Modulation of actin microfilament dynamics and fluid phase pinocytosis by phosphorylation of heat shock protein 27.

We recently reported that overexpression of heat shock protein 27 (HSP27) in rodent fibroblasts increases the stability of stress fibers during hyperthermia and partially prevents actin depolymerization during exposure to cytochalasin D (Lavoie, J.N., Gingras-Breton, G., Tanguay, R. M., and Landry, J. (1993) J. Biol. Chem. 268, 3420-3429). Because HSP27 is a ubiquitous target of phosphorylation upon cell stimulation with a variety of growth factors and agents that affect cellular differentiation, we examined the role of HSP27 phosphorylation in regulating actin filament dynamics. Here we show that HSP27 is enriched at the leading edge of polarized fibroblasts. HSP27 is localized in lamellipodia and membrane ruffles where most actin polymerization occurs. We developed Chinese hamster cell lines that constitutively overexpressed either human HSP27 or a nonphosphorylatable mutant form of the protein. Overexpression of HSP27 caused an increased concentration of filamentous actin (F-actin) at the cell cortex and elevated pinocytotic activity. In contrast, overexpression of the non-phosphorylatable mutant form of HSP27 reduced cortical F-actin concentration and decreased pinocytosis activity relative to control cells. Mitogenic stimulation of fibroblasts resulted in a rapid polymerization of submembranous actin filaments. HSP27 enhanced growth factor-induced F-actin accumulation, whereas mutant HSP27 exerted a dominant negative effect and inhibited this response to growth factors. Thus, HSP27 is a component of a signal transduction pathway that can regulate microfilament dynamics.