Aberrant activation of androgen receptor in a new neuropeptide-autocrine model of androgen-insensitive prostate cancer

Dasatinib inhibits site-specific tyrosine phosphorylation of androgen receptor by Ack1 and Src kinases

Activation of androgen receptor (AR) may play a role in the development of castration resistant prostate cancer. Two intracellular tyrosine kinases, Ack1 (activated cdc42-associated kinase) and Src, phosphorylate and enhance AR activity and promote prostate xenograft tumor growth in castrated animals. However, the upstream signals that activate these kinases and lead to AR activation are incompletely characterized. In this study, we investigated AR phosphorylation in response to non-androgen ligand stimulation using phospho-specific antibodies. Treatment of LNCaP and LAPC-4 cells with epidermal growth factor (EGF), heregulin, Gas6 (ligand binding to Mer receptor tyrosine kinase and activating Ack1 downstream), interleukin (IL)-6 or bombesin stimulated cell proliferation in the absence of androgen. Treatment of LNCaP and LAPC-4 cells with EGF, heregulin, or Gas6 induced AR phosphorylation at Tyr-267; IL-6 or bombesin treatment did not. AR phosphorylation at Tyr-534 was induced by treatment with EGF, IL-6 or bombesin, but not by heregulin or Gas6. siRNA-mediated knockdown of Ack1 or Src showed that Ack1 mediates heregulin- and Gas6-induced AR Tyr-267 phosphorylation whereas Src mediates Tyr-534 phosphorylation induced by EGF, IL-6, and bombesin. Dasatinib, a Src inhibitor, blocked EGF-induced Tyr-534 phosphorylation. In addition, we show dasatinib also inhibited Ack1 kinase. Dasatinib inhibited heregulin-induced Ack1 kinase activity and AR Tyr-267 phosphorylation. Dasatinib inhibited heregulin-induced AR-dependent reporter activity. Dasatinib also inhibited heregulin-induced expression of endogenous AR target genes. Dasatinib inhibited Ack1-dependent colony formation and prostate xenograft tumor growth in castrated mice. Interestingly, Ack1 or Src knockdown or dasatinib did not inhibit EGF-induced AR Tyr-267 phosphorylation or EGF-stimulated AR activity, suggesting the existence of an additional tyrosine kinase that phosphorylates AR at Tyr-267. These data suggest that specific tyrosine kinases phosphorylate AR at distinct sites and that dasatinib may exert anti-tumor activity in prostate cancer through inhibition of Ack1.

Activation of Rac1 is closely related to androgen-independent cell proliferation of prostate cancer cells both in vitro and in vivo

We and others previously showed that signaling through cSrc or atypical protein kinase C (aPKC) pathway regulates the proliferation of prostate cancer cells and is associated with their progression to castrate-resistance in vivo. However, the interrelation of these two kinases has been largely unexplored. In the present study, we show that androgen-induced activation of cSrc regulates the activity of aPKC through the small molecular weight G protein Rac1 in androgen-dependent LNCaP cells. Knockdown of cSrc in those cells reduces the phosphorylation of aPKC and the abundance of activated form of Rac1. Additionally, the treatment of those cells with Rac1 inhibitor repressed cell cycle progression at G(1)/S transition. In fact, forced expression of a constitutively active Rac1 mutant in LNCaP cells promoted cell proliferation under androgen-depleted conditions both in vitro and in vivo. Moreover, LNCaP C4-2 and AILNCaP cells, the syngeneic androgen-independent sublines from LNCaP cells, harbored abundant Rac1-GTP. Importantly, the inhibition of Rac1 suppressed cell proliferation and induced apoptotic cell death in all prostate cancer cell lines tested irrespective of their androgen-dependence. In immunohistochemical evaluation of tumor specimens from prostate cancer patients, Rac1 pathway appeared to be activated in the majority of castrate-resistant diseases. Collectively, our present results both in vitro and in vivo highly implicate that Rac1 can be a potential therapeutic target for patients with advanced prostate cancer, especially those with castrate-resistant status.

SRC kinase inhibition: targeting bone metastases and tumor growth in prostate and breast cancer

Prostate and breast cancer cells preferentially metastasize to bone, whereupon a complex interaction between metastatic tumor cells, osteoclasts, and osteoblasts results in the development of bone lesions that cause significant pain and patient morbidity. For patients with bone lesions, the goals of treatment are to decrease tumor growth, prevent further metastases, and inhibit tumor-associated bone pathology. Preclinical data suggest that SRC, a nonreceptor tyrosine kinase, is an important signaling molecule during the processes of osteoclast-mediated bone resorption, tumor growth, and metastasis, and that SRC has a role in hormone receptor signaling and resistance. As such, SRC represents a logical target for the treatment of advanced metastatic prostate or breast cancer. SRC-targeting agents, including dasatinib, saracatinib, and bosutinib, are currently in clinical development for patients with solid tumors. Preliminary data from phase 1/2 trials, including tumor responses and bone-specific activity in patients with prostate or breast cancer, demonstrate that SRC inhibitors have potential in the clinical setting. Data arising from ongoing and future clinical trials will confirm whether SRC inhibitors provide clinical benefits for patients with advanced disease.

The relationship of neuroendocrine carcinomas to anti-tumor therapies in TRAMP mice

BACKGROUND

Neuroendocrine differentiation and neuroendocrine carcinoma (NEC) have been linked to androgen deprivation in prostate cancers. No previous study has directly connected neuroendocrine phenotypes to chemotherapy. The pathogenesis of prostatic NEC has not yet been determined.

METHODS

Using the transgenic adenocarcinoma of mouse prostate (TRAMP) model, we studied tumor progression after hormone ablation (castration) and/or chemotherapy (docetaxel), and analyzed the incidence of NEC as a function of the anti-tumor therapies. Non-treated mice were used as controls. Protein expressions in tumor tissues were analyzed by Western blots and immunohistochemistry.

RESULTS

Although all animals developed prostate cancer, no NEC was found in control mice. However, over 30% of the mice that received an anti-tumor therapy developed NEC. A similar incidence of NEC was found in the castration-only and docetaxel-only treatment groups, while a higher incidence was observed in the combined treatment (castration and docetaxel) group. The NEC-bearing mice had smaller tumors in their prostates and lived longer than mice with adenocarcinoma (ADC-only). However, NEC tumors had a higher proliferative index and greater potential for metastasis and drug-resistance, as evidenced by significantly higher expression levels of PCNA, S100A4, and Pgp, but lower levels of E-cadherin. SV40 T-antigen was highly expressed in both NEC and ADC tumors.

CONCLUSIONS

Stress induced by anti-cancer treatments may play a role in NEC development. Although NEC and ADC differ in their expressions of many proteins, a high level of SV40 T-antigen in both tumor types suggest a common progenitor..

Neuroendocrine differentiation in prostate cancer

As any organ in the body human prostate is composed of many different types of cells as well as extracellular components. During prostate development, reciprocal cellular interactions between stromal cells and prostate epithelial cells ultimately lead to the development of a mature prostate. Normal prostate is composed of repeating cellular units that contain stromal and epithelial compartments. The epithelial compartment contains luminal epithelial cells, basal cells and a minor component of neuroendocrine cells whose function may be to regulate the growth, differentiation and secretory function of the prostate gland. Neuroendocrine cells are also evident in prostate cancer and numerous studies showed that its number increases in high grade and high stage tumors, particularly in hormonally treated and hormone-refractory (androgen-independent) prostate cancer. Although androgen withdrawal reduces the secretion of the andromedins from the prostate stromal cells that are critical for the survival for prostate epithelial cells, there is clear evidence that androgen receptor is also required for the tumorigenesis of human prostate cancer, and therefore androgen deprivation therapy likely works through inhibition of androgen receptor in the prostate epithelium. Because neuroendocrine cells lack androgen receptor and are likely androgen-independent, it is conceivable that hormonal therapy for advanced/metastatic prostate cancer, which consists of inhibiting androgen production and/or blocking androgen receptor function, will not eliminate neuroendocrine cancer cells. Instead, these cells may be enriched after the therapy and they may establish paracrine networks to stimulate androgen-independent proliferation of prostate cancer, leading to tumor recurrence. In this article, we will review the known functions of the neuroendocrine cells in prostate cancer, including stimulation of cancer proliferation and invasion, apoptosis resistance and angiogenesis as well as molecular pathways involved in neuroendocrine differentiation.