Phosphorylation of HDM2 by Akt

Abstract

The HDM2 protein is a key regulator of the tumour suppressor, p53. Control of HDM2 function is critical for normal cell proliferation and stress responses, and it is becoming evident that multiple modifications of HDM2 can regulate its function within cells. In this study we show that HDM2 associated with the serine-threonine kinase, Akt, in response to growth factor stimulation of human primary cells. This association was concurrent with phosphorylation of Akt (at Ser 473), and resulted in elevated expression of HDM2 and enhanced nuclear localization. However, analysis of HDM2 proteins mutated at the consensus Akt recognition sites at serines 166 and 186 indicated that modification at these residues was not sufficient for the increased expression of the protein, which was blocked by the PI3 kinase inhibitor LY294002. Tryptic peptide and mutational analyses revealed evidence for an Akt phosphorylation site in HDM2 additional to the two consensus sites.

Complex regulation of human androgen receptor expression by Wnt signaling in prostate cancer cells

“…The activation of Akt signaling was consistent with a similar increase in the phosphorylation (at ser 166) of the Akt downstream target, MDM2 (Ashcroft et al, 2002). Further evidence that Wnt mediates activation of Akt signaling is shown in the results of Figure 4b wherein siRNAs against PCDH-PC or b-catenin or dominant negative TCF-4 strongly suppressed Akt (and MDM2) phosphorylation in LNCaP cells maintained in androgen-free medium.…”

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.…”

“…Different DNA damaging agents induce varying degrees of p53 phosphorylation in various cell types, including neuroblastoma (Isaacs et al, 2001;Ashcroft et al, 2002;Wang et al, 2003). In our study, we found phosphorylation of p53 serine residue 15 following treatment with COX inhibitors (data not shown).…”

“…Specifically, we found that both COX inhibitors alone and in combination with chemotherapy decrease the levels of P-HDM2 (ser166), which correlates with the early increase in p53 nuclear localization. Previous studies demonstrate that nuclear accumulation following stress such as DNA damage involves the inhibition of HDM2-dependent nuclear export of p53 (Ashcroft et al, 2002;Oren, 2003;Inoue et al, 2005). 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).…”

“…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).…”