Targeted disruption of CDK4 delays cell cycle entry with enhanced p27(Kip1) activity

The mechanism by which cyclin-dependent kinase 4 (CDK4) regulates cell cycl e progression is not entirely clear. Cyclin D/CDK4 appears to initiate phos phorylation of retinoblastoma protein (Rb) leading to inactivation of the S -phase-inhibitory action of Rb. However, cyclin D/CDK4 has been postulated to act in a noncatalytic manner to regulate the cyclin E/CDK2-inhibitory ac tivity of p27(Kip1) by sequestration. In this study we investigated the rol es of CDK4 in cell cycle regulation by targeted disruption of the mouse CDK 4 gene. CDK4(-/-) mice survived embryogenesis and showed growth retardation and reproductive dysfunction associated with hypoplastic seminiferous tubu les in the testis and perturbed corpus luteum formation in the ovary. These phenotypes appear to be opposite to those of p27-deficient mice such as gi gantism and gonadal hyperplasia. A majority of CDK4(-/-) mice developed dia betes mellitus by 6 weeks, associated with degeneration of pancreatic islet s. Fibroblasts from CDK4(-/-) mouse embryos proliferated similarly to wild- type embryonic fibroblasts under conditions that promote continuous growth. However, quiescent CDK4(-/-) fibroblasts exhibited a substantial (similar to 6-h) delay in S-phase entry after serum stimulation. This cell cycle per turbation by CDK4 disruption was associated with increased binding of p27 t o cyclin E/CDK2 and diminished activation of CDK2 accompanied by impaired R b phosphorylation. Importantly, fibroblasts from CDK4(-/-) p27(-/-) embryos displayed partially restored kinetics of the G(0)-S transition, indicating the significance of the sequestration of p27 by CDK-4. These results sugge st that at least part of CDK4's participation in the rate-limiting mechanis m for the G(0)-S transition consists of controlling p27 activity.