SUMOylation modulates the stability and function of PI3K-p110ß.

Class I PI3K are heterodimers composed of a p85 regulatory subunit and a p110 catalytic subunit involved in multiple cellular functions. Recently, the catalytic subunit p110ß has emerged as a class I PI3K isoform playing a major role in tumorigenesis. Understanding its regulation is crucial for the control of the PI3K pathway in p110ß-driven cancers. Here we sought to evaluate the putative regulation of p110ß by SUMO. Our data show that p110ß can be modified by SUMO1 and SUMO2 in vitro, in transfected cells and under completely endogenous conditions, supporting the physiological relevance of p110ß SUMOylation. We identify lysine residue 952, located at the activation loop of p110ß, as essential for SUMOylation. SUMOylation of p110ß stabilizes the protein increasing its activation of AKT which promotes cell growth and oncogenic transformation. Finally, we show that the regulatory subunit p85ß counteracts the conjugation of SUMO to p110ß. In summary, our data reveal that SUMO is a novel p110ß interacting partner with a positive effect on the activation of the PI3K pathway.

A p85 isoform switch enhances PI3K activation on endosomes by a MAP4- and PI3P-dependent mechanism.

Phosphatidylinositol 3-kinase α (PI3Kα) is a heterodimer of p110α catalytic and p85 adaptor subunits that is activated by agonist-stimulated receptor tyrosine kinases. Although p85α recruits p110α to activated receptors on membranes, p85α loss, which occurs commonly in cancer, paradoxically promotes agonist-stimulated PI3K/Akt signaling. p110α localizes to microtubules via microtubule-associated protein 4 (MAP4), facilitating its interaction with activated receptor kinases on endosomes to initiate PI3K/Akt signaling. Here, we demonstrate that in response to agonist stimulation and p85α knockdown, the residual p110α, coupled predominantly to p85ß, exhibits enhanced recruitment with receptor tyrosine kinases to endosomes. Moreover, the p110α C2 domain binds PI3-phosphate, and this interaction is also required to recruit p110α to endosomes and for PI3K/Akt signaling. Stable knockdown of p85α, which mimics the reduced p85α levels observed in cancer, enhances cell growth and tumorsphere formation, and these effects are abrogated by MAP4 or p85ß knockdown, underscoring their role in the tumor-promoting activity of p85α loss.

FAK-mediated phosphorylation at Y464 regulates p85ß nuclear translocation to promote tumorigenesis of ccRCC by repressing RB1 expression.

PI3K regulatory subunit p85s normally stabilizes and regulates catalytic subunit p110s in the cytoplasm. Recent studies show that p110-free p85s in the nucleus plays important roles in biological processes. However, the mechanisms by which p85s translocate into the nucleus remain elusive. Here, we describe the mechanism by which p85ß translocates into the nucleus to promote ccRCC tumorigenesis. Phosphorylation of p85ß at the Y464 by FAK facilitates its nuclear translocation in the kidney through enhancing the binding of p85ß to KPNA1. PIK3R2/p85ß is highly expressed in ccRCC samples and associated with overall survival of ccRCC patients. Nuclear but not cytoplasmic p85ß performs oncogenic functions by repressing RB1 expression and regulating the G1/S cell cycle transition. Nuclear p85ß represses RB1 expression by stabilizing histone methyltransferase EZH1/EZH2 proteins. Last, the FAK inhibitor defactinib significantly suppresses the tumor growth of ccRCC with high p85ß Y464 levels.

The Opposing Roles of PIK3R1/p85α and PIK3R2/p85ß in Cancer.

Dysregulation of the PI3K/PTEN pathway is a frequent event in cancer, and PIK3CA and PTEN are the most commonly mutated genes after TP53. PIK3R1 is the predominant regulatory isoform of PI3K. PIK3R2 is an ubiquitous isoform that has been so far overlooked, but data from The Cancer Genome Atlas shows that increased expression of PIK3R2 is also frequent in cancer. In contrast to PIK3R1, which is a tumor-suppressor gene, PIK3R2 is an oncogene. We review here the opposing roles of PIK3R1 and PIK3R2 in cancer, the regulatory mechanisms that control PIK3R2 expression, and emerging therapeutic approaches targeting PIK3R2.

Phosphoinositide 3-kinase p85beta regulates invadopodium formation.

The acquisition of invasiveness is characteristic of tumor progression. Numerous genetic changes are associated with metastasis, but the mechanism by which a cell becomes invasive remains unclear. Expression of p85ß, a regulatory subunit of phosphoinositide-3-kinase, markedly increases in advanced carcinoma, but its mode of action is unknown. We postulated that p85ß might facilitate cell invasion. We show that p85ß localized at cell adhesions in complex with focal adhesion kinase and enhanced stability and maturation of cell adhesions. In addition, p85ß induced development at cell adhesions of an F-actin core that extended several microns into the cell z-axis resembling the skeleton of invadopodia. p85ß lead to F-actin polymerization at cell adhesions by recruiting active Cdc42/Rac at these structures. In accordance with p85ß function in invadopodium-like formation, p85ß levels increased in metastatic melanoma and p85ß depletion reduced invadopodium formation and invasion. These results show that p85ß enhances invasion by inducing cell adhesion development into invadopodia-like structures explaining the metastatic potential of tumors with increased p85ß levels.