Excessive activation of G-protein coupled receptor (GPCR) and receptor tyrosine kinase (RTK) pathways has been linked to prostate cancer metastasis. Rac activation by guanine-nucleotide exchange factors (GEFs) plays an important role in directional cell migration, a critical step of tumor metastasis cascades. We found that upregulation of P-Rex1, a Rac-selective GEF synergistically activated by Gβγ freed during GPCR signaling and PIP3 generated during either RTK or GPCR signaling, strongly correlates with metastatic phenotypes in both prostate cancer cell lines and human prostate cancer specimens. Silencing endogenous P-Rex1 in metastatic prostate cancer PC-3 cells selectively inhibited Rac activity and reduced cell migration and invasion in response to ligands of both epidermal growth factor receptor and G-protein coupled CXC chemokine receptor 4. Conversely, expression of recombinant P-Rex1, but not its “GEF-dead” mutant, in non-metastatic prostate cancer CWR22Rv1 cells increased cell migration and invasion via Rac-dependent lamellipodia formation. More importantly, using a mouse xenograft model, we demonstrated that expression of P-Rex1, but not its mutant, induced lymph node metastasis of CWR22Rv1 cells without an effect on primary tumor growth. Thus, by functioning as a coincidence detector of chemotactic signals from both GPCRs and RTKs, P-Rex1-dependent activation of Rac promotes prostate cancer metastasis.

No abstract

Steroid hormones play key roles in regulating cell proliferation and dierentiation in targeting tissues. However, in advanced cancers, the steroid hormone regulation is frequently attenuated through a yet unknown mechanism even in the presence of functional steroid hormone receptors. We investigate the functional role of tyrosine phosphorylation signaling in the hormone-refractory growth of human prostate tumors. Initial studies demonstrate that the androgen-responsive phenotype of human prostate cancer cells associates with a low phosphotyrosine (p-Tyr) level of ErbB-2, which is regulated by cellular prostatic acid phosphatase (PAcP), a protein tyrosine phosphatase. In prostate cancer cells, the p-Tyr level, but not the protein level, of ErbB-2 inversely correlates with the androgen-responsiveness of cell proliferation. Androgen-stimulated cell growth concurs with a down-regulation of cellular PAcP, an elevated p-Tyr level of ErbB-2, and the activation of mitogen-activated protein kinases. Furthermore, only the ErbB-2 inhibitor AG 879, but not the EGFR inhibitor AG 1478, abolishes androgen-induced cell proliferation. Forced expression of ErbB-2 can also attenuate androgen promotion of cell growth. Data taken collectively conclude that in human prostate cancer cells, the tyrosine phosphorylation of ErbB-2 regulated by cellular PAcP plays a key role in regulating androgen-mediated proliferation signaling.

Our LNCaP cell model closely recapitulates the progression of human prostate cancer from the androgen-responsive to the hormone-refractory state under the androgen nondeprived condition. This cell model may provide the opportunity to understand the molecular mechanisms associated with the acquisition of androgen independence during human prostate cancer progression.

Because of the heterogeneous nature of prostate cancer, identifying the molecular mechanisms involved during the transition from an androgen-sensitive to an androgen-independent phenotype is very complex. An LNCaP cell model that recapitulates prostate cancer progression, comprising early passage androgen-sensitive (LNCaP-C33) and late passage androgen-independent (LNCaP-C81) phenotypes, would help to provide a better understanding of such molecular events. In this study, we examined the genes expressed by LNCaP-C33 and LNCaP-C81 cells using cDNA microarrays containing 1176 known genes. This analysis demonstrated that 34 genes are up-regulated and eight genes are down-regulated in androgen-independent cells. Northern blot analysis confirmed the differences identified by microarrays on several candidate genes, including c-MYC, c-MYC purine-binding transcription factor (PuF), macrophage migration inhibitory factor (MIF), macrophage inhibitory cytokine-1 (MIC-1), lactate dehydrogenase-A (LDH-A), guanine nucleotide-binding protein Gi, alpha-1 subunit (NBP), cyclin dependent kinase-2 (CDK-2), prostate-specific membrane antigen (PSM), cyclin H (CCNH), 60S ribosomal protein L10 (RPL10), 60S ribosomal protein L32 (RPL32), and 40S ribosomal protein S16 (RPS16). These differentially-regulated genes are correlated with progression of human prostate cancer and may be of therapeutic relevance as well as an aid in understanding the molecular genetic events involved in the development of this disease's hormone-refractory behavior.

p66Shc is shown to negatively regulate the life span in mice through reactive oxygen species (ROS) production. Recent reports, however, revealed that p66Shc protein level is significantly elevated in several human cancer tissues and growth-stimulated carcinoma cells, suggesting a mitogenic and carcinogenic role for p66Shc. In this communication, we demonstrate for the first time that p66Shc mediates androgenic growth signals in androgen-sensitive human prostate cancer cells through mitochondrial ROS production. Growth stimulation of prostate cancer cells with 5a-dihydrotestosterone (DHT) is accompanied by increased p66Shc level and ROS production, which is abolished by antioxidant treatments. However, antioxidant treatments do not affect the transcriptional activity of androgen receptor (AR) as observed by its inability to block DHTinduced prostate-specific antigen expression, an AR-dependent correlate of prostate cancer progression. Elevated expression of p66Shc by cDNA transfection increases the basal cell proliferation and, thus, reduces additional DHTinduced cell proliferation. Furthermore, DHT increases the translocation of p66Shc into mitochondria and its interaction with cytochrome c. Conversely, both redox-negative p66Shc mutant (W134F), which is deficient in cytochrome c interaction, and p66Shc small interfering RNA decrease DHT-induced cell proliferation. These results collectively reveal a novel role for p66Shc-ROS pathway in androgeninduced prostate cancer cell proliferation and, thus, may play a role in early prostate carcinogenesis.

Expression of RGS2, but not other RGS proteins, abolished androgen-independent AR activity in androgenindependent LNCaP cells and CWR22Rv1 cells. In LNCaP cells, RGS2 inhibited G q -coupled GPCR signaling. Expression of exogenous wild-type RGS2, but not its GAP-deficient mutant, significantly reduced AR activation by constitutively activated G q Q209L mutant whereas silencing endogenous RGS2 by siRNA enhanced G q Q209L-stimulated AR activity. RGS2 had no effect on RGS-insensitive G q Q209L/G188S-induced AR activation. Furthermore, extracellular signal-regulated kinase 1/2 (ERK1/2) was found to be involved in RGS2-mediated regulation of androgen-independent AR activity. In addition, RGS2 functioned as a growth suppressor for androgen-independent LNCaP cells whereas androgensensitive LNCaP cells with RGS2 silencing had a growth advantage under steroid-reduced conditions. Finally, RGS2 expression level was significantly decreased in human prostate tumor specimens. Taken together, our results suggest RGS2 as a novel regulator of AR signaling and its repression may be an important step during prostate tumorigenesis and progression.

The growth and progression of prostate cancer are dependent on androgens and androgen receptor (AR), which act by modulating gene expression. Utilizing a gene microarray approach, we have identified the a1-subunit gene of soluble guanylyl cyclase (sGC) as a novel androgen-regulated gene. A heterodimeric cytoplasmic protein composed of one a and one b subunit, sGC mediates the widespread cellular effects of nitric oxide (NO). We report here that, in prostate cancer cells, androgens stimulate the expression of sGCa1. A cloned human sGCa1 promoter is activated by androgen in an AR-dependent manner, suggesting that sGCa1 may be a direct AR target gene. Disruption of sGCa1 expression severely compromises the growth of both androgendependent and androgen-independent AR-positive prostate cancer cells. Overexpression of sGCa1 alone is sufficient for stimulating prostate cancer cell proliferation. Interestingly, the major growth effect of sGCa1 is independent of NO and cyclic guanosine monophosphate, a major mediator of the sGC enzyme. These data strongly suggest that sGCa1 acts in prostate cancer via a novel pathway that does not depend on sGCb1. Tissue studies show that sGCa1 expression is significantly elevated in advanced prostate cancer. Thus, sGCa1 may be an important mediator of the procarcinogenic effects of androgens.