A phosphatidylinositol 3-kinase/Akt pathway promotes translocation of Mdm2 from the cytoplasm to the nucleus

Abstract

The Mdm2 oncoprotein promotes cell survival and cell cycle progression by inhibiting the p53 tumor suppressor protein. To regulate p53, Mdm2 must gain nuclear entry, and the mechanism that induces this is now identified. Mitogen-induced activation of phosphatidylinositol 3-kinase (PI3-kinase) and its downstream target, the Akt͞PKB serine-threonine kinase, results in phosphorylation of Mdm2 on serine 166 and serine 186. Phosphorylation on these sites is necessary for translocation of Mdm2 from the cytoplasm into the nucleus. Pharmacological blockade of PI3-kinase͞ Akt signaling or expression of dominant-negative PI3-kinase or Akt inhibits nuclear entry of Mdm2, increases cellular levels of p53, and augments p53 transcriptional activity. Expression of constitutively active Akt promotes nuclear entry of Mdm2, diminishes cellular levels of p53, and decreases p53 transcriptional activity. Mutation of the Akt phosphorylation sites in Mdm2 produces a mutant protein that is unable to enter the nucleus and increases p53 activity. The demonstration that PI3-kinase͞Akt signaling affects Mdm2 localization provides insight into how this pathway, which is inappropriately activated in many malignancies, affects the function of p53.

Cyclooxygenase inhibitors modulate the p53/HDM2 pathway and enhance chemotherapy-induced apoptosis in neuroblastoma

“…Furthermore, inhibition of HDM2 by antisense oligonucleotides resulted in accumulation of nuclear p53 in neuroblastoma, as well as other tumors with p53 cytoplasmic sequestration (Lu et al, 2000). In addition, phosphorylation of HDM2 at ser166 has been shown to promote HDM2 nuclear localization (Mayo and Donner, 2001;Ashcroft et al, 2002). Thus, the downregulation of HDM2 and P-HDM2 seen in this study is consistent with a role for COX inhibitors in blocking p53 nuclear export in a manner similar to that induced by some DNA damaging agents.…”

“…However, another plausible mechanism that may explain our findings is the downregulation of Akt/PDK1. Mitogeninduced activation of PI3-kinase and its downstream target Akt results in phosphorylation of HDM2 on ser166 and 168 and enhanced HDM2 nuclear localization (Mayo and Donner, 2001;Ashcroft et al, 2002). Interestingly, a number of studies have demonstrated that COX-2 inhibitors induce apoptosis by blocking Akt activation in adult carcinomas (Hsu et al, 2000;Arico et al, 2002).…”

“…In contrast, the levels of the E3 ligase HDM2, a critical regulator of p53 stability and subcellular localization (Brooks and Gu, 2004), were decreased at early time points following treatment with COX inhibitors (Figure 6a). Phosphorylation of HDM2 at serine residue 166 has previously been reported to enhance HDM2 translocation to the nucleus, where it binds to p53 and affects both p53 stability and localization (Roth et al, 1998;Mayo and Donner, 2001). Hence, we asked whether HDM2 phosphorylation could be affected by treatment with COX inhibitors.…”

FOXO transcription factor-dependent p15INK4b and p19INK4d expression

“…Akt also phosphorylates and translocates Mdm2 from the cytoplasm into the nucleus, resulting in promoting p53 degradation and cell-cycle arrest (Mayo and Donner, 2001). In the cells containing normal p53, suppression of Akt signaling pathway upregulates p53 expression and abrogates cell-cycle progression in this mechanism.…”

Effects of bufalin on the proliferation of human lung cancer cells and its molecular mechanisms of action

“…Rescue of p53-induced apoptosis by survival factors has been associated with the activation of Akt kinase (Sabbatini and McCormick 1999). Akt can phosphorylate and activate MDM2, thereby enhancing the degradation of p53 (Mayo and Donner 2001;Zhou et al 2001). Thus, up-regulation of WT p53 in A549 cells after treatment with either bufalin or inhibitor of PI3 K/ Akt (LY), as observed in our study, could be explained by down-regulation of the pAkt/MDM2 pathway.…”