A protein involved in bone formation and calcification has provided new insights into the development of smoking-related cardiovascular disease and may represent a new biomarker for assessing the harmful effects of tobacco smoke. The protein, osteoponin, is a noncollagenous bone matrix protein produced by macrophages and endothelial cells, and has already been implicated in inflammation, tissue remodelling, and cardiovascular disease via its activity as a soluble cytokine.
Researchers at British American Tobacco (BAT), and R.J. Reynolds Tobacco Company have now found that human umbilical vein endothelial cells (HUVECs) markedly upregulate expression, production, and secretion of osteoponin in response to exposure to the nontoxic levels of particulate matter from cigarette smoke. This production of osteoponin was likely linked to oxidative stress, as treating the cells with the antioxidant ascorbate reduced levels of the protein.
Soluble osteoponin interacts with cell surface integrins to regulate cell adhesion and migration, and can be differentially cleaved by thrombin and matrix metalloproteinases (MMPs), and in particular MMP-3, to generate fragments with different functional attributes. Interestingly, report BAT’s Ian M Fearon, Ph.D., and colleagues, MMP-3 was also upregulated in response to the tobacco smoke exposure, but unlike upregulation of osteoponin, increased MMP-3 expression wasn’t itself directly due to oxidative stress. Nevertheless, Western blotting studies confirmed that the enzyme acted to cleave full-length osteoponin into its active fragment. “Oxidative stress therefore may be differentially involved in the production of functionally-dissimilar osteopontin fragments, and functional changes in osteopontin effects can be mediated by cigarette smoke acting on distinct and nonoxidative stress-dependent parts of the pathway,” the researchers write in their published paper in BMC Cardiovascular Disorders.
Finally, to assess the link between smoking, oxidative stress, inflammation, and osteopontin expression, the team measured serum levels of the protein in healthy regular smokers both before and after they refrained from smoking for five days. The results confirmed that cessation from smoking rapidly led to reduced levels of both osteoponin and the inflammatory marker IL-6.
The researchers say as far as they know their results provide the first direct report of increased osteoponin production by endothelial cells in response to smoke exposure. "Furthermore, our studies highlight a potential role for MMP cleavage in regulating the levels of cleaved forms of osteopontin, which have different functional attributes," they state. "While further studies are needed to examine the longer-term effects of smoking cessation on both biomarkers of oxidative stress/inflammation and on the levels of osteopontin in serum, this study indicates that osteopontin can be a potential short-term biomarker of inflammation/oxidative stress."