Ectopic FGF23 production induces mineral changes, osteogenic transdifferentiation, and cancer associated microcalcifications

Testicular microcalcifications consist of hydroxyapatite and their demonstration by ultrasound has been associated with increased risk of testicular germ cell cancer (TGCT). Here, we show that fibroblast growth factor 23 (FGF23), a bone-specific regulator of phosphate homeostasis, is expressed in testicular germ cell neoplasia (GCNIS), embryonal carcinoma (EC), and human embryonic stem cells. FGF23 is not glycosylated in TGCTs and thus rapidly cleaved into a C-terminal fragment that serves as a competitive antagonist for full-length FGF23. High levels of C-terminal FGF23 occupy the receptor formed by Klotho and FGF receptor 1 (FGFR1) in the germ cells facilitating a shift in the expression of phosphate transport proteins from SLC34A2 to SLC34A1 in seminiferous tubules adjacent to GCNIS. knockout mice have a marked epididymal deposition of hydroxyapatite, while the testicular phenotype is milder with spermatogenic arrest and focal germ-cell-specific expression of the bone-like markers runt-related transcription factor 2 (RUNX2) and bone gamma-carboxyglutamic acid-containing protein (BGLAP). In accordance, mice with no functional androgen receptor and lack of circulating gonadotropins develop microcalcifications in 94% of cases and have lower and higher and expression. In accordance, human testicular specimens with microcalcifications also have lower , and focally germ cells express SLC34A1, BGLAP, and RUNX2. Importantly, calcium or phosphate induced osteogenic transdifferentiation of a spermatogonial cell line demonstrated by induction of alkaline phosphatase activity and deposition of hydroxyapatite, which could be fully rescued by pyrophosphate (PPi). Severe microcalcifications were also found in a mouse model with Sertoli-cell ablation particularly when Sertoli-ablation was conducted prepubertally where the germ cells retain stem cell potential. In conclusion, cancer-related microcalcifications may arise secondary to gonadal mineral alterations, which in combination with impaired Sertoli cell function and reduced PPi due to high alkaline phosphatase activity in GCNIS and TGCTs, facilitates osteogenic transdifferentiation of testicular germ cells and deposition of hydroxyapatite.