Binding of apolipoprotein A-I and acetaldehyde-modified apolipoprotein A-I to liver extracellular matrix

Abstract

Apolipoprotein A-I (Apo A-I), a protein produced mainly by hepatocytes, is decreased in the sera of alcoholic patients with liver fibrosis and cirrhosis. To explain this decrease, we investigated possible interactions between liver extracellular matrix (ECM) and Apo A-I. Using a solid-phase binding assay, we evaluated the binding of Apo A-I to the different liver matrix components. Apo A-I bound significantly to fibronectin (FN) (optical density [OD] = 1.11 +/- .26, P = .01) and collagen (C) I (OD = 0.91 +/- 0.22, P = .02) in comparison with bovine serum albumin (BSA) (OD = 0.26 +/- 0.16). Binding of Apo A-I to fibronectin was concentration dependent and saturable. Apo A-I bound also to ECM in vivo because Apo A-I was detected by immunofluorescence on fibrous septa in liver biopsy specimens of alcoholic patients. Because a negative correlation between Apo A-I and liver fibrosis is amplified in alcoholic patients, we investigated whether the in vitro formation of Apo A-I/acetaldehyde complex (adducts) increased the binding of Apo A-I to the ECM. We showed that the amount of Apo A-I that bound to FN was significantly higher with acetaldehyde-modified Apo A-I (OD = 2.18 +/- 0.19, P = .01) than with native Apo A-I. This increase was probably related to the formation and binding of Apo A-I dimers, because immunoblot of in vitro acetaldehyde-modified Apo A-I showed the formation of dimeric Apo A-I. In conclusion, FN binds both native and acetaldehyde-modified Apo A-I. Because FN is deposited early and in excess during liver fibrosis, a storage mechanism of Apo A-I on newly deposited fibronectin would explain, in part, the decrease observed in alcoholic patients with liver fibrosis.