TGF-β1-induced cell cycle arrest in renal mesangial cells involves inhibition of cyclin E-cdk 2 activation and retinoblastoma protein phosphorylation

TGF-β1-induced cell cycle arrest in renal mesangial cells involves inhibition of cyclin E-cdk 2 activation and retinoblastoma protein phosphorylation. In glomerular disease, transforming growth factor-β1 (TGF-β1) has been demonstrated to exert anti-mitogenic and anti-inflammatory as well as fibrogenic effects. To better understand the TGF-β1 action on glomerular cells at the molecular level, we investigated mechanisms of TGF-β1-induced growth suppression in primary cultures of rat mesangial cells (MCs). TGF-β1 (5 ng/ml) markedly inhibited proliferation of MCs incubated with PDGF, endothelin-1, bFGF, serotonin, or EGF, indicating that TGF-β1 interferes with post-receptor signals of mitogenesis. TGF-β1 did not affect mitogen-stimulated induction of the immediate early genes, c-fos, c-jun, and Egr-1 in MCs that occurred transiently at 30 to 120 minutes. Time-course studies revealed that TGF-β1 inhibited DNA synthesis and MC replication when added up to six to eight hours after MC stimulation with PDGF. FACS analysis demonstrated that MCs had reached middle to late G1 phase of cell cycle progression at this timepoint. PDGF stimulation of MCs induced protein expression of the G1 phase cyclin Dl as well as the cyclin-dependent kinases cdk 4 and cdk 2. This was not significantly altered when MCs were coincubated with both, PDGF and TGF-β1. However, TGF-β1 prevented PDGF-elicited phosphorylation of the retinoblastoma tumor suppressor (pRb), a negative cell cycle regulator. Moreover, TGF-β1 significantly reduced cyclin E-associated histone H1 kinase activity in the presence of PDGF. These results indicate that TGF-β1 inhibits mitogen-stimulated MC growth by causing cell cycle arrest in late G1 phase. While TGF-β1 does not alter the mitogen-induced expression and abundance of G1 phase cyclin D1 and cdks 4 and 2 in MCs, it inhibits cyclin E-cdk 2 activity, thus preventing mitogen-elicited phosphorylation and inactivation of pRb in G1 phase and transition to S phase.