ABSTRACT
Prostate cancer invariably recurs after androgen deprivation therapy. Growth of this recurrent/androgen-independent form of prostate cancer may be due to increased androgen receptor (AR) transcriptional activity in the absence of androgen. This ligand-independent AR activation is promoted by some growth factors but the mechanism is not well understood. Vav3, a Rho guanosine triphosphatase guanine nucleotide exchange factor, which is activated by growth factors, is up-regulated in human prostate cancer. We show here that Vav3 levels increase during in vivo progression of prostate cancer to androgen independence. Vav3 strikingly enhanced growth factor activation of AR in the absence of androgen. Because Vav3 may be chronically activated in prostate cancer by growth factor receptors, we examined the effects of a constitutively active (Ca) form of Vav3 on AR transcriptional activity. Ca Vav3 caused nuclear localization and ligand-independent activation of AR via the Rho guanosine triphosphatase, Rac1. Ca Rac1 activation of AR occurred, in part, through MAPK/ERK signaling. Expression of active Rac1 conferred androgen-independent growth of prostate cancer cells in culture, soft agar, and mice. These findings suggest that Vav3/Rac 1 signaling is an important modulator of ligand-independent AR transcriptional activity in prostate cancer progression.
Subject(s)
Guanine Nucleotide Exchange Factors/metabolism , Neoplasms, Hormone-Dependent/metabolism , Prostatic Neoplasms/metabolism , Proto-Oncogene Proteins c-vav/metabolism , Receptors, Androgen/metabolism , rac1 GTP-Binding Protein/metabolism , Animals , Cell Line, Tumor , Guanine Nucleotide Exchange Factors/biosynthesis , Guanine Nucleotide Exchange Factors/genetics , Humans , MAP Kinase Signaling System , Male , Mice , Mice, Nude , Mitogen-Activated Protein Kinases/metabolism , Neoplasm Transplantation , Neoplasms, Hormone-Dependent/enzymology , Prostatic Neoplasms/enzymology , Proto-Oncogene Proteins c-vav/biosynthesis , Proto-Oncogene Proteins c-vav/genetics , RNA, Messenger/biosynthesis , RNA, Messenger/genetics , Reverse Transcriptase Polymerase Chain Reaction , Signal Transduction , Transcription, Genetic , Transplantation, Heterologous , Up-Regulation , rac1 GTP-Binding Protein/biosynthesis , rac1 GTP-Binding Protein/geneticsABSTRACT
The progression of prostate cancer from androgen dependence to androgen independence is often accompanied by enhanced androgen receptor (AR) transcriptional activity. We observed a marked increase in the expression of Vav3, a Rho GTPase guanine nucleotide exchange factor (GEF), during the progression of human prostate cancer LNCaP cells to the androgen-independent derivative, LNCaP-R1. GEFs activate Rho family GTPases by promoting the exchange of GDP for GTP. Reporter gene assays showed that Vav3 potentiated AR transcriptional activity, and knock down of Vav3 resulted in decreased AR transactivation. Vav3 also increased androgen-induced levels of prostate-specific antigen mRNA. Furthermore, Vav3 enhanced AR activity at subnanomolar concentrations of androgen. This finding is particularly relevant because low androgen levels may be present in prostate tissue of patients undergoing androgen deprivation therapy. Enhancement of AR activity by Vav3 required amino terminal activation function 1 (AF1) of AR; however, Vav3 did not interact with AR or increase AR levels. Neither GEF function nor the C-terminal domains of Vav3 were required for Vav3-mediated enhancement of AR activity; however, the pleckstrin homology domain was obligatory. These data show that Vav3 levels rise during progression to androgen independence and support continued AR signaling (even under conditions of low androgen) by a novel GEF-independent cross-talk mechanism.
Subject(s)
Androgens/metabolism , Gene Expression Regulation, Neoplastic , Guanine Nucleotide Exchange Factors/metabolism , Prostatic Neoplasms/metabolism , Prostatic Neoplasms/pathology , Proto-Oncogene Proteins c-vav/metabolism , Receptors, Androgen/metabolism , Androgens/pharmacology , Cell Line, Tumor , Disease Progression , Guanine Nucleotide Exchange Factors/genetics , Humans , Male , Mutation , Phosphatidylinositol 3-Kinases/metabolism , Prostate-Specific Antigen/genetics , Prostatic Neoplasms/genetics , Protein Structure, Tertiary/genetics , Proto-Oncogene Proteins c-vav/genetics , RNA, Messenger/analysis , RNA, Messenger/metabolism , Receptors, Androgen/genetics , Transcription, Genetic , Up-RegulationABSTRACT
Abnormally suppressed levels of cyclin-dependent kinase inhibitors (CKIs) are associated with aggressive androgen-independent prostate cancer and contribute to uncontrolled proliferation. The androgen-independent human prostate cancer cell lines, LNCaP-104R1, ALVA31 and PC-3, express low levels of the CKI, p21(CIP1), compared to the less-malignant, androgen-dependent LNCaP cells. We investigated the mechanism underlying this suppression by examining the role of Rho GTPases, signaling proteins that play important roles in cell cycle progression, at least in part through regulation of CKIs. Inhibition of Rac1 induced p21 expression in androgen-independent lines but had no effect on the higher p21 levels characteristic of LNCaP cells. This induction of p21 was functionally significant as evidenced by inhibition of cyclin-dependent kinase 2 activity and decreased cell proliferation. Conversely, overexpression of constitutively active Rac1 suppressed the higher p21 levels seen in LNCaP cells. Thus, Rac1 activity is both necessary and sufficient for suppression of p21 in prostate cancer cells. Furthermore, Rac1 activity was significantly higher in all three androgen-independent cell lines compared to LNCaP cells. Thus in three models of aggressive human prostate cancer, hyperactivity of Rac1 corresponds to suppressed levels of p21. These results are unique in describing a role for Rac1 in p21 regulation and may implicate the Rac1 signaling pathway as a potential therapeutic target for controlling prostate cancer cell growth following progression to androgen independence.
