Aims/hypothesis  

Mesangial cell hypertrophy is one of the earliest morphological abnormalities of diabetic nephropathy. We have previously shown that high glucose induces p27^Kip1 by a post-transcriptional mechanism and that mesangial cell hypertrophy depends on G₁-phase arrest mediated by this CDK-inhibitor. However, it remains poorly understood how high glucose stimulates p27^Kip1 expression in mesangial cells.

Methods  

Mesangial cells were isolated from p27^Kip1 +/+ and –/– mice and characterized by light microscopy and immunohistochemistry. It was tested by Western blotting and autoradiography whether high glucose medium activates Erk 1,2 and whether this activation phosphorylates p27^Kip1. The three consensus phosphorylation sites of p27^Kip1 were mutated and these constructs were expressed in p27^Kip1 –/– mesangial cells. Hypertrophy was assessed by different methods.

Results  

High glucose stimulates phosphorylation of MAP kinases Erk 1,2 in p27^Kip1 +/+ and –/– mesangial cells. Activation of Erk 1,2 leads to phosphorylation of p27^Kip1 in vitro and in vivo . Mutations of serine¹⁰ or threonine¹⁸⁷ still supported high glucose-induced hypertrophy. In contrast, a mutation of serine¹⁷⁸ converted the hypertrophic response into a proliferative phenotype. Mutation of serine¹⁷⁸ leads to the attenuated expression of p27^Kip1 protein in the presence of high glucose.

Conclusions/interpretation  

Our study shows that high glucose stimulates Erk 1,2 that phosphorylate p27^Kip1 at serine¹⁷⁸ increasing its expression. This is an important molecular mechanism of high glucose-induced hypertrophy of mesangial cells.