Stress Kinase Signaling Regulates Androgen Receptor Phosphorylation, Transcription, and Localization

Abstract

Activation of signal transduction kinase cascades is known to alter androgen receptor (AR) activity, but the molecular mechanisms are still poorly defined. Here we show that stress kinase signaling regulates Ser 650 phosphorylation and AR nuclear export. In LNCaP prostate cancer cells, activation of either MAPK kinase (MKK) 4:c-Jun N-terminal kinase (JNK) or MKK6:p38 signaling pathways increased Ser 650 phosphorylation, whereas pharmacologic inhibition of JNK or p38 signaling led to a reduction of AR Ser 650 phosphorylation. Both p38alpha and JNK1 phosphorylated Ser 650 in vitro. Small interfering RNA-mediated knockdown of either MKK4 or MKK6 increased endogenous prostate-specific antigen (PSA) transcript levels, and this increase was blocked by either bicalutamide or AR small interfering RNA. Stress kinase inhibition of PSA transcription is, therefore, dependent on the AR. Similar experiments involving either activation or inhibition of MAPK/ERK kinase:ERK signaling had little effect on Ser 650 phosphorylation or PSA mRNA levels. Ser 650 is proximal to the DNA binding domain that contains a nuclear export signal. Mutation of Ser 650 to alanine reduced nuclear export of the AR, whereas mutation of Ser 650 to the phosphomimetic amino acid aspartate restored AR nuclear export. Pharmacologic inhibition of stress kinase signaling reduced wild-type AR nuclear export equivalent to the S650A mutant without affecting nuclear export of the S650D mutant. Our data suggest that stress kinase signaling and nuclear export regulate AR transcriptional activity.

Role of mitogen-activated protein kinase kinase 4 in cancer

“…Similar to the case of p53, both p38 and JNK can phosphorylate the androgen receptor (AR) at Ser650 and promote its nuclear export in prostate cancer cells thereby reducing AR-mediated transcription (Gioeli et al, 2006). It was also demonstrated that decreasing MKK4 or MKK6 expression levels by siRNA led to increased AR nuclear accumulation and transactivation.…”

“…It was also demonstrated that decreasing MKK4 or MKK6 expression levels by siRNA led to increased AR nuclear accumulation and transactivation. (Gioeli et al, 2006). As the AR plays a key role in the progression of prostate cancer, then loss of function mutants of MKK4 may hypersensitize the AR to androgen and promote the androgen-independent diseased state.…”

Degradation and beyond: Control of androgen receptor activity by the proteasome system

“…However, as in the case of Akt, other authors reported that GSK3β is required for AR gene expression [44]. The phosphorylation of Ser650 is also upregulated by stress kinase signalling and reduces AR transcription by augmentation of AR export to the cytoplasm [45]. So far, it has not been established which AR amino acid sequence is a target for Mdm2-driven ubiquitylation.…”

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

“…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.…”