on human development and disease. In animal studies Tbx20 is essential for the development of the atrioventricular channel, the outflow tract and valves, suggesting its potential causative role for the development of Tetralogy of Fallot (TOF) in humans. In the presented study, we analyzed TBX20 in cardiac biopsies derived from patients with TOF, ventricular septal defects (VSDs) and normal hearts. Mutation analysis did not reveal any disease causing sequence variation, however, TBX20 is significantly upregulated in tissue samples of patients with TOF, but not VSD. In depth analysis of TBX20 transcripts lead to the identification of two new exons 3' to the known TBX20 message resembling the mouse variant Tbx20a, as well as an extended 5'UTR. Functional analysis of the human TBX20 promoter revealed a 100 bp region that contains strong activating elements. Within this core promoter region we recognized functional binding sites for TFAP2 transcription factors and identified TFAP2 as repressors of the TBX20 gene in vitro and in vivo. Moreover, decreased TFAP2C levels in cardiac biopsies of TOF patients underline the biological significance of the pathway described. In summary, we provide first insights into the regulation of TBX20 and show its potential for human congenital heart diseases.

Expression and localization of Ski determine cell type-specific TGFbeta signaling effects on the cell cycle

Transforming growth factor β (TGFβ) promotes epithelial cell differentiation but induces Schwann cell proliferation. We show that the protooncogene Ski (Sloan-Kettering viral oncogene homologue) is an important regulator of these effects. TGFβ down-regulates Ski in epithelial cells but not in Schwann cells. In Schwann cells but not in epithelial cells, retinoblastoma protein (Rb) is up-regulated by TGFβ. Additionally, both Ski and Rb move to the cytoplasm, where they partially colocalize. In vivo, Ski and phospho-Rb (pRb) appear to interact in the Schwann cell cytoplasm of developing sciatic nerves. Ski overexpression induces Rb hyperphosphorylation, proliferation, and colocalization of both proteins in Schwann cell and epithelial cell cytoplasms independently of TGFβ treatment. Conversely, Ski knockdown in Schwann cells blocks TGFβ-induced proliferation and pRb cytoplasmic relocalization. Our findings reveal a critical function of fine-tuned Ski levels in the control of TGFβ effects on the cell cycle and suggest that at least a part of Ski regulatory effects on TGFβ-induced proliferation of Schwann cells is caused by its concerted action with Rb.

Expression of thymidylate synthase in human cells is an early G(1) event regulated by CDK4 and p16INK4A but not E2F

Thymidylate synthase (TS) is the enzyme that catalyses the last step in de novo thymidylate synthesis. It is of interest clinically because it is an effective target for drugs such as 5-fluorouracil, often used in combination therapy. Despite a number of earlier reports indicating that TS is a cell cycle-dependent enzyme, this remains equivocal. Here, we show that in HCT116 cells synchronised by serum starvation, there is a clear dissociation between the expression of cyclin E (a well-characterised cell-cycle protein) and TS. Although both cyclin E and TS mRNA and protein increased during G₁, TS upregulation was delayed. Moreover, TS levels did not decrease following S-phase completion while cyclin E decreased sharply. Similarly, clear differences were seen between cyclin E and TS as asynchronously growing HCT116 cells were growth-inhibited by low-serum treatment. In contrast to previous reports using rodent cells, adenovirus-mediated over-expression of E2F1 and cyclin E in three human cell lines had no effect on TS. Cell-cycle progression was blocked by treatment of cells with pharmacological inhibitors of CDK2 and CDK4 and by ectopic expression of p16INK4A. Whereas CDK2 inhibition had no effect on TS levels, inhibition of CDK4 was associated with decreased TS protein levels. These results provide the first evidence that drugs targeting CDK4 may be useful with anti-TS drugs as combination therapy for cancer.

Recombinant adenoviral vectors can induce expression of p73 via the E4-orf6/7 protein

Despite the utility of recombinant adenoviral vectors in basic research, their therapeutic promise remains unfulfilled. Most engineered adenoviral vectors use a heterologous promoter to transcribe a foreign gene. We show that adenoviruses containing the cytomegalovirus immediate-early promoter induce the expression of the proapoptotic cellular protein TAp73 via the cyclin-dependent kinase-retinoblastoma protein-E2F pathway in murine embryonic fibroblasts. Cells transduced with these vectors also expressed high levels of the adenoviral E4-orf6/7 and E2A proteins. By contrast, adenoviruses containing the ubiquitin C promoter failed to elicit these effects. E4-orf6/7 is necessary and sufficient for increased TAp73 expression, as shown by using retrovirus-mediated E4-orf6/7 expression and adenovirus with the E4-orf6/7 gene deleted. Activation of TAp73 likely occurs via E4-orf6/7-induced dimerization of E2F and subsequent binding to the inverted E2F-responsive elements within the TAp73 promoter. In addition, adenoviral vectors containing the cytomegalovirus immediate-early promoter, but not the ubiquitin C promoter, cooperated with chemotherapeutic agents to decrease cellularity in vitro. In contrast to murine embryonic fibroblasts, adenoviruses containing the ubiquitin C promoter, but not the cytomegalovirus immediate-early promoter, induced both E4-orf6/7 and TAp73 in human foreskin fibroblasts, emphasizing the importance of cellular context for promoter-dependent effects. Because TAp73 is important for the efficacy of chemotherapy, adenoviruses that increase TAp73 expression may enhance cancer therapies by promoting apoptosis. However, such adenoviruses may impair the long-term survival of transduced cells during gene replacement therapies. Our findings reveal previously unknown effects of foreign promoters in recombinant adenoviral vectors and suggest means to improve the utility of engineered adenoviruses by better controlling their impact on viral and cellular gene expression.

CDCA4 is an E2F transcription factor family-induced nuclear factor that regulates E2F-dependent transcriptional activation and cell proliferation

The TRIP-Br1/p34(SEI-1) family proteins participate in cell cycle progression by coactivating E2F1- or p53-dependent transcriptional activation. Here, we report the identification of human CDCA4 (also know as SEI-3/Hepp) as a novel target gene of transcription factor E2F and as a repressor of E2F-dependent transcriptional activation. Analysis of CDCA4 promoter constructs showed that an E2F-responsive sequence in the vicinity of the transcription initiation site is necessary for the E2F1-4-induced activation of CDCA4 gene transcription. Chromatin immunoprecipitation analysis demonstrated that E2F1 and E2F4 bound to an E2F-responsive sequence of the human CDCA4 gene. Like TRIP-Br1/p34(SEI-1) and TRIP-Br2 (SEI-2), the transactivation domain of CDCA4 was mapped within C-terminal acidic region 175-241. The transactivation function of the CDCA4 protein was inhibited by E2F1-4 and DP2, but not by E2F5-8. Inhibition of CDCA4 transactivation activity by E2F1 partially interfered with retinoblastoma protein overexpression. Conversely, CDCA4 suppressed E2F1-3-induced reporter activity. CDCA4 (but not acidic region-deleted CDCA4) suppressed E2F1-regulated gene promoter activity. These findings suggest that the CDCA4 protein functions as a suppressor at the E2F-responsive promoter. Small interfering RNA-mediated knockdown of CDCA4 expression in cancer cells resulted in up-regulation of cell growth rates and DNA synthesis. The CDCA4 protein was detected in several human cells and was induced as cells entered the G1/S phase of the cell cycle. Taken together, our results suggest that CDCA4 participates in the regulation of cell proliferation, mainly through the E2F/retinoblastoma protein pathway.

Cell cycle regulator E2F1 modulates angiogenesis via p53-dependent transcriptional control of VEGF

The transcription factor E2F1 is known to regulate cell proliferation and has been thought to modulate tumorigenesis via this mechanism alone. Here we show that mice deficient in E2F1 exhibit enhanced angiogenesis. The proangiogenic phenotype in E2F1 deficiency is the result of overproduction of vascular endothelial growth factor (VEGF) and is prevented by VEGF blockade. Under hypoxic conditions, E2F1 down-regulates the expression of VEGF promoter activity by associating with p53 and specifically down-regulating expression of VEGF but not other hypoxia-inducible genes, suggesting a promoter structure context-dependent regulation mechanism. We found that the minimum VEGF promoter mediating transcriptional repression by E2F1 features an E2F1- binding site with four Sp-1 sites in close proximity. These data disclose an unexpected function of endogenous E2F1: regulation of angiogenic activity via p53-dependent transcriptional control of VEGF expression.

E2F1 induces MRN foci formation and a cell cycle checkpoint response in human fibroblasts

Deregulation of the Rb/E2F pathway in human fibroblasts results in an E2F1-mediated apoptosis dependent on Atm, Nbs1, Chk2 and p53. Here, we show that E2F1 expression results in MRN foci formation, which is independent of the Nbs1 interacting region and the DNA-binding domain of E2F1. E2F1-induced MRN foci are similar to irradiation-induced foci (IRIF) that result from double-strand DNA breaks because they correlate with 53BP1 and gammaH2AX foci, do not form in NBS cells, do form in AT cells and do not correlate with cell cycle entry. In fact, we find that in human fibroblasts deregulated E2F1 causes a G1 arrest, blocking serum-induced cell cycle progression, in part through an Nbs1/53BP1/p53/p21(WAF1/CIP1) checkpoint pathway. This checkpoint protects against apoptosis because depletion of 53BP1 or p21(WAF1/CIP1) increases both the rate and extent of apoptosis. Nbs1 and p53 contribute to both checkpoint and apoptosis pathways. These results suggest that E2F1-induced foci generate a cell cycle checkpoint that, with sustained E2F1 activity, eventually yields to apoptosis. Uncontrolled proliferation due to Rb/E2F deregulation as well as inactivation of both checkpoint and apoptosis programs would then be required for transformation of normal cells to tumor cells.

A functional genetic screen identifies TFE3 as a gene that confers resistance to the anti-proliferative effects of the retinoblastoma protein and transforming growth factor-beta

The helix-loop-helix transcription factor TFE3 has been suggested to play a role in the control of cell growth by acting as a binding partner of transcriptional regulators such as E2F3, SMAD3, and LEF-1. Furthermore, translocations/TFE3 fusions have been directly implicated in tumorigenesis. Surprisingly, however, a direct functional role for TFE3 in the regulation of proliferation has not been reported. By screening retroviral cDNA expression libraries to identify cDNAs that confer resistance to a pRB-induced proliferation arrest, we have found that TFE3 overrides a growth arrest in Rat1 cells induced by pRB and its upstream regulator p16(INK4A). In addition, TFE3 expression blocks the anti-mitogenic effects of TGF-beta in rodent and human cells. We provide data supporting a role for endogenous TFE3 in the direct regulation of CYCLIN E expression in an E2F3-dependent manner. These observations establish TFE3 as a functional regulator of proliferation and offer a potential mechanism for its involvement in cancer.

Rb inactivation leads to E2F1-mediated DNA double-strand break accumulation

Although it is unclear which cellular factor(s) is responsible for the genetic instability associated with initiating and sustaining cell transformation, it is known that many cancers have mutations that inactivate the Rb-mediated proliferation pathway. We show here that pRb inactivation and the resultant deregulation of one E2F family member, E2F1, leads to DNA double-strand break (DSB) accumulation in normal diploid human cells. These DSBs occur independent of Atm, p53, caspases, reactive oxygen species, and apoptosis. Moreover, E2F1 does not contribute to c-Myc-associated DSBs, indicating that the DSBs associated with these oncoproteins arise through distinct pathways. We also find E2F1-associated DSBs in an Rb mutated cancer cell line in the absence of an exogenous DSB stimulus. These basal, E2F1-associated DSBs are not observed in a p16(ink4a) inactivated cancer cell line that retains functional pRb, unless pRb is depleted. Thus, Rb status is key to regulating both the proliferation promoting functions associated with E2F and for preventing DNA damage accumulation if E2F1 becomes deregulated. Taken together, these data suggest that loss of Rb creates strong selective pressure, via DSB accumulation, for inactivating p53 mutations and that E2F1 contributes to the genetic instability associated with transformation and tumorigenesis.

Involvement of E2F transcription factor family in cancer

The E2F family of transcription factors is a central modulator of important cellular events, including cell cycle progression, apoptosis and DNA damage response. The role of E2F family members in various human malignancies is yet unclear and may provide vital clues to the diagnosis, prognosis and therapy of cancer patients. In this review we provide a brief but concise overview of E2F function and its putative role in the most common human tumour types.

Role of the E2F1-p19-p53 pathway in ischemic acute renal failure

BACKGROUND

Cell cycle progression and arrest of renal tubular cells after acute injury is a reactive process of renal regeneration. The p16(INK4a)/p19(ARF) (alternative reading frame) locus encodes two proteins involved in cell cycle regulation. We investigated the transcriptional regulation and tissue distribution of p19(ARF) in ischemic acute renal failure (ARF).

METHODS

We examined the time course and immunohistochemistry of p19(ARF) in rat kidneys following the induction of ischemic ARF. We also examined the effect of p19(ARF) overexpression on p53 levels and cell cycle progression in MDCK cells.

RESULTS

The protein expression of p19(ARF) strongly increased 72 h after the ischemia. Immunohistochemical studies showed that the renal tubular cells in the outer medulla expressed p19(ARF) protein 72 h after ischemic injury. The time course of E2F1 induction was observed at 6-24 h, and it was found to precede p19(ARF) expression. In MDCK cells, the overexpression of E2F1 increased promoter activity and the protein level of p19(ARF) and induced apoptosis. Transfection of the p19(ARF) expression vector caused an increase in p53 protein, cell cycle arrest and apoptosis.

CONCLUSIONS

These data support the hypothesis that the E2F1-p19(ARF)-p53 pathway forms a negative feedback loop to regulate the cell cycle of renal tubular cells in the ischemic ARF.

Recent advances in understanding transforming growth factor beta regulation of orofacial development

Members of the transforming growth factor (TGF) family have emerged as critical contributors to the choreography of cellular and tissue interactions underlying morphogenesis of the orofacial region. The TGFs beta, and their downstream effector molecules, the Smads, play a pivotal role in normal as well as abnormal development of first branchial arch structures. Components of the TGFbeta signal transduction machinery are discussed in relation to regulation of transcription, cell division and tissue differentiation in developing orofacial tissue, as evidence for a functional linkage between the TGFbeta and retinoic acid signal transduction pathways during orofacial development.

The E2F transcriptional network: old acquaintances with new faces

The E2 factor (E2F) family of transcription factors are downstream targets of the retinoblastoma protein. E2F factors have been known for several years to be important regulators of S-phase entry. Recent studies have improved our understanding of the molecular mechanisms of action used by this transcriptional network. In addition, they have given us an appreciation of the fact that E2F has functions that reach beyond G1/S control and impact cell proliferation in several different ways. The discovery of new family members with unusual properties, the unexpected phenotypes of mutant animals, a diverse collection of biological activities, a large number of new putative target genes and the new modes of transcriptional regulation have all contributed to an increasingly complex view of E2F function. In this review, we will discuss these recent developments and describe how they are beginning to shape a new and revised picture of the E2F transcriptional program.

Endometrial cancer is a receptor-mediated target for Mullerian Inhibiting Substance

Mullerian Inhibiting Substance (MIS), a 140-kDa homodimer glycoprotein member of the TGF-beta superfamily of biological-response modifiers, causes regression of the Mullerian ducts in developing male embryos. MIS also can induce growth arrest and apoptosis in ovarian and cervical cancer cell lines. The embryonic progenitor of the ovarian and cervical epithelium is the coelomic epithelium, the same tissue that regresses under the direction of MIS in the male. The endometrium and uterus also arise from the coelomic epithelium and the Mullerian ducts. Here, we show that both normal human endometrium and endometrial cancers express the receptor for MIS and that MIS can inhibit the proliferation of a number of human endometrial cancer cell lines that express the MIS type II receptor. In the representative endometrial cancer cell line AN3CA, MIS affects the expression of key cell-cycle regulatory proteins. This work broadens the scope of tumors that MIS can potentially control and, by elucidating the MIS signaling pathway, identifies other potential avenues for intervention.

Differential regulation of expression of the mammalian DNA repair genes by growth stimulation

During DNA replication, DNA becomes more vulnerable to certain DNA damages. DNA repair genes involved in repair of the damages may be induced by growth stimulation. However, regulation of DNA repair genes by growth stimulation has not been analysed in detail. In this report, we analysed the regulation of expression of mammalian MSH2, MSH3 and MLH1 genes involved in mismatch repair, and Rad51 and Rad50 genes involved in homologous recombination repair, in relation to cell growth. Unexpectedly, we found a clear difference in regulation of these repair gene expression by growth stimulation even in the same repair system. The expression of MSH2, MLH1 and Rad51 genes was clearly growth regulated, whereas MSH3 and Rad50 genes were constitutively expressed, suggesting differential requirement of the repair gene products for cell proliferation. MSH3 gene is located in a bidirectionally divergent manner with DHFR gene that is regulated by growth stimulation, indicating that bidirectionally divergent promoters are not necessarily coordinately regulated. Promoter analysis showed that the growth-regulated expression of MLH1 and Rad51 genes was mainly mediated by E2F that plays crucial roles in regulation of DNA replication, suggesting close relation between some of the repair genes and DNA replication.

E2F-1 Regulates the Expression of a Subset of Target Genes during Skeletal Myoblast Hypertrophy

Cellular hypertrophy, or growth without division, is an adaptive response to various physiological and pathological stimuli in postmitotic muscle. We demonstrated previously that angiotensin II stimulates hypertrophy in C2C12 myoblasts by transient activation of the cyclin-dependent kinase 4 complex, subsequent phosphorylation of retinoblastoma protein, release of histone deacetylase 1 from the retinoblastoma protein inhibitory complex, and partial activation of the transcription factor E2F-1. These observations led us to propose a model in which partial inactivation of the retinoblastoma protein complex leads to the derepression of a subset of E2F-1 targets necessary for cell growth without division during hypertrophy. We now present data that support this model and suggest the mechanism by which E2F-1 regulates hypertrophy. We examined expression profiles of angiotensin II-stimulated myoblasts and identified a subset of E2F-1 target genes that are specifically regulated during the hypertrophic response. We showed that the expression of E2F-1 targets involved in G1/S transit, DNA replication, and mitosis is not altered during the hypertrophic response, while the expression of E2F-1-regulated genes controlling early G1 progression, cytoskeletal organization, protein synthesis, mitochondrial function, and programmed cell death is up-regulated. Furthermore, we demonstrated that activation of cytochrome c oxidase genes occurs during the development of hypertrophy and that cytochrome c oxidase IV is a direct transcriptional target of E2F-1. These studies demonstrated that E2F-1 activity at specific promoters is dependent on physiological circumstances and that E2F-1 should be considered a potential target in the treatment of pathologic hypertrophy.

Life and death decisions by E2F-1

Deregulation of the transcription factor E2F-1 is a common event in most human cancers. Paradoxically, E2F-1 has been shown to have the ability to induce both cell cycle progression and programmed cell death, leading potentially to both tumour-promoting as well as tumour-suppressive effects. Although the pathway to cell cycle progression seems straightforward with a number of growth-promoting E2F target genes having been described, the pathways to apoptosis are less well defined and more complex. The discovery that E2F-1 'knockout' mice are highly tumour prone has caused a recent surge in the number of reports relating to programmed cell death. This review focuses on these recent findings, highlighting the way in which they have increased our understanding of E2F-1-induced cell death, as well as indicating the questions that remain. Insight gained as to the role of this intriguing molecule in cancer and its potential for targeted therapy will also be discussed.

Protein Farnesyltransferase Inhibitor (SCH 66336) Abolishes NF-κB Activation Induced by Various Carcinogens and Inflammatory Stimuli Leading to Suppression of NF-κB-regulated Gene Expression and Up-regulation of Apoptosis

Ras farnesyltransferase inhibitor (FTI) exhibit antiproliferative and antiangiogenic effects through a mechanism that is poorly understood. Because of the known role of Ras in the activation of transcription factor NF-kappaB and because NF-kappaB-regulated genes can control cell survival and angiogenesis, we postulated that FTI mediates its effects in part by modulating NF-kappaB activation. Therefore, in the present study we investigated the effect of FTI, SCH 66336, on NF-kappaB and NF-kappaB-regulated gene expression activated by a variety of inflammatory and carcinogenic agents. We demonstrate by DNA-binding assay that NF-kappaB activation induced by tumor necrosis factor (TNF), phorbol 12-myristate 13-acetate, cigarette smoke, okadaic acid, and H(2)O(2) was completely suppressed by SCH 66336; the suppression was not cell type-specific. This FTI suppressed the activation of IkappaBalpha kinase (IKK), thus abrogating the phosphorylation and degradation of IkappaBalpha. Additionally, TNF-activated Ras and SCH 66336 inhibited the activation. Also, overexpression of Ras (V12) enhanced TNF-induced NF-kappaB activation, and adenoviral dominant-negative Ras (N17) suppressed the activation, thus suggesting the critical role of Ras in TNF signaling. SCH 66336 also inhibited the NF-kappaB-dependent reporter gene expression activated by TNF, TNFR1, TRADD, TRAF2, NIK, and IKK but not that activated by the p65 subunit of NF-kappaB. The TNF-induced NF-kappaB-regulated gene products cyclin D1, COX-2, MMP-9, survivin, IAP1, IAP2, XIAP, Bcl-2, Bfl-1/A1, TRAF1, and FLIP were all down-regulated by SCH 66336, which potentiated apoptosis induced by TNF and doxorubicin. Overall, our results indicate that SCH 66336 inhibited activation of NF-kappaB and NF-kappaB-regulated gene expressions induced by carcinogens and inflammatory stimuli, which may provide a molecular basis for the ability of SCH 66336 to suppress proliferation and angiogenesis.

Apoptosis associated with deregulated E2F activity is dependent on E2F1 and Atm/Nbs1/Chk2

The retinoblastoma protein (Rb)/E2F pathway links cellular proliferation control to apoptosis and is critical for normal development and cancer prevention. Here we define a transcription-mediated pathway in which deregulation of E2F1 by ectopic E2F expression or Rb inactivation by E7 of human papillomavirus type 16 signals apoptosis by inducing the expression of Chk2, a component of the DNA damage response. E2F1- and E7-mediated apoptosis are compromised in cells from patients with the related disorders ataxia telangiectasia and Nijmegen breakage syndrome lacking functional Atm and Nbs1 gene products, respectively. Both Atm and Nbs1 contribute to Chk2 activation and p53 phosphorylation following deregulation of normal Rb growth control. E2F2, a related E2F family member that does not induce apoptosis, also activates Atm, resulting in phosphorylation of p53. However, we found that the key commitment step in apoptosis induction is the ability of E2F1, and not E2F2, to upregulate Chk2 expression. Our results suggest that E2F1 plays a central role in signaling disturbances in the Rb growth control pathway and, by upregulation of Chk2, may sensitize cells to undergo apoptosis.

Transforming growth factor-beta induces Cdk2 relocalization to the cytoplasm coincident with dephosphorylation of retinoblastoma tumor suppressor protein

Background

The transforming growth factor-β (TGF-β) signaling pathway functions to prevent tumorigenesis, and loss of sensitivity to TGF-β-mediated cell cycle arrest is nearly ubiquitous among human cancers. Our previous studies demonstrated that rapamycin potentiates TGF-β-induced cell cycle arrest in nontransformed epithelial cells and partially restores TGF-β-induced growth arrest of some human cancer cell lines. Growth arrest correlated with increased binding of p21 and p27 to cyclin-dependent kinase-2 (Cdk2), and inhibition of Cdk2 kinase activity. However, it was unclear how TGF-β caused increased binding of p21 and p27 to Cdk2.

Methods

Cell fractionation and immunofluorescence microscopy experiments were performed to examine the effect of TGF-β on the intracellular localization of Cdk2, p21, and p27. Kinase assays were performed on cytoplasmic and nuclear extracts to determine how TGF-β altered Cdk2 activity in both subcellular compartments.

Results

In breast epithelial cells treatment with TGF-β induced a decrease in nuclear Cdk2 concentrations and relocalization of Cdk2 to the cytoplasm. Cdk2 relocalization to the cytoplasm correlated with dephosphorylation of nuclear retinoblastoma tumor suppressor protein and decreased nuclear Cdk2 activity. In these epithelial cell lines, p21 and p27 were localized primarily in the cytoplasm. Decreases in nuclear Cdk2 concentrations correlated with increased binding of Cdk2 to cytoplasmic p21 and p27.

Conclusion

Cooperative growth arrest induced by treatment with TGF-β + rapamycin causes inhibition of nuclear Cdk2 activity through multiple mechanisms, including Cdk2 relocalization to the cytoplasm, increased p27 and p21 binding to Cdk2, and increased phosphorylation of nuclear Cdk2 on its inhibitory site, Tyr¹⁵.

Mullerian Inhibiting Substance inhibits cervical cancer cell growth via a pathway involving p130 and p107

In addition to causing regression of the Mullerian duct in the male embryo, Mullerian Inhibiting Substance (MIS) inhibits the growth of epithelial ovarian cancer cells, which are known to be of Mullerian origin. Because the uterine cervix is derived from the same Mullerian duct precursor as the epithelium of the ovary, we tested the hypothesis that cervical cancer cells might also respond to MIS. A number of cervical cancer cell lines express the MIS type II receptor, and MIS inhibits the growth of both human papilloma virus-transformed and non-human papilloma virus-transformed cervical cell lines, with a more dramatic effect seen in the latter. As in the ovarian cancer cell line OVCAR8, suppression of growth of the C33A cervical cancer cell line by MIS is associated with induction of the p16 tumor suppressor protein. However, in contrast to OVCAR8 cells, induction of p130 and p107 appears to play an important role in the inhibition of growth of C33A cells by MIS. Finally, normal cervical tissue expresses the MIS type II receptor in vivo, supporting the idea that MIS could be a targeted therapy for cervical cancer.

P27Kip1 and p21Cip1 are not required for the formation of active D cyclin-cdk4 complexes

Our studies address questions pertaining to the regulation of D cyclin-cdk4 activity, and the following results were obtained. Conditions that increased the abundance of the D cyclins also increased the abundance of enzymatically active D cyclin-cdk4 complexes in mouse embryo fibroblasts (MEFs) lacking both p27(Kip1) and p21(Cip1) (p27/p21(-/-)). Such conditions included ectopic expression of cyclin D1 and inhibition of D cyclin degradation by the proteasome inhibitor MG132. However, as determined by treatment of wild-type MEFs with MG132, maximal accumulation of D cyclin-cdk4 complexes required p27(Kip1) and p21(Cip1) and coincided with the formation of inactive D cyclin-cdk4-p27(Kip1) or -p21(Cip1) complexes. p27(Kip1) or p21(Cip1) also increased the abundance of D cyclin-cdk4 complexes and reduced amounts of cdk4 activity when ectopically expressed in p27/p21(-/-) MEFs. Lastly, increases in the stability of the D cyclins accounted for their greater abundance in wild-type MEFs than in p27/p21(-/-) MEFs. We conclude that (i) D cyclin-cdk4 complexes are formed and become active in the absence of p27(Kip1) and p21(Cip1) and (ii) p27(Kip1) and p21(Cip1) maximize the accumulation but inhibit the activity of D cyclin-cdk4 complexes. We suggest that D cyclin-cdk4 complexes are more stable when bound to p27(Kip1) or p21(Cip1) and that formation of ternary complexes also stabilizes the D cyclins.

Tissue- and agonist-specific regulation of human and murine plasminogen activator inhibitor-1 promoters in transgenic mice

Numerous studies have described regulatory factors and sequences that control transcriptional responses in vitro. However, there is a paucity of information on the qualitative and quantitative regulation of heterologous promoters using transgenic strategies. In order to investigate the physiological regulation of human plasminogen activator inhibitor type-1 (hPAI-1) expression in vivo compared to murine PAI-1 (mPAI-1) and to test the physiological relevance of regulatory mechanisms described in vitro, we generated transgenic mice expressing enhanced green fluorescent protein (EGFP) driven by the proximal -2.9 kb of the hPAI-1 promoter. Transgenic animals were treated with Ang II, TGF-beta1 and lipopolysaccharide (LPS) to compare the relative activation of the human and murine PAI-1 promoters. Ang II increased EGFP expression most effectively in brain, kidney and spleen, while mPAI-1 expression was quantitatively enhanced most prominently in heart and spleen. TGF-beta1 failed to induce activation of the hPAI-1 promoter but potently stimulated mPAI-1 in kidney and spleen. LPS administration triggered robust expression of mPAI-1 in liver, kidney, pancreas, spleen and lung, while EGFP was induced only modestly in heart and kidney. These results indicate that the transcriptional response of the endogenous mPAI-1 promoter varies widely in terms of location and magnitude of response to specific stimuli. Moreover, the physiological regulation of PAI-1 expression likely involves a complex interaction of transcription factors and DNA sequences that are not adequately replicated by in vitro functional studies focused on the proximal -2.9 kb promoter.

Transforming growth factor-beta inhibition of proteasomal activity: a potential mechanism of growth arrest

Although the proteasome plays a critical role in the controlled degradation of proteins involved in cell cycle control, the direct modulation of proteasomal function by growth regulatory signaling has not yet been demonstrated. We assessed the effect of transforming growth factor (TGF)-beta, a potent inhibitor of cell growth, on proteasomal function. TGF-beta selectively decreased hydrolysis of the proteasomal substrate Cbz-Leu-Leu-Leu-7-amido-4-methyl-coumarin (z-LLL-AMC) in a concentration-dependent manner but did not inhibit hydrolysis of other substrates Suc-Leu-Leu-Val-Tyr-AMC (suc-LLVY-AMC) or Cbz-Leu-Leu-Glu-AMC (z-LLE-AMC). An increase in intracellular oxidative injury occurred during incubation with TGF-beta. Furthermore, in vitro hydrolysis of z-LLL-AMC, but not suc-LLVY-AMC, was decreased by hydrogen peroxide. TGF-beta did not increase cellular expression of heat shock protein (HSP)90, a potent inhibitor of z-LLL-AMC hydrolysis in vitro. The physiological relevance of TGF-beta inhibition of proteasomal activity was studied by assessing the role of z-LLL-AMC hydrolysis on cyclin-dependent kinase inhibitor expression and cell growth. TGF-beta increased expression of p27KIP1 but did not alter expression of p21WAF1 or p16INK4A. The peptide aldehyde Cbz-Leu-Leu-leucinal (LLL-CHO or MG132) potently inhibited z-LLL-AMC hydrolysis in cell extracts as well as increasing p27KIP1 and decreasing cell proliferation. Thus growth inhibition by TGF-beta decreases a specific proteasomal activity via an HSP90-independent mechanism that may involve oxidative inactivation or modulation of proteasomal subunit composition and results in altered cellular expression of key cell cycle regulatory proteins such as p27KIP1.

E2F and cell cycle control: a double-edged sword

The E2F family of transcription factors plays a central role in regulating cellular proliferation by controlling the expression of both the genes required for cell cycle progression, particularly DNA synthesis, and the genes involved with apoptosis. E2F is regulated in a cell cycle-dependent manner, principally through its temporal association with pocket protein family members, the prototype member being the retinoblastoma tumor suppressor protein. Pocket proteins are, in turn, regulated through phosphorylation by cyclin-dependent kinase (cdk). The kinase activity of cyclin/cdk complexes is negatively regulated by cdk inhibitors, and thus both positive and negative growth regulatory signals impinge on E2F activity. Different E2F family members exhibit distinct cell cycle and apoptotic activities. Thus, E2F appears to play a pivotal role in coordinating events connected with proliferation, cell cycle arrest, and apoptosis.

Rapamycin potentiates transforming growth factor beta-induced growth arrest in nontransformed, oncogene-transformed, and human cancer cells

Transforming growth factor beta (TGF-beta) induces cell cycle arrest of most nontransformed epithelial cell lines. In contrast, many human carcinomas are refractory to the growth-inhibitory effect of TGF-beta. TGF-beta overexpression inhibits tumorigenesis, and abolition of TGF-beta signaling accelerates tumorigenesis, suggesting that TGF-beta acts as a tumor suppressor in mouse models of cancer. A screen to identify agents that potentiate TGF-beta-induced growth arrest demonstrated that the potential anticancer agent rapamycin cooperated with TGF-beta to induce growth arrest in multiple cell lines. Rapamycin also augmented the ability of TGF-beta to inhibit the proliferation of E2F1-, c-Myc-, and (V12)H-Ras-transformed cells, even though these cells were insensitive to TGF-beta-mediated growth arrest in the absence of rapamycin. Rapamycin potentiation of TGF-beta-induced growth arrest could not be explained by increases in TGF-beta receptor levels or rapamycin-induced dissociation of FKBP12 from the TGF-beta type I receptor. Significantly, TGF-beta and rapamycin cooperated to induce growth inhibition of human carcinoma cells that are resistant to TGF-beta-induced growth arrest, and arrest correlated with a suppression of Cdk2 kinase activity. Inhibition of Cdk2 activity was associated with increased binding of p21 and p27 to Cdk2 and decreased phosphorylation of Cdk2 on Thr(160). Increased p21 and p27 binding to Cdk2 was accompanied by decreased p130, p107, and E2F4 binding to Cdk2. Together, these results indicate that rapamycin and TGF-beta cooperate to inhibit the proliferation of nontransformed cells and cancer cells by acting in concert to inhibit Cdk2 activity.

Increased hepatic Forkhead Box M1B (FoxM1B) levels in old-aged mice stimulated liver regeneration through diminished p27Kip1 protein levels and increased Cdc25B expression

Recent liver regeneration studies indicate that maintaining hepatic Forkhead Box M1B (FoxM1B) expression in 12-month-old (old-aged) Transthyretin-FoxM1B transgenic mice increases hepatocyte proliferation and expression of cell cycle regulatory genes. Because these transgenic CD-1 mice maintain FoxM1B levels during the aging process, we conducted the current study to determine whether adenovirus delivery of the FoxM1B gene (AdFoxM1B) is sufficient to stimulate liver regeneration in old-aged Balb/c mice. Here we show that AdFoxM1B infection of old-aged mice caused a significant increase in FoxM1B expression, hepatocyte DNA replication, and mitosis following partial hepatectomy. This stimulation in hepatocyte S-phase progression was associated with diminished protein expression and perinuclear localization of cyclin-dependent kinase (Cdk) inhibitor p27(Kip1) (p27) protein following partial hepatectomy. In contrast, old-aged mice infected with control virus displayed high hepatocyte levels of p27 protein, which had been localized to the nucleus prior to S-phase. Furthermore, we found that restoring FoxM1B expression did not influence p27 mRNA levels, and this new finding implicates FoxM1B in regulation of p27 protein levels. Likewise, AdFoxM1B-infected regenerating livers displayed elevated S-phase levels of Cdk2 kinase activity compared with old-aged mice infected with control virus. Furthermore, restoring FoxM1B expression in old-aged mice caused elevated levels of Cyclin B1, Cyclin B2, Cdc25B, Cdk1, and p55CDC mRNA as well as stimulating Cdc25B nuclear localization during liver regeneration, all of which are required for mitosis. These studies indicated that an acute delivery of the FoxM1B gene in old-aged mice is sufficient to re-establish proliferation of regenerating hepatocytes, suggesting that FoxM1B can be used for therapeutic intervention to alleviate the reduction in cellular proliferation observed in the elderly.

Direct coupling of the cell cycle and cell death machinery by E2F

Unrestrained E2F activity forces S phase entry and promotes apoptosis through p53-dependent and -independent mechanisms. Here, we show that deregulation of E2F by adenovirus E1A, loss of Rb or enforced E2F-1 expression results in the accumulation of caspase proenzymes through a direct transcriptional mechanism. Increased caspase levels seem to potentiate cell death in the presence of p53-generated signals that trigger caspase activation. Our results demonstrate that mitogenic oncogenes engage a tumour suppressor network that functions at multiple levels to efficiently induce cell death. The data also underscore how cell cycle progression can be coupled to the apoptotic machinery.

P130 and its truncated form mediate p53-induced cell cycle arrest in Rb(-/-) Saos2 cells

In the present study, we investigate the mechanism of how p53 induces growth arrest in Rb-defective Saos2 cells that express temperature-sensitive mutant p53 (ts p53). The activation of p53 at a permissive temperature (32.5 degrees C) induces the cell cycle arrest at both the G1 and G2 stages. The induction of several p53-responsive genes as well as a small form of p130 (S-p130) was detected upon p53 activation. S-p130 retained the functions as a pocket protein and was dominant over p130 at the protein level after 36 h at 32.5 degrees C. A canonical p53 binding site was identified in intron 4 of p130. Furthermore, a novel p53-inducible transcript containing a partial intron 4 sequence downstream of the p53 binding site and exon 5 of p130 was detected by RT-PCR, suggesting S-p130 is induced by p53 at transcriptional level. The results from gel shift assay and immunoprecipitation showed that S-p130 as well as p130 formed complexes with both E2F1 and E2F4 at a permissive temperature. Moreover, the transient expression of E1A (12S) and E2F1 effectively abrogated p53-induced cell cycle arrest. These results strongly suggested that p130 and its truncated form might substitute Rb in mediating p53-induced cell cycle arrest in Rb(-/-) Saos2 cells.

A 63-base pair DNA segment containing an Sp1 site but not a canonical E2F site can confer growth-dependent and E2F-mediated transcriptional stimulation of the human ASK gene encoding the regulatory subunit for human Cdc7-related kinase

Cdc7-Dbf4 kinase complexes, conserved widely in eukaryotes, play essential roles in initiation and progression of the S phase. Cdc7 kinase activity fluctuates during cell cycle, and this is mainly the result of oscillation of expression of the Dbf4 subunit. Therefore, it is crucial to understand the mechanisms of regulation of Dbf4 expression. We have isolated and characterized the promoter region of the human ASK gene encoding Dbf4-related regulatory subunit for human Cdc7 kinase. We have identified a 63-base pair ASK promoter segment, which is sufficient for mediating growth stimulation. This minimal promoter segment (MP), containing an Sp1 site but no canonical E2F site, can be activated by ectopic E2F expression as well. Within the 63-base pair region, the Sp1 site as well as other elements are essential for stimulation by growth signals and by E2F, whereas an AT-rich sequence proximal to the coding region may serve as an element required for suppression in quiescence. Gel shift assays in the presence of an antibody demonstrate the presence of E2F1 in the protein-DNA complexes generated on the MP segment. However, the complex formation on MP was not competed by a DHFR promoter fragment, known to bind to E2F, nor by a consensus E2F binding oligonucleotide. Gel shift assays with point mutant MP fragments indicate that a non-canonical E2F site in the middle of this segment is critical for generation of the E2F complex. Our results suggest that E2F regulates the ASK promoter through an atypical mode of recognition of the target site.

E2F1 induces phosphorylation of p53 that is coincident with p53 accumulation and apoptosis

It has been proposed that the E2F1 transcription factor serves as a link between the Rb/E2F proliferation pathway and the p53 apoptosis pathway by inducing the expression of p19ARF, a protein that regulates p53 stability. We find that although p19ARF contributes to p53 accumulation in response to E2F expression, p19ARF is not required for E2F1-mediated apoptosis. E2F1 can signal p53 phosphorylation in the absence of p19ARF, similar to the observed modifications to p53 in response to DNA damage. These modifications are not observed in the absence of p19ARF following expression of E2F2, an E2F family member that does not induce apoptosis in mouse embryo fibroblasts but can induce p19ARF and p53 protein expression. p53 modification is found to be crucial for E2F1-mediated apoptosis, and this apoptosis is compromised when E2F1 is coexpressed with a p53 mutant lacking many N- and C-terminal phosphorylation sites. Additionally, E2F1-mediated apoptosis is abolished in the presence of caffeine, an inhibitor of phosphatidylinositol 3-kinase-related kinases that phosphorylate p53. These findings suggest that p53 phosphorylation is a key step in E2F1-mediated apoptosis and that this modification can occur in the absence of p19ARF.

Interview with the retinoblastoma family members: do they help each other?

The ultimate destiny of a cell to undergo division, differentiation, survival, and death results from an intricate balance between multiple regulators including oncogenes, tumor suppressor genes, and cell cycle associated proteins. Deregulation of the cell cycle machinery switches the phenotype from a normal cell to a cancerous cell. Fundamental alterations of tumor suppressor genes may result in an unregulated cell cycle with the accumulation of mutations and eventual neoplastic transformation. As such, one may define cancer as a genetic disease of the cell cycle. In this review, we will emphasize our current understanding of how the cell cycle machinery maintains cellular homeostasis by studying the consequences of its deregulation.