Calpain-mediated cleavage negatively regulates the expression level of ABCG1.

The ATP-binding cassette transporter ABCG1 mediates cholesterol efflux from macrophages, and prevents the progression of macrophage foam-cell formation. Much less is known about the regulatory mechanism of ABCG1, although its physiological importance is becoming clearer. Here, we show the role of calpain in ABCG1 degradation. Purified μ-calpain cleaved ABCG1 in crude membrane fractions prepared from human ABCG1-expressing HEK293 (ABCG1-HEK) cells. In ABCG1-HEK cells, calpeptin treatment, a calpain inhibitor, inhibited ABCG1 degradation, and thereby increased the expression and cholesterol efflux function of ABCG1. Biotinylation study demonstrated greater ABCG1 induction with calpeptin treatment in cell surface than that in whole cell lysates. Together with the result that increased ABCG1 expression with calpeptin treatment was observed under clathrin heavy-chain (CHC) knockdown conditions, where ABCG1 internalization was prevented, calpain is considered to catalyze ABCG1 cleavage on the plasma membrane. In mouse peritoneal macrophages as well as in ABCG1-HEK cells, calpeptin treatment inhibited ABCG1 degradation and enhanced ABCG1 expression, even under CHC-depleted conditions. These observations indicate that calpain promotes ABCG1 degradation by cleaving cell surface-resident ABCG1, and consequently reduces the expression and cholesterol efflux function of ABCG1. Inhibition of ABCG1 cleavage by calpain could be a novel approach to suppress the progression of atherosclerosis.