Reverse effect of mammalian hypocalcemic cortisol in fish: cortisol stimulates Ca2+ uptake via glucocorticoid receptor‐mediated vitamin D3 metabolism

Abstract

Cortisol was reported to downregulate body‐fluid Ca2+ levels in mammals but was proposed to show hypercalcemic effects in teleostean fish. Fish, unlike terrestrial vertebrates, obtain Ca2+ from the environment mainly via the gills and skin rather than by dietary means, and have to regulate the Ca2+ uptake functions to cope with fluctuating Ca2+ levels in aquatic environments. Cortisol was previously found to regulate Ca2+ uptake in fish; however, the molecular mechanism behind this is largely unclear. Zebrafish were used as a model to explore this issue. Acclimation to low‐Ca2+ fresh water stimulated Ca2+ influx and expression of epithelial calcium channel (ecac), 11β‐hydroxylase and the glucocorticoid receptor (gr). Exogenous cortisol increased Ca2+influx and the expressions of ecac and hydroxysteroid 11‐beta dehydrogenase 2 (hsd11b2), but downregulated 11β‐hydroxylase and the gr with no effects on other Ca2+ transporters or the mineralocorticoid receptor (mr). Morpholino knockdown of the GR, but not the MR, was found to impair zebrafish Ca2+ uptake function by inhibiting the ecac expression. To further explore the regulatory mechanism of cortisol in Ca2+ uptake, the involvement of vitamin D3 was analyzed. Cortisol stimulated expressions of vitamin D‐25hydroxylase (cyp27a1), cyp27a1 like (cyp27a1l), 1α‐OHase (cyp27b1) at 3 dpf through GR, the first time to demonstrate the relationship between cortisol and vitamin D3 in fish. In conclusion, cortisol stimulates ecac expression to enhance Ca2+ uptake functions, and this control pathway is suggested to be mediated by the GR. Lastly, cortisol also could mediate vitamin D3 signaling to stimulate Ca2+ uptake in zebrafish.