Background: Vitamin D deficiency, determined by blood levels of 25-hydroxyvitamin D [25(OH) D, i.e. the major vitamin D form in blood], has been shown to associate with all-cause mortalities. We recently demonstrated that blood levels of 1,25-dihydroxyvitamin D [1,25(OH)2D, i.e. the active vitamin D] were significantly lower in non-survivors compared to survivors among sepsis patients. Unexpectedly, despite the well documented roles of 1,25(OH)2D in multiple biological functions such as regulation of immune responses, stimulation of antimicrobials, and maintenance of barrier function, 1,25(OH)2D supplementation failed to improve disease outcomes. These previous findings suggest that, in addition to 1,25(OH)2D deficiency, disorders leading to the 1,25(OH)2D deficiency also contribute to mortality among sepsis patients. Therefore, this study investigated the mechanisms leading to sepsis-associated 1,25(OH)2D deficiency.
Methods: We studied mechanisms known to regulate kidney 25-hydroxylvitamin D 1α-hydroxylase which physiologically catalyzes the conversion of 25(OH) D into 1,25(OH)2D. Such mechanisms included parathyroid hormone (PTH), insulin-like growth factor 1 (IGF-1), fibroblast growth factor 23 (FGF-23), and kidney function.
Results: We demonstrated in both human subjects and mice that sepsis-associated 1,25(OH)2D deficiency could not be overcome by increased production of PTH which stimulates 1α-hydroxylase. Further studies showed that this failure of PTH to maintain blood 1,25(OH)2D levels was associated with decreased blood levels of IGF-1, increased blood levels of FGF-23, and kidney failure. Since the increase in blood levels of FGF-23 is known to associate with kidney failure, we further investigated the mechanisms leading to sepsis-induced decrease in blood levels of IGF-1. Our data showed that blood levels of growth hormone, which stimulates IGF-1 production in liver, were increased but could not overcome the IGF-1 deficiency. Additionally, we found that the inability of growth hormone to restore the IGF-1 deficiency was associated with suppressed expression and signaling of growth hormone receptor in liver.
Conclusions: Because FGF-23 and IGF-1 have multiple biological functions besides their role in regulating kidney 1α-hydroxylase, our data suggest that FGF-23 and IGF-1 are warranted for further investigation as potential agents for the correction of 1,25(OH)2D deficiency and for the improvement of survival among sepsis patients.
Keywords: 1,25-dihydroxyvitamin D; 25-hydroxyvitamin D 1α-hydroxylase; Fibroblast growth factor 23; Insulin-like growth factor 1; Parathyroid hormone; Sepsis.