The association of phosphoinositide 3-kinase enhancer A with hepatic insulin receptor enhances its kinase activity

Tumor Necrosis Factor-α Promotes Phosphoinositide 3-Kinase Enhancer A and AMP-Activated Protein Kinase Interaction to Suppress Lipid Oxidation in Skeletal Muscle

Tumor necrosis factor-α (TNF-α) is an inflammatory cytokine that plays a central role in obesity-induced insulin resistance. It also controls cellular lipid metabolism, but the underlining mechanism is poorly understood. We report in this study that phosphoinositide 3-kinase enhancer A (PIKE-A) is a novel effector of TNF-α to facilitate its metabolic modulation in the skeletal muscle. Depletion of PIKE-A in C2C12 myotubes diminished the inhibitory activities of TNF-α on mitochondrial respiration and lipid oxidation, whereas PIKE-A overexpression exacerbated these cellular responses. We also found that TNF-α promoted the interaction between PIKE-A and AMP-activated protein kinase (AMPK) to suppress its kinase activity in vitro and in vivo. As a result, animals with PIKE ablation in the skeletal muscle per se display an upregulation of AMPK phosphorylation and a higher preference to use lipid as the energy production substrate under high-fat diet feeding, which mitigates the development of diet-induced hyperlipidemia, ectopic lipid accumulation, and muscle insulin resistance. Hence, our data reveal PIKE-A as a new signaling factor that is important for TNF-α-initiated metabolic changes in skeletal muscle.

Fyn-phosphorylated PIKE-A binds and inhibits AMPK signaling, blocking its tumor suppressive activity

The AMP-activated protein kinase, a key regulator of energy homeostasis, has a critical role in metabolic disorders and cancers. AMPK is mainly regulated by cellular AMP and phosphorylation by upstream kinases. Here, we show that PIKE-A binds to AMPK and blocks its tumor suppressive actions, which are mediated by tyrosine kinase Fyn. PIKE-A directly interacts with AMPK catalytic alpha subunit and impairs T172 phosphorylation, leading to repression of its kinase activity on the downstream targets. Mutation of Fyn phosphorylation sites on PIKE-A, depletion of Fyn, or pharmacological inhibition of Fyn blunts the association between PIKE-A and AMPK, resulting in loss of its inhibitory effect on AMPK. Cell proliferation and oncogenic assays demonstrate that PIKE-A antagonizes tumor suppressive actions of AMPK. In human glioblastoma samples, PIKE-A expression inversely correlates with the p-AMPK levels, supporting that PIKE-A negatively regulates AMPK activity in cancers. Thus, our findings provide additional layer of molecular regulation of the AMPK signaling pathway in cancer progression.

PIKE is essential for oligodendroglia development and CNS myelination

Oligodendrocyte (OL) differentiation and myelin development are complex events regulated by numerous signal transduction factors. Here, we report that phosphoinositide-3 kinase enhancer L (PIKE-L) is required for OL development and myelination. PIKE-L expression is up-regulated when oligodendrocyte progenitor cells commit to differentiation. Conversely, depleting phosphoinositide-3 kinase enhancer (PIKE) expression by shRNA prevents oligodendrocyte progenitor cell differentiation. In both conventional PIKE knockout (PIKE(-/-)) and OL-specific PIKE knockout mice, the number of OLs is reduced in the corpus callosum. PIKE(-/-) OLs also display defects when forming myelin sheath on neuronal axons during neonatal development, which is partially rescued when PTEN is ablated. In addition, Akt/mTOR signaling is impaired in OL-enriched tissues of the PIKE(-/-) mutant, leading to reduced expression of critical proteins for myelin development and hypomyelination. Moreover, myelin repair of lysolecithin-induced lesions is delayed in PIKE(-/-) brain. Thus, PIKE plays pivotal roles to advance OL development and myelinogenesis through Akt/mTOR activation.

The roles of PIKE in tumorigenesis

Tumorigenesis is the process by which normal cells evolve the capacity to evade and overcome the constraints usually placed upon their growth and survival. To ensure the integrity of organs and tissues, the balance of cell proliferation and cell death is tightly maintained. The proteins controlling this balance are either considered oncogenes, which promote tumorigenesis, or tumor suppressors, which prevent tumorigenesis. Phosphoinositide 3-kinase enhancer (PIKE) is a family of GTP-binding proteins that possess anti-apoptotic functions and play an important role in the central nervous system. Notably, accumulating evidence suggests that PIKE is a proto-oncogene involved in tumor progression. The PIKE gene (CENTG1) is amplified in a variety of human cancers, leading to the resistance against apoptosis and the enhancement of invasion. In this review, we will summarize the functions of PIKE proteins in tumorigenesis and discuss their potential implications in cancer therapy.

Fyn regulates adipogenesis by promoting PIKE-A/STAT5a interaction

Fyn is a tyrosine kinase with multiple roles in a variety of cellular processes. Here we report that Fyn is a new kinase involved in adipocyte differentiation. Elevated Fyn protein is detected specifically in the adipocytes of obese mice. Moreover, Fyn expression increases progressively in 3T3-L1 cells during in vitro adipogenesis, which correlates with its kinase activity. Inhibition of Fyn by either genetic or pharmacological manipulation restrains the 3T3-L1 preadipocytes from fully differentiating into mature adipocytes. Mechanistically, Fyn regulates the activity of the adipogenic transcription factor signal transducer and activator of transcription 5a (STAT5a) through enhancing its interaction with the GTPase phosphoinositide 3-kinase enhancer A (PIKE-A). The STAT5a activity is therefore reduced in Fyn- or PIKE-ablated adipose tissues, leading to diminished expression of adipogenic markers and adipocyte differentiation. Our data thus demonstrate a novel functional interaction between Fyn, PIKE-A, and STAT5a in mediating adipogenesis.

Disrupting the PIKE-A/Akt interaction inhibits glioblastoma cell survival, migration, invasion and colony formation

The cyclin-dependent kinase 4 (CDK4) amplicon is frequently amplified in numerous human cancers including gliomas. PIKE-A, a proto-oncogene that is one of the important components of the CDK4 amplicon, binds to and enhances the kinase activity of Akt, thereby promoting cancer progression. To define the roles of the PIKE-A/Akt interaction in glioblastoma multiform (GBM) progression, we used biochemical protein/protein interaction (PPI) assays and live cell fluorescence-based protein complementation assays to search for small peptide antagonist from these proteins that were able to block their interaction. Here, we show that disruption of the interaction between PIKE-A and Akt by the small peptides significantly reduces glioblastoma cell proliferation, colony formation, migration and invasion. Disruption of PIKE-A/Akt association potently suppressed GBM cell proliferation and sensitized the cells to two clinical drugs that are currently used to treat GBM. Interestingly, GBM cells containing the CDK4 amplicon were more responsive to the inhibition of the PIKE-A/Akt interaction than GBM cells lacking this amplicon. Taken together, our findings provide proof-of-principle that blocking a PPI that is essential for cancer progression provides a valuable strategy for therapeutic discovery.

Activation of receptor protein-tyrosine kinases from the cytoplasmic compartment

It is widely accepted that receptor protein-tyrosine kinases (RTKs) are activated upon dimerization by binding to their extracellular ligands. However, EGF receptor (EGFR) dimerization per se does not require ligand binding. Instead, its cytoplasmic kinase domains have to form characteristic head-to-tail asymmetric dimers to become active, where one 'activator' domain activates the other 'receiver' domain. The non-catalytic, cytoplasmic regions of RTKs, namely the juxtamembrane and carboxy terminal portions, also regulate kinase activity. For instance, the juxtamembrane region of the RTK MuSK inhibits the kinase domain probably together with a cellular factor(s). These findings suggest that RTKs could be activated by cytoplasmic proteins. Indeed, Dok-7 and cytohesin have recently been identified as such activators of MuSK and EGFR, respectively. Given that failure of Dok-7 signaling causes myasthenia, and inhibition of cytohesin signaling reduces the proliferation of EGFR-dependent cancer cells, cytoplasmic activators of RTKs may provide new therapeutic targets.

Phosphoinositide 3-kinase enhancer (PIKE) in the brain: is it simply a phosphoinositide 3-kinase/Akt enhancer?

Since its discovery in 2000, phosphoinositide 3-kinase enhancer (PIKE) has been recognized as a class of GTPase that controls the enzymatic activities of phosphoinositide 3-kinase (PI3K) and Akt in the central nervous system (CNS). However, recent studies suggest that PIKEs are not only enhancers to PI3K/Akt but also modulators to other kinases including insulin receptor tyrosine kinase and focal adhesion kinases. Moreover, they regulate transcription factors such as signal transducer and activator of transcription and nuclear factor κB. Indeed, PIKE proteins participate in multiple cellular processes including control of cell survival, brain development, memory formation, gene transcription, and metabolism. In this review, we have summarized the functions of PIKE proteins in CNS and discussed their potential implications in various neurological disorders.

What we have learnt about PIKE from the knockout mice

Phosphoinositide 3-kinase enhancer (PIKE) is a group of GTPase that belongs to the centaurin superfamily. These proteins have been discovered for more than a decade but our understandings on their functions are still limited. Studies from our research group and others have revealed some of their functions in a cellular context but their roles in organ development or systemic homeostasis just begin to unveil. The generation of PIKE knockout mice thus provides the valuable model to delineate the physiological roles of PIKE. In addition to being a PI3K/Akt enhancer, phenotypic characterization of the PIKE knockout mice demonstrates that the proteins are involved in multiple signaling cascades including Janus kinase (JAK)/ Signal Transducer and Activator of Transcription (STAT), AMP-activated protein kinase (AMPK)/Acetyl-CoA carboxylase (ACC) and insulin receptor (IR)/Akt. In this article, we will review the current findings from the PIKE knockout mice studies and will discuss how these in vivo observations lead to the identifications of novel signaling cascades regulated by PIKE.