Background: Angiotensin II has been reported to induce renal tubular hypertrophy, but the mechanisms of this hypertrophy are not well known. We evaluated the roles of cyclin-dependent kinase (CDK) inhibitors in renal tubular hypertrophy.
Methods: To elucidate whether CDK inhibitors cause renal tubular hypertrophy, we produced adenovirus vectors containing coding sequences of the CDK inhibitors p27Kip1 (AxCAp27), p21CIP1 (AxCAp21), and p16INK4 (AxCAp16), and we investigated the effect of these gene transfers on epidermal growth factor (EGF)-induced proliferation in LLC-PK1 cells. We evaluated the cell cycle and hypertrophy by measurements of the [3H]-leucine and [3H]-thymidine incorporation, the protein:DNA ratio, flow cytometry, and CDK4 and CDK2 kinase assays.
Results: AxCAp27 and AxCAp21 caused significant increases in [3H]-leucine incorporation and the protein:DNA ratio but did not change the [3H]-thymidine incorporation. Conversely, AxCAp16 inhibited EGF-stimulated [3H]-thymidine incorporation but did not change the [3H]-leucine incorporation. AxCAp27, AxCAp21, and AxCAp16 all inhibited EGF-stimulated CDK4 kinase activity (to 15.6, 14.1, and 21.9% of control, respectively). Forward light-scatter analysis demonstrated that AxCAp27 and AxCAp21 increased the cell size but that AxCAp16 effected no change in cell size.
Conclusion: These findings suggest that p27Kip1 and p21CIP1 may play an important role in hypertrophy of renal tubule cells by reducing pRb phosphorylation. On the other hand, p16INK4 was not found to cause hypertrophic changes in EGF-treated LLC-PK1 cells.