AbstractThere are several reports describing participation of small heat shock proteins (sHsps) in cellular protein quality control. In this study, we estimated the endoplasmic reticulum (ER) stress‐induced response of Hsp27 and αB‐crystallin in mammalian cells. Treatment targeting the ER with tunicamycin or thapsigargin induced the phosphorylation of Hsp27 but not of αB‐crystallin in U373 MG cells, increase being observed after 2–10 h and decline at 24 h. Similar phosphorylation of Hsp27 by ER stress was also observed with U251 MG and HeLa but not in COS cells and could be blocked using SB203580, an inhibitor of p38 MAP kinase. Other protein kinase inhibitors, like Gö6983, PD98059, and SP600125, inhibitors of protein kinase C (PKC), p44/42 MAP kinase, and JNK, respectively, were without major influence. Prolonged treatment with tunicamycin but not thapsigargin for 48 h caused the second induction of the phosphorylation of Hsp27 in U251 MG cells. Under these conditions, the intense perinuclear staining of Hsp27, with some features of aggresomes, was observed in 10%–20% of the cells. © 2005 Wiley‐Liss, Inc.