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Akt-phosphorylated PIKE-A inhibits UNC5B-induced apoptosis in cancer cell lines in a p53-dependent manner.

ABSTRACT: UNC5B acts as a tumor suppressor, and it induces apoptosis in the absence of its cognate ligand netrins. UNC5B is a direct transcriptional target of p53 upon UV stimulation. Here we show that Akt phosphorylates PIKE-A and regulates its association with UNC5B and inhibits UNC5B-provoked apoptosis in a p53-dependent manner. PIKE-A GTPase binds active Akt and stimulates its kinase activity in a guanine-nucleotide-dependent way. Akt feeds back and phosphorylates PIKE-A on Ser-472 and subsequently enhances its stimulatory effect on Akt kinase activity. Akt activity is significantly reduced in PIKE -/- Mouse Embryonic Fibroblast (MEF) cells as compared to wild-type cells. PIKE-A directly interacts with UNC5B, which is regulated by netrin-1-activated Akt. Overexpression of PIKE-A diminishes UNC5B expression through down-regulation of p53. Knocking down PIKE-A stabilizes p53, increases UNC5B, and escalates UV-triggered apoptosis. Depletion of Akt abrogates PIKE-A's inhibitory effect on both p53 and UNC5B. Hence our findings support the notion that Akt--phosphorylated PIKE-A inhibits UNC5B-elicited apoptosis and reduces its expression level through inactivation of p53.

SUBMITTER: He K

PROVIDER: S-EPMC3103409 | biostudies-literature | 2011 Jun

REPOSITORIES: biostudies-literature

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Akt-phosphorylated PIKE-A inhibits UNC5B-induced apoptosis in cancer cell lines in a p53-dependent manner.

He Kunyan K Jang Sung-Wuk SW Joshi Jayashree J Yoo Min-Heui MH Ye Keqiang K

Molecular biology of the cell 20110401 11

UNC5B acts as a tumor suppressor, and it induces apoptosis in the absence of its cognate ligand netrins. UNC5B is a direct transcriptional target of p53 upon UV stimulation. Here we show that Akt phosphorylates PIKE-A and regulates its association with UNC5B and inhibits UNC5B-provoked apoptosis in a p53-dependent manner. PIKE-A GTPase binds active Akt and stimulates its kinase activity in a guanine-nucleotide-dependent way. Akt feeds back and phosphorylates PIKE-A on Ser-472 and subsequently en ...[more]

PMID: 21460185

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Project description:UNC5B is a dependence receptor that promotes survival in the presence of its ligand, netrin-1, while inducing cell death in its absence. The receptor has an important role in the development of the nervous and vascular systems. It is also involved in the normal turnover of intestinal epithelium. Netrin-1 and UNC5B are deregulated in multiple cancers, including colorectal, neuroblastoma, and breast tumors. However, the detailed mechanism of UNC5B function is not fully understood. We have utilized the murine polyomavirus small T antigen (PyST) as a tool to study UNC5B-mediated apoptosis. PyST is known to induce mitotic arrest followed by extensive cell death in mammalian cells. Our results show that the expression of PyST increases mRNA levels of UNC5B by approximately 3-fold in osteosarcoma cells (U2OS) and also stabilizes UNC5B at the posttranslational level. Furthermore, UNC5B is upregulated predominantly in those cells that undergo mitotic arrest upon PyST expression. Interestingly, although its expression was previously reported to be regulated by p53, our data show that the increase in UNC5B levels by PyST is p53 independent. The posttranslational stabilization of UNC5B by PyST is regulated by the interaction of PyST with PP2A. We also show that netrin-1 expression, which is known to inhibit UNC5B apoptotic activity, promotes survival of PyST-expressing cells. Our results thus suggest an important role of UNC5B in small-T antigen-induced mitotic catastrophe that also requires PP2A.IMPORTANCE UNC5B, PP2A, and netrin-1 are deregulated in a variety of cancers. UNC5B and PP2A are regarded as tumor suppressors, as they promote apoptosis and are deleted or mutated in many cancers. In contrast, netrin-1 promotes survival by inhibiting dependence receptors, including UNC5B, and is upregulated in many cancers. Here, we show that UNC5B-mediated apoptosis can occur independently of p53 but in a PP2A-dependent manner. A substantial percentage of cancers arise due to p53 mutations and are insensitive to chemotherapeutic treatments that activate p53. Unexpectedly, treatment of cancers having functional p53 with many conventional drugs leads to the upregulation of netrin-1 through activated p53, which is counterintuitive. Therefore, understanding the p53-independent mechanisms of the netrin-UNC5B axis, such as those involving PP2A, assumes greater clinical significance. Anticancer strategies utilizing anti-netrin-1 antibody treatment are already in clinical trials.

Project description:Cell cycle checkpoints, activated by stressful events, halt the cell cycle progression, and prevent the transmission of damaged DNA. These checkpoints prompt cell repair but also trigger cell death if damage persists. Decision-making between these responses is multifactorial and context-dependent, with the tumor suppressor p53 playing a central role. In many tumor cells, p53 alterations lead to G1/S checkpoint loss and the weakening of the G2 checkpoint, rendering cell viability dependent on the strength of the latter through mechanisms not fully characterized. Cells with a strong pro-survival drive can evade cell death despite substantial DNA lesions. Deciphering the integration of survival pathways with p53-dependent and -independent mechanisms governing the G2/M transition is crucial for understanding G2 arrest functionality and predicting tumor cell response to chemotherapy. The serine/threonine kinase GRK2 emerges as a signaling node in cell cycle modulation. In cycling cells, but not in G2 checkpoint-arrested cells, GRK2 protein levels decline during G2/M transition through a process triggered by CDK2-dependent phosphorylation of GRK2 at the S670 residue and Mdm2 ubiquitination. We report now that this downmodulation in G2 prevents the unscheduled activation of the PI3K/AKT pathway, allowing cells to progress into mitosis. Conversely, higher GRK2 levels lead to tyrosine phosphorylation by the kinase c-Abl, promoting the direct association of GRK2 with the p85 regulatory subunit of PI3K and AKT activation in a GRK2 catalytic-independent manner. Hyperactivation of AKT is conditioned by p53's scaffolding function, triggering FOXO3a phosphorylation, impaired Cyclin B1 accumulation, and CDK1 activation, causing a G2/M transition delay. Upon G2 checkpoint activation, GRK2 potentiates early arrest independently of p53 through AKT activation. However, its ability to overcome the G2 checkpoint in viable conditions depends on p53. Our results suggest that integrating the GRK2/PI3K/AKT axis with non-canonical functions of p53 might confer a survival advantage to tumor cells.

Dataset Information

Akt-phosphorylated PIKE-A inhibits UNC5B-induced apoptosis in cancer cell lines in a p53-dependent manner.

Publications

Akt-phosphorylated PIKE-A inhibits UNC5B-induced apoptosis in cancer cell lines in a p53-dependent manner.

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