Cofilin plays a critical role in IL-8-dependent chemotaxis of neutrophilic HL-60 cells through changes in phosphorylation

Cofilin is a ubiquitous, actin‐binding protein. Only unphosphorylated cofilin binds actin and severs or depolymerizes filamentous actin (F‐actin), and the inactive form of cofilin is phosphorylated at Ser 3. We reported recently that cofilin plays a regulatory role in superoxide production and phagocytosis by leukocytes, and in the present study, we investigated the role of cofilin in the chemotaxis of neutrophilic HL‐60 cells. IL‐8 is a potent, physiological chemokine, and it triggers a rapid, transient increase in F‐actin beneath the plasma membrane and rapid dephosphorylation and subsequent rephosphorylation of cofilin. In this study, cofilin phosphorylation was found to be inhibited by S3‐R peptide, which consists of a peptide corresponding to part of the phosphorylation site of cofilin and a membrane‐permeable arginine polymer. When S3‐R peptide was introduced into the neutrophilic cells, their chemotactic activity was enhanced, whereas a control peptide that contained an inverted sequence of the phosphorylation site of cofilin had no enhancing effect. Cofilin small interfering RNA (siRNA) decreased cofilin expression by about half and inhibited chemotaxis. In IL‐8‐stimulated cells, unphosphorylated cofilin accumulated around F‐actin, and colocalization of F‐actin and phosphorylated cofilin was observed, but these changes in cofilin localization were less prominent in cofilin siRNA‐treated cells. The inhibitors of PI‐3K wortmannin and LY294002 inhibited the chemotaxis and suppressed IL‐8‐evoked dephosphorylation and rephosphorylation of cofilin. These results suggested that unphosphorylated cofilin plays a critical role in leukocyte chemotaxis and that PI‐3K is involved in the control of the phosphorylation/dephosphorylation cycle of cofilin.