Calcifying human aortic smooth muscle cells express different bone alkaline phosphatase isoforms, including the novel B1x isoform.

BACKGROUND

Vascular calcification, causing cardiovascular morbidity and mortality, is associated with hyperphosphatemia in chronic kidney disease (CKD). In vitro, phosphate induces transdifferentiation of vascular smooth muscle cells to osteoblast-like cells that express alkaline phosphatase (ALP). In vivo, raised serum ALP activities are associated with increased mortality. A new bone ALP isoform (B1x) has been identified in serum from CKD patients. The present study investigated the different ALP isoforms in calcifying human aortic smooth muscle cells (HAoSMCs).

METHODS

HAoSMCs were cultured for 30 days in medium containing 5 or 10 mmol/l β-glycerophosphate in the presence or absence of the ALP-specific inhibitor tetramisole.

RESULTS

All known bone-specific ALP (BALP) isoforms (B/I, B1x, B1 and B2) were identified in HAoSMCs. β-Glycerophosphate stimulated calcification of HAoSMCs, which was associated with increased BALP isoforms B/I, B1x and B2. Tetramisole inhibited the β-glycerophosphate-induced HAoSMC calcification, which was paralleled by the inhibition of the B1x and B/I, but not the other isoforms.

CONCLUSIONS

HAoSMCs express the four known BALP isoforms. B/I, B1x and B2 could be essential for soft tissue calcification. B/I and B1x were more affected by tetramisole than the other isoforms, which suggests different biological functions during calcification of HAoSMCs.