Ribociclib is a specific cyclin dependent kinase (Cdk) 4/6 inhibitor that induces G<sub>1</sub> arrest by blocking the formation of cyclin D1-Cdk4/6 complex and inhibiting retinoblastoma (RB) phosphorylation.
Forty-four B-chronic lymphocytic leukemias (CLL) were studied by Southern blot analysis using probes for the Ig genes and bcl-1, bcl-2 (major, minor and 5' breakpoint region), bcl-3, c-myc, and retinoblastoma (Rb) loci.
Gene and protein transfer experiments demonstrated that concurrent alterations of cyclin D1 and p16 levels cooperate to (de)regulate G1 control in diploid fibroblasts, and that both events influence growth of retinoblastoma (RB)-positive, but not RB-deficient cancer cells.
Conversely, overexpression of Wt1 attenuated the decrease in S phase induced by ORG 2058 at 48-96 h. This was accompanied by the sustained expression of cyclin D1 despite progestin treatment, and increased levels of retinoblastoma (Rb) phosphorylation at sites targeted by cyclin D1-Cdk4 (Ser249/Thr252).
Alterations of the Retinoblastoma (Rb) pathway are frequent in ovarian cancer, typically resulting from <i>CDKN2A</i> down-regulation, <i>CCNE1</i> amplification, <i>CCND1/2</i> amplification, and <i>RB1</i> loss.
These immunochemical and molecular analyses of the RB1 tumor suppressor gene and cyclin D1 proto-oncogene in a large series of human pancreatic cancers and cell lines indicate that RB1 and cyclin D1 alterations occur during the development of some human DPCAs.
We further demonstrated that ABT-263 treatment markedly increased the expression of p21<sup>Waf1/Cip1</sup> and decreased the expression of cyclin D1 and phospho-Rb (retinoblastoma tumor suppressor protein) (Ser780) proteins that contributed to the G<sub>1</sub>/G<sub>0</sub>-phase arrest.
Taken together, these data suggest that concurrent overexpression of cyclin D1 and functional elimination of p16CDKN2 and p15CDKN2B may characterize certain cases of mantle cell NHL, and that cooperation of the abnormalities is likely to provide a growth advantage of the tumour cells through more efficient inactivation of the RB tumor suppressor.
To investigate the role of the cell cycle regulators p21(Waf1), p27(Kip1), retinoblastoma (Rb), and cyclin D1 in Richter's transformation of chronic lymphocytic leukemia (CLL), we analyzed 19 CLL and eight Richter's syndrome (RS) tumors, previously characterized for p53 and ARF/INK4a abnormalities. p21(Waf1)immunohistochemical expression was negative in 12 of 15 CLL (80%), whereas it was moderate or strong in three of seven RS (43%). p21(Waf1) gene was in germline configuration in all the tumors analyzed.
These data suggest that inhibition of cyclin D1 expression contributes to the growth inhibition induced by the decoy oligonucleotide in MCF7 cells through a cyclin D1/Cdk4/pRB signaling pathway.
Because the PRAD1, or cyclin D1, gene, a cell-cycle regulator, has been implicated in a subgroup of benign parathyroid tumors, we examined the possibility that another cell-cycle regulator with possible functional links to PRAD1, the retinoblastoma tumor-suppressor gene (RB), might be involved in the molecular pathogenesis of parathyroid carcinoma.
The mammalian cell cycle is controlled by regulators of the G1 to S transition such as tumor suppressor proteins, p53 and retinoblastoma (RB); cyclin D1 and cyclin-dependent kinase 4; and inhibitor of cyclin dependent kinase, p16INK4A.
On the other hand, knockdown of p16(ink4a) sensitizes cancer cells to TSC2 knockdown induced cell death in a manner that is likely dependant on serum induction of Cyclin D1 to inactivate the Rb function.
UV-C, as well as EGFR kinase inhibitors, decreased the expression level of cyclin D1 and the phosphorylated level of retinoblastoma, indicating that EGFR down-regulation is correlated to cell cycle arrest.
In skin and tumour biopsies, dinaciclib reduced Rb phosphorylation at CDK2 phospho-sites and modulated expression of cyclin D1 and p53, suggestive of CDK9 inhibition.
These findings reveal an unexpected function of a core cell cycle protein in DNA repair and suggest that targeting cyclin D1 may be beneficial also in retinoblastoma-negative cancers which are currently thought to be unaffected by cyclin D1 inhibition.
We find that stoichiometric inhibition of cyclin D1-CDK4 activity by p21 controls the retinoblastoma (Rb) and E2F transcription program in an ultrasensitive manner.