Regulation of the mesangial cell myofibroblast phenotype by actin polymerization

Mesangial cells in diverse glomerular diseases become myofibroblast-like, characterized by activation of smooth muscle α-actin (α-SMA) expression. In cultured mesangial cells, serum-deprivation markedly increases α-SMA expression, cell size, and stress fiber formation. Since stress fibers are assembled from actin monomers, we investigated the hypothesis that alterations in stress fiber formation regulate α-SMA expression and hypertrophy. Human mesangial cells were treated with agents that disrupt or stabilize actin stress fibers. Depolymerization of actin stress fibers in serum-deprived cells with actin-depolymerizing agents, cytochalasin B (CytB) and latrunculin B (LatB), or with inhibitors of Rho-kinase, Y-27632 and HA-1077 decreased α-SMA mRNA as judged by Northern blot analysis. Western blot analysis showed that CytB also reduced α-SMA protein levels. In serum-fed cells, agents that stabilized actin stress fibers, jasplakinolide Jas) and phalloidin, increased α-SMA mRNA and protein. Treatment of human or rat mesangial cells with CytB, LatB, or Y-27632 decreased α-SMA promoter activity. In contrast, Jas increased promoter activity 5.6-fold in rat mesangial cells. The presence of an RNA polymerase inhibitor blocked degradation of α-SMA mRNA in cells treated with CytB suggesting that destabilization of this message is dependent on a newly transcribed or rapidly degraded factor. Inhibition of actin polymerization by CytB, LatB, Y-27623, and HA-1077 inhibited incorporation of ³[H]-leucine into newly synthesized protein. Additionally, CytB and LatB decreased cell volume as determined by flow cytometry. Collectively, these results indicate that the state of polymerization of the actin cytoskeleton regulates α-SMA expression, hypertrophy, and myofibroblast differentiation in mesangial cells.