Multiple Functions of D-Type Cyclins Can Antagonize pRb-Mediated Suppression of Proliferation

Abstract

The most well understood function of the D-type cyclins is to activate the G 1 kinases, cdk4 and cdk6, and target the retinoblastoma gene product (pRb) for phosphorylation and inactivation. pRb can suppress S phase entry, cause a transient G 1 arrest following DNA damage, and is critical in establishing terminal cell cycle withdrawal in cells exposed to differentiation or senescence-inducing signals. Each of these functions of pRb can be demonstrated in cultured cells derived from human tumors that have suffered RB1 gene inactivation. In such in vitro assays, coexpression of D type cyclins has been shown to inhibit the function of pRb, likely reflecting an oncogenic role of cyclin D1 in vivo. Two regions of cyclin D, the LxCxE pRb-binding motif, and the cyclin box, are thought to be critical for the proper function of cyclin D. Here we show that the LxCxE motif is dispensable in cyclin D1 for all functions tested, but is required by cyclin D2. This observation suggests that there is a functional difference between cyclins D1 and D2 in pRb regulation, and argues against complete functional redundancy of these D cyclins. In addition, the ability of cyclins D1 and D2 to activate cdk partners is required for induction of pRb phosphorylation and S phase entry. However, mutant forms of cyclins D1 and D2 that are incapable of activating kinase partners were still able to prevent pRb-induced senescence. Thus, D cyclins have both kinase-dependent and kinase-independent mechanisms of interfering with proliferation arrest and senescence. www.landesbioscience.com Cell Cycle 331 Multiple Functions of D-Type Cyclins

Src kinase inhibition decreases thrombin-induced injury and cell cycle re-entry in striatal neurons

“…In the studies of cell cycle related genes, PP2 significantly down-regulated Cdkn2b, Ccnd1, and Ccnd2 mRNAs that were up regulated by thrombin. Ccnd1 and Ccnd2 code for cyclin D1 and cyclin D2 proteins, respectively, and these cyclin D proteins activate Cdks which are required for induction of Rb phosphorylation and S phase entry (Nurse, 2000, Baker et al, 2005. This suggests that PP2 might block thrombin-induced cell cycle reactivation at the transcriptional level, in part, by down-regulating mRNA expression of cyclin D1 and cyclin D2.…”

Posttranslational Protein Modifications

“…Phosphorylation of pRB1 is thus the key regulatory step in this pathway controlling cell division. Amplification of the cyclin D1 gene or mutations in p16 are often found in tumors contributing to a hyperphosphorylated state of pRB1 and a consequential commitment to continued cell growth (16,17).…”

Functional high-throughput screening identifies the miR-15 microRNA family as cellular restriction factors for Salmonella infection

“…For overexpression of HA-tagged cyclin D1, HeLa cells (5 Â 10 5 cells) were seeded into 6-cm dishes 24 h before transfection. The cells were transfected using FuGENE HD Transfection reagent (Promega, E2311) with either 2 mg of pRc/CMV-CyclinD1-HA 58 (Addgene, plasmid 8948) or 2 mg of pAS1B-HA 59 plasmid in combination with 0.2 mg of pBabe/GEM2 (ref. 60) plasmid, which expresses an internal membrane-anchored GFP, as a transfection marker.…”