The cysteine protease legumain is upregulated by vitamin D and regulates vitamin D metabolism

Legumain is a lysosomal cysteine protease that has been implicated in an increasing amount of physiological and pathophysiological processes. However, the upstream mechanisms regulating the expression and function of legumain are not well understood. Here, we provide in vitro and in vivo data showing that vitamin D ₃ (VD ₃) enhances legumain expression and function. In turn, legumain alters VD ₃ bioavailability, possibly through proteolytic cleavage of vitamin D binding protein (VDBP). Active VD ₃ (1,25(OH) ₂D ₃) increased legumain expression, activity, and secretion in osteogenic cultures of human bone marrow stromal cells. Upregulation of legumain was also observed in vivo, evidenced by increased legumain mRNA in the liver and spleen, as well as increased legumain activity in kidneys from wild-type mice treated with 25(OH)D ₃ (50 µg/kg, subcutaneously) for 8 days compared to a control. In addition, the serum level of legumain was also increased. We further showed that active legumain cleaved purified VDBP (55 kDa) in vitro, forming a 45 kDa fragment. In vivo, no VDBP cleavage was found in kidneys or liver from legumain-deficient mice (Lgmn ^−/−), whereas VDBP was cleaved in wild-type control mice (Lgmn ^+/+). Finally, legumain deficiency resulted in increased plasma levels of 25(OH)D ₃ and total VD ₃ and altered expression of key renal enzymes involved in VD ₃ metabolism (CYP24A1 and CYP27B1). In conclusion, a regulatory interplay between VD ₃ and legumain is suggested.