Phosphatidylinositol-4-phosphate 5-kinase 1α Mediates Extracellular Calcium-induced Keratinocyte Differentiation

Extracellular calcium (Cao) is a major regulator of keratinocyte differentiation, but the mechanism is unclear. Phosphatidylinositol-4-phosphate 5-kinase 1 alpha (PIP5K1 alpha) is critical in synthesizing phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)]. In this study, we sought to determine whether PIP5K1 alpha plays a role in mediating the ability of Cao to induce keratinocyte differentiation. We found that treatment of human keratinocytes in culture with Cao resulted in increased PIP5K1 alpha level and activity, as well as PI(4,5)P(2) level, binding of phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P(3)] to and activation of phospholipase C-gamma 1 (PLC-gamma 1), with the resultant increase in inositol 1,4,5-trisphosphate (IP(3)) and intracellular calcium (Cai). Knockdown of PIP5K1 alpha in human keratinocytes blocked Cao-induced increases in the binding of PI(3,4,5)P(3) to PLC-gamma 1; PLC-gamma 1 activity; levels of PI(4,5)P(2), IP(3), and Cai; and induction of keratinocyte differentiation markers. Coimmunoprecipitation and confocal studies revealed that Cao stimulated PIP5K1 alpha recruitment to the E-cadherin-catenin complex in the plasma membrane. Knockdown of E-cadherin or beta-catenin blocked Cao-induced activation of PIP5K1 alpha. These results indicate that after Cao stimulation PIP5K1 alpha is recruited by the E-cadherin-catenin complex to the plasma membrane where it provides the substrate PI(4,5)P(2) for both PI3K and PLC-gamma 1. This signaling pathway is critical for Cao-induced generation of the second messengers IP(3) and Cai and keratinocyte differentiation.