Michael W. Starbuck

Androgen receptor-negative human prostate cancer cells induce osteogenesis in mice through FGF9-mediated mechanisms.

In prostate cancer, androgen blockade strategies are commonly used to treat osteoblastic bone metastases. However, responses to these therapies are typically brief, and the mechanism underlying androgen-independent progression is not clear. Here, we established what we believe to be the first human androgen receptor-negative prostate cancer xenografts whose cells induced an osteoblastic reaction in bone and in the subcutis of immunodeficient mice. Accordingly, these cells grew in castrated as well as intact male mice. We identified FGF9 as being overexpressed in the xenografts relative to other bone-derived prostate cancer cells and discovered that FGF9 induced osteoblast proliferation and new bone formation in a bone organ assay. Mice treated with FGF9-neutralizing antibody developed smaller bone tumors and reduced bone formation. Finally, we found positive FGF9 immunostaining in prostate cancer cells in 24 of 56 primary tumors derived from human organ-confined prostate cancer and in 25 of 25 bone metastasis cases studied. Collectively, these results suggest that FGF9 contributes to prostate cancer-induced new bone formation and may participate in the osteoblastic progression of prostate cancer in bone. Androgen receptor-null cells may contribute to the castration-resistant osteoblastic progression of prostate cancer cells in bone and provide a preclinical model for studying therapies that target these cells.

Prostate cancer cell-stromal cell crosstalk via FGFR1 mediates antitumor activity of dovitinib in bone metastases.

Dovitinib is therapeutically active in a subset of patients with prostate cancer bone metastases, partly due to blockade of FGFR-mediated stromal-epithelial interactions in the bone microenvironment. Effective to the Bone Bone is the most common site of metastatic spread for prostate cancer, and tumors that have spread to the bone are usually very difficult to treat. Dovitinib is a recently developed drug that inhibits the fibroblast growth factor receptor. Now, Wan et al. have demonstrated that dovitinib is effective for some patients with prostate cancer that has spread to bone. The authors also identified an explanation for some of the observed antitumor effects by showing that the drug interferes with interactions between the prostate cancer cells and surrounding stromal cells in the bone microenvironment. Bone is the most common site of prostate cancer (PCa) progression to a therapy-resistant, lethal phenotype. We found that blockade of fibroblast growth factor receptors (FGFRs) with the receptor tyrosine kinase inhibitor dovitinib has clinical activity in a subset of men with castration-resistant PCa and bone metastases. Our integrated analyses suggest that FGF signaling mediates a positive feedback loop between PCa cells and bone cells and that blockade of FGFR1 in osteoblasts partially mediates the antitumor activity of dovitinib by improving bone quality and by blocking PCa cell–bone cell interaction. These findings account for clinical observations such as reductions in lesion size and intensity on bone scans, lymph node size, and tumor-specific symptoms without proportional declines in serum prostate-specific antigen concentration. Our findings suggest that targeting FGFR has therapeutic activity in advanced PCa and provide direction for the development of therapies with FGFR inhibitors.

Activation of β-Catenin Signaling in Androgen Receptor–Negative Prostate Cancer Cells

Purpose: To study Wnt/β-catenin in castrate-resistant prostate cancer (CRPC) and understand its function independently of the β-catenin–androgen receptor (AR) interaction. Experimental Design: We carried out β-catenin immunocytochemical analysis, evaluated TOP-flash reporter activity (a reporter of β-catenin–mediated transcription), and sequenced the β-catenin gene in MDA prostate cancer 118a, MDA prostate cancer 118b, MDA prostate cancer 2b, and PC-3 prostate cancer cells. We knocked down β-catenin in AR-negative MDA prostate cancer 118b cells and carried out comparative gene-array analysis. We also immunohistochemically analyzed β-catenin and AR in 27 bone metastases of human CRPCs. Results: β-Catenin nuclear accumulation and TOP-flash reporter activity were high in MDA prostate cancer 118b but not in MDA prostate cancer 2b or PC-3 cells. MDA prostate cancer 118a and MDA prostate cancer 118b cells carry a mutated β-catenin at codon 32 (D32G). Ten genes were expressed differently (false discovery rate, 0.05) in MDA prostate cancer 118b cells with downregulated β-catenin. One such gene, hyaluronan synthase 2 (HAS2), synthesizes hyaluronan, a core component of the extracellular matrix. We confirmed HAS2 upregulation in PC-3 cells transfected with D32G-mutant β-catenin. Finally, we found nuclear localization of β-catenin in 10 of 27 human tissue specimens; this localization was inversely associated with AR expression (P = 0.056, Fishers exact test), suggesting that reduced AR expression enables Wnt/β-catenin signaling. Conclusion: We identified a previously unknown downstream target of β-catenin, HAS2, in prostate cancer, and found that high β-catenin nuclear localization and low or no AR expression may define a subpopulation of men with bone metastatic prostate cancer. These findings may guide physicians in managing these patients. Clin Cancer Res; 18(3); 726–36. ©2011 AACR.

Effect of transforming growth factor beta (TGF-β) receptor I kinase inhibitor on prostate cancer bone growth

Transforming growth factor beta 1 (TGF-β1) has been implicated in the pathogenesis of prostate cancer (PCa) bone metastasis. In this study, we tested the antitumor efficacy of a selective TGF-β receptor I kinase inhibitor, LY2109761, in preclinical models. The effect of LY2109761 on the growth of MDA PCa 2b and PC-3 human PCa cells and primary mouse osteoblasts (PMOs) was assessed in vitro by measuring radiolabeled thymidine incorporation into DNA. In vivo, the right femurs of male SCID mice were injected with PCa cells. We monitored the tumor burden in control- and LY2109761-treated mice with MRI analysis and the PCa-induced bone response with X-ray and micro-CT analyses. Histologic changes in bone were studied by performing bone histomorphometric evaluations. PCa cells and PMOs expressed TGF-β receptor I. TGF-β1 induced pathway activation (as assessed by induced expression of p-Smad2) and inhibited cell growth in PC-3 cells and PMOs but not in MDA PCa 2b cells. LY2109761 had no effect on PCa cells but induced PMO proliferation in vitro. As expected, LY2109761 reversed the TGF-β1-induced pathway activation and growth inhibition in PC-3 cells and PMOs. In vivo, LY2109761 treatment for 6weeks resulted in increased volume in normal bone and increased osteoblast and osteoclast parameters. In addition, LY2109761 treatment significantly inhibited the growth of MDA PCa 2b and PC-3 in the bone of SCID mice (p<0.05); moreover, it resulted in significantly less bone loss and change in osteoclast-associated parameters in the PC-3 tumor-bearing bones than in the untreated mice. In summary, we report for the first time that targeting TGF-β receptors with LY2109761 can control PCa bone growth while increasing the mass of normal bone. This increased bone mass in nontumorous bone may be a desirable side effect of LY2109761 treatment for men with osteopenia or osteoporosis secondary to androgen-ablation therapy, reinforcing the benefit of effectively controlling PCa growth in bone. Thus, targeting TGF-β receptor I is a valuable intervention in men with advanced PCa.

Dasatinib inhibits both osteoclast activation and prostate cancer PC-3 cell-induced osteoclast formation

Purpose: Therapies to target prostate cancer bone metastases have only limited effects. New treatments are focused on the interaction between cancer cells, bone marrow cells and the bone matrix. Osteoclasts play an important role in the development of bone tumors caused by prostate cancer. Since Src kinase has been shown to be necessary for osteoclast function, we hypothesized that dasatinib, a Src family kinase inhibitor, would reduce osteoclast activity and prostate cancer (PC-3) cell-induced osteoclast formation. Experimental Design: We performed in vitro experiments utilizing the Src family kinase inhibitor dasatinib to target osteoclast activation as a means of inhibiting prostate cancer bone metastases. Results: Dasatinib inhibited RANKL-induced osteoclast differentiation of bone marrow-derived monocytes with an EC50 of 7.5 nM. PC-3 cells, a human prostate cancer cell line, were able to differentiate RAW 264.7 cells, a murine monocytic cell line, into osteoclasts, and dasatinib inhibited this differentiation. In addition, conditioned medium from PC-3 cell cultures was able to differentiate RAW 264.7 cells into osteoclasts and this too, was inhibited by dasatinib. Even the lowest concentration of dasatinib, 1.25 nmol, inhibited osteoclast differentiation by 29%. Moreover, dasatinib inhibited osteoclast activity by 58% as measured by collagen 1 release. Conclusion: Dasatinib inhibits osteoclast differentiation of mouse primary bone marrow-derived monocytes and PC-3 cell-induced osteoclast differentiation. Dasatinib also inhibits osteoclast degradation activity. Inhibiting osteoclast differentiation and activity may be an effective targeted therapy in patients with prostate cancer bone metastases.

Inhibition of prostate cancer osteoblastic progression with VEGF121/rGel, a single agent targeting osteoblasts, osteoclasts, and tumor neovasculature

Purpose: A hallmark of prostate cancer (PCa) progression is the development of osteoblastic bone metastases, which respond poorly to available therapies. We previously reported that VEGF121/rGel targets osteoclast precursors and tumor neovasculature. Here we tested the hypothesis that targeting nontumor cells expressing these receptors can inhibit tumor progression in a clinically relevant model of osteoblastic PCa. Experimental Design: Cells from MDA PCa 118b, a PCa xenograft obtained from a bone metastasis in a patient with castrate-resistant PCa, were injected into the femurs of mice. Osteoblastic progression was monitored following systemic administration of VEGF121/rGel. Results: VEGF121/rGel was cytotoxic in vitro to osteoblast precursor cells. This cytotoxicity was specific as VEGF121/rGel internalization into osteoblasts was VEGF121 receptor driven. Furthermore, VEGF121/rGel significantly inhibited PCa-induced bone formation in a mouse calvaria culture assay. In vivo, VEGF121/rGel significantly inhibited the osteoblastic progression of PCa cells in the femurs of nude mice. Microcomputed tomographic analysis revealed that VEGF121/rGel restored the bone volume fraction of tumor-bearing femurs to values similar to those of the contralateral (non–tumor-bearing) femurs. VEGF121/rGel significantly reduced the number of tumor-associated osteoclasts but did not change the numbers of peritumoral osteoblasts. Importantly, VEGF121/rGel-treated mice had significantly less tumor burden than control mice. Our results thus indicate that VEGF121/rGel inhibits osteoblastic tumor progression by targeting angiogenesis, osteoclastogenesis, and bone formation. Conclusions: Targeting VEGF receptor (VEGFR)-1- or VEGFR-2–expressing cells is effective in controlling the osteoblastic progression of PCa in bone. These findings provide the basis for an effective multitargeted approach for metastatic PCa. Clin Cancer Res; 17(8); 2328–38. ©2011 AACR.

Thymidine phosphorylase exerts complex effects on bone resorption and formation in myeloma

Myeloma-expressed thymidine phosphorylase enhances bone lesion formation by acting on osteoclasts and osteoblasts. Myeloma enzyme makes way for metastasis Bone tissue is built up by osteoblasts and broken down by osteoclasts in a balanced remodeling process. In metastatic cancer, however, the balance is tipped, leading to the formation of cancerous growths in the bone. Attempts to prevent metastasis have not been successful in the clinic; thus, Liu and colleagues set out in search of a new pathway to target. The authors found that an enzyme produced by myeloma cells, called thymidine phosphorylase (TP), suppressed osteoblast activity (new bone formation) and enhanced osteoclast activity (bone resorption). Inhibiting TP reduced the incidence of myeloma-induced osteolytic bone lesions, suggesting a new target for translation to the clinic, especially because certain TP inhibitors are already approved for human use. Myelomatous bone disease is characterized by the development of lytic bone lesions and a concomitant reduction in bone formation, leading to chronic bone pain and fractures. To understand the underlying mechanism, we investigated the contribution of myeloma-expressed thymidine phosphorylase (TP) to bone lesions. In osteoblast progenitors, TP up-regulated the methylation of RUNX2 and osterix, leading to decreased bone formation. In osteoclast progenitors, TP up-regulated the methylation of IRF8 and thereby enhanced expression of NFATc1 (nuclear factor of activated T cells, cytoplasmic 1 protein), leading to increased bone resorption. TP reversibly catalyzes thymidine into thymine and 2-deoxy-d-ribose (2DDR). Myeloma-secreted 2DDR bound to integrin αVβ3/α5β1 in the progenitors, activated PI3K (phosphoinositide 3-kinase)/Akt signaling, and increased DNMT3A (DNA methyltransferase 3A) expression, resulting in hypermethylation of RUNX2, osterix, and IRF8. This study elucidates an important mechanism for myeloma-induced bone lesions, suggesting that targeting TP may be a viable approach to healing resorbed bone in patients. Because TP overexpression is common in bone-metastatic tumors, our findings could have additional mechanistic implications.

Intravital microscopy of osteolytic progression and therapy response of cancer lesions in the bone

A skin window enables noninvasive, longitudinal monitoring of cancer growth and therapy response in tissue-engineered bone in mice. Bone tumors revealed Noninvasive imaging can help monitor cancer metastasis and tumor-stroma interactions but is challenging for thick, dense tissues such as bone. Dondossola et al. studied prostate cancer metastasis to bone using tissue engineering and intravital multiphoton microscopy in mice. A skin window overlaying implanted engineered bone constructs injected with cancer cells allowed for observation of osteolysis in the bone constructs, with osteoclasts localized at the tumor-bone interface. Treatment with zoledronic acid slowed osteoclast activity (bone resorption) without affecting cancer growth. This engineered bone and imaging method gives a glimpse into tumor-bone interactions that could be useful to test therapies for bone remodeling and cancer metastasis. Intravital multiphoton microscopy (iMPM) in mice provides access to cellular and molecular mechanisms of metastatic progression of cancers and the underlying interactions with the tumor stroma. Whereas iMPM of malignant disease has been performed for soft tissues, noninvasive iMPM of solid tumor in the bone is lacking. We combined miniaturized tissue-engineered bone constructs in nude mice with a skin window to noninvasively and repetitively monitor prostate cancer lesions by three-dimensional iMPM. In vivo ossicles developed large central cavities containing mature bone marrow surrounded by a thin cortex and enabled tumor implantation and longitudinal iMPM over weeks. Tumors grew inside the bone cavity and along the cortical bone interface and induced niches of osteoclast activation (focal osteolysis). Interventional bisphosphonate therapy reduced osteoclast kinetics and osteolysis without perturbing tumor growth, indicating dissociation of the tumor-stroma axis. The ossicle window, with its high cavity-to-cortex ratio and long-term functionality, thus allows for the mechanistic dissection of reciprocal epithelial tumor-bone interactions and therapy response.

Abstract A026: Expression of fibroblast growth factor receptor 1 isoforms and activation of different pathways in prostate cancer progression

Bone metastases typically develop in patients with advanced prostate cancer (PCa). Androgen deprivation is commonly used as treatment, but responses are short, and eventually the disease progresses to a castration-resistant form (CRPC). The fibroblast growth factor (FGF) axis is commonly altered during PCa progression. Our group and others have implicated the FGF axis in the pathogenesis of PCa progression in bone, identified it as a candidate target for therapy, and suggested that FGF receptor (FGFR)-1 mediates a positive feedback loop between PCa cells and bone cells. RNA sequencing studies of 183 human PCa samples indicated that the mean expression of FGFR-1 is the highest of all the FGFR family genes studied. Analyses of these FGFR-1 transcripts identified eight different protein-coding transcripts to be the most abundantly expressed with diverse human PCa tissue samples expressing different FGFR-1 isoforms. The overall goal of this project is to investigate implications of FGFR1/FGFR1 isoforms expression in the pathogenesis of PCa bone metastases. When we assessed the expression of the two best-characterized FGFR-1 isoforms (alpha (NM_023110.2) and beta (NM_023105.2), which represent the most abundant protein coding transcripts found in PCa, we observed that all patient-derived xenografts (PDXs) studied express significantly higher levels of FGFR-1 alpha than beta while PCa cell lines mostly express the beta isoform. Mining The Cancer Genome Atlas (TCGA) PCa data, we identified distinct patterns of gene expression associated with each FGFR-1 isoform (alpha and beta). Briefly, FGFR-1 beta (but not alpha) is significantly associated with pathways including the MAP-kinase signaling cascade, signaling by FGFR in disease and pathways in cancer. In vitro studies of FGF signaling activation in PCa cells expressing FGFR-1 isoforms alpha, beta or empty vector, confirmed these results. Male SCID mice injected intracardially with PC3-FGFR-1 alpha exhibited higher death rates compared to animals injected with either PC3-FGFR-1 beta or PC3-empty vector. Furthermore, higher number of metastases per mice as well as higher number of mice with bone metastases were observed in animals injected with PC3-FGFR-1 alpha. The fact that different prostate tumors express different FGFR-1 isoforms suggests that FGFR1 alpha and beta isoforms activate different genes or pathways in PCa cells and this may underlie, at least in part, PCa heterogeneity, pattern of progression, and differences in response to FGFR1 inhibitors. Further studies will warrant the understanding of the implications of FGFR-1 isoforms expression in the pathogenesis of PCa. Citation Format: Estefania Labanca, Jun Yang, Peter Shepherd, Justin Roberts, Michael Starbuck, Bradley Broom, Matthew Iyer, Christopher Logothetis, Arul Chinnaiyan, Nora Navone. Expression of fibroblast growth factor receptor 1 isoforms and activation of different pathways in prostate cancer progression [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A026.

Abstract 1767: Targeting prostate cancer osteoblastic progression with VEGF121/rGel, a single agent targeting osteoblasts, osteoclasts, and tumor neovasculature

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Purpose: A hallmark of prostate cancer (PCa) progression is the development of osteoblastic bone metastases, which respond poorly to available therapies. We previously reported that VEGF121/rGel targets osteoclast precursors and tumor neovasculature. Here we tested the hypothesis that targeting non-tumor cells expressing the receptors for VEGF121, namely VEGFR-1 and VEGFR-2, can inhibit tumor progression in a clinically relevant model of osteoblastic PCa. Experimental Design: We examined the effect of VEGF121/rGel on osteoblast precursors and several PCa cell lines in vitro; on osteoid formation in a mouse calvaria culture assay ex vivo; and on PCa osteoblastic progression in the femurs of mice injected with MDA PCa 118b, a PCa xenograft obtained from a bone metastasis in a patient with castrate-resistant PCa. Results: VEGF121/rGel was cytotoxic in vitro to osteoblast precursor cells. This cytotoxicity was specific as VEGF121/rGel internalization into osteoblasts was VEGF121 receptor driven. Furthermore, VEGF121/rGel significantly inhibited PCa-induced bone formation in a mouse calvaria culture assay. In vivo, systemic administration of VEGF121/rGel significantly inhibited the osteoblastic progression of PCa cells in the femurs of nude mice. Microcomputed tomography analysis revealed that VEGF121/rGel restored the bone volume fraction of tumor-bearing femurs to values similar to those of the contralateral (non-tumor bearing) femurs. VEGF121/rGel significantly reduced the number of tumor-associated osteoclasts but did not change the numbers of peritumoral osteoblasts. Importantly, VEGF121/rGel-treated mice had significantly less tumor burden than control mice did. Our results thus indicate that VEGF121/rGel inhibits osteoblastic tumor progression by targeting angiogenesis, osteoclastogenesis, and bone formation. Conclusions: Targeting VEGFR-1- or VEGFR-2-expressing cells is effective in controlling the osteoblastic progression of PCa in bone. These findings provide the basis for an effective multitargeted approach for metastatic PCa. We believe that VEGF121/rGel in combination with tumor cell-targeting therapies (e.g., chemotherapy) constitutes a novel strategy for advanced PCa. Research conducted, in part, by the Clayton Foundation for Research. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1767. doi:10.1158/1538-7445.AM2011-1767