Overexpression of mouse D-type cyclins accelerates G1 phase in rodent fibroblasts.

TLDR

Mammalian D‑type cyclins are growth‑factor‑regulated early response genes that control G1 progression by modulating cyclin‑dependent kinase activity. Overexpression of cyclin D1 or D2 in rodent fibroblasts shortens G0‑to‑S and G1‑to‑S intervals, lowers serum dependence, and for D1 reduces cell size, whereas antibody inhibition of cyclin D1 blocks S‑phase entry, demonstrating that cyclin D1 (and likely D2) are rate‑limiting for G1 progression.

Abstract

Mammalian D-type cyclins are growth factor-regulated, delayed early response genes that are presumed to control progression through the G1 phase of the cell cycle by governing the activity of cyclin-dependent kinases (cdks). Overexpression of mouse cyclin D1 in serum-stimulated mouse NIH-3T3 and rat-2 fibroblasts increased their rates of G0 to S- and G1- to S-phase transit by several hours, leading to an equivalent contraction of their mean cell generation times. Although such cells remained contact inhibited and anchorage dependent, they manifested a reduced serum requirement for growth and were smaller in size than their normal counterparts. Ectopic expression of cyclin D2 in rodent fibroblasts, either alone or together with exogenous cdk4, shortened their G0- to S-phase interval and reduced their serum dependency, but cyclin D2 alone did not alter cell size significantly. When cells were microinjected during the G1 interval with a monoclonal antibody specifically reactive to cyclin D1, parental rodent fibroblasts and derivatives overexpressing this cyclin were inhibited from entering S phase, but cells injected near the G1/S phase transition were refractory to antibody-induced growth suppression. Thus, cyclin D1, and most likely D2, are rate limiting for G1 progression.