Cell cycle regulation in diabetic nephropathy

Renal hypertrophy is one of the earliest abnormalities of diabetic nephropathy. Although selected cell populations, such as tubulointerstitial fibroblasts, may undergo sustained proliferation in the diabetic environment, most renal cells such as mesangial cells are arrested in the G(1)-phase of the cell cycle after actively leaving G(0)-phase and some self-limited early proliferation. High glucose, transforming growth factor-beta (TGF-beta), angiotensin II, and probably other factors induce inhibitors of cyclin-dependent kinases (CDK) including p21(Cip1) and p27(Kip1). These CDK-inhibitors bind to and inactivate G(1)-phase cyclin/CDK complexes. The consequence is a lack in kinase activity, underphosphorylation of the retinoblastoma gene protein, and a failure tu initiate the G(1)-S-phase transit. The half-life of CDK-inhibitors may also be increased by serine phosphorylation mediated through activated MAP kinases. Treatment of diabetic rats with angiotensin-converting enzyme inhibitors attenuates glomerular hypertrophy and abolishes the glomerular expression of the CDK-inhibitors p16(INK4) and p27(Kip1), thus indicating that the cell cycle arrest can be therapeutically influenced. Cell cycle proteins may also be involved in these molecular events, leading to a limited degree of tubular apoptosis, which is a feature of diabetic nephropathy. Although not definitively proven, accumulating evidence suggests that early hypertrophy of renal cells may act as pacemaker for subsequent irreversible structural changes, such as glomerulosclerosis and tubulointerstitial fibrosis. Therefore, a better understanding of altered processes of cell cycle regulation is necessary to develop novel therapeutic strategies to prevent diabetic nephropathy. The recent observation that glomerular hypertrophy and proteinuria do not develop in diabetic p21(Cip1) knockout mice indicates that this approach is feasible.