Abstract 4141690: Bisphosphonate therapy improves bone deficits but worsens arterial calcification in <i>Mgp</i> -null mice

Background: Arterial calcification often occurs with bone mineral loss in osteoporosis, for which bisphosphonates are a standard therapy. Mice lacking the matrix GLA protein (Mgp) exhibit lethal arterial calcification and cranial bone formation deficits causing dental malocclusion. Mgp not only binds to hydroxyapatite but also inhibits the bone morphogenic protein activation of Runx2 transcription factor, which drives the osteogenic conversion of arterial smooth muscle cells (SMCs). Prior studies showed bisphosphate therapy ameliorates warfarin-induced arterial calcification in rats. This study investigated whether bisphosphonate (Alendronate,ALN) treatment could address bone deficits and arterial calcification in Mgp-null mice. Methods: Mgp-null mice were subjected to various doses of Alendronate therapy starting at weaning or postnatal day 3. Dental malocclusion severity was assessed by incisor trimming frequency. Arterial calcification was quantified by Alizarin red staining and µ-CT. Results: In Mgp-null mice, susceptibility to aortic artery calcification was not uniform but varied by location, with calcification highest at the proximal aorta. Of note, recurring nodes of calcification were found to correspond to branch points of vertebral arteries, with internodal regions showing less calcification. Alendronate therapy started at 3 weeks of age ameliorated dental malocclusion in Mgp-null mice of both sexes and improved but did not normalize body weight gain in males and females. However, no improvement in arterial calcification was observed at 7 weeks. Similarly, Alendronate therapy at lower doses from postnatal day 3 for 5 weeks ameliorated dental malocclusion and improved but did not normalize body weight gain in both sexes, while at higher doses, Alendronate prevented incisor development and caused growth retardation in both wild type and Mgp-null mice. However, Alendronate accelerated the appearance of Runx2 in SMCs of the aorta and worsened internodal arterial calcification in Mgp-null mice in a dose-dependent fashion, but not in Mgp-hemizygous or wild type mice. Conclusion: Our study reveals an important dichotomy in Mgp's function in bone and arteries. The bone deficits in Mgp-null mice, highlighted by incisor malocclusion, could be rescued by bisphosphonate treatment, whereas the arterial calcification could not. Thus, Mgp performs an essential function to prevent arterial calcification that cannot be restored by bisphosphonate therapy.