Fibronectin (Fn) is a major adhesive protein found in the hepatic extracellular matrix (ECM). In adult rats, the in vivo turnover of plasma Fn (pFn) incorporated into the liver ECM is relatively rapid, i.e., <24 h, but the regulation of its turnover has not been defined. We previously reported that cellular Fn (cFn) and enzymatically desialylated plasma Fn (aFn), both of which have a high density of exposed terminal galactose residues, rapidly interact with hepatic asialoglycoprotein receptors (ASGP-R) in association with their plasma clearance after intravenous infusion. With the use of adult male rats (250–350 g) and measurement of the deoxycholate (DOC)-insoluble125I-labeled Fn in the liver, we determined whether the ASGP-R system can also influence the hepatic matrix retention of various forms of Fn. There was a rapid deposition of 125I-pFn,125I-aFn, and125I-cFn into the liver ECM after their intravenous injection. Although125I-pFn was slowly lost from the liver matrix over 24 h, more than 90% of the incorporated125I-aFn and125I-cFn was cleared within 4 h ( P < 0.01). Intravenous infusion of excess nonlabeled asialofetuin to competitively inhibit the hepatic ASGP-R delayed the rapid turnover of both aFn and cFn already incorporated within the ECM of the liver. ECM retention of both125I-aFn and125I-cFn was also less than125I-pFn ( P < 0.01) as determined in vitro using liver slices preloaded in vivo with either tracer form of Fn. The hepatic ASGP-R appears to participate in the turnover of aFn and cFn within the liver ECM, whereas a non-ASGP-R-associated endocytic pathway apparently influences the removal of normal pFn incorporated within the hepatic ECM, unless it becomes locally desialylated.
A New Exposed-terminal-free MAC Protocol for Multi-hop Wireless Networks
