Phosphoinositide 3-Kinase-independent Non-genomic Signals Transit from the Androgen Receptor to Akt1 in Membrane Raft Microdomains

Abstract

The serine-threonine kinase, Akt1/protein kinase B␣ is an important mediator of growth, survival, and metabolic signaling. Recent studies have implicated cholesterol-rich, lipid raft microdomains in survival signals mediated by Akt1. Here we address the role of lipid raft membranes as a potential site of intersection of androgenic and Akt1 signaling. A subpopulation of androgen receptor (AR) was found to localize to a lipid raft subcellular compartment in LNCaP prostate cancer cells. Endogenous AR interacted with endogenous Akt1 preferentially in lipid raft fractions and androgen substantially enhanced the interaction between the two proteins. The association of AR with Akt1 was inhibited by the anti-androgen, bicalutamide, but was not affected by inhibition of phosphoinositide 3-kinase (PI3K). Androgen promoted endogenous Akt1 activity in lipid raft fractions, in a PI3K-independent manner, within 10 min of treatment. Fusion of a lipid raft targeting sequence to AR enhanced localization of the receptor to rafts, and stimulated Akt1 activity in response to androgen, while reducing the cells' dependence on constitutive signaling through PI3K for cell survival. These findings suggest that signals channeled through AR and Akt1 intersect by a mechanism involving formation within lipid raft membranes of an androgen-responsive, extranuclear AR/Akt1 complex. Our results indicate that cholesterol-rich membrane microdomains play a role in transmitting nongenomic signals involving androgen and the Akt pathway in prostate cancer cells. Androgen receptor (AR),3 a member of the nuclear receptor superfamily of transcription factors, is an important regulator of reproductive physiology as a mediator of the biological activities of androgen (1). AR also plays a role in the pathogenesis of prostate cancer (PCa), including progression to the androgen-independent state and metastasis (2, 3). Upon androgen binding, the AR undergoes conformational changes and translocates from the cytosol to the nucleus (4), where the receptor-hormone complex regulates the expression of target genes (5).In addition to this well recognized genomic function, AR has been demonstrated to activate intracellular signaling pathways by a rapid (occurring in minutes) non-genomic process activated in response to hormonal stimulation (6, 7). The mechanisms underlying the non-canonical pathways involving the AR and other nuclear receptors are still poorly understood, however, possible nongenomic roles for the AR have been described (8 -12). For example, androgen promotes oocyte maturation, in both Xenopus (10) and mouse (12), by a transcription-independent mechanism that involves the classical AR (12,14). Another report showed that cells treated with 17␤-estradiol (E2) or androgen resulted in cell proliferation mediated by the rapid formation of a cytosolic signaling complex containing estrogen receptor-␣ or -␤ (ER␣ or ER␤ (depending on the cell type)), AR, and the non-receptor tyrosine kinase, Src (8). Similarly, Kousteni et al. (9) showed that ER␣, ER␤, or AR...

Phosphoinositide 3-kinase signaling in the vertebrate retina

“…These studies suggest a mechanism of signal downregulation of IRS-1 that is mediated by monomeric p85 through the formation of a sequestration complex between p85 and IRS-1 ( 187 ). Modulation of PI3K-Akt signaling components in the membrane raft microdomains have been reported (188)(189)(190). Consistent with these reports, we have also demonstrated the localization of the IR and PI3K in detergentresistant membrane rafts of rod photoreceptor outer segments ( 191 ).…”

Androgen Receptor Survival Signaling Is Blocked by Anti-β2-microglobulin Monoclonal Antibody via a MAPK/Lipogenic Pathway in Human Prostate Cancer Cells

“…In this study, we showed that ␤2M mAb inhibited a large number of cell signaling networks, including MAPK, SREBP-1, AR, and PI3K/Akt. It is conceivable that these signaling networks are interconnected through lipid raft complexes (47,48). The inhibitory action exerted by ␤2M mAb could affect the lipid composition of the raft structures, hence altering domain interactions and how downstream cell signal networks can be assembled and interact in a coordinated manner (45,49).…”

Phosphoinositide 3-OH kinase p85α and p110β are essential for androgen receptor transactivation and tumor progression in prostate cancers

“…However, we (Liao et al, 2004) and other (Mellinghoff et al, 2004) found that Src kinase inhibitor did not suppress androgen-induced gene expression, indicating that Src kinase is not involved in androgen-induced PI3K activation or AR genomic effect. In addition, recent studies showed that after androgen stimulation the AR was transiently translocated to the lipid rafts on the plasma membrane (Lu et al, 2001;Cinar et al, 2007), where multiple signaling molecules such as G proteins are located (reviewed in Chini and Parenti, 2004). In fact, G protein a-subunit has been shown to activate the AR in the absence or presence of androgens (Kasbohm et al, 2005).…”