雄激素受体信号转导途径及其在前列腺癌发展中的作用

前列腺癌是西方男性发病率最高的癌症之一,在采用雄激素阻断疗法后,大部分患者的病情可得到控制,但经过一段时间又会转变为雄激素非依赖型前列腺癌。雄激素受体(AR)在前列腺细胞中扮演重要的角色,它可调节大量基因的表达。在前列腺癌由雄激素依赖型向雄激素非依赖型的转变过程中,AR及其信号途径通过多种方式发挥作用,AR基因的扩增、AR的突变,以及与共激活子之间作用的改变都可能使细胞获得雄激素非依赖型的生长能力。此外,AR还受到多肽生长因子和细胞因子等的调节,表现激素非依赖型的转录激活活性。AR在前列腺癌中作用的阐明对前列腺癌的诊断与治疗有着重大的意义。     

Androgens modulate autophagy and cell death via regulation of the endoplasmic reticulum chaperone glucose-regulated protein 78/BiP in prostate cancer cells

Pro-survival signalling mediated by the androgen receptor (AR) is implicated as a key contributor to prostate carcinogenesis. As prostate tumours are characterized by nutrient-poor, hypoxic and acidified microenvironments, one mechanism whereby AR signalling may contribute to survival is by promoting adaptation to cellular stress. Here we have identified a novel role for AR in the inhibition of autophagy induced by serum withdrawal. This blockade is attributed to AR-mediated upregulation of the endoplasmic reticulum (ER) chaperone glucose-regulated protein 78/BiP (Grp78/BiP), and occurs independently of ER stress response pathway activation. Interestingly, AR activation did not affect serum starvation-induced mammalian target of rapamycin inhibition, illustrating that the adaptive role for androgens lies not in the ability to modulate nutrient sensing, but in the promotion of ER stability. Finally, we show that the adaptive advantage conferred by AR-mediated Grp78/BiP upregulation is temporary, as upon chronic serum starvation, AR activation delayed but did not suppress the onset of autophagy and cell death. This study reveals a novel mechanism whereby maintained AR signalling promotes temporary adaptation to cellular stress and in turn may contribute to the evasion of prostate tumour cell death.     

Loss of Androgen Receptor-Dependent Growth Suppression by Prostate Cancer Cells Can Occur Independently from Acquiring Oncogenic Addiction to Androgen Receptor Signaling

The conversion of androgen receptor (AR) signaling as a mechanism of growth suppression of normal prostate epithelial cells to that of growth stimulation in prostate cancer cells is often associated with AR mutation, amplification and over-expression. Thus, down-regulation of AR signaling is commonly therapeutic for prostate cancer. The E006AA cell line was established from a hormone naïve, localized prostate cancer. E006AA cells are genetically aneuploid and grow equally well when xenografted into either intact or castrated male NOG but not nude mice. These cells exhibit: 1) X chromosome duplication and AR gene amplification, although paradoxically not coupled with increased AR expression, and 2) somatic, dominant-negative Serine-599-Glycine loss-of-function mutation within the dimerization surface of the DNA binding domain of the AR gene. No effect on the growth of E006AA cells is observed using targeted knockdown of endogenous mutant AR, ectopic expression of wild-type AR, or treatment with androgens or anti-androgens. E006AA cells represent a prototype for a newly identified subtype of prostate cancer cells that exhibit a dominant-negative AR loss-of-function in a hormonally naïve patient. Such loss-of-function eliminates AR-mediated growth suppression normally induced by normal physiological levels of androgens, thus producing a selective growth advantage for these malignant cells in hormonally naïve patients. These data highlight that loss of AR-mediated growth suppression is an independent process, and that, without additional changes, is insufficient for acquiring oncogene addiction to AR signaling. Thus, patients with prostate cancer cells harboring such AR loss-of-function mutations will not benefit from aggressive hormone or anti-AR therapies even though they express AR protein.     

Serum/glucocorticoid-induced protein kinase-1 facilitates androgen receptor-dependent cell survival

Androgen receptor (AR) is a critical factor in the development and progression of prostate cancer. We and others recently demonstrated that eliminating AR expression leads to apoptotic cell death in AR-positive prostate cancer cells. To understand the mechanisms of AR-dependent survival, we performed a genome-wide search for AR-regulated survival genes. We found that serum/glucocorticoid-induced protein kinase-1 (SGK-1) mRNA levels were significantly upregulated after androgen stimulation, which was confirmed to be AR dependent. Promoter analysis revealed that the AR interacted with the proximal and distal regions of the sgk1 promoter, leading to sgk-1 promoter activation after androgen stimulation. Functional assays demonstrated that SGK-1 was indispensable for the protective effect of androgens on cell death induced by serum starvation. SGK-1 overexpression not only rescued cells from AR small-interfering RNA (siRNA)-induced apoptosis, but also enhanced AR transactivation, even in the absence of androgen. Additionally, SGK-1 siRNA reduced AR transactivation, indicating a positive feedback effect of SGK-1 expression on AR-mediated gene expression and cellular survival. Taken together, our data suggest that SGK-1 is an androgen-regulated gene that plays a pivotal role in AR-dependent survival and gene expression.     

Pharmacology of an antiandrogen, anandron, used as an adjuvant therapy in the treatment of prostate cancer

To improve the inhibition of prostate cancer growth obtained by surgical or chemical castration (estrogens or LHRH analogs), blockade of the action of residual androgens of adrenal origin has been proposed. Among antiandrogens acting through the androgen receptor (AR), the nonsteroid anandron (RU 23908) has several advantages over available compounds: megestrol acetate and cyproterone acetate, both steroids, bind substantially to other hormone receptors (progestin, gluco- and mineralocorticoid); and anandron binds only to AR. The nonsteroid flutamide is a prodrug converted to the active metabolite, hydroxyflutamide; anandron is well absorbed on oral administration of an active dose and intact compound disappears slowly from plasma. This may explain why, although in vitro anandron interacts very transiently with AR, in vivo a high level of untransformed anandron is present at the receptor site to induce its antiandrogenic activity. Animal experiments confirm that anandron can counteract the effect of adrenal androgens and inhibit the LHRH analog-induced initial increase in androgen ("flare-up"). Thus, in rats castrated either surgically or by buserelin or DES and supplemented with adrenal androgens (since endogenous adrenal secretion is very low in this species compared to man), anandron decreased prostate weight to control levels. The administration of buserelin to intact rats over 15 days resulted in a significant increase in prostate weight between Days 1 and 5. The addition of anandron to the buserelin inhibited this increase and, furthermore, led to a far greater decrease in prostate weight than that due to buserelin alone at 15 days, indicating a synergy of action.     

Suppression of androgen receptor-mediated transactivation and cell growth by the glycogen synthase kinase 3 beta in prostate cells

Androgens play important roles in the growth of normal prostate and prostate cancer via binding to the androgen receptor (AR). In addition to androgens, AR activity can also be modulated by selective growth factors and/or kinases. Here we report a new kinase signaling pathway by showing that AR transactivation was repressed by wild type glycogen synthase kinase 3beta (GSK3 beta) or constitutively active S9A-GSK3 beta in a dose-dependent manner. In contrast, the catalytically inactive kinase mutant GSK3 beta showed little effect on the AR transactivation. The suppression of AR transactivation by GSK3 beta was abolished by the GSK3 beta inhibitor lithium chloride. The in vitro kinase assay showed that GSK3 beta prefers to phosphorylate the amino terminus of AR that may lead to the suppression of activation function 1 activity located in the NH(2)-terminal region of AR. GSK3 beta interrupted the interaction between the NH(2) and COOH termini of AR, and overexpression of the constitutively active form of GSK3 beta, S9A-GSK3 beta, reduced the androgen-induced prostate cancer cell growth in stably transfected CWR22R cells. Together, our data demonstrated that GSK3 beta may function as a repressor to suppress AR-mediated transactivation and cell growth, which may provide a new strategy to modulate the AR-mediated prostate cancer growth.     

The evolutionary impact of androgen levels on prostate cancer in a multi-scale mathematical model

Background  

Androgens bind to the androgen receptor (AR) in prostate cells and are essential survival factors for healthy prostate epithelium. Most untreated prostate cancers retain some dependence upon the AR and respond, at least transiently, to androgen ablation therapy. However, the relationship between endogenous androgen levels and cancer etiology is unclear. High levels of androgens have traditionally been viewed as driving abnormal proliferation leading to cancer, but it has also been suggested that low levels of androgen could induce selective pressure for abnormal cells. We formulate a mathematical model of androgen regulated prostate growth to study the effects of abnormal androgen levels on selection for pre-malignant phenotypes in early prostate cancer development.     

Molecular determinants of resistance to antiandrogen therapy

Using microarray-based profiling of isogenic prostate cancer xenograft models, we found that a modest increase in androgen receptor mRNA was the only change consistently associated with the development of resistance to antiandrogen therapy. This increase in androgen receptor mRNA and protein was both necessary and sufficient to convert prostate cancer growth from a hormone-sensitive to a hormone-refractory stage, and was dependent on a functional ligand-binding domain. Androgen receptor antagonists showed agonistic activity in cells with increased androgen receptor levels; this antagonist-agonist conversion was associated with alterations in the recruitment of coactivators and corepressors to the promoters of androgen receptor target genes. Increased levels of androgen receptor confer resistance to antiandrogens by amplifying signal output from low levels of residual ligand, and by altering the normal response to antagonists. These findings provide insight toward the design of new antiandrogens.     

Stimulatory effects of antiandrogens on LNCaP human prostate tumor cell growth, EGF-receptor level and acid phosphatase secretion

LNCaP cells (derived from a lymph node carcinoma of the human prostate) show androgen responsive growth. Progestagens, estradiol and antiandrogens competed with androgens for binding to the androgen receptor in the cells to a higher extent than in other androgen-sensitive systems. Optimal growth (3–4 fold increase in DNA content of 6 day cell cultures vs controls) was observed after addition of the synthetic androgen R1881 (0.1 nM). Both steroidal and non-steroidal antiandrogens did not suppress the androgen responsive growth. At a concentration of 10 nM cyproterone acetate or 100 nM RU 23908, growth was even stimulated to an extent comparable to that observed after addition of androgen. Cyproterone acetate and RU 23908 also increased the number of epidermal growth factor receptors expressed at the cell surface to a comparable level as did the androgen. Like androgens, cyproterone acetate, RU 23908 or estradiol stimulated the secretion per cell of prostate specific acid phosphatase in the culture fluid. In conclusion, antiandrogens can exert striking stimulatory effects on the proliferation of LNCaP cells probably due to a defective androgen receptor system. It is discussed that comparable changes in the specificity of the androgen receptor in prostate cancer cells may give these cells an advantage in growth rate and may contribute to development of tumors characterized as hormone independent.