Platelet-derived growth factor-BB-induced hypertrophy of peritubular smooth muscle cells is mediated by activation of p38 MAP-kinase and of Rho-kinase.

Peritubular smooth muscle cells (PSMC) from rat testis in primary serum-free cultures unexpectedly undergo contraction and subsequent cell hypertrophy in response to the growth factor PDGF-BB, remaining stationary. The present study investigates the transduction pathways involved in the observed paradoxical upregulation of the differentiated phenotype and induction of hypertrophy in PSMC. PI3K, ERK, JNK, and p38 kinases, known to mediate PDGF-BB signaling in the canonic dedifferentiative and proliferative response of smooth muscle cells (SMC) were rapidly activated by PDGF-BB but only p38 remained activated after 2-day stimulation. Immunofluorescence and immunoblotting experiments showed that in 4-day treatment: (i) continuous inhibition of PI3K, of ERK, of JNK, failed to inhibit either cell enlargement and formation of prominent alpha-SM actin containing stress fibers or the typical increase in alpha-SM actin; (ii) when stimulated in the presence of the p38 inhibitor SB203580 both responses were significantly inhibited and cytofluorimetric analysis of cell size showed a remarkable reduction of the hypertrophic response. PDGF-BB was also found to activate the small GTPase RhoA and inhibition of Rho-dependent kinase ROCK by Y27632 counteracted the effects of PDGF-BB similarly to SB203580. Both the transcription factor ATF2 and the nucleosomal kinase MSK1, downstream targets of p38, were activated by PDGF-BB, but p38 inhibitor SB203580 inhibited only the phosphorylation of MSK1 which appeared unaffected by ROCK inhibitor Y27632. In concluding, p38 and the Rho-ROCK system were found to play prominent, probably independent roles in the upregulation of PSMC differentiated phenotype and induction of hypertrophy by PDGF-BB.