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 these in the extraglom erular mesangium (ECM) region of the juxtagomerular apparatus, express high amounts of heat shock protein 25 (HSP25). Because MCs are contractile in v ivo and HSP25 is known to modulate polymerization/depolymerization of F-act in and to be involved in smooth muscle contraction, it is possible that HSP 25 participates in the contraction process of MCs. We analyzed a permanent mouse MC line using Northern and Western blot analyses, and observed that s imilar 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 phosp horylation, while the cytoplasmic fraction of HSP25 was transiently reduced . Because phosphorylation of HSP25 is essential for its actin-modulating fu nction, we suppressed the activity of p38 MAP kinase, the major upstream ac tivator of HSP25 phosphorylation, with the specific inhibitor SE 203580. Th is maneuver reduced HSP25 phosphorylation dramatically, abolished cell cont raction, and prevented the decrease of the cytoplasmic HSP25 content. This suggests that HSP25 might be a component of the contraction machinery in MC s and that this process depends on p38 MAP kinase-mediated HSP25 phosphoryl ation. The decrease of cytoplasmic HSP25 content observed after ANC II expo sure is probably the result of a transient redistribution of HSP25 into a b uffer-insoluble fraction, because the whole cell content of HSP25 did not c hange, 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 SE 203580-pret reated cells, (C) 1999 Wiley-Liss, Inc.