PI(3,5)P 2 Controls the Signaling Activity of Class I PI3K.

Abstract

3-Phosphoinositides are ubiquitous cellular lipids that play pivotal regulatory roles in health and disease. Among 3-phosphoinositides, phosphatidylinositol-3,5-bisphosphate (PI(3,5)P ₂ ) remains the least understood species in terms of its spatiotemporal dynamics and physiological function due to the lack of a specific sensor that allows spatiotemporally resolved quantitative imaging of PI(3,5)P ₂ . Using a newly developed ratiometric PI(3,5)P ₂ sensor engineered from the C-terminal SH2 domain of Class I phosphoinositide 3-kinases (PI3K)-p85α subunit we demonstrate that a unique pool of PI(3,5)P ₂ is generated on lysosomes and late endosomes in response to growth factor stimulation. This PI(3,5)P ₂ , the formation of which is mediated sequentially by Class II PI3KC2β and PIKfyve, plays a crucial role in terminating the activity of growth factor-stimulated Class I PI3K, one of the most frequently mutated proteins in cancer, via specific interaction with its regulatory p85 subunit. A small molecule inhibitor of p85α-PI(3,5)P ₂ binding specifically blocks the feedback inhibition of Class I PI3K by PI(3,5)P ₂ and thus serves as a PI3K activator that promotes neurite growth. Furthermore, cancer-causing mutations of the Class I PI3K-p85 subunit inhibit p85-PI(3,5)P ₂ interaction and thereby induce sustained activation of Class I PI3K. Our results unravel a hitherto unknown spatiotemporally specific regulatory function of PI(3,5)P ₂ that links Class I and II PI3Ks and modulates the magnitude of PI3K-mediated growth factor signaling. These results also suggest new therapeutic possibilities for treating cancer patients with p85 mutations and promoting wound healing and tissue regeneration.