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Objective: Renal 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) enables selective access of aldosterone to the mineralocorticoid receptor (MR). Impaired 11β-HSD2 activity has been suggested in patients with hypertension as well as in patients with renal disease, where it may contribute to sodium retention, oedema and hypertension. To date, these studies have relied upon urinary cortisol (F) metabolite levels as surrogate markers of renal 11β-HSD2 activity.

Methods: We have directly analysed renal 11β-HSD2 mRNA expression in 95 patients undergoing kidney biopsy using TaqMan real-time PCR. Serum and 24-h urine samples were used to document underlying renal function and endocrine parameters. Urinary F and cortisone (E) metabolites were analysed using gas chromatography/mass spectrometry.

Results: Expression of 11β-HSD2 did not correlate with blood pressure or urinary Na/K ratio, but a significant positive correlation with creatinine clearance was observed (r = 0.284; P < 0.01). Immunofluorescence and confocal laser microscopy confirmed decreased 11β-HSD2 expression in patients with impaired renal function. For the first time, we showed that 11β-HSD2 mRNA expression correlated negatively with the urinary free (UF) F/E (UFF/UFE) ratio (r = 0.276; P < 0.05) as well as with the urinary tetrahydrocortisol + 5α-tetrahydrocortisol/tetrahydrocortisone ((THF + αTHF)/THE) ratio (r = 0.256; P < 0.05). No difference in 11β-HSD2 mRNA expression or in the UFF/UFE ratio was found between groups with no proteinuria, microalbuminuria, moderate or severe proteinuria. In contrast, the urinary (THF + αTHF)/THE ratio increased significantly (P < 0.05) in patients with severe albuminuria, suggesting increased hepatic 11β-HSD1 in those patients.

Conclusions: These data suggest that renal 11β-HSD2 expression may be represented only marginally better, if at all, by the UFF/UFE than by the (THF + αTHF)/THE ratio. Reduced renal 11β-HSD2 expression may lead to occupancy of the MR by glucocorticoids such as cortisol and may contribute to the increased sodium retention seen in patients with impaired renal function.

The SARS-CoV-2 virus responsible for the COVID-19 pandemic has generated an explosion of interest both in the mechanisms of infection leading to dissemination and expression of this disease, and in potential risk factors that may have a mechanistic basis for disease propagation or control. Vitamin D has emerged as a factor that may be involved in these two areas. The focus of this article is to apply our current understanding of vitamin D as a facilitator of immunocompetence both with regard to innate and adaptive immunity and to consider how this may relate to COVID-19 disease. There are also intriguing potential links to vitamin D as a factor in the cytokine storm that portends some of the most serious consequences of SARS-CoV-2 infection, such as the acute respiratory distress syndrome. Moreover, cardiac and coagulopathic features of COVID-19 disease deserve attention as they may also be related to vitamin D. Finally, we review the current clinical data associating vitamin D with SARS-CoV-2 infection, a putative clinical link that at this time must still be considered hypothetical.