Phosphorylation of inositol 1,4,5-trisphosphate receptors by protein kinase B/Akt inhibits Ca ²⁺ release and apoptosis

Abstract

Imbalance of signals that control cell survival and death results in pathologies, including cancer and neurodegeneration. Two pathways that are integral to setting the balance between cell survival and cell death are controlled by lipid-activated protein kinase B (PKB)/Akt and Ca 2؉ . PKB elicits its effects through the phosphorylation and inactivation of proapoptotic factors. Ca 2؉ stimulates many prodeath pathways, among which is mitochondrial permeability transition. We identified Ca 2؉ release through inositol 1,4,5-trisphosphate receptor (InsP3R) intracellular channels as a prosurvival target of PKB. We demonstrated that in response to survival signals, PKB interacts with and phosphorylates InsP3Rs, significantly reducing their Ca 2؉ release activity. Moreover, phosphorylation of InsP3Rs by PKB reduced cellular sensitivity to apoptotic stimuli through a mechanism that involved diminished Ca 2؉ flux from the endoplasmic reticulum to the mitochondria. In glioblastoma cells that exhibit hyperactive PKB, the same prosurvival effect of PKB on InsP3R was found to be responsible for the insensitivity of these cells to apoptotic stimuli. We propose that PKB-mediated abolition of InsP3-induced Ca 2؉ release may afford tumor cells a survival advantage.signaling ͉ cell death ͉ cancer P rotein kinase B (PKB) is a central player in regulating many signaling pathways controlling cell metabolism, growth, and survival (1, 2). PKB elicits these effects by phosphorylating and regulating the activity of downstream targets such as glycogen synthase kinase 3␤ and Bad, or via transcription factors such as Forkhead (1, 3). Because of this critical role of PKB, gain or loss of function is manifest in major disease phenotypes such as cancer and type 2 diabetes (1, 4-6).Ca 2ϩ released from the endoplasmic reticulum (ER) through inositol 1,4,5-trisphosphate (InsP 3 ) receptors (InsP 3 Rs) plays a key role in regulating physiological processes (7). However, under pathological conditions, InsP 3 -induced Ca 2ϩ release (IICR) can be subverted to promote cell death pathways (8-10). The importance of IICR in cell death is underlined by the uncovering of functional interactions with a number of proteins with known proapoptotic and antiapoptotic activity. Notable among these are Bcl-2, Bcl-X L , and cytochrome c (11)(12)(13)(14). PKB has also recently been shown to phosphorylate the InsP 3 R, with consequences for cell survival (15).We investigated whether cross-talk between the phosphatidylinositol 3-kinase (PI3K)/PKB and InsP 3 /Ca 2ϩ signaling pathways regulated how cells responded to death-inducing stimuli. We determined that PKB-mediated phosphorylation of InsP 3 R results in a decrease in the magnitude of IICR and resultant flux of Ca 2ϩ from the ER to mitochondria. Moreover, we show that this decrease in Ca 2ϩ flux caused by PKB-mediated phosphorylation of InsP 3 Rs contributes to protection from the effects of apoptotic stimuli. This prosurvival action of PKB was also apparent in a glioblastoma cell line (U87) that exhibits increase...

Constitutive Akt1 signals attenuate B‐cell receptor signaling and proliferation, but enhance B‐cell migration and effector function

“…In this respect, the slightly but consistently higher expression of CD22, an inhibitory coreceptor involved in Ca 2+ -flux [40], on unstimulated Akt1 Tg B cells could also contribute to their reduced Ca 2+ -release. It is also possible that enhanced Akt1 signals more directly influence Ca 2+ -release since cross-regulation between PI3K/Akt and Ca 2+ -signaling and modulation of the release and effects of calcium by Akt, thereby attenuating signaling, has been shown for other cell types [41].In contrast to NFATc1 and NF-κB/p65, expression and activation of STAT5 was strongly enhanced in α-IgM-stimulated Akt1 Tg B cells. In response to IL-7 signals, STAT5 regulates the survival and proliferation of B-cell progenitors and plays an important role in the generation of B-cell leukemia and lymphoma [36,42].…”

“…In this respect, the slightly but consistently higher expression of CD22, an inhibitory coreceptor involved in Ca 2+ -flux [40], on unstimulated Akt1 Tg B cells could also contribute to their reduced Ca 2+ -release. It is also possible that enhanced Akt1 signals more directly influence Ca 2+ -release since cross-regulation between PI3K/Akt and Ca 2+ -signaling and modulation of the release and effects of calcium by Akt, thereby attenuating signaling, has been shown for other cell types [41].…”

Identification of PTEN at the ER and MAMs and its regulation of Ca2+ signaling and apoptosis in a protein phosphatase-dependent manner

“…By using specific subcellular-targeted PTEN chimeras, we demonstrated that the effects of PTEN on Ca 2 þ homeostasis could not be ascribed to its regulation of PIP3 levels and subsequent Akt activation at the plasma membrane. 23,24,36,37 Indeed, the plasma membrane-targeted snap25-PTEN chimera displayed the higher reduction in Akt phosphorylation but no difference in [Ca 2 þ ] m . We demonstrated that ER-targeted PTEN markedly increases the [Ca 2 þ ] m transients, and we found that this correlate with a more effective reduction of pAkt Ser473 levels at the ER, in comparison with overexpression of wild-type PTEN.…”

“…Akt can phosphorylate all IP3R isoforms, 36,37 and a specific activity of Akt on IP3R3 leads to diminished ER-to-mitochondria Ca 2 þ transfer and protection from apoptosis. 19,24 The functional importance of PTEN interaction with IP3R3 is highlighted by the demonstration that, during apoptotic stimulation, the enhanced ER localization of PTEN positively correlated with an increased interaction with IP3R3, whereas Akt amount and phosphorylation levels as well as IP3R3 phosphorylation were reduced.…”

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

“…The interaction of specific accessory proteins has been demonstrated to either positively or negatively regulate IP 3 R opening, with consequent alteration of cellular Ca 2+ signals (e.g. [25,26]). Although most of the data exploring IP 3 R-binding proteins has been derived from studies of neuronal IP 3 Rs, it is likely that cardiac IP 3 Rs form similar macromolecular complexes.…”