Abstract 1871: Alpha and beta isoforms of fibroblast growth factor receptor 1 in prostate cancer

Castrate-resistant progression and bone metastases are hallmarks of advanced prostate cancer (PCa). The fibroblast growth factor (FGF)/FGF receptor (FGFR) complex mediates tumor-stromal interactions and is commonly altered in PCa. FGFR1, FGFR2, FGFR3, and FGFR4 genes encode alternatively spliced variants of FGFRs that vary in the extracellular ligand-binding and intracellular kinase domains. A published study from our group implicated FGFR1 as a therapy target for PCa bone metastases (STM 2014; 6:252ra122). Further, our studies of FGFR1 transcripts by RNA sequencing of 183 human PCas identified eight different protein coding transcripts as the most abundantly expressed with different human PCas expressing different FGFR1 isoforms (Abstract #3913 AACR 2015). These results suggest that different FGFR1 isoforms in PCa cells may partially underlie the biological heterogeneity of PCa. The studies presented here focus in two of the best-characterized isoforms: FGFR1alpha (R1alpha), with 3 Ig-like domains, and 822 aa in length; and FGFR1beta (R1beta), with only 2 Ig-like domains and 733 aa in length. We assessed whether these isoforms induce activation of the same pathways using PC3 cells transiently transfected with empty vector (EV), R1alpha (NM_023110.2) or R1beta (NM_023105.2) and treated with vehicle, FGF2 or FGF9. By Western blot analysis we found that total FGFR1 expression (relative to a loading control) was similar in cells transfected with R1beta or R1alpha. Levels of p-FGFR1 were high in untreated cells transfected with R1alpha, but no further induction was observed after treatment with FGF2 or FGF9. However, p-FGFR1 expression was almost undetectable in untreated cells expressing R1beta and was slightly induced by FGF2 but not by FGF9. p-PLCγ expression was found only in cells expressing R1alpha. We subsequently stably transfected these isoforms in PC3 cells and discovered that while no significant difference in R1alpha and R1beta transcript levels was detected, the levels of R1alpha protein were higher than those of R1beta suggesting that these isoforms may undergo different translational regulation, We also performed in vivo studies by subcutaneous injection of R1alpha or R1beta expressing PC3 cells in immunocompromised mice and weekly monitored tumor volume. We found that PC3 cells expressing R1alpha developed significantly larger tumors than PC3 cells expressing R1beta. We are now in the process of analyzing at the molecular level these tissue specimens. In conclusion, R1alpha and R1beta isoforms trigger different biological effects in PCa cells. Because different isoforms are expressed in different prostate tumors, the expression of these isoforms could be associated with the typical PCa heterogeneity and could explain differences in therapy responses to FGFR1 blockade. These results warrant further studies to fully understand the biological implications of FGFR1 isoforms in the pathogenesis of PCa. Citation Format: Estefania Labanca, Xinhai Wan, Jun Yang, Matthew Iyer, Christopher Logothetis, Arul Chinnaiyan, Nora Navone. Alpha and beta isoforms of fibroblast growth factor receptor 1 in prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1871.