Vitamin D receptor activators can protect against vascular calcification

Abstract

An apparent conflict exists between observational studies that suggest that vitamin D receptor (VDR) activators provide a survival advantage for patients with ESRD and other studies that suggest that they cause vascular calcification. In an effort to explain this discrepancy, we studied the effects of the VDR activators calcitriol and paricalcitol on aortic calcification in a mouse model of chronic kidney disease (CKD)-stimulated atherosclerotic cardiovascular mineralization. At dosages sufficient to correct secondary hyperparathyroidism, calcitriol and paricalcitol were protective against aortic calcification, but higher dosages stimulated aortic calcification. At protective dosages, the VDR activators reduced osteoblastic gene expression in the aorta, which is normally increased in CKD, perhaps explaining this inhibition of aortic calcification. Interpreting the results obtained using this model, however, is complicated by the adynamic bone disorder; both calcitriol and paricalcitol stimulated osteoblast surfaces and rates of bone formation. Therefore, the skeletal actions of the VDR activators may have contributed to their protection against aortic calcification. We conclude that low, clinically relevant dosages of calcitriol and paricalcitol may protect against CKD-stimulated vascular calcification.

Figure 1.

Effects of CKD and VDR activators on aortic Ca levels. High-fat feeding of LDLR−/− mice (sham high fat) increased aortic calcification. CKD induced at 12 wk of age stimulated aortic calcification in the group killed at 28 wk of age (untreated). Compared with the untreated group, both calcitriol 20 ng/kg intraperitoneally three times per week and paricalcitol 100 ng/kg intraperitoneally three times per week administered from 22 to 28 wk of age decreased aortic Ca accumulation. The levels of aortic Ca were not different from the baseline CKD group studied at 22 wk, indicating reduction in aortic Ca accumulation. Paricalcitol, 400 ng/kg intraperitoneally three times per week increased aortic Ca accumulation compared with the untreated group.

Figure 2.

Effects of VDR activators on CKD-induce gene expression in aortas of LDLR−/− high fat–fed mice: (A) osterix. (B) Msx2. (C) CBFA1/RUNX2. (D) osteocalcin. Calcitriol 20 ng/kg and paricalcitol 50 and 100 ng/kg all decreased CKD-induced gene expression. Paricalcitol 400 ng/kg had no effect on CKD-stimulated RUNX2 expression, although it inhibited osteocalcin probably by inhibition of osterix and Msx2 expression. Note that the scale of the y axis was adjusted for the high levels of osteocalcin.

Figure 3.

Effects of VDR activators on metaphyseal trabecular bone volume (bv/tv) in LDLR−/− high fat–fed mice with CKD and ABD. Bone volume was decreased in the baseline and untreated CKD groups compared with the sham high-fat group. Paricalcitol 50 and 100 ng/kg increased bone volume.

Figure 4.

Effects of VDR activators on bone surfaces covered by osteoblasts (obs/bs) in LDLR−/− high fat–fed mice with CKD and ABD. Osteoblast surfaces were decreased in the sham high-fat group and the baseline and untreated CKD groups compared with LDLR−/− chow-fed (sham chow group) animals. Both of the VDR activators increased osteoblast surfaces.

Figure 5.

Effects of VDR activators on osteoid volume (ov/tv) in LDLR−/− high fat–fed mice with CKD and ABD. Osteoid volume was decreased in the baseline and untreated CKD groups compared with sham-operated animals. Calcitriol at 20 ng/kg and all dosages of paricalcitol increased osteoid surfaces back to normal levels.

Figure 6.

Effects of VDR activators on bone surfaces covered by osteoclasts (ocs/bs) in LDLR−/− high fat–fed mice with CKD and ABD. Osteoclast surfaces were not affected by the induction of CKD (untreated group) compared with the sham high-fat group. Osteoclast surfaces were significantly reduced in the 50-ng/kg paricalcitol group.

Figure 7.

Effects of VDR activators on bone formation rates (bfr/bs) in mm³/m² per yr in LDLR−/− high fat–fed mice with CKD and ABD. Bone formation rates were decreased in the sham-operated, high fat–fed group and the baseline and untreated CKD groups compared with sham-operated chow-fed animals. Calcitriol at 10 ng/kg and paricalcitol at 100 ng/kg increased bone formation rates.

Figure 8.

Effects of VDR activators on mineralizing surfaces (ms/bs). Mineralizing surfaces tended to be decreased in the vehicle and calcitriol CKD animals compared with sham high fat–fed animals. The decrease was significant in the 20-ng/kg calcitriol group. Mineralizing surfaces were significantly increased in the 100-ng/kg paricalcitol group compared with the untreated group.

Figure 9.

Effects of VDR activators on the serum Pi in LDLR−/− high fat–fed mice with CKD. CKD produced hyperphosphatemia in the baseline (22 wk) and untreated groups (28 wk). The hyperphosphatemia tended to be ameliorated by treatment with the VDR activators, and calcitriol 20 ng/kg and paricalcitol 100 ng/kg significantly reduced serum Pi levels.

Figure 10.

Effects of VDR activators on the serum Ca in LDLR−/− high fat–fed mice with CKD. A transient increase in the serum Ca was observed in the baseline CKD group that was not present in the untreated group at 28 wk. Serum Ca levels were not different among the various treatment groups.