Expression, transport, and storage of fetuin-B in human granulosa cells

Fetuin-B is crucial in fertilization through inhibiting ovastacin, an enzyme that hardens the zona pellucida. Primarily synthesized in the liver, fetuin-B is also found in the placenta, tongue, and follicular fluid produced by granulosa cells. This study investigated fetuin-B localization and production in granulosa cells. Samples from 45 women undergoing intracytoplasmic sperm injection therapy at the University Medical Center Mainz were analyzed. Serum and follicular fluid samples were retrieved during egg collection. Granulosa cells were cultured and sampled at different time points. FETUB gene expression was quantified by RT-qPCR, with HepG2 cells as a positive control. Fetuin-B levels in granulosa cells and supernatants were measured by ELISA. Fetuin-B was localized using immunocytochemical staining. HepG2 cells showed strong FETUB expression, while granulosa cells exhibited minimal transcription. Fetuin-B levels were highest in serum and significantly lower in follicular fluid (p < 0.001). Supernatant fetuin-B peaked on day 1 (p = 0.004), while intracellular fetuin-B exceeded extracellular (p = 0.008). Immunocytochemistry revealed a largely uniform intracellular distribution, with notable accumulations in filopodia. These findings indicate that granulosa cells store, release, and re-internalize fetuin-B but do not synthesize it. The early release at culture onset suggests a regulatory role in supporting fertilization conditions.