Glycogen synthase kinase 3 has a limited role in cell cycle regulation of cyclin D1 levels

β-Ionone enhances the inhibitory effects of 5-fluorouracil on the proliferation of gastric adenocarcinoma cells by the GSK-3β signaling pathway

5-Fluorouracil (5-FU) is widely used in the treatment of gastric cancer, and the emergence of drug resistance and toxic effects has limited its application. Therefore, there is an urgent need for safe and effective novel drugs or new therapies. β-Ionone (BI) is found in vegetables and fruits and possesses an inhibitory proliferation of tumor cells in vitro and in vivo. In this study, we investigated whether BI could enhance the inhibitory effects of 5-FU on the proliferation of gastric adenocarcinoma cells and the growth of gastric cancer cell xenografts in a mouse model. The effects of BI and 5-FU alone or their combination on the cell viability, apoptosis, and mitochondrial membrane potential, the cell cycle, and its related proteins-Cyclin D1, and CDK4 as well as PCNA and GSK-3β were evaluated in SGC-7901 cells and MKN45 cells by MTT, MB, flow cytometry and Western blot. In addition, the effects of BI and 5-FU alone or their combination on the growth of SGC-7901 cell xenografts in nude mice were investigated. The results showed that BI significantly enhanced the sensitivity of gastric adenocarcinoma cells to 5-FU in vitro and in vivo, i.e. proliferation inhibited, apoptosis induced and GSK-3β protein activated. Therefore, our results suggest that BI increases the antitumor effect of 5-FU on gastric adenocarcinoma cells, at least partly from an activated GSK-3β signaling pathway.

Rare A360T Mutation Alters GSK3β(Ser9) Binding in the Cytosolic Loop of Presenilin 1, Influencing β-Catenin Nuclear Localization and Pro-Death Gene Expression in Alzheimer's Disease Case

Presenilin 1 (PS1) forms, via its large cytosolic loop, a trimeric complex with N-cadherin and β-catenin, which is a key component of Wnt signaling. PS1 undergoes phosphorylation at 353 and 357 serines upon enhanced activity and elevated levels of the GSK3β isoform. PS1 mutations surrounding these serines may alter the stability of the β-catenin complex. Such mutations are found in some cases of familial early-onset Alzheimer's disease (fEOAD), but their functional impact remains obscure. One of such variants of PS1, the A360T substitution, is located close to GSK3β-targeted serine residues. This variant was recently demonstrated in the French population, but more detail is needed to understand its biological effects. To assess the significance of this variant, we employed functional studies using a fibroblast cell line from an Alzheimer's disease case (a female proband) carrying the A360T mutation. Based on functional transcriptomic, cellular, and biochemical assays, we demonstrated atypically impaired β-catenin/GSK3β signaling in the A360T patient's fibroblasts. In detail, this was characterized by a decreased level of active cytosolic β-catenin and bound by PS1, an increased level of nuclear β-catenin, an increased level of inhibited GSK3β phosphorylated on Ser9, and enhanced interaction of GSK3β(Ser9) with PS1. Based on the transcriptomic profile of the A360T fibroblasts, we proposed a dysregulated transcriptional activity of β-catenin, exemplified by increased expression of various cyclin-dependent kinases and cyclins, such as cyclin D1, potentially inducing neurons' cell cycle re-entry followed by apoptosis. The A360T cells did not exhibit significant amyloid pathology. Therefore, cell death in this PS1 cytosolic loop mutation may be attributed to impaired β-catenin/GSK3β signaling rather than amyloid deposition per se. We further estimated the biological and clinical relevance of the A360T variant by whole exome sequencing (WES). WES was performed on DNA from the blood of an A360T female proband, as well as an unrelated male patient carrying the A360T mutation and his mutation-free daughter (both unavailable for the derivation of the fibroblast cell lines). WES confirmed the highest-priority AD causality of the A360T variant in PS1 and also profiled the pathways and processes involved in the A360T case, highlighting the greatest importance of altered Wnt signaling.

Comparative LC-LTQ-MS-MS Analysis of the Leaf Extracts of Lantana camara and Lantana montevidensis Growing in Egypt with Insights into Their Antioxidant, Anti-Inflammatory, and Cytotoxic Activities

Lantana camara L. and Lantana montevidensis Briq. (F. Verbenaceae) are invasive ornamental weeds native to the tropical regions of Africa and America. The leaves of both species have been traditionally used as infusions for treating fever, rheumatism, and cancer. LC-MS-MS-guided profiling of the methanolic extracts of the leaves of L. camara and L. montevidensis growing in Egypt led to the putative identification of 59 compounds belonging to terpenoids, flavonoids, iridoid glycosides, phenolic acids, and their derivatives. The in-vitro antioxidants and anti-inflammatory and anticancer activities of the two extracts were investigated. L. camara and L. montevidensis inhibited DPPH^• (IC₅₀ = 34.01 ± 1.32 and 47.43 ± 1.74 µg/mL), ABTS⁺ (IC₅₀ = 30.73 ± 1.42 and 40.37 ± 1.51 µg/mL), and superoxide anion (IC₅₀ = 1.57 ± 0.19 and 1.31 ± 0.14 μg/mL) free radicals. A potent anti-inflammatory effect was observed for both species through the inhibition of elastase release in fMLF/CB-induced human neutrophils (IC₅₀ = 2.40 ± 0.16 and 1.90 ± 0.07 μg/mL). The extracts showed significant cytotoxic activity against a panel of cancer cell lines with the most potent activity against Caco cells (IC₅₀ = 45.65 ± 1.64 and 40.67 ± 1.52 µg/mL for L. camara and L. montevidensis, respectively). Western blotting supported by FACS analysis revealed that the extracts inhibited cancer cell proliferation, reduced metastasis, and induced apoptosis resulting in cell cycle arrest. This was achieved via increasing mRNA and protein expressions of p53 and GSK-3β as well as decreasing the expression of PI3K, Akt, and cyclin D1.

The Long-Lost Ligase: CRL4AMBRA1 Regulates the Stability of D-Type Cyclins

D-type cyclins (cyclin D1, D2, and D3, together cyclin D) are central drivers of the cell division cycle and well-described proto-oncoproteins. Rapid turnover of cyclin D is critical for its regulation, but the underlying mechanism has remained a matter of debate. Recently, AMBRA1 was identified as the major regulator of the stability of all three D-type cyclins. AMBRA1 serves as the substrate receptor for one of ∼40 CUL4-RING E3 ubiquitin ligase (CRL4) complexes to mediate the polyubiquitylation and subsequent degradation of cyclin D. Consequently, AMBRA1 regulates cell proliferation to impact tumor growth and the cellular response to cell cycle-targeted cancer therapies. Here we discuss the findings that implicate AMBRA1 as a core member of the cell cycle machinery.

Targeted Protein Profiling of In Vivo NIPP-Treated Tissues Using DigiWest Technology

Non-invasive physical plasma (NIPP) is a novel therapeutic tool, currently being evaluated for the treatment of cancer and precancerous lesions in gynecology and other disciplines. Additionally, patients with cervical intraepithelial neoplasia (CIN) may benefit from NIPP treatment due to its non-invasive, side-effect-free, and tissue-sparing character. However, the molecular impact of in vivo NIPP treatment needs to be further investigated. For this purpose, usually only very small tissue biopsies are available after NIPP treatment. Here, we adapted DigiWest technology, a high-throughput bead-based Western blot, for the analysis of formalin-fixed paraffin-embedded (FFPE) cervical punch biopsies with a minimal sample amount. We investigated the molecular effects of NIPP treatment directly after (0 h) and 24 h after in vivo application. Results were compared to in vitro NIPP-treated human malignant cervical cells. NIPP effects were primarily based on an inhibitory impact on the cell cycle and cell growth factors. DigiWest technology was suitable for detailed protein profiling of small, primary FFPE biopsies.

VS-5584, a PI3K/mTOR dual inhibitor, exerts antitumor effects on neuroblastomas in vitro and in vivo

BACKGROUND

The phosphatidylinositol-3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway is closely related to oncogenesis. PI3K/mTOR inhibitors are considered capable of counteracting the feedback mechanisms within the pathway. In this study, we investigated the antitumor effects of VS-5584, an orally administered PI3K/mTOR dual inhibitor, on neuroblastomas.

METHODS

The effects of VS-5584 on proliferation, cell cycle distribution, and related signaling molecules were examined in neuroblastoma cells using the (3-[4,5-dimethylthiazole-2-yl]-2,5-diphenyltetrazolium bromide)-based colorimetric assay, flow cytometry, and western blotting, respectively. Nude mice were subcutaneously inoculated with human neuroblastoma cells, followed by VS-5584 treatment for two weeks. Tumor growth was tracked and tumor tissues were subjected to immunohistochemical investigations.

RESULTS

In neuroblastoma cells, VS-5584 significantly inhibited proliferation and induced G0/G1 cell cycle arrest. Additionally, VS-5584 decreased the expression of phospho-S6 kinase 1 (p-S6K1), p-retinoblastoma protein, p-cyclin-dependent kinase 2, and cyclin E1, and increased the expression of p21 and p27 in neuroblastoma cells. In mice, VS-5584 significantly suppressed tumor growth in neuroblastomas and downregulated the expression of p-mTOR and p-S6K1 in tumor tissues.

CONCLUSIONS

VS-5584 blocks the PI3K/mTOR pathway, induces a G0/G1 cell cycle arrest, and exerts antitumor effects on neuroblastomas both in vitro and in vivo.

CHANGES OF CYCLIN D1-DEPENDENT REGULATION OF CELL CYCLE IN PERIPHERAL BLOOD LYMPHOCYTES OF CHORNOBYL CLEAN-UP WORKERS AT A REMOTE PERIOD AFTER RADIATION EXPOSURE

OBJECTIVE

To study proliferative potential of peripheral blood lymphocytes of Chornobyl clean-up workers by levelof expression of cyclin D1 and quantitative parameters of cell cycle at a remote period after radiation exposure.

MATERIALS AND METHODS

The research subject was the peripheral blood lymphocytes (PB) of Chornobyl clean-upworkers 30-33 years after radiation exposure. A total of 207 men were surveyed, 164 of them were clean-up workers exposed in the dose range 10.43-3623.31 mSv and 43 persons of the control group. Analysis of proliferationpotential (cell cycle initiation) and cyclin D1 expression in PB lymphocytes were performed in vitro by a micro methodof whole blood leukocytes culture with phytohemagglutinine-P (PHA). Sample preparation was performed by a standard immunofluorescent assay for intracellular proteins using the FITC labelled Mouse Anti-Human Cyclin D1Antibody Set. Cell distribution by cell cycle phases studied by propidium iodide DNA staining and analysis onFACSCalibur laser flow cytometer in histogram mode with separation of G0/G1-, S- and G2/M-regions and Sub-G0/G1-region (apoptotic cells).

RESULTS AND CONCLUSIONS

An increase in the level of spontaneous сyclin D1 expression and disturbance of сyclinD1-dependent regulation of cell cycle of PB lymphocytes after mitogen activation were determined in a remote period after radiation exposure. An increase in the level of cyclin D1 expression was accompanied by increase in pool ofcells in the S- and G2/M-phases of cell cycle which characterizes the high proliferative potential of PB lymphocytes.Mitogen-induced delay of cell cycle of lymphocytes in G1/S check point and reduction of S-phase was revealed.These changes are a manifestation of genomic instability caused by the effect of radiation and depend on the radiation dose. The results confirm the hypothesis about the significance of levels of cyclin D1 expression, as a criterion for manifestations of genome instability and risks of oncogenesis in a remote period after irradiation.

MST4 inhibits human hepatocellular carcinoma cell proliferation and induces cell cycle arrest via suppression of PI3K/AKT pathway

Objective: MST4 has exhibited functions in regulating cell polarity, Golgi apparatus, cell migration, and cancer. Mechanistically, it affects the activity of p-ERK, Hippo-YAP pathway and autophagy. The aim of this study is to further examine the functions of MST4 in hepatocellular carcinoma (HCC) and the underlying mechanism. Methods: The expression level of MST4 in HCC and noncancer adjacent liver tissues was determined by qRT-PCR and immunohistochemistry staining. Wild-type MST4 (MST4) and a dominant-negative mutant of MST4 (dnMST4) were overexpressed in HCC cell lines, respectively. CCK-8 assay, EdU incorporation assay, and soft agar assay were used to determine cell proliferation in vitro. The xenograft mouse model was employed to determine HCC cell growth in vivo. Cell cycle analysis was performed by PI staining and flow cytometry. The expression of key members in PI3K/AKT pathway was detected by Western blot analysis. Results: In our study, we reported new evidence that MST4 was frequently down-regulated in HCC tissues. Gain-of-function and loss-of-function experiments demonstrated that MST4 negatively regulated in vitro HCC cell proliferation. Additionally, MST4 overexpression suppressed Bel-7404 cell tumor growth in nude mice. Further experiments revealed that the growth-inhibitory effect of MST4 overexpression was partly due to a G1-phase cell cycle arrest. Importantly, mechanistic investigations suggested that dnMST4 significantly elevated the phosphorylation levels of key members of PI3K/AKT pathway, and the selective PI3K inhibitor LY294002 can reverse the proliferation-promoting effect of dnMST4. Conclusions: Overall, our results provide a new insight into the clinical significance, functions and molecular mechanism of MST4 in HCC, suggesting that MST4 might have a potential therapeutic value in the HCC clinical treatment.

Morpholine as ubiquitous pharmacophore in medicinal chemistry: Deep insight into the structure-activity relationship (SAR)

Morpholine is a versatile moiety, a privileged pharmacophore and an outstanding heterocyclic motif with wide ranges of pharmacological activities due to different mechanisms of action. The ability of morpholine to enhance the potency of the molecule through molecular interactions with the target protein (kinases) or to modulate the pharmacokinetic properties propelled medicinal chemists and researchers to synthesize morpholine ring by the efficient ways and to incorporate this moiety to develop various lead compounds with diverse therapeutic activities. The present review primarily focused on discussing the most promising synthetic leads containing morpholine ring along with structure-activity relationship (SAR) to reveal the active pharmacophores accountable for anticancer, anti-inflammatory, antiviral, anticonvulsant, antihyperlipidemic, antioxidant, antimicrobial and antileishmanial activity. This review outlines some of the recent effective chemical synthesis for morpholine ring. The review also highlighted the metabolic liability of some clinical drugs containing this nucleus and various researches on modified morpholine to enhance the metabolic stability of drugs as well. Drugs bearing morpholine ring and those under clinical trials are also mentioned with the role of morpholine and their mechanism of action. This review will provide the necessary knowledge base to the medicinal chemists in making strategic structural changes in designing morpholine derivatives.

Vanadyl complexes discriminate between neuroblastoma cells and primary neurons by inducing cell-specific apoptotic pathways

Vanadium compounds have arisen as potential therapeutic agent for the treatment of cancers over the past decades. A few studies suggested that vanadyl complexes may discriminate between the cancerous and the normal cells. Here, we reported the investigation on the pro-apoptotic effect and the underlying mechanism of bis(acetylacetonato) oxovanadium(IV) ([VO(acac)₂]) on SH-SY5Y neuroblastoma cells in comparison with that of mouse primary cortex neurons. The experimental results revealed that [VO(acac)₂] showed about 10-fold higher cytotoxicity (IC50 ~16 μM) on the neuroblastoma cells than on normal neurons (IC50 ~250 μM). Further analysis indicated that the vanadyl complex suppressed the growth of neuroblastoma cells via different pathways depending on its concentration. It induced a special cyclin D-mediated and p53-independent cell apoptosis at <50 μM but cell cycle arrests at >50 μM. In contrast, [VO(acac)₂] promoted cell viability of primary neurons in the concentration range of 0-150 μM; while [VO(acac)₂] at hundreds of μM would cause neuronal death possibly via the reactive oxygen species (ROS)-mediated signal pathways. The extraordinary discrimination between neuroblastoma cells and primary neurons suggests potential application of vanadyl complexes for therapeutic treatment of neuroblastoma. In addition, the p53-independent apoptotic pathways induced by vanadyl complexes may provide new insights for future discovery of new anticancer drugs overcoming the chemo-resistance due to p53 mutation.

The GSK3 kinase inhibitor lithium produces unexpected hyperphosphorylation of β-catenin, a GSK3 substrate, in human glioblastoma cells

Lithium salt is a classic glycogen synthase kinase 3 (GSK3) inhibitor. Beryllium is a structurally related inhibitor that is more potent but relatively uncharacterized. This study examined the effects of these inhibitors on the phosphorylation of endogenous GSK3 substrates. In NIH-3T3 cells, both salts caused a decrease in phosphorylated glycogen synthase, as expected. GSK3 inhibitors produce enhanced phosphorylation of Ser9 of GSK3β via a positive feedback mechanism, and both salts elicited this enhancement. Another GSK3 substrate is β-catenin, which has a central role in Wnt signaling. In A172 human glioblastoma cells, lithium treatment caused a surprising increase in phospho-Ser33/Ser37-β-catenin, which was quantified using an antibody-coupled capillary electrophoresis method. The β-catenin hyperphosphorylation was unaffected by p53 RNAi knockdown, indicating that p53 is not involved in the mechanism of this response. Lithium caused a decrease in the abundance of axin, a component of the β-catenin destruction complex that has a role in coordinating β-catenin ubiquitination and protein turnover. The axin and phospho-β-catenin results were reproduced in U251 and U87MG glioblastoma cell lines. These observations run contrary to the conventional view of the canonical Wnt signaling pathway, in which a GSK3 inhibitor would be expected to decrease, not increase, phospho-β-catenin levels.

This article has an associated First Person interview with the first author of the paper.

Summary: GSK3 inhibitors have potential use against Alzheimer's disease and other conditions. In this study, a classic inhibitor produced unexpected molecular effects on key components of the Wnt signaling pathway.

Tomato leaf curl Yunnan virus-encoded C4 induces cell division through enhancing stability of Cyclin D 1.1 via impairing NbSKη -mediated phosphorylation in Nicotiana benthamiana

The whitefly-transmitted geminiviruses induce severe developmental abnormalities in plants. Geminivirus-encoded C4 protein functions as one of viral symptom determinants that could induce abnormal cell division. However, the molecular mechanism by which C4 contributes to cell division induction remains unclear. Here we report that tomato leaf curl Yunnan virus (TLCYnV) C4 interacts with a glycogen synthase kinase 3 (GSK3)/SHAGGY-like kinase, designed NbSKη, in Nicotiana benthamiana. Pro32, Asn34 and Thr35 of TLCYnV C4 are critical for its interaction with NbSKη and required for C4-induced typical symptoms. Interestingly, TLCYnV C4 directs NbSKη to the membrane and reduces the nuclear-accumulation of NbSKη. The relocalization of NbSKη impairs phosphorylation dependent degradation on its substrate-Cyclin D1.1 (NbCycD1;1), thereby increasing the accumulation level of NbCycD1;1 and inducing the cell division. Moreover, NbSKη-RNAi, 35S::NbCycD1;1 transgenic N. benthamiana plants have the similar phenotype as 35S::C4 transgenic N. benthamiana plants on callus-like tissue formation resulted from abnormal cell division induction. Thus, this study provides new insights into mechanism of how a viral protein hijacks NbSKη to induce abnormal cell division in plants.

PSAT1 is regulated by ATF4 and enhances cell proliferation via the GSK3β/β-catenin/cyclin D1 signaling pathway in ER-negative breast cancer

Background

A growing amount of evidence has indicated that PSAT1 is an oncogene that plays an important role in cancer progression and metastasis. In this study, we explored the expression and function of PSAT1 in estrogen receptor (ER)-negative breast cancer.

Method

The expression level of PSAT1 in breast cancer tissues and cells was analyzed using real-time-PCR (RT-PCR), TCGA datasets or immunohistochemistry (IHC). The overall survival of patients with ER-negative breast cancer stratified by the PSAT1 expression levels was evaluated using Kaplan-Meier analysis. The function of PSAT1 was analyzed using a series of in vitro assays. Moreover, a nude mouse model was used to evaluate the function of PSAT1 in vivo. qRT-PCR and western blot assays were used to evaluate gene and protein expression, respectively, in the indicated cells. In addition, we demonstrated that PSAT1 was activated by ATF4 by chromatin immunoprecipitation (ChIP) assays.

Results

mRNA expression of PSAT1 was up-regulated in ER-negative breast cancer. A tissue microarray that included 297 specimens of ER-negative breast cancer was subjected to an immunohistochemistry assay, which demonstrated that PSAT1 was overexpressed and predicted a poor clinical outcome of patients with this disease. Our data showed that PSAT1 promoted cell proliferation and tumorigenesis in vitro and in vivo. We further found that PSAT1 induced up-regulation of cyclin D1 via the GSK3β/β-catenin pathway, which eventually led to the acceleration of cell cycle progression. Furthermore, ATF4 was also overexpressed in ER-negative breast cancers, and a positive correlation between the ATF4 and PSAT1 mRNA levels was observed in ER-negative breast cancers. We further demonstrated that knockdown of ATF4 by siRNA reduced PSAT1 expression. Finally, chromatin immunoprecipitation (ChIP) assays showed that PSAT1 was a target of ATF4.

Conclusions

PSAT1, which is overexpressed in ER-negative breast cancers, is activated by ATF4 and promotes cell cycle progression via regulation of the GSK3β/β-catenin/cyclin D1 pathway.

Electronic supplementary material

The online version of this article (doi: 10.1186/s13046-017-0648-4) contains supplementary material, which is available to authorized users.

Encorafenib (LGX818), a potent BRAF inhibitor, induces senescence accompanied by autophagy in BRAFV600E melanoma cells

Encorafenib (LGX818) is a new-generation BRAF inhibitor that is under evaluation in clinical trials. However, the underlying mechanism remains to be elucidated. Here we show that LGX818 potently decreased ERK phosphorylation and inhibited proliferation in BRAFV600E melanoma cell lines. Moreover, LGX818 downregulated CyclinD1 in a glycogen synthase kinase 3β-independent manner and induced cell cycle arrest in the G1 phase, Surprisingly, LGX818 triggered cellular senescence in BRAFV600E melanoma cells, as evidenced by increased β-galactosidase staining, while no appreciable induction of apoptosis was detected, as determined by Annexin V and propidium iodide staining and immunoblot analysis of caspase-3 processing and poly (ADP-ribose) polymerase cleavage. Increased p27KIP1 expression and retinoblastoma protein activation were detected during LGX818-induced senescence. Additionally, inhibition of dual-specificity tyrosine phosphorylation-regulated kinase 1B by AZ191 reversed LGX818-induced CyclinD1 turnover and senescence. Interestingly, autophagy is triggered through inhibition of the mTOR/70S6K pathway during LGX818-induced senescence. Moreover, autophagy inhibition by pharmacological and genetic regulation attenuates LGX818-induced senescence. Notably, combining LGX818 with autophagy modulators has anti-proliferative effect in LGX818-resistant BRAF mutant melanoma cells. Altogether, we uncovered a mechanism by which LGX818 exerts its anti-tumor activity in BRAFV600E melanoma cells.

A novel DYRK1B inhibitor AZ191 demonstrates that DYRK1B acts independently of GSK3β to phosphorylate cyclin D1 at Thr(286), not Thr(288)

DYRK1B (dual-specificity tyrosine phosphorylation-regulated kinase 1B) is amplified in certain cancers and may be an oncogene; however, our knowledge of DYRK1B has been limited by the lack of selective inhibitors. In the present study we describe AZ191, a potent small molecule inhibitor that selectively inhibits DYRK1B in vitro and in cells. CCND1 (cyclin D1), a key regulator of the mammalian G1-S-phase transition, is phosphorylated on Thr(286) by GSK3β (glycogen synthase kinase 3β) to promote its degradation. DYRK1B has also been proposed to promote CCND1 turnover, but was reported to phosphorylate Thr(288) rather than Thr(286). Using in vitro kinase assays, phospho-specific immunoblot analysis and MS in conjunction with AZ191 we now show that DYRK1B phosphorylates CCND1 at Thr(286), not Thr(288), in vitro and in cells. In HEK (human embryonic kidney)-293 and PANC-1 cells (which exhibit DYRK1B amplification) DYRK1B drives Thr(286) phosphorylation and proteasome-dependent turnover of CCND1 and this is abolished by AZ191 or DYRK1B RNAi, but not by GSK3β inhibitors or GSK3β RNAi. DYRK1B expression causes a G1-phase cell-cycle arrest, but overexpression of CCND1 (wild-type or T286A) fails to overcome this; indeed, DYRK1B also promotes the expression of p21CIP1 (21 kDa CDK-interacting protein 1) and p27KIP1 (CDK-inhibitory protein 1). The results of the present study demonstrate for the first time that DYRK1B is a novel Thr(286)-CCND1 kinase that acts independently of GSK3β to promote CCND1 degradation. Furthermore, we anticipate that AZ191 may prove useful in defining further substrates and biological functions of DYRK1B.

Glycogen synthase kinase 3β inhibition as a therapeutic approach in the treatment of endometrial cancer

Alternative strategies beyond current chemotherapy and radiation therapy regimens are needed in the treatment of advanced stage and recurrent endometrial cancers. There is considerable promise for biologic agents targeting the extracellular signal-regulated kinase (ERK) pathway for treatment of these cancers. Many downstream substrates of the ERK signaling pathway, such as glycogen synthase kinase 3β (GSK3β), and their roles in endometrial carcinogenesis have not yet been investigated. In this study, we tested the importance of GSK3β inhibition in endometrial cancer cell lines and in vivo models. Inhibition of GSK3β by either lithium chloride (LiCl) or specific GSK3β inhibitor VIII showed cytostatic and cytotoxic effects on multiple endometrial cancer cell lines, with little effect on the immortalized normal endometrial cell line. Flow cytometry and immunofluorescence revealed a G2/M cell cycle arrest in both type I (AN3CA, KLE, and RL952) and type II (ARK1) endometrial cancer cell lines. In addition, LiCl pre-treatment sensitized AN3CA cells to the chemotherapy agent paclitaxel. Administration of LiCl to AN3CA tumor-bearing mice resulted in partial or complete regression of some tumors. Thus, GSK3β activity is associated with endometrial cancer tumorigenesis and its pharmacologic inhibition reduces cell proliferation and tumor growth.

The role of cyclin D1 in response to long-term exposure to ionizing radiation

The health-related hazards resulting from long-term exposure to radiation remain unknown. Thus, an appropriate molecular marker is needed to clarify these effects. Cyclin D1 regulates the cell cycle transition from the G 1 phase to the S phase. Cyclin D1 is degraded as a G1/S checkpoint after 10 Gy of single acute radiation exposure, whereas conversely, cyclin D1 is stabilized when human tumor cells are exposed to fractionated radiation (FR) with 0.5 Gy of x-rays for 31 d. In this article, we review new findings regarding cyclin D1 overexpression in response to long-term exposure to FR. Cyclin D1 overexpression is associated with induction of genomic instability in irradiated cells. Therefore, repression of cyclin D1 expression is likely to cancel the harmful effects of long-term exposure to FR. Thus cyclin D1 may be a marker of long-term exposure to radiation and is a putative molecular radioprotection target for radiation safety.

Cyclin d1 downregulation contributes to anticancer effect of isorhapontigenin on human bladder cancer cells

Isorhapontigenin (ISO) is a new derivative of stilbene compound that was isolated from the Chinese herb Gnetum Cleistostachyum and has been used for treatment of bladder cancers for centuries. In our current studies, we have explored the potential inhibitory effect and molecular mechanisms underlying isorhapontigenin anticancer effects on anchorage-independent growth of human bladder cancer cell lines. We found that isorhapontigenin showed a significant inhibitory effect on human bladder cancer cell growth and was accompanied with related cell cycle G(0)-G(1) arrest as well as downregulation of cyclin D1 expression at the transcriptional level in UMUC3 and RT112 cells. Further studies identified that isorhapontigenin downregulated cyclin D1 gene transcription via inhibition of specific protein 1 (SP1) transactivation. Moreover, ectopic expression of GFP-cyclin D1 rendered UMUC3 cells resistant to induction of cell-cycle G(0)-G(1) arrest and inhibition of cancer cell anchorage-independent growth by isorhapontigenin treatment. Together, our studies show that isorhapontigenin is an active compound that mediates Gnetum Cleistostachyum's induction of cell-cycle G(0)-G(1) arrest and inhibition of cancer cell anchorage-independent growth through downregulating SP1/cyclin D1 axis in bladder cancer cells. Our studies provide a novel insight into understanding the anticancer activity of the Chinese herb Gnetum Cleistostachyum and its isolate isorhapontigenin.

Phosphoinositide-3-kinases p110α and p110β mediate S phase entry in astroglial cells in the marginal zone of rat neocortex

In cells cultured from neocortex of newborn rats, phosphoinositide-3-kinases of class I regulate the DNA synthesis in a subgroup of astroglial cells. We have studied the location of these cells as well as the kinase isoforms which facilitate the S phase entry. Using dominant negative (dn) isoforms as well as selective pharmacological inhibitors we quantified S phase entry by nuclear labeling with bromodeoxyuridine (BrdU). Only in astroglial cells harvested from the marginal zone (MZ) of the neocortex inhibition of phosphoinositide-3-kinases reduced the nuclear labeling with BrdU, indicating that neocortical astroglial cells differ in the regulation of proliferation. The two kinase isoforms p110α and p110β were essential for S phase entry. p110α diminished the level of the p27^Kip1 which inactivates the complex of cyclin E and CDK2 necessary for entry into the S phase. p110β phosphorylated and inhibited glycogen synthase kinase-3β which can prevent S-phase entry. Taken together, both isoforms mediated S phase in a subgroup of neocortical astroglial cells and acted via distinct pathways.

Adenosine-5'-triphosphate up-regulates proliferation of human cardiac fibroblasts

BACKGROUND AND PURPOSE

ATP is a potent signalling molecule that regulates biological activities including increasing or decreasing proliferation in different types of cells. The aim of the present study was to investigate how ATP regulates the proliferation of human cardiac fibroblasts.

EXPERIMENTAL APPROACH

Reverse transcription (RT)-PCR, Western blot analysis, cell proliferation and migration assays were employed to investigate the effects of ATP on human adult ventricular fibroblasts.

KEY RESULTS

ATP increased cell proliferation in a concentration-dependent manner. Similarly, the P2X receptor agonist α,β-methylene ATP and P2Y receptor agonist ATP-γS also up-regulated cell proliferation. The P2 receptor antagonists suramin and reactive blue-2 prevented the ATP-induced increase in proliferation and RT-PCR and Western blot analysis revealed that mRNAs of P2X(4/7) and P2Y(2) are abundant in cardiac fibroblasts. ATP increased phosphorylated PKB (Akt) and ERK1/2 levels; an effect antagonized by suramin, reactive blue-2, the PI3-kinase inhibitor, wortmannin, PKB inhibitor, API-2, and MAPK inhibitor, PD98059. These kinase inhibitors also prevented the ATP-induced increase in proliferation. In addition, ATP enhanced the progression of cells from the G0/G1 phase to the S phase by increasing the expression of proteins for cyclin D1 and cyclin E. Silencing the P2X(4/7) and P2Y(2) receptors with siRNA targeting the corresponding receptor diminished ATP-stimulated proliferation and migration of the cardiac fibroblasts.

CONCLUSION AND IMPLICATION

ATP activates P2X(4/7) and P2Y(2) receptors and up-regulates the proliferation of human cardiac fibroblasts by promoting cell cycling progression. It also increases the migration of these cells. These effects of ATP may be involved in cardiac remodelling of injured hearts.

Differential cellular and molecular effects of butyrate and trichostatin a on vascular smooth muscle cells

The histone deacetylase (HDAC) inhibitors, butyrate and trichostatin A (TSA), are epigenetic histone modifiers and proliferation inhibitors by downregulating cyclin D1, a positive cell cycle regulator, and upregulating p21Cip1 and INK family of proteins, negative cell cycle regulators. Our recent study indicated cyclin D1 upregulation in vascular smooth muscle cells (VSMC) that are proliferation-arrested by butyrate. Here we investigate whether cyclin D1 upregulation is a unique response of VSMC to butyrate or a general response to HDAC inhibitors (HDACi) by evaluating the effects of butyrate and TSA on VSMC. While butyrate and TSA inhibit VSMC proliferation via cytostatic and cytotoxic effects, respectively, they downregulate cdk4, cdk6, and cdk2, and upregulate cyclin D3, p21Cip1 and p15INK4B, and cause similar effects on key histone H3 posttranslational modifications. Conversely, cyclin D1 is upregulated by butyrate and inhibited by TSA. Assessment of glycogen synthase 3-dependent phosphorylation, subcellular localization and transcription of cyclin D1 indicates that differential effects of butyrate and TSA on cyclin D1 levels are linked to disparity in cyclin D1 gene expression. Disparity in butyrate- and TSA-induced cyclin D1 may influence transcriptional regulation of genes that are associated with changes in cellular morphology/cellular effects that these HDACi confer on VSMC, as a transcriptional modulator.

Three Observations That Have Changed Our Understanding of Cyclin D1 and p27 in Cell Cycle Control

Our understanding of cell cycle control has been based largely upon studies of synchronized cultures, often focused upon the early stages of the cell cycle following stimulation of quiescent cultures. These studies showed that cyclin D1 and p27(Kip1) (p27) each respond to the growth environment of the cell and together control entry into the cell cycle. In contrast, all cell cycle phases were considered in these studies of actively growing cultures, including events leading to withdrawal from the cell cycle. This approach relies upon the techniques of microinjection, quantitative image analysis, and time-lapse microscopy. The results provide critical new detail to our understanding of the roles of cyclin D1 and p27 in cell cycle regulation. Three critical observations resulting from this work will be described here to demonstrate that 1) cyclin D1 levels oscillate through the normal cell cycle, 2) checkpoint kinases are able to suppress cyclin D1 during S phase, and 3) the level of p27 is determined by a dynamic interaction between cyclin D1 and p27 so as to determine the rate of cell cycle progression. Based upon these observations, a model of cell cycle control is presented in which ras activity stimulates cyclin D1 during G2 phase, resulting in commitment of the cell to continued cell cycle progression. During G1 phase, ras activity suppresses the level of p27 protein, most of which is bound to cyclin D1, resulting in regulation of the rate of proliferation. This model predicted the involvement of checkpoint kinases in regulating cyclin D1 and the role of checkpoint kinases in the protection of neural cells against reactive oxygen. The substantiation of these 2 predictions serves as general validation of the model.

Examining the role of cyclin D1 in breast cancer

Cyclin D1 overexpression is found in more than 50% of human breast cancers and causes mammary cancer in transgenic mice. Dysregulation of cyclin D1 gene expression or function contributes to the loss of normal cell cycle control during tumorigenesis. Recent studies have demonstrated that cyclin D1 conducts additional specific functions to regulate gene expression in the context of local chromatin, promote cellular migration and inhibit mitochondrial metabolism. It is anticipated that these additional functions contribute to the pathology associated with dysregulated cyclin D1 abundance. This article discusses evidence that examines the significance of cyclin D1 in breast cancer with emphasis on its role in breast cancer stem cell expansion.

The role of Skp2 in hematopoietic stem cell quiescence, pool size, and self-renewal

Although the maintenance of HSC quiescence and self-renewal are critical for controlling stem cell pool and transplantation efficiency, the mechanisms by which they are regulated remain largely unknown. Understanding the factors controlling these processes may have important therapeutic potential for BM failure and cancers. Here, we show that Skp2, a component of the Skp2 SCF complex, is an important regulator for HSC quiescence, frequency, and self-renewal capability. Skp2 deficiency displays a marked enhancement of HSC populations through promoting cell cycle entry independently of its role on apoptosis. Surprisingly, Skp2 deficiency in HSCs reduces quiescence and displays increased HSC cycling and proliferation. Importantly, loss of Skp2 not only increases HSC populations and long-term reconstitution ability but also rescues the defect in long-term reconstitution ability of HSCs on PTEN inactivation. Mechanistically, we show that Skp2 deficiency induces Cyclin D1 gene expression, which contributes to an increase in HSC cycling. Finally, we demonstrate that Skp2 deficiency enhances sensitivity of Lin(-) Sca-1(+) c-kit(+) cells and leukemia cells to chemotherapy agents. Our findings show that Skp2 is a novel regulator for HSC quiescence and self-renewal and that targeting Skp2 may have therapeutic implications for BM transplantation and leukemia stem cell treatment.

Impaired nuclear export of tumor-derived c-terminal truncated cyclin D1 mutant in ESCC cancer

Cyclin D1 is a significant regulator of the G1- to S-phase transition and is often aberrant in human tumors of various origins. Although cancer-derived cyclin D1 mutants are potent oncogenes in vitro and in vivo, the mechanisms by which they contribute to neoplasia remaind to be elucidated. We previously identified a cyclin D1 mutation (Δ266-295) in esophageal cancer with deleted codons from 266 to 295 of wild-type cyclin D1, the critical COOH-terminal regulatory sequences necessary for cyclin D1 nuclear export. In the present study, this cancer-derived cyclin D1-Δ266-295 was shown to be a constitutively nuclear cyclin D1 protein with a significantly increased oncogenic potential. Moreover, the cancer-derived cyclin D1-Δ266-295 mutant was found to retain its ability to bind to and activate CDK4, which in turn phosphorylates and inactivates the pRb protein and promotes cell cycle progression. In comparison to wild-type cyclin D1a, D1-Δ266-295 exhibited enforced nuclear accumulation. In addition, the transient transfection and ectopic expression of this nuclear localized D1-Δ266-295 up-regulated endogenous Notch1 expression, indicating that the mutant retained its ability as a transcriptional regulator. Furthermore, data from the flow cytometry assay showed that D1-Δ266-295 fractionally increased >4N cell accumulation, and further analysis suggested the retriggering of DNA replication relevant to its inhibition of Cdt1 proteolysis. Therefore, the inappropriate nuclear localization of this cyclin D1 mutant may interfere with DNA replication in cultured cells, thereby contributing to genomic instability.

What Are the bona fide GSK3 Substrates?

Nearly 100 proteins are proposed to be substrates for GSK3, suggesting that this enzyme is a fundamental regulator of almost every process in the cell, in every tissue in the body. However, it is not certain how many of these proposed substrates are regulated by GSK3 in vivo. Clearly, the identification of the physiological functions of GSK3 will be greatly aided by the identification of its bona fide substrates, and the development of GSK3 as a therapeutic target will be highly influenced by this range of actions, hence the need to accurately establish true GSK3 substrates in cells. In this paper the evidence that proposed GSK3 substrates are likely to be physiological targets is assessed, highlighting the key cellular processes that could be modulated by GSK3 activity and inhibition.

Silica induces cell cycle changes through PI-3K/AP-1 pathway in human embryo lung fibroblast cells

Exposure to silica is associated with progressive pulmonary inflammation and fibrosis. Our previous study had demonstrated silica exposure could cause cell cycle alternation and activator protein-1 (AP-1) activation. This study showed that silica exposure induced phosphorylation of p70S6 kinase (p70S6K) and Akt in human embryo lung fibroblasts (HELFs). These changes were blocked by overexpression of dominant-negative mutants of phosphatidylinositol-3 kinase (Δp85) or Akt (DN-Akt), respectively. Moreover, pretreatment of cells with rapamycin, a specific p70S6K inhibitor, could inhibit silica-induced cell cycle alteration, AP-1 activation, and phosphorylation of p70S6K, but had no effect on Akt phosphorylation. This suggested that phosphatidylinositol-3 kinase (PI-3K)/AP-1 pathway was likely responsible for cell cycle changes. Furthermore, we observed the effect of the pathway on cell cycle regulatory proteins. Our results indicated that inactivation of PI-3K, Akt, or p70S6K could inhibit silica-induced overexpression of cyclin D1 and cyclin-dependent kinase 4 (CDK4) and decreased expression of E2F-4. Taken together, silica could induce cell cycle changes through PI-3K/ AP-1 pathway in HELFs.

Clinical and biological significance of GSK-3β inactivation in breast cancer-an immunohistochemical study

Glycogen synthase kinase 3β, recently found to be functionally abnormal in various types of human disease, is negatively regulated by the PI3K/Akt signaling pathway. As Akt is constitutively activated in a subset of breast cancer, we hypothesized that glycogen synthase kinase 3β is inappropriately inactivated in these cases. In this study, we aimed to assess (1) the overall frequency of glycogen synthase kinase 3β inactivation in breast cancer; (2) whether there is an association between Akt activation and glycogen synthase kinase 3β inactivation; and (3) whether there is a correlation between glycogen synthase kinase 3β inactivation and various pathologic and clinical parameters. The phosphorylated form of glycogen synthase kinase 3β (pGSK-3β) and Akt (pAkt) were used as surrogate markers for glycogen synthase kinase 3β inactivation and Akt activation, respectively. Immunohistochemistry applied to paraffin-embedded tissues was used to assess 72 consecutive invasive mammary carcinomas, of which 50 were estrogen receptor positive. Overall, pGSK-3β and pAkt were positive in 34 (47.2%) and 35 (48.6%) cases, respectively. These 2 markers were significantly correlated with each other in the overall group and in the estrogen receptor-positive subgroup (P = .01 and .003, Spearman, respectively). Importantly, pGSK-3β, but not pAkt, significantly correlated a worse clinical outcome in this cohort (P = .004, log rank). In summary, evidence of glycogen synthase kinase 3β inactivation was found in approximately half of the invasive mammary carcinomas. Our data suggest that this abnormality is likely attributed to Akt activation and that glycogen synthase kinase 3β inactivation confers a worse clinical outcome.

Constitutive activation of glycogen synthase kinase-3beta correlates with better prognosis and cyclin-dependent kinase inhibitors in human gastric cancer

Background

Aberrant regulation of glycogen synthase kinase-3β (GSK-3β) has been implicated in several human cancers; however, it has not been reported in the gastric cancer tissues to date. The present study was performed to determine the expression status of active form of GSK-3β phosphorylated at Tyr²¹⁶ (pGSK-3β) and its relationship with other tumor-associated proteins in human gastric cancers.

Methods

Immunohistochemistry was performed on tissue array slides containing 281 human gastric carcinoma specimens. In addition, gastric cancer cells were cultured and treated with a GSK-3β inhibitor lithium chloride (LiCl) for immunoblot analysis.

Results

We found that pGSK-3β was expressed in 129 (46%) of 281 cases examined, and was higher in the early-stages of pathologic tumor-node-metastasis (P < 0.001). The expression of pGSK-3β inversely correlated with lymphatic invasion (P < 0.001) and lymph node metastasis (P < 0.001) and correlated with a longer patient survival (P < 0.001). In addition, pGSK-3β expression positively correlated with that of p16, p21, p27, p53, APC, PTEN, MGMT, SMAD4, or KAI1 (P < 0.05), but not with that of cyclin D1. This was confirmed by immunoblot analysis using SNU-668 gastric cancer cells treated with LiCl.

Conclusions

GSK-3β activation was frequently observed in early-stage gastric carcinoma and was significantly correlated with better prognosis. Thus, these findings suggest that GSK-3β activation is a useful prognostic marker for the early-stage gastric cancer.

Unique and overlapping functions of GSK-3 isoforms in cell differentiation and proliferation and cardiovascular development

Intensive study over the past 30 years has helped define the role of the GSK-3 (glycogen synthase kinase-3) family in a variety of physiological and pathophysiological processes. However, the majority of these studies have relied upon overexpression approaches or nonselective small molecule inhibitors. Herein, we examine recent data derived from studies in gene-targeted embryonic stem cells and knock-out mice in an attempt to define the role these protein kinases play in critical decisions made by stem/progenitor cells and by early lineage-committed cardiomyocytes during development.

Deletion of GSK-3beta in mice leads to hypertrophic cardiomyopathy secondary to cardiomyoblast hyperproliferation

Based on extensive preclinical data, glycogen synthase kinase-3 (GSK-3) has been proposed to be a viable drug target for a wide variety of disease states, ranging from diabetes to bipolar disorder. Since these new drugs, which will be more powerful GSK-3 inhibitors than lithium, may potentially be given to women of childbearing potential, and since it has controversially been suggested that lithium therapy might be linked to congenital cardiac defects, we asked whether GSK-3 family members are required for normal heart development in mice. We report that terminal cardiomyocyte differentiation was substantially blunted in Gsk3b(-/-) embryoid bodies. While GSK-3alpha-deficient mice were born without a cardiac phenotype, no live-born Gsk3b(-/-) pups were recovered. The Gsk3b(-/-) embryos had a double outlet RV, ventricular septal defects, and hypertrophic myopathy, with near obliteration of the ventricular cavities. The hypertrophic myopathy was caused by cardiomyocyte hyperproliferation without hypertrophy and was associated with increased expression and nuclear localization of three regulators of proliferation - GATA4, cyclin D1, and c-Myc. These studies, which we believe are the first in mammals to examine the role of GSK-3alpha and GSK-3beta in the heart using loss-of-function approaches, implicate GSK-3beta as a central regulator of embryonic cardiomyocyte proliferation and differentiation, as well as of outflow tract development. Although controversy over the teratogenic effects of lithium remains, our studies suggest that caution should be exercised in the use of newer, more potent drugs targeting GSK-3 in women of childbearing age.

Lithium inhibits proliferation of human esophageal cancer cell line Eca-109 by inducing a G2/M cell cycle arrest

AIM: To investigate the effect of lithium on proliferation of esophageal cancer (EC) cells and its preliminary mechanisms.

METHODS: Eca-109 cells were treated with lithium chloride, a highly selective inhibitor of glycogen synthase kinase 3β (GSK-3β), at different concen-trations (2-30 mmol/L) and time points (0, 2, 4, 6 and 24 h). Cell proliferative ability was evaluated by 3-(4,5-dimethylthiazole-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay, and cell cycle distribution was examined by flow cytometry. Expressions of p-GSK-3β, β-catenin, cyclin B1, cdc2 and cyclin D1 protein were detected by Western blotting, and the subcellular localization of β-catenin was determined by immunofluorescence. The mRNA level of cyclin B1 was detected by reverse transcription polymerase chain reaction (RT-PCR).

RESULTS: Lithium could inhibit the proliferation of Eca-109 cells. Lithium at a concentration of 20 mmol/L lithium for 24 h produced obvious changes in the distribution of cell cycle, and increased the number of cells in G₂/M phase (P < 0.05 vs control group). Western blotting showed that lithium inhibited GSK-3β by Ser-9 phosphorylation and stabilized free β-catenin in the cytoplasm. Immunofluorescence further confirmed that free β-catenin actively translocated to the nucleus. Moreover, lithium slightly elevated cyclin D1 protein expression, whereas lowered the cyclin B1 expression after 24 h lithium exposure and no obvious change was observed for cdc2 protein.

CONCLUSION: Lithium can inhibit the proliferation of human esophageal cancer cell line Eca-109 by inducing a G₂/M cell cycle arrest, which is mainly mediated through the inhibition of lithium-sensitive molecule, GSK-3β, and reduction of cyclin B1 expression.

Cell cycle studies based upon quantitative image analysis

When cell cycle studies are performed following cell cycle synchronization, it is possible that critical properties of an actively cycling cell will be overlooked. For this reason past studies have not revealed critical aspects of cell cycle control; such as how a cell determines when to exit the cell cycle, or how rapidly it should cycle. To address these challenging questions we have developed a procedure to quantitate fluorescent stains in a monolayer culture, where nuclear fluorescence and cell cycle history can be assessed with accuracy on a cell by cell basis. The cell cycle position of each cell can be determined by analyzing DNA and BrdU levels. The behavior of cells in a given cell cycle position can then be studied by quantitating up to two other stained markers. When the microinjection of siRNA, neutralizing antibodies, and expression plasmids are coupled with quantitative image analysis, these cell cycle studies can be conducted following alterations in the expression levels of selected cellular targets. With these techniques we have discovered critical aspects of cell cycle control; including how cyclin D1 levels vary through the cell cycle, the molecular mechanisms governing these changes, and the biological implications of changes in cyclin D1 concentration in various cell cycle stages. Our studies with cyclin D1, coupled with similar studies of p27Kip1, form the basis of an entirely new model of cell cycle control proposed here. This model explains how cell cycle progression is terminated, and how the length of the cell cycle is regulated.

Glycogen synthase kinase-3beta -- actively inhibiting hypertrophy

A number of signaling pathways are involved in the regulation of cardiac hypertrophy and remodeling. One that serves as an integrator of upstream inputs, and as a transducer to downstream effectors, is the protein kinase, glycogen synthase kinase-3beta. In this work we review the role of this putative "nodal point" in the response of the heart to growth stimuli, both physiologic and pathologic.

Glycogen synthase kinase-3beta and p38 phosphorylate cyclin D2 on Thr280 to trigger its ubiquitin/proteasome-dependent degradation in hematopoietic cells

Cyclin D2 plays an important role in regulation of hematopoietic cell proliferation by cytokines and is implicated in oncogenesis of various hematopoietic malignancies. However, mechanisms regulating cyclin D2 stability and its expression level have remained to be known. Here, we demonstrate that interleukin-3 signaling stabilizes cyclin D2 by inhibition of glycogen synthase kinase-3beta (GSK3beta) through Janus kinase2-dependent activation of phosphatidylinositol 3'-kinase (PI3K)/Akt signaling pathway in hematopoietic 32Dcl3 cells. On the other hand, osmotic stress was shown to induce a rapid proteasomal degradation of cyclin D2, which was mediated by activation of p38. GSK3beta and p38 was demonstrated to phosphorylate cyclin D2 on Thr280 in vitro, while a cyclin D2 mutant with this residue substituted with Ala was found to be resistant to ubiquitination and proteasome-dependent degradation in 32Dcl3 cells. Inhibition of the PI3K pathway or induction of osmotic stress also caused a rapid proteasomal degradation of cyclin D2 in primary leukemic or myeloma cells. These results indicate that cyclin D2 expression in normal and malignant hematopoietic cells is regulated by ubiquitin/proteasome-dependent degradation that is triggered by Thr280 phosphorylation by GSK3beta or p38, which is induced by inhibition of the PI3K pathway or by osmotic stress, respectively.

The regulation of cyclin D1 degradation: roles in cancer development and the potential for therapeutic invention

Cyclin D1 is an important regulator of cell cycle progression and can function as a transcriptionl co-regulator. The overexpression of cyclin D1 has been linked to the development and progression of cancer. Deregulated cyclin D1 degradation appears to be responsible for the increased levels of cyclin D1 in several cancers. Recent findings have identified novel mechanisms involved in the regulation of cyclin D1 stability. A number of therapeutic agents have been shown to induce cyclin D1 degradation. The therapeutic ablation of cyclin D1 may be useful for the prevention and treatment of cancer. In this review, current knowledge on the regulation of cyclin D1 degradation is discussed. Novel insights into cyclin D1 degradation are also discussed in the context of ablative therapy. A number of unresolved questions regarding the regulation of cellular cyclin D1 levels are also addressed.

Variations in cyclin D1 levels through the cell cycle determine the proliferative fate of a cell

We present evidence that variations in cyclin D1 levels through the cell cycle are essential for continuing proliferation. Cyclin D1 levels must be high during G1 phase for a cell to initiate DNA synthesis, but then must be suppressed to low levels during S phase to allow for efficient DNA synthesis. This suppression during S phase is apparently regulated by cell cycle position alone and occurs automatically during each cell cycle. If the cell is to continue proliferating, cyclin D1 levels must be induced once again during G2 phase. This induction depends upon the activity of proliferative signaling molecules, and ensures that the extracellular environment continues to be conducive for growth. We propose that the suppression of cyclin D1 levels during each S phase ensures that the subsequent induction during G2 phase, and the resulting commitment to continuing proliferation, is closely linked to the cellular growth environment.