Abstract 4022: Fibroblast-derived PEDF inhibits tumor growth and angiogenesis and is suppressed by TGF-β and PDGF

Abstract

Abstract Rationale and Objective: PEDF (pigment epithelium-derived factor) is an endogenous angiogenesis inhibitor and a type II tumor suppressor whose expression is often reduced/ lost in a variety of tumors, including malignant melanoma. To date, the mechanisms and consequences of PEDF loss on tumor progression have been investigated in neoplastic cells but not in the tumor-associated non-neoplastic cells. Fibroblasts are extremely abundant in the tumor microenvironment and can switch to the tumor-promoting phenotype in response to the tumor-associated factors. Here, we investigated how the loss of fibroblast-derived PEDF in the tumor microenvironment alters melanoma progression and the mechanisms, by which melanoma cells attenuate PEDF expression in fibroblasts. Methods: We used lentiviral vectors for PEDF knockdown (shPEDF and control). Tumor growth was evaluated in immunodeficient mice (flank injections). Angiogenesis and proliferation in the tumor were measured by immunohistochemistry (IHC) for CD31 and Ki-67, respectively. Gene expression was evaluated by q-PCR and protein levels were detected by western blot/ IHC. The contribution of growth factors tested using specific pathway inhibitors. Results: We have measured tumorigenesis of highly aggressive melanoma cell line C8161-HA co-mixed with shPEDF or control fibroblasts. PEDF loss in fibroblasts promoted tumor growth/ angiogenesis in xenografted human melanoma. Seeking mechanisms by which tumors attenuate PEDF expression in fibroblasts we found that it is mildly attenuated by hypoxia and potently down-regulated by the soluble factors produced by melanoma cells. Using recombinant growth factors and tumor conditioned media in combination with receptor and pathway-specific inhibitors we have identified TGF-β and PDGF as the main antagonists of PEDF expression. We have also discovered that PDGF directly blocks PEDF expression while regulation by TGF-β is indirect, whereby it elevates PDGFR expression by fibroblasts and thus increases their sensitivity to both paracrine and autocrine PDGF. Conclusions: Together, our data demonstrate the key role of PEDF loss in non-neoplastic cells in the tumor microenvironment and the mechanisms by which tumors overcome its anti-cancer effects. Our data also indicate potential utility of PDGF inhibitors to restore PEDF in the tumor microenvironment. Citation Format: Nkechiyere G. Nwani. Fibroblast-derived PEDF inhibits tumor growth and angiogenesis and is suppressed by TGF-β and PDGF. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4022. doi:10.1158/1538-7445.AM2015-4022