PROTEIN-KINASE-C-DELTA INHIBITS THE PROLIFERATION OF VASCULAR SMOOTH-MUSCLE CELLS BY SUPPRESSING G(1) CYCLIN EXPRESSION

To elucidate the physiological role of protein kinase C (PKC) delta, a ubiquitously expressed isoform in vascular smooth muscle cells (VSMC) , PKC delta was stably overexpressed in A7r5 cells, rat clonal VSMC. T he [H-3]thymidine incorporation in A7r5 overexpressed with PKC delta ( DVs) was suppressed to 37.1 +/- 16.3% (mean +/- S.D.) of the level in control or A7r5 transfected with vector alone (EVs). The reduction of [H-3]thymidine incorporation was strongly correlated with overexpresse d PKC levels. Moreover, transient transfection of a dominant negative mutant of PKC delta restored the reduced proliferation in DVs. Flow cy tometry analysis demonstrated that DVs were arrested in the G(0)/G(1), phase of the cell cycle. Expression of cyclins D1 and E and retinobla stoma protein phosphorylation were reduced, while the protein levels o f p27 were elevated in DVs as compared with EVs. There were no signifi cant differences in the expression of c-fos, c-jun, c-myc, cyclin D2, D3, cyclin dependent kinase 2, cyclin-dependent kinase 4, and p21 amon g the clones. We conclude that PKC delta inhibits the proliferation of VSMC by arresting cells in G(1) via mainly inhibiting the expression of cyclin D1 and cyclin E.