Activation of cyclin D1-kinase in murine fibroblasts lacking both p21Cip1 and p27Kip1

Abstract

Deregulation of D-type cyclin-dependent kinases (CDK4 and 6) is widely observed in various human cancers, illustrating their importance in cell cycle control. Like other cyclin-dependent kinases (CDKs), assembly with cyclins is the most critical step for activation of CDK4/ 6. As previously reported elsewhere, we observed that the level of cyclinD1-CDK4 complex and its associated kinase activity were significantly low in asynchronously proliferating mouse embryo fibroblasts lacking both p21 Cip1 and p27 Kip1 (p21/p27-null MEFs). These evidences imply that p21 Cip1 and p27 Kip1 CDK inhibitors are 'essential activators' of cyclin D-kinases. We, however, discovered here that both the assembly and activation of cyclin D1-CDK4 complex occur when quiescent p21/p27-null MEFs were stimulated to re-enter the cell cycle. This mitogen-induced cyclin D1-kinase activity was blocked by overexpression of p16 INK4a and resulted in the inhibition of S phase entry in p21/p27-null MEFs. Furthermore, ectopic expression of p34 SEI-1 , a mitogen-induced CDK4 binding protein, increased the levels of active cyclinD1-CDK4 complex in asynchronously proliferating p21/p27-null MEFs. Together, our results suggest that there are several independent ways to stimulate the assembly of cyclin D1-CDK4 kinases. Although p21 Cip1 and p27 Kip1 play a role in this process, our results demonstrate that additional mechanisms must occur in G0 to S phase transition.

Decreased p21 Levels Are Required for Efficient Restart of DNA Synthesis after S Phase Block

“…Modulation of p21 functions depends on its final concentration in cells; lower levels of p21 initially promote cell cycle progression by stimulation of cyclin-CDK assembly (52,53), whereas higher levels will inhibit cyclin-CDK activity. Our data suggest that only when there are no p21-free cyclin A-CDK2 complexes will p21 interact with PCNA.…”

Proliferation control strategies to improve productivity and survival during CHO based production culture

“…Cyclin dependent kinases (CDKs) are key components of the pathways that control cell cycle transition (Hengst et al 1994;Sugimoto et al 2002) and therefore are an important target for achieving control of cell proliferation. Cyclins, phosphorylation and the formation of ternary complexes with cyclin dependent kinase inhibitors (CKI), such as p21 CIP1 and p27 KIP1 , direct CDK activity (Grana and Reddy 1995).…”

The CDK4/CDK6 inhibitor PD0332991 paradoxically stabilizes activated cyclin D3-CDK4/6 complexes

“…16,6 Less stable cyclin D3-CDK4 complexes in p21/p27 null cells are hyperactive. [17][18][19] How can p21 and p27 shift from an inhibitory to an activation mode is still poorly understood. One debated possibility is related to different stoichiometries of the binding of these proteins to cyclin-CDK complexes.…”