SUMMARY Members of the mammalian phosphoinositide-3-OH kinase (PI3K) family of proteins are critical regulators of various cellular process including cell survival, growth, proliferation and motility. Oncogenic activating mutations in the p110α catalytic subunit of the heterodimeric p110/p85 PI3K enzyme are frequent in human cancers. Here we show the presence of frequent mutations in p85α in colon cancer, a majority of which occurs in the inter-Src homology-2 (iSH2) domain. These mutations uncouple and retain p85α's p110-stabilizing activity, while abrogating its p110-inhibitory activity. The p85α mutants promote cell survival, Akt activation, anchorage independent cell growth, and oncogenesis in a p110-dependent manner. SIGNIFICANCE Somatic mutations in the catalytic p110α subunit of PI3K are common in cancers. In this study, we show the occurrence of frequent mutations in the regulatory p85α subunit of PI3K in human cancers. Our data demonstrate an alternate mechanism for PI3K-pathway activation and oncogenesis resulting from the impaired regulation of p110 activity by mutant p85α. Further, p85α mutations are likely to be useful as diagnostic markers for identification of p110-dependent tumors that may not carry an activating p110α mutation, but are candidates for targeted treatment with PI3K pathway inhibitors that are in development.

Connexins constitute a large family of trans-membrane proteins that allow intercellular communication and the transfer of ions and small signaling molecules between cells. Recent studies have revealed complex translational and post-translational mechanisms that regulate connexin synthesis, maturation, membrane transport and degradation that in turn modulate gap junction intercellular communication. With the growing myriad of connexin interacting proteins, including cytoskeletal elements, junctional proteins, and enzymes, gap junctions are now perceived, not only as channels between neighboring cells, but as signaling complexes that regulate cell function and transformation. Connexins have also been shown to form functional hemichannels and have roles altogether independent of channel functions, where they exert their effects on proliferation and other aspects of life and death of the cell through mostly-undefined mechanisms. This review provides an updated overview of current knowledge of connexins and their interacting proteins, and it describes connexin modulation in disease and tumorigenesis.

Phosphoinositide 3-kinase gamma (PI3Kγ) has profound roles downstream of G-protein-coupled receptors in inflammation, cardiac function, and tumor progression. To gain insight into how the enzyme's activity is shaped by association with its p101 adaptor subunit, lipid membranes, and Gβγ heterodimers, we mapped these regulatory interactions using hydrogen-deuterium exchange mass spectrometry. We identify residues in both the p110γ and p101 subunits that contribute critical interactions with Gβγ heterodimers, leading to PI3Kγ activation. Mutating Gβγ-interaction sites of either p110γ or p101 ablates G-protein-coupled receptor-mediated signaling to p110γ/p101 in cells and severely affects chemotaxis and cell transformation induced by PI3Kγ overexpression. Hydrogen-deuterium exchange mass spectrometry shows that association with the p101 regulatory subunit causes substantial protection of the RBD-C2 linker as well as the helical domain of p110γ. Lipid interaction massively exposes that same helical site, which is then stabilized by Gβγ. Membrane-elicited conformational change of the helical domain could help prepare the enzyme for Gβγ binding. Our studies and others identify the helical domain of the class I PI3Ks as a hub for diverse regulatory interactions that include the p101, p87 (also known as p84), and p85 adaptor subunits; Rab5 and Gβγ heterodimers; and the β-adrenergic receptor kinase.

Synergistic activation by heterotrimeric guanine nucleotide–binding protein (G protein)-coupled receptors (GPCRs) and receptor tyrosine kinases distinguishes p110β from other class IA phosphoinositide 3-kinases (PI3Ks). Activation of p110β is specifically implicated in various physiological and pathophysiological processes, such as the growth of tumors deficient in phosphatase and tensin homolog deleted from chromosome 10 (PTEN). To determine the specific contribution of GPCR signaling to p110β-dependent functions, we identified the site in p110β that binds to the Gβγ subunit of G proteins. Mutation of this site eliminated Gβγ-dependent activation of PI3Kβ (a dimer of p110β and the p85 regulatory subunit) in vitro and in cells, without affecting basal activity or phosphotyrosine peptide–mediated activation. Disrupting the p110β-Gβγ interaction by mutation or with a cell-permeable peptide inhibitor blocked the transforming capacity of PI3Kβ in fibroblasts, and reduced proliferation, chemotaxis, and invasiveness of PTEN-null tumor cells in culture. Our data suggest that specifically targeting GPCR signaling to PI3Kβ could provide a therapeutic approach for tumors that depend on p110β for growth and metastasis.

Summary Autophagy is an evolutionarily conserved membrane trafficking process. Induction of autophagy in response to nutrient limitation or cellular stress occurs by similar mechanisms in organisms from yeast to mammals. Unlike yeast, metazoan cells rely more on growth factor signaling for a wide variety of cellular activities including nutrient uptake. How growth factor availability regulates autophagy is poorly understood. Here we show that, upon growth factor limitation, the p110β catalytic subunit of the Class IA phosphoinositide 3-kinases (PI3Ks) dissociates from growth factor receptor complexes, and increases its interaction with the small GTPase Rab5. This p110β-Rab5 association maintains Rab5 in its GTP-bound state and enhances the Rab5-Vps34 interaction that promotes autophagy. p110β mutants that fail to interact with Rab5 are defective in autophagy promotion. Hence, in mammalian cells, p110β acts as a molecular sensor for growth factor availability and induces autophagy by activating a Rab5-mediated signaling cascade.

The with no lysine (K) (WNK) family of enzymes is best known for control of blood pressure through regulation of the function and membrane localization of ion cotransporters. In mice, global as well as endothelial-specific WNK1 gene disruption results in embryonic lethality due to angiogenic and cardiovascular defects. WNK1−/− embryos can be rescued by endothelial-specific expression of a constitutively active form of the WNK1 substrate protein kinase OSR1 (oxidative stress responsive 1). Using human umbilical vein endothelial cells (HUVECs), we explored mechanisms underlying the requirement of WNK1-OSR1 signaling for vascular development. WNK1 is required for cord formation in HUVECs, but the actions of the two major WNK1 effectors, OSR1 and its close relative SPAK (STE20/SPS1-related proline-, alanine-rich kinase), are distinct. SPAK is important for endothelial cell proliferation, whereas OSR1 is required for HUVEC chemotaxis and invasion. We also identified the zinc-finger transcription factor Slug in WNK1-mediated control of endothelial functions. Our study identifies a separation of functions for the WNK1-activated protein kinases OSR1 and SPAK in mediating proliferation, invasion, and gene expression in endothelial cells and an unanticipated link between WNK1 and Slug that is important for angiogenesis.endothelium | angiogenesis | migration | EMT | Slug

Autophagy is an important catabolic cellular process that eliminates damaged and unnecessary cytoplasmic proteins and organelles. Basal autophagy occurs during normal physiological conditions, but the activity of this process can be significantly altered in human diseases. Thus, defining the regulatory inputs and signals that control autophagy is essential. Nutrients are key modulators of autophagy. While autophagy is generally accepted to be regulated in a cell autonomous fashion, recent studies suggest nutrients can modulate autophagy in a systemic manner by inducing the secretion of hormones and neurotransmitters that regulate G protein-coupled receptors (GPCRs). Emerging studies show that GPCRs also regulate autophagy by directly detecting extracellular nutrients. We review the role of GPCRs in autophagy regulation, highlighting their potential as therapeutic drug targets.

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