Associate Professor Florida International University Miami, Florida, United States
Introduction:: Though vascular smooth muscle cells (VSMCs) adopt an osteogenic phenotype during pathological vascular calcification, clinical studies note an inverse correlation between bone mineral density and arterial mineral— a phenomenon known as the calcification paradox. The mineralization processes of each cell type share some similarities, as both are mediated by extracellular vesicles (EVs) that sequester calcium and phosphate. Previous studies associate calcification paradox with an imbalanced phosphate/calcium content due to bone loss and/or increased systemic inflammation. However, modulation of caveolin-1 (CAV1), a membrane scaffolding protein essential to calcifying EV formation in vasculature, also results in inverse effects on mineralization in bone and vascular tissues. Mice with global CAV1 knockout exhibit increased bone mineral density, while knockdown of CAV1 in VSMC cultures decreases in-vitro calcification. These preliminary findings indicate that the calcification paradox may not be fully attributed to systemic distribution of mineral components or inflammation, highlighting a need for more research into the underlying mechanisms of these analogous mineralization pathways.
Materials and Methods:: To further investigate this divergent role of CAV1 in mineralization, CAV1 expression was knocked down by siRNA in primary human coronary artery VSMC and human osteoblast (HOB) cell cultures, grown in both control and osteogenic media. Additionally, to assess the functional role of CAV1 in both cell types, we supplemented osteogenic media with either Methyl β-cyclodextrin (MβCD) or calpain inhibitor to induce a disruptive or stabilizing effect in the caveolar domains, respectively. VSMCs and HOBs were grown in control, osteogenic, or supplemented osteogenic media for up to 28 days. At the study endpoint, calcification was measured via Alizarin Red staining and quantification. Total cell lysate was collected on day 14 of culture and separated by density via ultracentrifugation in an OptiPrep density gradient column. Each layer of cell lysate was then isolated and western blotted to examine the migration of CAV1 to locations of different density during the above experimental conditions.
Results, Conclusions, and Discussions:: In vitro siRNA knockdown of CAV1 abrogated calcification production in VSMCs, while having no effect on osteoblast mineralization. MβCD-mediated caveolae disruption led to a 3-fold increase of calcification in VSMCs treated with osteogenic media (p< 0.05) but hindered osteoblast mineralization (p< 0.001). Conversely, stabilizing caveolae by calpain inhibition prevented VSMC calcification (p< 0.01) but increased osteoblast mineralization (p< 0.01). Osteogenic culture conditions resulted in translocation of CAV1 from low density caveolar lipid raft domains to high density non-caveolar domains in VSMCs. In HOBs, we found no significant differences in the amount of CAV1 in any layer between control and OS groups.
All manipulations of CAV1 trafficking analyzed in this study had opposite outcomes on endpoint calcification, supporting the idea that these two pathways are individually distinct. Density gradient results suggest that CAV1 traffics from caveolar to non-caveolar domains during VSMC mediated calcification, while the same trafficking pattern is not observed in HOB mineralization. Altogether, our data indicate fundamental cellular-level differences in physiological and pathophysiological mineralization mediated by CAV1 dynamics in VSMC and HOB cultures.
In this study, we observe fundamental mechanistic divergence in HOB and VSMC mineralization that may provide further insight into the calcification paradox. The unique role of CAV1 in the formation of calcific vesicles in both cell types should be investigated further, and future work should focus on identifying additional mechanistic differences in these two processes. Future studies are also needed to clarify the relative contributions of systemic pressures and mechanistic differences leading to the calcification paradox.
