*5.1. Paracrine FGFs*

Upregulated expression of FGF1, FGF2, FGF8, FGF9, and FGF10 were shown in human PCa [37]. Murine studies demonstrated the epithelial and mesenchymal interaction using a FGF/FGFR complex in PCa [38]. Pecqueux et al. demonstrated the association between strong expression of FGF2 in tumor stroma and a high postoperative recurrence rate and that exogenous FGF2 can drive genomic instability to promote PCa progression through enhancement of DNA damage [39]. Cuevas et al. reported the linkage between altered micro-environmental signaling that includes FGF2 overexpression and mitotic instability [40]. In the study focused on a bone metastatic site, FGF2 was upregulated in osteoblast and promoted the proliferation of PCa cells under the loss of TGFβ signaling [41]. These reports indicated the significance of FGF/FGFR signaling through paracrine FGFs (especially FGF2) in a cancer micro-environment for the development and progression of PCa.

FGF9 is an abundant molecule in nervous tissue and soft tissue, while it has been reported to be a key molecule of epithelial-stromal interaction [42]. Several studies have demonstrated that FGF9 was associated with the proliferation of glia [43], regulation of the differentiation of astrocytes [44] and oligodendrocytes [45], and the regulation of joint formations [46,47]. FGF9's involvement in malignant neoplasms has been reported in glioma [48], ovarian cancer [49], and lung cancer [50]. In prostate epithelial cells, overexpression of FGF9 lead the augmentation of reactive stroma formation and promoted initiation and progression of PCa [51]. In our study, cell viability and invasion of LNCaP was significantly enhanced, and expression of MMP2, N-cadherin, and VEGF-A were induced in LNCaP incubated in medium with FGF9. In immuno-histochemical staining, the prevalence of both VEGF-A and N-cadherin-positive cells was significantly higher compared to FGF9-negative cases [52]. The biochemical relapse-free survival (bRFS) rate in cases with FGF9-positive cases was significantly lower than that in FGF9-negative cases [53]. "FGF9-positive" in this study was determined based on the findings of immunohistochemistry staining on just one representative section. And even in FGF9-positive cases, just a small population of FGF9-positive cells with very aggressive pathological features was present. In other words, FGF9 was only positive in especially high-grade cancer cells in cases with localized PCa. Furthermore, several studies have reported FGF9 in PCa at an advanced stage. Accumulation of FGF9 to the region of bone metastasis formed by AR-negative PCa cells indicated that FGF9 was a key factor in formation of bone metastasis [54]. In AR-negative CRPC cases, neuroendocrine differentiation (NE) is one of the most representative phenotypes. In a further dedifferentiated phenotype, "AR-negative and NE-negative", expression of FGF1, FGF8, and FGF9 increased, and MAPK pathway was activated [55]. The results of these studies indicated that FGF9 might be a key molecule for an advanced PCa that include CRPC rather than a localized one.
