High glucose-induced hypertrophy of mesangial cells requires p27Kip1, an inhibitor of cyclin-dependent kinases

Hypertrophy of mesangial cells is one of the earliest morphological alterations in the kidney after the onset of diabetes mellitus. We have previously shown that cultured mesangial cells exposed to high ambient glucose arrest in the G(1) phase of the cell cycle and that this is associated with an increased expression of inhibitors of the cyclin-dependent kinase (CDK)-inhibitors p21(Cip) and p27(Kip1). TO further investigate a potential role of p27(Kip1) in the development of glucose-induced hypertrophy, mesangial cells from p27(Kip1) wild-type (+/+) and knockout (-/-) mice mere established, High glucose medium (450 mg/dl) increased p21(Cip1) protein in p27(Kip1)+/+ and -/- mesangial cells, and increased p27(Kip1) protein levels in p27(Kip1)+/+ cells. In contrast to high glucose increasing de novo protein synthesis in p27(Kip1)+/+ cells, high glucose did not increase protein synthesis in p27(Kip1)-/- cells. High glucose also reduced DNA synthesis and caused cell cycle arrest in p27(Kip1)+/+ cells. In contrast, despite an increase in transforming growth factor (TGF)-beta mRNA and protein expression, DNA synthesis and cell cycle progression were increased by high glucose in p27(Kip1)-/- cells. Exogenous TGF-beta comparably induced fibronectin mRNA in p27(Kip1)+/+ and -/- cells suggesting intact TGF-beta receptor transduction. In addition, high glucose failed to increase the total protein/cell number ratio in p27(Kip1)-/- cells. However, in the presence of high glucose, reconstituting p27(Kip1) expression by transient or stable transfection in p27(Kip1)-/- cells, using an inducible expression system, increased the de novo protein synthesis and restored G(1)-phase arrest. These results show that p27(Kip1) is required for glucose-induced mesangial cell hypertrophy and cell cycle arrest.