PI3K signaling in cancer: beyond AKT

Abstract

The phosphoinositide 3-kinase (PI3K) signaling pathway is one of the most frequently altered pathways in human cancer and has a critical role in driving tumor initiation and progression. Although PI3K and its lipid product phosphatidylinositol-3,4,5-trisphosphate (PIP₃) have been shown to activate multiple downstream signaling proteins, the vast majority of studies have focused on the protein kinase AKT as the dominant effector of PI3K signaling. However, recent studies have demonstrated many contexts under which other PIP₃-dependent signaling proteins critically contribute to cancer progression, illustrating the importance of understanding AKT-independent signaling downstream of PI3K. Here, we highlight three PI3K-dependent, but AKT-independent, signaling branches that have recently been shown to have important roles in promoting phenotypes associated with malignancy. First, the PDK1-mTORC2-SGK axis can substitute for AKT in survival, migration, and growth signaling and has emerged as a major mechanism of resistance to PI3K and AKT inhibitors. Second, Rac signaling mediates the reorganization of the actin cytoskeleton to regulate cancer cell migration, invasion, and metabolism. Finally, the TEC family kinase BTK has a critical role in B cell function and malignancy and represents a recent example of an effective therapeutic target in cancer. These mechanisms highlight how understanding PI3K-dependent, but AKT-independent, signaling mechanisms that drive cancer progression will be crucial for the development of novel and more effective approaches for targeting the PI3K pathway for therapeutic benefit in cancer.

Figure 1. The PDK1-mTORC2-SGK signaling axis

SGK3 is activated at either the plasma membrane or the endosome, where its PX domain binds to PI-3-P. PI-3-P is produced both at the plasma membrane through the sequential dephosphorylation of PIP₃ by SHIP1/2 and INPP4B and at the endosome by the Class III PI3K hVps34. After PI-3-P binding, SGK3 is first phosphorylated on the hydrophobic motif (HM) by mTORC2. PDK1 then binds to the phosphorylated HM through its PIF-binding pocket and phosphorylates SGK3 on its activation loop. SGK1 is not regulated by PI-3-P because it lacks a PX domain, but it is phosphorylated by mTORC2 and PDK1 in the same manner as SGK3. Upon full activation, SGK1/3 phosphorylates its substrates with an optimal consensus motif of R-X-R-X-X-S/T to regulate cellular processes including cell growth, proliferation, migration, invasion, and resistance to PI3K and AKT inhibitors.

Figure 2. PI3K-Rac signaling regulates actin cytoskeleton reorganization

Upon binding to PIP₃ via their PH domains, PIP₃-sensitive Rac-GEFs catalyze the exchange of GDP for GTP on Rac. Rac activation is negatively regulated by Rac-GAPs, which facilitate GTP hydrolysis by Rac. When Rac-GTP is activated, it regulates cytoskeleton reorganization by binding and activating relevant target proteins, most notably PAK and WAVE. Remodeling of the actin cytoskeleton modulates cellular processes such as cell migration, invasion, and metabolism.

Figure 3. PI3K and BTK Signaling in Response to BCR activation in B cells

Antigen engagement of the BCR receptor induces clustering of the BCR, associated CD79A/CD79B heterodimers, and the non-receptor tyrosine kinase (NRTK) LYN which is tethered to the membrane by dual lipid modifications. LYN phosphorylates tyrosine motifs on CD79A/CD79B and the co-receptor CD19 to which the NRTK SYK and PI3K dock respectively and become activated. PIP₃ produced by PI3K recruits BTK to the membrane where it is phosphorylated and activated by SYK and LYN. BTK also forms a complex with the adaptor protein BLNK, the Rho family GEF Vav, and the phospholipase PLCγ. Following its BTK-dependent activation, Vav promotes GTP loading and activation of Cdc42 and Rac which then trigger actin cytoskeleton remodeling through a variety of effectors. Simultaneously, BTK phosphorylates and activates PLCγ which cleaves PI-4,5-P₂ to diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). DAG is bound by PKCβ which triggers IKK signaling and NFkB-dependent transcription plus activation of Ras-Raf-MEK-ERK signaling. IP3 stimulates calcium (Ca²⁺) mobilization which results in NFAT-dependent transcription. These signaling events cooperate to control survival, proliferation, and adhesion signals that underlie B cell development and activation.