Induction of p21(WAF1/CIP1) and cyclin D1 expression by the Src oncoprotein in mouse fibroblasts: Role of activated STAT3 signaling

While the activated viral Src oncoprotein, v-Src, induces uncontrolled cell growth, the mechanisms underlying cell cycle deregulation by v-Src have no t been fully defined. Previous studies demonstrated that v-Src induces cons titutively active STAT3 signaling that is required for cell transformation and recent data have implicated STAT3 in the transcriptional control of cri tical cell cycle regulators. Here we show in mouse fibroblasts stably trans formed by v-Src that mRNA and protein levels of p21 (WAF1/CIP1), cyclin D1, and cyclin E are elevated. Using reporter constructs in transient-transfec tion assays, the cyclin D1 and p21 promoters were both found to be transcri ptionaly induced by v-Src in a STAT3-dependent manner. The kinase activitie s of cyclin D/CDK4, 6 and cyclin E/CDK2 complexes were only slightly elevat ed, consistent with the findings that coordinate increases in p21, cyclin D 1 and cyclin E resulted in an increase in cyclin/CDK/p21 complexes. Similar results were obtained in NTH3T3 and BALB/c 3T3 cells stably transformed by v-Src, indicating that these regulatory events associated with STAT3 signa ling represent common mechanisms independent of cell line or clonal variati on, These findings suggest that STAT3 has an essential role in the regulati on of critical cell cycle components in v-Src transformed mouse fibroblasts .