No abstract

The precise molecular mechanisms by which prostate cancer cells progress from androgen-sensitive to androgen-insensitive status still remain largely unclear.

The Wnt signaling pathway plays a critical role in embryogenesis and tumorigenesis. However, biological roles of Wnt growth factors have not been fully characterized in prostate development and the pathogenesis of prostate cancer. In this study, we used Wnt3a-conditioned medium (Wnt3a-CM) and purified Wnt3a proteins to investigate whether there is a direct effect of Wnt3a on androgen receptor (AR)-mediated transcription and to determine its role in the growth of prostate cancer cells. We demonstrated that Wnt3a-CM either induces AR activity in the absence of androgens or enhances AR activity in the presence of low concentrations of androgens, whereas purified Wnt3a showed a pronounced effect in the presence of low concentrations of ligands. We also showed that Wnt3a-CM and the purified Wnt3a enhance the level of cytosolic and nuclear ␤-catenin, suggesting an involvement of ␤-catenin in this regulation. Moreover, treatment of LNCaP cells with Wnt3a-CM and purified Wnt3a significantly enhances cell growth in the absence of androgens. Our findings demonstrate that Wnt3a plays an important role in androgenmediated transcription and cell growth. These results suggest a novel mechanism for the progression of prostate cancer.

Poorly formed tumor blood vessels lead to regions of microenvironmental stress due to depletion of oxygen and glucose and accumulation of waste products (acidosis). These conditions contribute to tumor progression and correlate with poor patient prognosis. Here we show that the microenvironmental stresses found in the solid tumor are able to inhibit the canonical Wnt/␤-catenin signaling pathway. However, tumor cells harboring common ␤-catenin pathway mutations, such as loss of adenomatous polyposis coli, are insensitive to this novel hypoxic effect. The underlying mechanism responsible is hypoxia-induced endoplasmic reticulum (ER) stress that inhibits normal Wnt protein processing and secretion. ER stress causes dissociation between GRP78/BiP and Wnt, an interaction essential for its correct posttranslational processing. Microenvironmental stress can therefore block autocrine and paracrine signaling of the Wnt/␤-catenin pathway and negatively affect tumor growth. This study provides a general paradigm relating oxygen status to ER function and growth factor signaling.Tumor hypoxia is found in regions that are distant from the supporting tumor vasculature (6). These microenvironments can also suffer from low levels of nutrients (hypoglycemia) and high levels of waste products (acidosis). The net result is a stressful environment that can adversely effect tumor cell proliferation and survival and select for aggressive clonogens (12). Cells respond to these stresses through the induction of both HIF-1-dependent and HIF-1-independent mechanisms (8). Recent data from model tumors has identified how the endoplasmic reticulum (ER) can be stressed by severe hypoxia and how an incomplete response to this stress can inhibit tumor cell growth and survival (5,19,30). One potential mechanism by which ER stress could inhibit tumor formation would be through the inhibition of secreted growth factors, blocking autocrine and paracrine signaling of many molecules promoting proliferation and survival.Wnt genes encode a family of secreted cysteine-rich glycoproteins that plays a significant role in development and homeostasis (21). Wnt proteins can bind to the Frizzled transmembrane receptors and activate different intracellular signaling pathways, such as the "canonical pathway." Here, the key element is stabilization and nuclear translocation of ␤-catenin which then dimerizes with T-cell-specific transcription factor/lymphoid enhancer-binding factor 1 (TCF/LEF) transcription factors to transactivate target genes. In the absence of Wnt ligand, ␤-catenin is phosphorylated by a cytosolic multiprotein complex, which includes adenomatous polyposis coli (APC), axin, and glycogen synthase kinase 3 (GSK-3) and is thus targeted for degradation by the proteasome. Loss of APC, or even mutation of the phosphorylation sites in ␤-catenin itself, leads to a constitutively stable and active ␤-catenin (11, 29). In colorectal cancer, more than 90% of tumor samples have canonical Wnt pathway activating mutations. However, it is extremely rare to fin...

The androgen-signaling pathway is important for the growth and progression of prostate cancer cells. IGF-I and other polypeptide growth factors have been shown to be capable of induction of androgen receptor (AR) activation in the absence of, or at low levels of, ligand. It has been shown that IGF-I increases the cellular level of beta-catenin, an AR coactivator. In this study, we performed several experiments to test whether beta-catenin is involved in IGF-I-induced AR-mediated transcription. We demonstrate that IGF-I enhances the expression of endogenous prostate-specific antigen, an AR target gene, and elevates the level of cytoplasmic and nuclear beta-catenin in prostate cancer cells. Transfection of either wild-type or a constitutively active mutant of the IGF-I receptor augments AR-mediated transcription. An antisense construct of beta-catenin that decreases the cellular level of beta-catenin can reduce IGF-1 receptor-mediated enhancement of AR activity. Moreover, using a pulse-chase experiment, we showed that IGF-I enhances the stability of beta-catenin in prostate cancer cells. Our findings delineate a novel pathway for IGF-I in modulating androgen signaling through beta-catenin.