The transition from androgen-dependent to castration-resistant prostate cancer (CRPC) is a lethal event of uncertain molecular etiology. Comparing gene expression in isogenic androgen-dependent and CRPC xenografts, we found a reproducible increase in N-cadherin expression, which was also elevated in primary and metastatic tumors of individuals with CRPC. Ectopic expression of N-cadherin in nonmetastatic, androgen-dependent prostate cancer models caused castration resistance, invasion and metastasis. Monoclonal antibodies against the ectodomain of N-cadherin reduced proliferation, adhesion and invasion of prostate cancer cells in vitro. In vivo, these antibodies slowed the growth of multiple established CRPC xenografts, blocked local invasion and metastasis and, at higher doses, led to complete regression. N-cadherin–specific antibodies markedly delayed the time to emergence of castration resistance, markedly affected tumor histology and angiogenesis, and reduced both AKT serine-threonine kinase activity and serum interleukin-8 (IL-8) secretion. These data indicate that N-cadherin is a major cause of both prostate cancer metastasis and castration resistance. Therapeutic targeting of this factor with monoclonal antibodies may have considerable clinical benefit.

The glucocorticoid receptor (GR) is phosphorylated at three major sites on its N terminus (S203, S211, and S226), and phosphorylation modulates GR-regulatory functions in vivo. We examined the phosphorylation site interdependence, the contribution of the receptor C-terminal ligand-binding domain, and the participation of protein phosphatases in GR N-terminal phosphorylation and gene expression. We found that GR phosphorylation at S203 was greater when S226 was not phosphorylated and vice versa, indicative of intersite dependency. We also observed that a GR derivative lacking the ligand-binding domain, which no longer binds the heat shock protein 90 (Hsp90) complex, exhibits increased GR phosphorylation at all three sites as compared with the full-length receptor. A GR mutation (F602S) that produces a receptor less dependent on Hsp90 for function as well as treatment with the Hsp90 inhibitor geldanamycin also increased basal GR phosphorylation at a subset of sites. Pharmacological inhibition of serine/threonine protein phosphatases increased GR basal phosphorylation. Likewise, a reduction in protein phosphatase 5 protein levels enhanced GR phosphorylation at a subset of sites and selectively reduced the induction of endogenous GR target genes. Together, our findings suggest that GR undergoes a phosphorylation/dephosphorylation cycle that maintains steady-state receptor phosphorylation at a low basal level in the absence of ligand. Our findings also suggest that the ligand-dependent increase in GR phosphorylation results, in part, from the dissociation of a ligand-binding domain-linked protein phosphatase(s), and that changes in the intracellular concentration of protein phosphatase 5 differentially affect GR target gene expression.

Prostate stem cell antigen (PSCA), a 123-amino acid cell surface glycoprotein, is highly expressed in both local and metastatic prostate cancers as well as in a large proportion of bladder and pancreatic cancers. PSCA overexpression correlates with a high risk of recurrence after primary therapy for prostate cancer. We have reported previously that anti-PSCA monoclonal antibody (mAb) 1G8 inhibits tumor growth, prevents metastasis, and prolongs the survival of mice inoculated with human prostate cancer cell lines and xenografts. The current study was undertaken to elucidate the mechanism of action of anti-PSCA antibody therapy. In particular, we asked whether antitumor activity resulted from recruitment of an immune response or a direct effect on the tumor cell itself. In vitro assays show that both intact 1G8 and F(abV ) 2 fragments of 1G8 induce prostate cancer cell death. The anti-PSCA antibody-induced cell death is caspase independent and requires antigen cross-linking. These results were confirmed in in vivo models in which both 1G8 and F(abV ) 2 fragments were able to inhibit prostate tumor formation and growth equally. These results suggest that the anti-PSCA mAb 1G8 acts by a direct, Fc-independent mechanism to inhibit prostate tumor growth both in vitro and in vivo. (Cancer Res 2005; 65(20): 9495-500)

These results show a potential therapeutic role for anti-PSCA antibodies in the treatment of pancreatic cancer. Furthermore, PSCA might serve as a novel marker in the diagnosis of pancreatic cancer.

Purpose: The use of high-dose per fraction radiotherapy delivered as stereotactic body radiotherapy is a standard of care for prostate cancer. It is hypothesized that high-dose radiotherapy may enhance or suppress tumor-reactive immunity. The objective of this study was to assess both antitumor and immunosuppressive effects induced by high-dose radiotherapy in prostate cancer coclinical models, and ultimately, to test whether a combination of radiotherapy with targeted immunotherapy can enhance antitumor immunity. Experimental Design: We studied the effects of high-dose per fraction radiotherapy with and without anti-Gr-1 using syngeneic murine allograft prostate cancer models. The dynamic change of immune populations, including tumor-infiltrating lymphocytes (TIL), T regulatory cells (Treg), and myeloid-derived suppressive cells (MDSC), was evaluated using flow cytometry and IHC. Results: Coclinical prostate cancer models demonstrated that high-dose per fraction radiotherapy induced a rapid increase of tumor-infiltrating MDSCs and a subsequent rise of CD8 TILs and circulating CD8 T effector memory cells. These radiation-induced CD8 TILs were more functionally potent than those from nonirradiated controls. While systemic depletion of MDSCs by anti-Gr-1 effectively prevented MDSC tumor infiltration, it did not enhance radiotherapy-induced antitumor immunity due to a compensatory expansion of Treg-mediated immune suppression. Conclusions: In allograft prostate cancer models, high-dose radiotherapy induced an early rise of MDSCs, followed by a transient increase of functionally active CD8 TILs. However, systemic depletion of MDSC did not augment the antitumor efficacy of high-dose radiotherapy due to a compensatory Treg response, indicating blocking both MDSCs and Tregs might be necessary to enhance radiotherapy-induced antitumor immunity.