Publication Date

2010-01-20

Availability

Open access

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Molecular and Cellular Pharmacology (Medicine)

Date of Defense

2009-12-15

First Committee Member

Abigail S. Hackam - Committee Chair

Second Committee Member

Amjad Farooq - Committee Member

Third Committee Member

Sandra Lemmon - Committee Member

Fourth Committee Member

Kerry L. Burnstein - Mentor

Fifth Committee Member

Irina U. Agoulnik - Outside Committee Member

Abstract

Most patients undergoing androgen deprivation therapy relapse eventually and progress to androgen-independent (AI) prostate cancer. Although the mechanisms underlying progression to AI prostate cancer are not well understood, studies suggest that androgen receptor (AR) is still required for AI prostate cancer. Our lab found that Vav3, a Rho GTPase guanine nucleotide exchange factor (GEF), is up-regulated during the progression of androgen-dependent human prostate cancer cells to androgen-independence in vivo and in cell-based experiments. Since Vav3 significantly increases ligand-dependent AR transcriptional activity and this action requires the Vav3 pleckstrin homology (PH) domain but not Vav3 GEF activity, we explored the role of the Vav3 PH domain in ligand-dependent AR coactivation by Vav3. We found that targeting the Vav3 PH mutant into nuclei but not the plasma membrane restored Vav3 PH mutant in AR coactivation. Targeting Vav3 to the plasma membrane eliminated the capacity of Vav3 to coactivate AR. In agreement with nuclear targeting of Vav3 via its PH domain, chromatin immunoprecipitation assays showed that Vav3 enhancement of AR transcriptional activity was accompanied by Vav3 recruitment to AR transcriptional complexes at an AR target gene enhancer. Further, Vav3 increased AR occupancy at the target gene enhancer upon androgen treatment and this may underlie the capacity of Vav3 to enhance AR transcriptional activity. Because Vav3 can also be activated by growth factors (GFs) and GFs activate AR in the absence of androgen (ligand-independent), we investigated the crosstalk between Vav3 and GF activation of AR and found Vav3 strongly enhanced AR transcriptional activity induced by GFs. GEF function and the downstream Rho GTPase, Rac1 were required for constitutively active (Ca) Vav3 activation of AR, which differs from Vav3 activation of AR in the presence of androgen. We also investigated the possible signal pathways contributing to AR activation by Ca Rac1. Ca Rac1 caused ligand-independent activation of AR in part through MAPK/ERK signaling and conferred prostate cancer growth in the absence of androgen in cell culture, soft agar and mouse tumor xenografts. Thus, our findings indicate that Vav3 activates AR in the presence or absence of ligand through two distinct mechanisms, which supports a versatile regulatory effect of Vav3 in AR signaling and prostate cancer progression.

Keywords

GEF; PH Domain; Androgen-independent Prostate Cancer; ChIP; Rac1

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