Nuclear but Not Cytosolic Phosphoinositide 3-Kinase Beta Has an Essential Function in Cell Survival

Abstract

Class I A phosphoinositide 3-kinases (PI3Ks) are heterodimeric enzymes composed of a p85 regulatory and a p110 catalytic subunit that induce the formation of 3-polyphosphoinositides, which mediate cell survival, division, and migration. There are two ubiquitous PI3K isoforms p110␣ and p110␤ that have nonredundant functions in embryonic development and cell division. However, whereas p110␣ concentrates in the cytoplasm, p110␤ localizes to the nucleus and modulates nuclear processes such as DNA replication and repair. At present, the structural features that determine p110␤ nuclear localization remain unknown. We describe here that association with the p85␤ regulatory subunit controls p110␤ nuclear localization. We identified a nuclear localization signal (NLS) in p110␤ C2 domain that mediates its nuclear entry, as well as a nuclear export sequence (NES) in p85␤. Deletion of p110␤ induced apoptosis, and complementation with the cytoplasmic C2-NLS p110␤ mutant was unable to restore cell survival. These studies show that p110␤ NLS and p85␤ NES regulate p85␤/p110␤ nuclear localization, supporting the idea that nuclear, but not cytoplasmic, p110␤ controls cell survival.The phosphoinositide 3-kinase (PI3K) family is divided into four groups (I A , I B , II, and III) according to structural features and substrate specificity. Of these, only class I enzymes catalyze the production of PI(3,4,5)P 3 and PI(3,4)P 2 in vivo. Class I A PI3Ks are heterodimeric proteins consisting of a p110 catalytic subunit (p110␣, p110␤, and p110␦) and an associated p85 regulatory subunit (p85␣, p85␤, and p55␥) (14,18,21,22,53). p110␥ (class I B PI3K) is structurally similar but associates with a distinct class of regulatory subunits. The catalytic subunits p110␣ and p110␤ are expressed ubiquitously, whereas p110␦ and p110␥ are more abundant in hematopoietic cells (14,44,53).Despite the similarity in sequence, expression patterns, and regulatory subunits, p110␣ and p110␤ have distinct functions in cell proliferation, cell cycle progression, and development (5,6,12,26,(32)(33)(34)(35)47). p110␣ has a key role in insulin action and cell cycle entry (12, 13), whereas p110␤ is reported to play a pivotal role in DNA replication, S phase progression, and DNA repair (32,34,35). Activating mutations of p110␣, but not of p110␤, have been found in human cancer; nonetheless, p110␤ drives tumorigenesis in PTEN-defective cells and induces focus formation in fibroblasts (8,9,26,29). Moreover, overexpression of p110␤ is found in specific tumor types (7,54,58). Previous studies showed that part of the specific functions of p110␣ and p110␤ result from their distinct subcellular localization and activation requirements (34, 35), highlighting the emergence of subcellular localization as a major mechanism to govern cell responses (30). Previous reports showed that p85/p110 complex can translocate to the nucleus regulating cell survival, particularly in neuronal cell lines (37). In addition, p110␤, but not p110␣, localizes to the nucleus in several cell types. The...

Classes of phosphoinositide 3-kinases at a glance

“…There are also emerging descriptions of PI3K localization and/or roles in the nucleus (Kumar et al, 2011). In addition, there are many indirect consequences of these diverse PI3K functions.…”

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

“…p110␣ is activated before p110␤ at the G 0 /G 1 transition, as well as in the G 1 /S and G 2 /M transitions (8,9,13,14). We analyzed whether p110␣ associated with p110␤ and regulated its activity.…”

“…p110␤ activity was less dependent on p110␣ in cells activated via GPCR with S1P, a less potent stimulus than serum, which alone does not trigger cell cycle entry (34); this suggests that p110␤ can also be activated (at least by GPCR) independently of p110␣. The need for p110␣/ p110␤ association for optimal p110␤ activation explains the late p110␤ activation kinetics compared to p110␣ in the G 1 and S phases and in mitosis (8,9,13,14).…”

“…p110␣ is activated again at the beginning of mitosis, followed by that of p110␤ (14). Activation of p110␣ thus precedes that of p110␤ at several points in cell cycle progression (8,9,13,14), although it is not known whether p110␣ regulates p110␤ activation. The PI3K activation-induced increase in PI(3,4,5)P 3 levels is subsequently reduced to basal levels by .…”

“…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)(9)(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)(10)(11)(12)(13). p110␣ is activated again at the beginning of mitosis, followed by that of p110␤ (14).…”