MESANGIAL CELL-PROLIFERATION MEDIATED BY PDGF AND BFGF IS DETERMINED BY LEVELS OF THE CYCLIN KINASE INHIBITOR P27(KIP1)

Mesangial cell proliferation in vitro is regulated by many cytokines. Platelet-derived growth factor (PDGF) and basic fibroblast growth fact or (bFGF) are potent mesangial cell mitogens, whereas transforming gro wth factor-beta 1 (TGF-beta 1) reduces their effects. We examined how these cytokines regulate rat mesangial cell proliferation at the level of the cell-cycle. Quiescent mesangial cells in vitro express the cyc lin kinase inhibitor, p27(Kip1) (p27), and PDGF- and bFGF-induced mesa ngial cell proliferation is associated with a substantial decrease in p27 levels. Consequently there is a marked increase in expression (Wes tern blot analysis, immunostaining) of cyclin A and CDK2. The decline in p27 levels was prevented by TGF-beta 1 during inhibition of PDGF- a nd bFGF induced mesangial cell proliferation. To determine the functio nal role of p27 during cytokine-mediated mesangial cell proliferation, the expression of p27 was reduced with specific p27(Kip1) antisense o ligodeoxynucleotides. Reducing the levels of p27 resulted in an increa sed magnitude of mesangial cell proliferation (BrdU and H-3-thymidine incorporation) induced by PDGF and bFGF compared to non-transfected me sangial cells and mesangial cells transfected with control mismatch ol igodeoxynucleotides. Furthermore, the onset of maximal proliferation o ccurred earlier in mesangial cells transfected with antisense compared to control. The reduction in proliferation by TGF-beta 1 were not alt ered by decreased p27 expression. Reducing p27 expression in the absen ce of mitogens was not associated with entry into the cell-cycle. Thes e results suggest cytokine mediated mesangial cell proliferation is as sociated with specific cell-cycle proteins, and that the levels of p27 may be important in determining the mesangial cell's proliferative re sponse to PDGF and bFGF in vitro.