CAMP‐mediated processing of FGF23 : involvement of O‐glycosylation and furin.

Abstract

The phosphate and vitamin‐D regulating hormone, FGF23, circulates in both an active intact (iFGF23) and an inactive C‐terminal (cFGF23) form. Post‐translational processing of FGF23 involves furin and the glycosyltransferase, GalNac‐T3. Subjects with fibrous dysplasia of bone (FD), which is caused by activating mutations in the cAMP‐regulating protein Gsalpha, have elevated levels of c‐ vs. iFGF23. To test the hypothesis that Gsalpha/cAMP signaling regulates FGF23 processing, we evaluated FGF23 levels, FGF23 glycosylation, and GalNac‐T3 and furin activity. HEKF, a HEK293 clonal cell‐line that is over‐expressing FGF23 and MLO‐Y4 cells, a mouse osteocyte cell line were used. Cells were transfected with mutant Gsalpha; and/or treated with dibutyryl cAMP (db‐cAMP) and/or forskolin. Both db‐cAMP and forskolin treatment resulted in increased c‐ vs: iFGF23 levels. Results from deglycosylation experiments indicated that i‐ and cFGF23 are glycosylated, and this process is modulated by db‐cAMP and forskolin. cAMP treatment resulted in decreased GalNac‐T3/FGF23 association in co‐immunoprecipitation experiments, and increased furin levels. No FGF23 cleavage was observed when furin (−/−) LoVo cells were transfected with FGF23, indicating the involvements of furin activity in FGF23 processing. Our results demonstrate that FGF23 processing can be regulated by Gsalpha/cAMP and both furin and GalNac‐T3 activities are regulated and important for FGF23 processing.