Possible involvement of heat shock protein 25 in the angiotensin II-induced glomerular mesangial cell contraction via p38 MAP kinase.

In the rat kidney, mesangial cells (MCs), especially those in the extraglomerular mesangium (EGM) region of the juxtagomerular apparatus, express high amounts of heat shock protein 25 (HSP25). Because MCs are contractile in vivo and HSP25 is known to modulate polymerization/depolymerization of F-actin and to be involved in smooth muscle contraction, it is possible that HSP25 participates in the contraction process of MCs. We analyzed a permanent mouse MC line using Northern and Western blot analyses, and observed that similar to the MCs in the glomerulus, these cells also express high amounts of HSP25 constitutively. Exposure of these cells to angiotensin II (ANG II: 2 x 10(-7) M) evoked contraction and a concomitant increase in HSP25 phosphorylation, while the cytoplasmic fraction of HSP25 was transiently reduced. Because phosphorylation of HSP25 is essential for its actin-modulating function, we suppressed the activity of p38 MAP kinase, the major upstream activator of HSP25 phosphorylation, with the specific inhibitor SB 203580. This maneuver reduced HSP25 phosphorylation dramatically, abolished cell contraction, and prevented the decrease of the cytoplasmic HSP25 content. This suggests that HSP25 might be a component of the contraction machinery in MCs and that this process depends on p38 MAP kinase-mediated HSP25 phosphorylation. The decrease of cytoplasmic HSP25 content observed after ANG II exposure is probably the result of a transient redistribution of HSP25 into a buffer-insoluble fraction, because the whole cell content of HSP25 did not change, a phenomenon known to be related to the actin-modulating activity of HSP25. The fact that this function requires phosphorylation of HSP25 would explain the observation that HSP25 does not redistribute in SB 203580-pretreated cells.