Requirement of cyclin D1 in mesangial cell mitogenesis.

Abstract. Hyperplasia of mesangial cells (MC) is a frequent finding in glomerulonephritis. The control and function of cyclin D1, a regulator of cell cycle progression, in MC proliferation in vivo and in vitro were investigated. In a rat model of mesangioproliferative glomerulonephritis, increases in the number of cyclin D1-positive MC nuclei were prominent on day 5 of the disease, preceding the peak of MC hyperplasia. In growth-arrested rat MC in culture, mitogenic stimulation with serum or platelet-derived growth factor (PDGF) led to rapid increases in cyclin D1 protein expression. Transforming growth factor-beta1 inhibited PDGF induction of cyclin D1 protein at 12 h. In an examination of the subcellular distribution of cyclin D1, it was observed that stimulation of MC with PDGF for 6 h caused translocation of cyclin D1 from the cytoplasm into the nucleus. Coincubation with PDGF and transforming growth factor-beta1 completely inhibited this effect, without altering the cellular cyclin D1 protein abundance at that time point. To test whether reduction of cyclin D1 protein levels was sufficient to inhibit mitogenesis, MC were transfected with antisense oligonucleotides (ODN) complementary to rat cyclin D1 mRNA. Antisense ODN against cyclin D1 reduced the serum- or PDGF-induced protein expression of cyclin D1 to 27 or 10% of control levels, respectively. These inhibitory effects were correlated with diminished cyclin-dependent kinase 4 activity. Antisense ODN against cyclin D1 also decreased the PDGF-induced increase in p21(Waf-1) protein levels. The MC proliferation caused by serum or PDGF was markedly inhibited by antisense ODN against cyclin D1, as measured by [(3)H]thymidine uptake and cell counts. It is concluded that increased cyclin D1 protein expression of MC is required for MC proliferation. Targeting cyclin D1 expression may represent an effective means to inhibit MC proliferation in vitro and in vivo.