ANG II induces secretion and activation of transforming growth factor-beta (TGF-beta) by glomerular mesangial cells. However, the mechanisms that operate this are unclear. Thrombospondin-1 (TSP-1), which is produced by mesangial cells in damaged glomeruli, is one of several molecules known to activate the latent TGF-beta1 complex. Therefore, we examined whether the ANG II-induced activation of latent TGF-beta1 in human mesangial cells (HMC) operates via TSP-1. The addition of ANG II (1-100 nM) to HMC significantly increased TSP-1 mRNA within 6 h, followed by an increase in TSP-1 protein production as shown by Western blot analysis of cells and immunoassay of the culture supernatant. Production of ANG II-induced TSP-1 mRNA and protein was completely inhibited by an ANG II type 1 (AT1)-receptor antagonist but was unaffected by an AT2-receptor antagonist. Use of a TSP-1-specific blocking peptide demonstrated that the ANG II-induced activation of latent TGF-beta1 operates via TSP-1. Next, we investigated the role of ERK1/2, p38 MAPK, and JNK in ANG II-induced TSP-1 production in HMC. The addition of the upstream ERK1/2 inhibitor PD-98059 did not affect ANG II-induced TSP-1 production, whereas addition of either the p38 MAPK inhibitor SB-203580 or the JNK inhibitor SP-600125 significantly reduced TSP-1 production. In conclusion, this study has demonstrated that ANG II-induced activation of latent TGF-beta1 in HMC operates via TSP-1. Furthermore, ANG II-induced TSP-1 production is dependent on p38 MAPK and JNK signaling.