Endocrine Abstracts

Medial calcification is a major cause of premature cardiovascular mortality in chronic kidney disease. Vascular smooth muscle cells (VSMC) play a key role in this process and studies have suggested a protective role for vitamin D. The exact mechanism for this is unclear but local synthesis of 1,25-dihydroxyvitamin D (1,25D) may be important. Production of 1,25D from 25-hydroxvitamin D (25D) is catalyzed by 25-hydroxyvitamin D-1α-hydroxylase (1α-OHase). 1,25D is metabolized by 24-hydroxylase (24-OHase). This study used normal arterial tissue (kidney donors; ethical approval obtained) and primary cultures of human aortic smooth muscle cells (HAoSMC) to examine the expression and regulation of 1α-OHase, 24-OHase and vitamin D receptor (VDR) mRNA (real-time PCR), protein (western blotting/immunohistochemistry) and synthesis of 1,25D. HAoSMC expressed 1α-OHase and 24-OHase mRNA. The PCR transcripts were similar to those in HKC-8 cells (a kidney cell line expressing both hydroxylases). Western blot analyses of 1α-OHase and 24-OHase identified single protein bands (~56 kDa) in HAoSMC and arteries, which were identical to those seen in HKC-8 cells and human kidney tissue. VDR mRNA and protein was detected in all samples. Immunohistochemistry of arteries indicated expression of 1α-OHase, 24-OHase and VDR in the medial layer. Incubation of HAoSMC with 1,25D (6 h) resulted in a dose dependent increase in VDR protein (1–100 nM). 1α-OHase mRNA and protein were significantly decreased (1 nM; P<0.05). Conversely, 24-OHase mRNA was significantly increased (10 nM; Plt;0.05). Importantly 24-OHase mRNA was also increased and 1α-OHase protein decreased by 25D3 (100 and 10 nM respectively; 6 h; Plt;0.05) indicating 1α-OHase activity in HAoSMC. These data demonstrate that arteries and VSMC cells express functional vitamin D signaling. We believe this is the first demonstration of 24-OHase in human arteries and VSMC. We have also shown that VSMC express 1α-OHase and are able to synthesize sufficient 1,25D to stimulate a local response.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.