Tumor suppression by IFN regulatory factor-1 is mediated by transcriptional down-regulation of cyclin D1

IFNs have been ascribed to mediate antitumor effects. IFN regulatory factor-1 (IRF-1) is a major target gene of IFNs. It inhibits cell proliferation and oncogenic transformation. Here, we show that 60% of all mRNAs deregulated by oncogenic transformation mediated by c-myc and H-ras are reverted to the expression levels of nontransformed cells by IRF-1. These include cell cycle-regulating genes. An indirect target is cyclin D1. Activation of IRF-1 decreased cyclin D1 expression and cyclin-dependent kinase 4 kinase activity concomitant with change in the levels of hyperphosphorylated retinoblastoma protein. These effects are mediated by inhibition of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway and a transcriptional repression of cyclin D1. As shown by in vitro assays and tumor growth in nude mice, IRF-1-mediated effects on cell cycle progression were found to be overridden by ectopic expression of cyclin D1. Conversely, decrease of cyclin D1 by RNA interference experiments prevents transformation and tumor growth. The data show that cyclin D1 is a key target for IRF-1-mediated tumor-suppressive effects.

p38-mediated inactivation of cyclin D1/cyclin-dependent kinase 4 stimulates nucleolar translocation of RelA and apoptosis in colorectal cancer cells

Aberrant nuclear factor-kappaB (NF-kappaB) signaling plays a role in cancer initiation and progression; thus, it represents a potential therapeutic target. We previously identified a mechanism of repression of NF-kappaB transcriptional activity and induction of apoptosis in colon cancer cells involving nuclear/nucleolar translocation of the RelA (p65) component of NF-kappaB. This response was stimulated by cellular stress-inducing agents, including aspirin, but not by tumor necrosis factor. Here, we investigate the upstream molecular mechanisms responsible for nucleolar targeting of RelA and show that aspirin activates the p38 mitogen-activated protein kinase (MAPK) pathway in colorectal cancer cells. We also show that aspirin causes rapid, ubiquitin-dependent degradation of cyclin D1, a known p38 target. Aspirin-induced p38 activation preceded cyclin D1 degradation, which was then followed by activation of the NF-kappaB pathway, suggesting a causative link. Indeed, chemical p38 inhibition (PD169316) and small interfering RNA directed against p38 blocked aspirin-induced cyclin D1 degradation, nucleolar translocation of RelA, and apoptosis. Furthermore, chemical inhibition of the cyclin D1/cyclin-dependent kinase 4 (CDK4) kinase complex, used as a surrogate for cyclin D1 degradation, caused nucleolar translocation of RelA, repression of kappaB-driven transcription, and apoptosis, thereby reproducing the effects of aspirin. In addition, we found that aspirin and the CDK4 inhibitor induced nucleolar translocation of RelA and apoptosis through a common mechanism involving the NH(2)-terminal nucleolar localization signal. Collectively, these data suggest that aspirin causes inhibition of cyclin D1/CDK4 through the p38 MAPK pathway. This inhibition stimulates the NF-kappaB pathway to induce nucleolar translocation of RelA and apoptosis. These novel findings have considerable relevance to the rational design of novel chemotherapeutic and chemopreventative strategies.

Cigarette tar phenols impede T cell cycle progression by inhibiting cyclin-dependent kinases

Cigarette smoking causes profound suppression of pulmonary T cell responses, which is associated with increased susceptibility to respiratory tract infections and decreased tumor surveillance. We previously demonstrated that the phenolic compounds in cigarette tar inhibit blastogenesis and interfere with human T cell cycle progression. To identify the mechanism by which cell cycle arrest occurs, we examined the effects of these compounds on cyclin-dependent kinases (Cdk) that control the G0/G1 transition. We found that hydroquinone inhibited induction of Cdk4 and Cdk6 kinase activities by >80%, while catechol and phenol were markedly less potent. HQ did not affect mitogenic induction of the Cdk6 protein, but inhibited expression of cyclin D3 by >90% resulting in a dramatic reduction in proper Cdk6/Cyclin D3 complex formation.

Differential Utilization of Cyclin D1 and Cyclin D3 in the Distinct Mitogenic Stimulations by Growth Factors and TSH of Human Thyrocytes in Primary Culture

Two distinct mitogenic modes coexist in thyroid epithelial cells. TSH via cAMP induces proliferation and differentiation expression, whereas growth factors including epidermal growth factor (EGF) induce proliferation and dedifferentiation. Divergent models of TSH/cAMP-dependent mitogenesis have emerged from different thyroid cell culture systems. In the FRTL-5 rat cell line, cAMP cross-signals with transduction pathways of growth factors to induce cyclin D1 and p21(cip1) and down-regulate p27(kip1). By contrast, in canine primary cultures, mitogenic pathways of cAMP and growth factors are fully distinct. cAMP does not induce D-type cyclins and p21, it up-regulates p27, and it stimulates the formation and activity of cyclin D3-cyclin-dependent kinase (CDK) 4 complexes. In primary cultures of normal human thyrocytes, EGF + serum increased cyclin D1 and p21 accumulation, and it stimulated the assembly and activity of cyclin D1-CDK4-p21 complexes. By contrast, TSH repressed or did not induce cyclin D1 and p21, and it rather up-regulated p27. TSH did not increase cyclin D1-CDK4 activity, but it stimulated the activating phosphorylation of CDK4 and the pRb-kinase activity of preexisting cyclin D3-CDK4 complexes. As recently demonstrated in dog thyrocytes and other systems, cyclin D1 and cyclin D3 differently oriented the site specificity of CDK4 pRb-kinase activity, which might differently impact some pRb functions. Cyclin D1 or cyclin D3 are thus differentially used in the distinct mitogenic stimulations by growth factors or TSH, and potentially in hyperproliferative diseases generated by the overactivation of their respective signaling pathways. At variance with dog thyroid primary cultures, rat thyroid cell lines might not be valid models of TSH-dependent mitogenesis of human thyrocytes.

3D-QSAR CoMFA study on indenopyrazole derivatives as cyclin dependent kinase 4 (CDK4) and cyclin dependent kinase 2 (CDK2) inhibitors

Cyclin dependent kinases (CDKs) have appeared as an important drug targets over the years with diverse therapeutic potentials. With the objective of designing new chemical entities with enhanced inhibitory potencies against CDK 2 (CDK2) and CDK 4 (CDK4), the 3D-QSAR CoMFA study carried out on indenopyrazole derivatives as inhibitors of these kinases is presented here. The developed model showed a strong correlative and predictive capability having a cross validated correlation co-efficient of 0.747 for CDK4 and 0.755 for CDK2 inhibitions. The conventional and predictive correlation co-efficients were, respectively, found to be 0.913 and 0.760 for CDK4, 0.941 and 0.765 for CDK2. The models could be employed to design ligands with enhanced inhibitory potencies and/or to predict the potencies of analogues to guide synthesis.

Dissecting the determinants of cyclin-dependent kinase 2 and cyclin-dependent kinase 4 inhibitor selectivity

Cyclin dependent kinases are a key family of kinases involved in cell cycle regulation and are an attractive target for cancer chemotherapy. The roles of four residues of the cyclin-dependent kinase active site in inhibitor selectivity were investigated by producing cyclin-dependent kinase 2 mutants bearing equivalent cyclin-dependent kinase 4 residues, namely F82H, L83V, H84D, and K89T. Assay of the mutants with a cyclin-dependent kinase 4-selective bisanilinopyrimidine shows that the K89T mutation is primarily responsible for the selectivity of this compound. Use of the cyclin-dependent kinase 2-selective 6-cyclohexylmethoxy-2-(4'-sulfamoylanilino)purine (NU6102) shows that K89T has no role in the selectivity, while the remaining three mutations have a cumulative influence. The results indicate that certain residues that are not frequently considered in structure-aided kinase inhibitor design have an important role to play.

Inhibition of cancer cell growth by cyclin dependent kinase 4 inhibitors synthesized based on the structure of fascaplysin

Tryptamine derivatives, a new structural class of cyclin dependent kinase 4 inhibitors, have been identified during extensive biological screening of synthetic molecules. The molecules were synthesized based on the structure of fascaplysin, which is not only a specific inhibitor of the Cdk4-cyclin D1 enzyme but also a relatively toxic molecule, probably because it binds and intercalates DNA. Interestingly, the new structural analogues of fascaplysin do not interact or intercalate with double-stranded DNA, although they inhibit Cdk4-cyclin D1 specifically. We found that compound CA199 was the most potent molecule, showing at least 25-fold specificity towards Cdk4-cyclin D1 (IC50 for Cdk4-cyclin D1 = 20 microM, Cdk2 > 500 microM). CA199 inhibits the growth of different cancer cell lines at concentrations ranging from 10-40 microM. It blocks growth of asynchronous cells at G0/G1 in a retinoblastoma protein (pRb) dependent manner. Moreover, CA199 blocks growth only at early G1 in synchronised cells released from a mimosine-induced G1/S block. These observations are reminiscent of a true Cdk4 inhibitor.

Structure-based drug design of a highly potent CDK1,2,4,6 inhibitor with novel macrocyclic quinoxalin-2-one structure

The design of a novel series of cyclin-dependent kinase (CDK) inhibitors containing a macrocyclic quinoxaline-2-one is reported. Structure-based drug design and optimization from the starting point of diarylurea 2, which we previously reported as a moderate CDK1,2,4,6 inhibitor [J. Biol.Chem.2001, 276, 27548], led to the discovery of potent CDK1,2,4,6 inhibitor that were suitable for iv administration for in vivo study.

A novel orally active small molecule potently induces G1 arrest in primary myeloma cells and prevents tumor growth by specific inhibition of cyclin-dependent kinase 4/6

Cell cycle deregulation is central to the initiation and fatality of multiple myeloma, the second most common hematopoietic cancer, although impaired apoptosis plays a critical role in the accumulation of myeloma cells in the bone marrow. The mechanism for intermittent, unrestrained proliferation of myeloma cells is unknown, but mutually exclusive activation of cyclin-dependent kinase 4 (Cdk4)-cyclin D1 or Cdk6-cyclin D2 precedes proliferation of bone marrow myeloma cells in vivo. Here, we show that by specific inhibition of Cdk4/6, the orally active small-molecule PD 0332991 potently induces G(1) arrest in primary bone marrow myeloma cells ex vivo and prevents tumor growth in disseminated human myeloma xenografts. PD 0332991 inhibits Cdk4/6 proportional to the cycling status of the cells independent of cellular transformation and acts in concert with the physiologic Cdk4/6 inhibitor p18(INK4c). Inhibition of Cdk4/6 by PD 0332991 is not accompanied by induction of apoptosis. However, when used in combination with a second agent, such as dexamethasone, PD 0332991 markedly enhances the killing of myeloma cells by dexamethasone. PD 0332991, therefore, represents the first promising and specific inhibitor for therapeutic targeting of Cdk4/6 in multiple myeloma and possibly other B-cell cancers.

Direct modulation of rheumatoid inflammatory mediator expression in retinoblastoma protein-dependent and -independent pathways by cyclin-dependent kinase 4/6

OBJECTIVE

It is known that the cyclin-dependent kinase inhibitor (CDKI) gene p21(Cip1) suppresses rheumatoid inflammation by down-modulating type I interleukin-1 receptor (IL-1RI) expression and inhibiting JNK activity. The purpose of this study was to determine whether CDK activity directly modulates the production of inflammatory molecules in patients with rheumatoid arthritis (RA).

METHODS

Genes for the CDKIs p16(INK4a) and p18(INK4c), a constitutively active form of retinoblastoma (RB) gene product, cyclin D1, and CDK-4, were transferred into RA synovial fibroblasts (RASFs). RASFs were also treated with a synthetic CDK-4/6 inhibitor (CDK4I). Levels of matrix metalloproteinase 3 (MMP-3), monocyte chemoattractant protein 1 (MCP-1), and IL-1RI expression were determined by Northern blotting, real-time polymerase chain reaction analysis, and enzyme-linked immunosorbent assay. CDKIs were immunoprecipitated to reveal their association with JNK.

RESULTS

Transfer of the p16(INK4a) and p18(INK4c) genes and CDK4I suppressed the production of MMP-3 and MCP-1. Unlike p21(Cip1), neither CDKI gene inhibited IL-1RI or JNK. The expression of MMP-3 was up-regulated when CDK-4 activity was augmented. This regulation functioned at the messenger RNA (mRNA) level in MMP-3, but not in MCP-1. Transfer of active RB suppressed the production of MMP-3 and MCP-1 without changing their mRNA levels.

CONCLUSION

CDK-4/6 modulated the production of MMP-3 and MCP-1. MMP-3 production was regulated primarily at the mRNA level in an RB-independent manner, whereas MCP-1 production was controlled posttranscriptionally by RB. These results show that cell cycle proteins are associated with control of mediators of inflammation through multiple pathways.

Pharmacologic inhibition of cyclin-dependent kinase 4/6 activity arrests proliferation in myoblasts and rhabdomyosarcoma-derived cells

Myoblast cell cycle exit and differentiation are mediated in part by down-regulation of cyclin D1 and associated cyclin-dependent kinase (Cdk) activity. Because rhabdomyosarcoma may represent a malignant tumor composed of myoblast-like cells failing to exit the cell cycle and differentiate, we considered whether excess Cdk activity might contribute to this biology. Cyclin D-dependent Cdk4 and Cdk6 were expressed in most of a panel of six human rhabdomyosarcoma-derived cell lines. Cdk4 was expressed in 73% of alveolar and embryonal rhabdomyosarcoma tumors evaluated using a human tissue microarray. When challenged to differentiate by mitogen deprivation in vitro, mouse C2C12 myoblasts arrested in G(1) phase of the cell cycle, whereas four in the panel of rhabdomyosarcoma cell lines failed to do so. C2C12 myoblasts maintained in mitogen-rich media and exposed to a Cdk4/Cdk6 inhibitor PD 0332991 accumulated in G(1) cell cycle phase. Similar treatment of rhabdomyosarcoma cell lines caused G(1) arrest and prevented cell accumulation in vitro, and it delayed growth of rhabdomyosarcoma xenografts in vivo. Consistent with a role for Cdk4/Cdk6 activity as a regulator of myogenic differentiation, we observed that PD 0332991 exposure promoted morphologic changes and enhanced the expression of muscle-specific proteins in cultured myoblasts and in the Rh30 cell line. Our findings support the concept that pharmacologic inhibition of Cdk4/Cdk6 may represent a useful therapeutic strategy to control cell proliferation and possibly promote myogenic differentiation in rhabdomyosarcoma.

New potential inhibitors of cyclin-dependent kinase 4: Design and synthesis of pyrido[2,3-d]pyrimidine derivatives under microwave irradiation

A simple and efficient synthesis of 2-amino pyrido[2,3-d]pyrimidine derivatives was accomplished via a three-component reaction under microwave irradiation without catalyst. This method had many dramatic advantages such as the short reaction time, high yield, and broad substrate scope, as well as convenient operation. We provide new series of potential biologically active compounds as inhibitors of Cdk4.

Fascaplysin, a selective CDK4 inhibitor, exhibit anti-angiogenic activity in vitro and in vivo

PURPOSE

This study was to evaluate the correlation of two important strategies, namely, cell cycle proliferation arrest and anti-angiogenesis. We chose fascaplysin, a marine natural product with selective CDK4 selective inhibition activity, to study its potential anti-angiogenesis effects in vivo and in vitro.

METHODS

Chorioallantoic membrane (CAM) assay was initially used as an in vivo approach to evaluate anti-angiogenic activity of fascaplysin. In addition, human umbilical vein endothelial cell (HUVEC) line was used to further confirm the anti-angiogenic activity of fascaplysin in vitro. To explore the mechanism of anti-angiogenesis, we examined the effect of fascaplysin on vascular endothelial growth factor (VEGF) expression and secretion by hepatocarcinoma cells BeL-7402.

RESULTS

The results of CAM assay suggested fascaplysin inhibited capillary plexus formation in a dose-dependent manner and suppressed VEGF in cross section. Moreover, the in vitro assay also confirmed that fascaplysin provided selective inhibition of endothelial cells proliferation towards tumor cells in low concentration. The immunocytochemical staining and ELISA verified fascaplysin could inhibit VEGF expression and secretion by BeL-7402.

CONCLUSIONS

These findings strongly suggest that fascaplysin is a natural angiogenesis inhibitor.

Toward Understanding the Structural Basis of Cyclin-Dependent Kinase 6 Specific Inhibition

Cyclin-dependent kinases (CDKs) are key players in cell cycle control, and genetic alterations of CDKs and their regulators have been linked to a variety of cancers. Hence, CDKs are obvious targets for therapeutic intervention in various proliferative diseases, including cancer. To date, drug design efforts have mostly focused on CDK2 because methods for crystallization of its inhibitor complexes have been well established. CDK4 and CDK6, however, may be at least as important as enzymes for cell cycle regulation and could provide alternative treatment options. We describe here two complex structures of human CDK6 with a very specific kinase inhibitor, PD0332991, which is based on a pyrido[2,3-d]pyrimidin-7-one scaffold, and with the less specific aminopurvalanol inhibitor. Analysis of the structures suggests that relatively small conformational differences between CDK2 and CDK6 in the hinge region are contributing to the inhibitor specificity by inducing changes in the inhibitor orientation that lead to sterical clashes in CDK2 but not CDK6. These complex structures provide valuable insights for the future development of CDK-specific inhibitors.

Mantle cell lymphoma cells express predominantly cyclin D1a isoform and are highly sensitive to selective inhibition of CDK4 kinase activity

The prognosis for patients with mantle cell lymphoma (MCL) is poor, and at present there is no truly effective therapy. Gene translocation-mediated constitutive expression of cyclin D1 seems to play the key role in the pathogenesis of MCL. Here we report that although 3 of 4 MCL cell lines expressed the recently identified, highly oncogenic cyclin D1b isoform, as well as the canonical cyclin D1a, 8 MCL patient samples expressed only the cyclin D1a protein despite expressing detectable cyclin D1b mRNA. Cell lines and tissue samples displayed constitutive activation of the cyclin D1 signaling cascade, as evidenced by strong expression of CDK4, Rb phosphorylation, and cyclin D1/CDK4 coassociation. All MCL cell lines and tissues examined displayed nondetectable to diminished expression of the cyclin D1 inhibitor p16. Novel small molecule CDK4/CDK6 inhibitor PD0332991 profoundly suppressed--at low nanomolar concentrations--Rb phosphorylation, proliferation, and cell cycle progression at the G0/G1 phase of MCL cells. These findings provide evidence that MCL should be very sensitive to targeted therapy aimed at functional inhibition of the cyclin D1/CDK4 complex.

Identification of potent 5-pyrimidinyl-2-aminothiazole CDK4, 6 inhibitors with significant selectivity over CDK1, 2, 5, 7, and 9

5-Pyrimidinyl-2-aminothiazole 1 was identified as an inhibitor of cyclin-dependent kinases (CDKs) by a screening of the Merck sample repository. The introduction of a methyl group at the C-5 or C-6 position on the pyrimidine ring, directed toward the gate keeper residue of CDK4 (Phe93), led to significant enhancement of selectivity for CDK4 over other CDKs. Compound 3 exhibited more than 300-fold selectivity for CDK4 over CDK1, 2, 5, 7, and 9. Subsequent improvements in aqueous solubility afforded compound 4, which is available for further in vivo studies and this compound inhibited pRb phosphorylation and BrdU incorporation in tumor models.

Structure-based discovery and optimization of potential cancer therapeutics targeting the cell cycle

Progress has been made recently in the structure-based optimization of novel cell cycle antitumor therapeutics based on cyclin-dependent kinase (CDK) inhibition. A novel inhibitor series based on the 2-amino-4-heteroaryl-pyrimidine scaffold was discovered using the LIDAEUS high-throughput docking methodology, and was subsequently optimized for CDK2 potency through information provided by crystallographic complex structures. A computational study of CDK4 inhibitors led to the incorporation of selectivity determinants into a pyrimidine pharmacophore to generate isoform-specific inhibitors. In addition, molecules from the inhibitor series have been crystallized in complex structures with both monomeric inactive CDK2 and an active complex of CDK2 bound to cyclin A or E. This crystallization revealed that significant differences exist in the affinity of the inhibitors for active and inactive states of CDK2. Information on differences in affinity facilitates the prediction of experimental binding of inhibitors and allows for the further development of structure-guided design.

Recombinant human prothrombin kringle-2 induces bovine capillary endothelial cell cycle arrest at G0-G1 phase through inhibition of cyclin D1/CDK4 complex: modulation of reactive oxygen species generation and up-regulation of cyclin-dependent kinase inhibitors

Prothrombin is a plasma glycoprotein involved in blood coagulation and, as we have previously reported, prothrombin kringles inhibit BCE (bovine capillary endothelial) cell proliferation. To reveal the mechanism, we investigated the influence of rk-2 (recombinant human prothrombin kringle-2) on the BCE cell cycle progression and ROS (reactive oxygen species) generation using FACS (fluorescence-activated cell sorter) analysis. Cell cycle analysis showed a decrease of G(1) phase cells in cells treated with bFGF (basic fibroblast growth factor) and an increase in cells treated with rk-2, as compared with the control cells. But, the portion of the S phase was reversed. In Western blot analysis, bFGF induced cytoplasmic translocation of p21(Waf1/Cip1) and p27(Kip1) and phosphorylation of p27(Kip1) but rk-2 treatment inhibited translocation of p21(Waf1/Cip1) and p27(Kip1) from nucleus to cytoplasm and phosphorylation of p27(Kip1). Also, rk-2 induced up-regulation of p53 and nuclear p21(Waf1/Cip1) and inhibited the cyclin D1/CDK4 (cyclin-dependent kinase 4) complex. The ROS level of rk-2-treated BCE cells was increased 2-fold when compared with the control, but treatment with NAC (N-Acetyl-L: -cysteine), an anti-oxidant, decreased ROS generation about 55% as compared with the rk-2 treatment. NAC treatment also restored cell cycle progression inhibited by rk-2 and down-regulated p53 and nuclear p21(Waf1/Cip1) expression induced by rk-2.These data suggest that rk-2 induces the BCE cell cycle arrest at G(0)-G(1) phase through inhibition of the cyclin D1/CDK4 complex caused by increase of ROS generation and nuclear cyclin-dependent kinase inhibitors.

AZ703, an Imidazo[1,2-a]Pyridine Inhibitor of Cyclin-Dependent Kinases 1 and 2, Induces E2F-1-Dependent Apoptosis Enhanced by Depletion of Cyclin-Dependent Kinase 9

Preclinical studies were performed of a novel selective imidazopyridine cyclin-dependent kinase (cdk) inhibitor, AZ703. In vitro kinase assays showed that IC50 values for AZ703 against purified cyclin E/cdk2 and cyclin B/cdk1 were 34 and 29 nmol/L, respectively. In contrast, the IC50 against cdk4 was 10 micromol/L. AZ703 also inhibited cdk7 and cdk9 with IC50 values of 2.1 micromol/L and 521 nmol/L, respectively. Treatment of U2OS, NCI-H1299, and A549 cells for 24 hours resulted in growth arrest involving multiple cell cycle phases. At low drug concentrations (< 2 micromol/L), G2 arrest predominated, whereas at higher concentrations (> or = 2 micromol/L), S-G2 arrest was observed. When cells were synchronized in G1 by starvation and released into AZ703, a block in G1 occurred that was not evident in exponentially growing cells. Cell cycle arrest was associated with reduced phosphorylation of the retinoblastoma protein and p27(Kip1) at cdk2 phospho-sites. Following longer exposures, apoptosis was evident. Cells were further sensitized to AZ703 following recruitment to S phase by synchronization. Consistent with the inhibition of cdks during S and G2 that modulate the activity and stability of E2F-1, AZ703 treatment induced E2F-1 expression. In U2OS and NCI-H1299 cells engineered to inducibly express the dominant-negative mutant E2F-1 (1-374), expression of the mutant decreased AZ703-mediated apoptosis, indicating dependence on E2F-1 transcriptional targets. AZ703-induced apoptosis in NCI-H1299 cells was enhanced by small interfering RNA-mediated depletion of cdk9, which caused reduced levels of Mcl-1 and XIAP, suggesting that cdk2, cdk1, and cdk9 represent a rational subset of family members for drug targeting.

Design, synthesis and biological activity of new CDK4-specific inhibitors, based on fascaplysin

We present the design, synthesis, and biological activity of three classes of tryptamine derivatives, which are non-planar analogues of the toxic anti-cancer agent fascaplysin. We show these compounds to be selective inhibitors of CDK4 over CDK2, the most active compound has an IC50 for the inhibition of CDK4 of 6 microM.

Synthesis, crystal structure and biological activity of β-carboline based selective CDK4-cyclin D1 inhibitors

The design, synthesis and biological activity of a series of non-planar dihydro-beta-carboline and beta-carboline-based derivatives of the toxic anticancer agent fascaplysin is presented. We show these compounds to be selective inhibitors of CDK4 over CDK2 with an IC50 (CDK4-cyclin D1) = 11 micromol for the best compound in the series 4d. The crystallographic analysis of some of the compounds synthesised (3b/d and 4a-d) was carried out, in an effort to estimate the structural similarities between the designed inhibitors and the model compound fascaplysin.

Validation of cyclin D1/CDK4 as an anticancer drug target in MCF-7 breast cancer cells: Effect of regulated overexpression of cyclin D1 and siRNA-mediated inhibition of endogenous cyclin D1 and CDK4 expression

We have examined the role of cyclin D1 and cyclin-dependent kinase-4 (CDK4) in the cell cycle progression and proliferation of MCF-7 breast cancer cells. Forced expression of cyclin D1 using a tetracycline-regulated expression system, and suppression of endogenous cyclin D1 and CDK4 using small interfering RNA (siRNA) were used to validate this protein complex as a drug target in cancer drug discovery. Overexpression of cyclin D1 increased both phosphorylation of the retinoblastoma gene product (RB) and passage through the G1-S phase transition, resulting in increased proliferation of cells. When cyclin D1 expression was shut off, growth rates fell below those seen in control cell lines transfected with the vector, indicating an increased dependence on this protein for proliferation. Inhibition of endogenous cyclin D1 or CDK4 expression by RNA interference resulted in hypophosphorylation of RB and accumulation of cells in G1. These results support the prevailing view that pharmacological inhibition of cyclin D1/CDK4 complexes is a useful strategy to inhibit the growth of tumors. Furthermore, since MCF-7 cells appear to be dependent on this pathway for their continued proliferation, it is a suitable cell line to test novel cyclin D1/CDK4 inhibitors.

Cdk4 promotes adipogenesis through PPARgamma activation

Cell cycle regulators such as E2F1 and retinoblastoma (RB) play crucial roles in the control of adipogenesis, mostly by controlling the transition between preadipocyte proliferation and adipocyte differentiation. The serine-threonine kinase cyclin-dependent kinase 4 (cdk4) works in a complex with D-type cyclins to phosphorylate RB, mediating the entry of cells into the cell cycle in response to external stimuli. Because cdk4 is an upstream regulator of the E2F-RB pathway, we tested whether cdk4 was a target for new factors that regulate adipogenesis. Here we find that cdk4 inhibition impairs adipocyte differentiation and function. Disruption of cdk4 or activating mutations in cdk4 in primary mouse embryonic fibroblasts results in reduced and increased adipogenic potential, respectively, of these cells. We show that the effects of cdk4 are not limited to the control of differentiation; cdk4 also participates in adipocyte function through activation of PPARgamma.