Targeting the stroma to reverse prostate cancer resistant to androgen ablation therapy

Mechanisms of androgen dependence of the prostate are critical to understanding prostate cancer progression to androgen independence associated with disease mortality. Transient elevation of transforming growth factor-beta (TGF-ß) occurs after androgen ablation. To determine the role of TGF-ß on prostate response to androgen ablation, conditional TGF-ß type II receptor knockout mouse models of the epithelia (Tgfbr2NKX3.1KO) and stromal fibroblasts (Tgfbr2fspKO) were used. Following castration, the prostates of Tgfbr2NKX3.1KO mice had apoptosis levels similar to those expected for control Tgfbr2floxE2/floxE2 mice. Prostates of Tgfbr2fspKO mice, however, had reduced regression and high levels of proliferation associated with canonical Wnt activity throughout the glandular epithelia regardless of androgen status.  In contrast, Tgfbr2floxE2/floxE2 prostates had epithelial canonical Wnt activity only in the surviving proximal ducts following castration. In vitro studies showed that androgen antagonist, bicalutamide, transiently elevated both Tgfbr2floxE2/floxE2 and Tgfbr2fspKO stromal expression of Wnt-2, Wnt-3a, and Wnt-5a. The neutralization of Wnt signaling by the expression of SFRP-2 resulted in decreased LNCaP prostate epithelial cell proliferation in stromal conditioned media transfer experiments. In vivo tissue recombination studies using Tgfbr2fspKO prostatic stromal cells in combination with wild type or SV40 large T antigen expressing epithelia resulted in prostates that were refractile to androgen ablation. The expression of SFRP-2 restored the Tgfbr2fspKO-associated prostate responsiveness to androgen ablation. These studies reveal a novel TGF-ß, androgen, and Wnt paracrine signaling axis that enables prostatic regression of the distal ducts following androgen ablation while supporting proximal duct survival.

Inhibiting TGF-ß signaling in the prostatic stroma results in constitutive Wnt signaling throughout the prostatic epithelia associated with survival following androgen ablation. Six week old TOPGal mice (Wnt signaling reporter) were stained for ß-galactosidase activity and anterior and dorsolateral prostate lobes from intact and three day castrated mice were analyzed. (A) Tgfbr2floxE2/floxE2/TOPGal and Tgfbr2fspKO/TOPGal prostates from intact and three days following castration were stained for ß-galactosidase activity (blue) and imaged as whole mounts (n=8) to show areas of canonical Wnt signaling activity.

Immunohistochemistry for TGF-ß type II receptor (TßRII) expression is not detectable in stromal cells of human prostate adenocarcinomas. The pathologic grade of the representative immunohistochemistry images is indicated as benign or Gleason score.  Note TßRII was consistently expressed in epithelial cells, but often lost in stromal cells of neoplastic tissues. Scale bar represents 50 µm. The table indicates the distribution of tissue pathology with positive histochemical TßRII staining in the stromal compartment.

Why is this research important?

                   Since the prostate is an androgen-dependent organ, a major mechanism for prostate cancer treatment includes the inhibition of androgen signaling. Regardless of the initial positive response to androgen ablation, the prostate cancer invariably overcomes its dependence on androgens and results in a drug-resistant cancer with few options for treatment. Although androgen ablation therapy is intended to target the prostate epithelia, the influence of the prostatic stroma on androgen responsiveness of the adjacent epithelia is likely to be critical. Mature and differentiated prostate tissue is formed and maintained by effects on androgen receptors within the stromal compartment.

Our progress:

Prostate tumor progression is mediated

by a paracrine TGF-ß/Wnt3a signaling axis