Protein kinase Cδ blocks immediate-early gene expression in senescent cells by inactivating serum response factor

Fibroblasts lose the ability to replicate in response to growth factors and become unable to express growth-associated immediate-early genes, including c-fos and egr-1, as they become senescent. The serum response factor (SRF), a major transcriptional activator of immediate-early gene promoters, loses the ability to bind to the serum response element (SRE) and becomes hyperphosphorylated in senescent cells. We identify protein kinase C delta (PKCdelta) as the kinase responsible for inactivation of SRF both in vitro and endogenously in senescent cells. This is due to a higher level of PKCdelta activity as cells age, production of the PKCdelta catalytic fragment, and its nuclear localization in senescent but not in low-passage-number cells. The phosphorylation of T160 of SRF by PKCdelta in vitro and in vivo led to loss of SRF DNA binding activity. Both the PKCdelta inhibitor rottlerin and ectopic expression of a dominant negative form of PKCdelta independently restored SRE-dependent transcription and immediate-early gene expression in senescent cells. Modulation of PKCdelta activity in vivo with rottlerin or bistratene A altered senescent- and young-cell morphology, respectively. These observations support the idea that the coordinate transcriptional inhibition of several growth-associated genes by PKCdelta contributes to the senescent phenotype.