To remove residual VitD in the culture medium, fetal bovine serum (FBS, Thermo Scientific) was subjected to stripping according to literature 25 (link). Briefly, FBS was incubated with 2.5% activated charcoal powder (Sigma-Aldrich) and incubate at 4 °C overnight with gentle shaking. FBS was then centrifuged at 2000g at 4 °C for 15 min. Supernatant (C-FBS) was collected, filtered through a 0.22μm syringe filter and stored at -20 °C.
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