Cell Activation-Induced Phosphoinositide 3-Kinase Alpha/Beta Dimerization Regulates PTEN Activity

Abstract

The phosphoinositide 3-kinase (PI3K)/PTEN (phosphatase and tensin homolog) pathway is one of the central routes that enhances cell survival, division, and migration, and it is frequently deregulated in cancer. PI3K catalyzes formation of phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P 3 ] after cell activation; PTEN subsequently reduces these lipids to basal levels. Activation of the ubiquitous p110␣ isoform precedes that of p110␤ at several points during the cell cycle. We studied the potential connections between p110␣ and p110␤ activation, and we show that cell stimulation promotes p110␣ and p110␤ association, demonstrating oligomerization of PI3K catalytic subunits within cells. Cell stimulation also promoted PTEN incorporation into this complex, which was necessary for PTEN activation. Our results show that PI3Ks dimerize in vivo and that PI3K and PTEN activities modulate each other in a complex that controls cell PI(3,4,5)P 3 levels. The phosphoinositide 3-kinase (PI3K)/PTEN axis regulates cell survival, division, and migration. PI3Ks are lipid kinases conserved throughout evolution that catalyze phosphorylation of the third position of the inositol ring of phosphatidylinositol (PI). The PI3K family is subdivided into three classes, of which only class I generates phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P 3 ] and, after SH2 domain-containing inositol 5=-phosphatase action, PI(3,4)P 2 (1-3). These plasma membrane components initiate signaling pathways that lead to cell activation and are downregulated by the PTEN phosphatase (1-5). Class IA PI3Ks are heterodimers composed of a 110-kDa catalytic subunit and a regulatory subunit: p85␣ (and its alternative spliced forms, p55␣ or p50␣), p85␤, or p55␥ (1-5). p85␣ and p85␤ are ubiquitous, whereas p55␥ is expressed only in certain tissues (1-5). Although four genes encode p110 subunits, only p110␣ and p110␤ are expressed ubiquitously in mammals and are more important in cancer (1-5). p110␣ and p110␤ are activated by receptor tyrosine kinases, although p110␤ can also be activated through G proteincoupled receptors (GPCRs) (6, 7). After activation, PI3K regulates processes such as cell cycling and survival (8-10).PI3K activity increases after stimulation of growth factor (GF) receptors in early G 1 phase and again in advanced G 1 . The first PI3K peak corresponds mainly to p110␣ activation, followed by a minor p110␤ activity peak; p110␣ is also activated before p110␤ in late G 1 -S-phase entry (8, 9). p110␣ is found in the cytoplasm, whereas p110␤ shuttles between the cytosol and nucleus and accumulates in the nucleus in S phase (8-10). The distinct p110␣ and p110␤ localizations and activation kinetics might reflect nonredundant roles for each isoform. p110␣ controls early G 1 events, whereas p110␤ regulates S-phase progression (8-10); p110␣ is involved in cell responses to insulin and angiogenesis (11,12), whereas p110␤ affects nuclear processes such as DNA replication and repair (9-13). p110␣ is activated again at the beginning of mitosis, followed b...

Assembly and Molecular Architecture of the Phosphoinositide 3-Kinase p85α Homodimer

“…Although there is one report of p85/p110 heterotetramers in cultured cells (63), these higher order complexes have not been reported during purification and gel filtration analysis of recombinant p85/p110 dimers. Structures of the nSH2-iSH2/p110␣ and iSH2-cSH2/p110␤ heterodimers show that the adaptor-binding domain, C2 domain, and kinase domain of p110 make extensive contacts with the iSH2 domain, whereas the nSH2 and cSH2 domains contact the helical, C2, and kinase domains (83,84).…”

Cystic Fibrosis Transmembrane Conductance Regulator Attaches Tumor Suppressor PTEN to the Membrane and Promotes Anti Pseudomonas aeruginosa Immunity

“…Both the catalytic p110 (δ and γ) and regulatory p85α subunits of class I PI3K interact with PTEN and facilitate its activation (Chagpar et al, 2010; Perez-Garcia et al, 2014; Rabinovsky et al, 2009), which includes dephosphorylation of its C-terminus, dimerization at the plasma membrane and PIP 2 production (Papa et al, 2014; Rahdar et al, 2009; Vazquez et al, 2000). Given this cooperative and likely immune regulatory PI3K-PTEN interaction, and because PTEN null mutations are embryonic lethal (Di Cristofano et al, 1998), PTEN has been studied mainly in myeloid (Huang et al, 2012; Kral et al, 2016; Schabbauer et al, 2010; Zaidi and Manna, 2016) and airway cells (Kwak et al, 2003).…”

Nuclear upregulation of PI3K p110β correlates with increased rRNA transcription in endometrial cancer cells

“…Consistent with previous studies in other cell types [27,28], p110α is cytoplasmic while p110β is both cytoplasmic and nuclear. This would suggest that, in the cytoplasm, p110α and p110β isoforms can share some of the functions attributed to PI3K signalling operating perhaps due to their reported cross-activation [53]. In cancer cells, the presence of genetic mutations affecting PIK3CA or PTEN would influence PtdIns(3,4,5)P3mediated downstream functions induced by p110α and p110β respectively in this compartment.…”