Differential Roles for Cyclin-Dependent Kinase Inhibitors p21 and p16 in the Mechanisms of Senescence and Differentiation in Human Fibroblasts

Gretchen H Stein , Linda F Drullinger , Alexandre Soulard (1) , Vjekoslav Dulić (2)

1 LGMMIC - Laboratoire de Génétique Moléculaire des Microorganismes et des Interactions Cellulaires
2 CRBM - Centre de recherches de biochimie macromoléculaire

The irreversible G1 arrest in senescent human diploid fibroblasts is probably caused by inactivation of theG1 cyclin–cyclin-dependent kinase (Cdk) complexes responsible for phosphorylation of the retinoblastomaprotein (pRb). We show that the Cdk inhibitor p21Sdi1,Cip1,Waf1, which accumulates progressively in aging cells,binds to and inactivates all cyclin E-Cdk2 complexes in senescent cells, whereas in young cells only p21-freeCdk2 complexes are active. Furthermore, the senescent-cell-cycle arrest occurs prior to the accumulation of theCdk4-Cdk6 inhibitor p16Ink4a, suggesting that p21 may be sufficient for this event. Accordingly, cyclin D1-associated phosphorylation of pRb at Ser-780 is lacking even in newly senescent fibroblasts that have a lowamount of p16. Instead, the cyclin D1-Cdk4 and cyclin D1-Cdk6 complexes in these cells are associated withan increased amount of p21, suggesting that p21 may be responsible for inactivation of both cyclin E- and cyclinD1-associated kinase activity at the early stage of senescence. Moreover, even in the late stage of senescencewhen p16 is high, cyclin D1-Cdk4 complexes are persistent, albeit reduced by <50% compared to young cells.We also provide new evidence that p21 may play a role in inactivation of the DNA replication factor proliferatingcell nuclear antigen during early senescence. Finally, because p16 accumulates in parallel with theincreases in senescence-associated b-Gal activity and cell volume that characterize the senescent phenotype, wesuggest that p16 upregulation may be part of a differentiation program that is turned on in senescent cells.Since p21 decreases after senescence is achieved, this upregulation of p16 may be essential for maintenance ofthe senescent-cell-cycle arrest.

Domaines: Sciences du Vivant [q-bio]

Gretchen H Stein, Linda F Drullinger, Alexandre Soulard, Vjekoslav Dulić. Differential Roles for Cyclin-Dependent Kinase Inhibitors p21 and p16 in the Mechanisms of Senescence and Differentiation in Human Fibroblasts. Molecular and Cellular Biology, 1999, 19 (3), pp.270 - 7306. ⟨hal-01632700⟩