Background: High plasma concentration of glucose indicates diabetes and excessive administration of insulin can cause hypoglycemia. Macrophages have been shown to respond to differences in glucose concentration by regulating their expressions and activities of cysteine cathepsins. Since these enzymes and the cysteine protease legumain have been suggested to play a role in destabilization of atherosclerotic plaques, there might be a link between fluctuating concentration of glucose and atherosclerosis.
Methods: Human THP-1 monocytes were stimulated with PMA to become adherent and macrophage-like before incubating with different concentrations of glucose. The cells were harvested after 3 or 7 days to measure protease activities and mRNA levels. E64 and a newly developed specific legumain inhibitor were used to differentiate between cysteine cathepsins and legumain activities. Legumain levels in conditioned media or plasma samples from a cohort of patients with carotid artery atherosclerosis were measured by ELISA.
Results: This study shows that low concentrations of glucose caused reduced legumain activity but not reduced activities of cysteine cathepsins in macrophages. Low activity and expression of cellular legumain was accompanied by increased secretion to the culture medium. Quantitative PCR indicated reduced expression of legumain at low glucose concentrations in contrast to cathepsin B and S (but not L), which showed significantly increased expression.
Increased expression of legumain in atherosclerotic plaques has been described in patients with unstable plaques. Here we describe for the first time elevated plasma concentrations of legumain in a cohort of patients diagnosed with carotid artery disease compared to healthy controls.
Conclusion: This study shows that the concentration of glucose has opposing effects on the regulation of cysteine cathepsins and legumain. Taking into account that cysteine cathepsins are substrates for legumain, unregulated plasma glucose could alter this proteolytic balance leading to atherosclerotic disease progression.