Phosphorylation-dependent ubiquitylation and degradation of androgen receptor by Akt require Mdm2 E3 ligase

Abstract

The androgen receptor (AR) controls several biological functions including prostate cell growth and apoptosis. However, the mechanism by which AR maintains its stability to function properly remains largely unknown. Here we show that Akt and Mdm2 form a complex with AR and promote phosphorylation-dependent AR ubiquitylation, resulting in AR degradation by the proteasome. The effect of Akt on AR ubiquitylation and degradation is markedly impaired in a Mdm2-null cell line compared with the wild-type cell line, suggesting that Mdm2 is involved in Akt-mediated AR ubiquitylation and degradation. Furthermore, we demonstrate that the E3 ligase activity of Mdm2 and phosphorylation of Mdm2 by Akt are essential for Mdm2 to affect AR ubiquitylation and degradation. These results suggest that phosphorylation-dependent AR ubiquitylation and degradation by Akt require the involvement of Mdm2 E3 ligase activity, a novel mechanism that provides insight into how AR is targeted for degradation.

Isosilybin B causes androgen receptor degradation in human prostate carcinoma cells via PI3K-Akt-Mdm2-mediated pathway

“…Isosilybin B increases Mdm2 phosphorylation and interaction with both Akt and AR Mdm2 (90 kDa) is a ring-finger protein whose E3-ligase activity plays a crucial role in ubiquitination and degradation of a number of proteins including AR (Zhou et al, 2001;Lin et al, 2002;Chen et al, 2005). Accordingly, we next examined the effect of isosilybin B on Mdm2 phosphorylation at Ser-166 site, which is associated with its activation and enhancement of ubiquitin ligase activity (Zhou et al, 2001;Lin et al, 2002;Paajarvi et al, 2005 (Figure 6a).…”

“…Isosilybin B increases the phosphorylation of Akt Akt (60 kDa) is one such kinase whose role has been described in the phosphorylation and degradation of AR (Lin et al, 2001(Lin et al, , 2002. Therefore, we next examined the effect of isosilybin B treatment on Akt phosphorylation.…”

“…As we observed that isosilybin B causes Akt phosphorylation and increases its kinase activity as well, which is involved in AR degradation, and since Akt is known to phosphorylate and degrade AR (Lin et al, 2001(Lin et al, , 2002, we next examined the effect of isosilybin B on phosphorylation status of AR at Ser-210/213 site in presence and absence of PI3K inhibitor. Indeed isosilybin B resulted in an increased phosphorylation of AR at Ser-210/213 site, which was inhibited in the presence of PI3K inhibitor (Figure 6a).…”

“…Accordingly, we next examined the effect of isosilybin B on Mdm2 phosphorylation at Ser-166 site, which is associated with its activation and enhancement of ubiquitin ligase activity (Zhou et al, 2001;Lin et al, 2002;Paajarvi et al, 2005 (Figure 6a). Further, isosilybin B treatment significantly increased the binding between Mdm2 and Akt (Figure 6c).…”

“…Various studies have shown the role of glycogen synthase kinase 3b (GSK3b), stress kinase, mammalian target of rapamycin (mTOR) kinase and cyclin-dependent kinase 1 in regulating phosphorylation, stability and localization of AR (Wang et al, 2004;Cinar et al, 2005;Chen et al, 2006;Gioeli et al, 2006). Further, there are numerous studies suggesting the importance of Akt/PKB (a serine/threonine kinase) activation in AR degradation (Lin et al, 2001(Lin et al, , 2002Liao et al, 2005) as well as AR activation (Miyamoto et al, 2005;Reddy et al, 2006); though the interaction of Akt with AR largely remains unclear. Accordingly, we also examined the effect of isosilybin B on Akt phosphorylation and its possible role in AR degradation.…”