Hypertonic stress increases phosphatidylinositol 4,5-bisphosphate levels by activating PIP5KIbeta.

ABSTRACT

Hyperosmotic stress increases phosphoinositide levels, reorganizes the actin cytoskeleton, and induces multiple acute and adaptive physiological responses. Here we showed that phosphatidylinositol 4,5-bisphosphate (PIP(2)) level increased rapidly in HeLa cells during hypertonic treatment. Depletion of the human type I phosphatidylinositol 4-phosphate 5-kinase beta isoform (PIP5KIbeta) by RNA interference impaired both the PIP(2) and actin cytoskeletal responses. PIP5KIbeta was recruited to membranes and was activated by hypertonic stress through Ser/Thr dephosphorylation. Calyculin A, a protein phosphatase 1 inhibitor, blocked the hypertonicity-induced PIP5KIbeta dephosphorylation/activation as well as PIP(2) increase in cells. Urea, which raises osmolarity without inducing cell shrinkage, did not promote dephosphorylation nor increase PIP(2) levels. Disruption or stabilization of the actin cytoskeleton, or inhibition of the Rho kinase, did not block the PIP(2) increase nor PIP5KIbeta dephosphorylation. Therefore, PIP5KIbeta is dephosphorylated in a volume-dependent manner by a calyculin A-sensitive protein phosphatase, which is activated upstream of actin remodeling and independently of Rho kinase activation. Our results establish a cause-and-effect relation between PIP5KIbeta dephosphorylation, lipid kinase activation, and PIP(2) increase in cells. This PIP(2) increase can orchestrate multiple downstream responses, including the reorganization of the actin cytoskeleton.