The lipid kinase PI4KIIIβ is highly expressed in breast tumors and activates Akt in cooperation with Rab11a

Abstract

Emerging evidence now implicates phosphatidylinositol 4-kinases (PI4K), enzymes that generate PI(4)P from phosphatidylinositol (PtdIns), in cancer. In this study, we investigate the role of PI4KIIIβ, one of four mammalian PI4Ks, in breast cancer. Although PI4KIIIβ protein levels are low in normal breast tissue, we find that approximately 20% of primary human breast tumors overexpress it. Expression of PI4KIIIβ in breast carcinoma cells leads to increased Akt activation, dependent on increased PI(3,4,5)P3 production. However, a kinase-inactive version of PI4KIIIβ also led to increased Akt activation, and no changes in PI(4)P or PI(4,5)P2 lipid abundance were detected in the PI4KIIIβ-overexpressing cells. This implies that PI4KIIIβ regulates PI(3,4,5)P3 and Akt independent of PI(4)P production. We find that the PI4KIIIβ-binding protein, Rab11a, a small GTPase that regulates endosomal recycling, is involved in PI4KIIIβ-mediated activation of Akt, as RNAi depletion of Rab11a impairs Akt activation. Furthermore, ectopic PI4KIIIβ expression alters cellular Rab11a distribution and enhances recruitment of PI4KIIIβ and Rab11a to recycling endosomes. This work suggests that PI4KIIIβ affects PI3K/Akt signaling through Rab11a and endosomal trafficking, independent of its lipid kinase activity. Thus, PI4KIIIβ likely plays a role in breast oncogenesis and that cooperation between Rab11a and PI4KIIIβ represents a novel Akt activation pathway.