The Hypertrophic Effect of Transforming Growth Factor-β is Reduced in the Absence of Cyclin-Dependent Kinase-Inhibitors p21 and p27

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Abstract
Transforming growth factor-β (TGF-β) has both antiproliferative and hypertrophic effects on mesangial cells (MC). However, it is not known if these processes are independent or if they share common signaling pathways. Proliferation and hypertrophy are regulated by specific cell-cycle regulatory proteins, where the cyclin-dependent kinase (CDK) inhibitors inhibit target cyclin-CDK complexes. This study examined whether the growth regulatory effects of TGF-β were determined by the CDK inhibitors p21 and p27. Accordingly, cultured MC from wild type (+/+) and single and double null (−/−) p21 and p27 mice were grown in 5% serum in the presence or absence of TGF-β1 (2 ng/ml). Proliferation ([3H]-thymidine incorporation, cell number, cell cycle) and hypertrophy ([3H]-leucine incorporation, total protein content, forward light scatter) were measured after 24 h, 48 h, and 96 h. TGF-β inhibited proliferation in +/+ and p21/p27 double −/− MC to a similar extent. TGF-β induced hypertrophy in +/+ MC (18.0% increase at 48 h), and to lesser extent in p21 −/− (12.8%) and p27 −/− MC (11.5%) measured by forward light scatter analysis. In p21/p27 double −/−, the hypertrophic effects of TGF-β were significantly reduced (3.9% at 48 h). Similar results were obtained by measuring hypertrophy by total protein and [3H]-leucine incorporation. In conclusion, the CDK inhibitors p21 and p27 are not required for the antiproliferative effects of TGF-β. However, the hypertrophic growth effects of TGF-β are reduced in the absence of both p21 and p27. These data suggest that the regulation of the antiproliferative and hypertrophic effects of TGF-β may be distinct processes.