Abstract IA21: Mitogenic signaling and the RB/p53 network
Authors: Sherr Charles J. 
Published: November 2016

Abstract
Cells entering the division cycle from a quiescent state (G0) in response to mitogenic cues synthesize one or more D-type cyclins (D1, D2, D3) that assemble in G1 phase with cyclin-dependent kinases CDK4 and/or CDK6. Notably, the transcription, assembly with CDK 4/6, and stability of D-type cyclins are each mitogen-dependent steps. Accumulation of cyclin D-dependent kinases in G1 phase leads to the progressive phosphorylation of the retinoblastoma protein (RB), facilitating transcription of E2F-responsive genes, including cyclins E and A, whose CDK2-dependent functions are normally required in S phase. Moreover, the stoichiometric sequestration of CDK2 inhibitors, p27Kip1 and p21Cip1, by cyclin D-dependent kinases, further enables CDK2 activation as well as SCFSkp2-dependent p27 degradation as cells approach the G1/S boundary. In G0, RB remains unphosphorylated, and E2F-responsive genes are repressed by the DREAM complex, which includes E2F-4/5, at least one retinoblastoma family member (RBL2/p130), and the MuvB complex. An open question is whether CDK4 and CDK6 are responsible, at least in part, for dissolution of the DREAM complex or relief of p130/E2F repression in early G1 phase. Cells in G0 and G1 express the APCCDH1 E3 ubiquitin ligase that prevents accumulation of the mitotic cyclins, A and B, prior to S phase entry. As cells near the G1/S transition, APCCDH1 is inactivated by CDK2 in conjunction with the E2F-responsive early mitotic inhibitor EMI1.
Cyclin D-CDK4/6 complexes accumulate maximally in the nucleus at G1/S, where their phosphorylation by GSK-3β triggers their subsequent nuclear export, ubiquitination, and proteasomal degradation in S phase. In continuously cycling cells, Ras signaling in G2 phase restores nuclear cyclin D, enabling its re-accumulation before M phase; cyclin D-CDK4/6 activity in G2/M shortens the subsequent G1 interval and cell cycle length. Mitogen withdrawal, leads to rapid cyclin D turnover and ultimately to G1/G0 arrest. Similar effects can result from oncogene-mediated induction of the specific CDK4/6 inhibitor p16INK4A, or by CDK4/6 inhibitory drugs (CDK4/6i's) that act as chemical p16 mimetics. Notably, continuous CDK4/6 inhibition by CDK4/6i's or p16 can induce reversible (quiescence) or irreversible (senescence) cell cycle exit; understanding the factors that distinguish these outcomes continues to represent a challenge. Although monotherapy with orally available, potent, and specific CDK4/6i's has not generally yielded durable clinical benefits in cancer treatment, these drugs can induce significant progression-free survival with minimal dose limiting side effects (principally clinically manageable neutropenia) when combined with targeted inhibitors of mitogenic signaling pathways that limit the accumulation of D-type cyclins. Perhaps, an “addiction” of cancer cells to mitogenic pathways constitutively activated by oncogenic mutations increases their vulnerability to CDK4/6i's while sparing normal cycling cells.
Expression of the CDKN2A/B gene cluster (encoding p16, p15INK4B, and ARF) is driven by an upstream super-enhancer. Canonical super-enhancer histone “marks” (H3K4me1 and H3K27Ac) are detected in cells that are able to express CDKN2A/B in response to hyperproliferative signals but are absent from their reprogrammed iPS derivatives or from ES cells. More than 30 independent GWAS studies have fingered SNPs within the CDKN2A/B super-enhancer region that are associated not only with various cancers but also with common degenerative diseases of aging populations, including coronary artery disease, aortic aneurysm, and type II diabetes. These findings suggest that while downregulation of CDKN2A/B results in a loss of tumor suppression, its increased expression during aging leads to cellular senescence and a loss of tissue regenerative capacity.
Citation Format: Charles J. Sherr. Mitogenic signaling and the RB/p53 network. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr IA21.