Hepatic processing of transforming growth factor β in the rat: Uptake, metabolism, and biliary excretion

Abstract

Transforming growth factor beta (TGFβ), a recently discovered polypeptide, modulates growth of normal and neoplastic cells. Since little is known concerning in vivo disposition of TGFβ, we performed studies to examine the hepatic processing of biologically active 125I-TGFβ in the rat. After intravenous injection, 125I-TGFβ disappeared from the plasma with an initial t( 1/2 ) of 2.2 min; partial hepatectomy delayed the plasma disappearance of 125I-TGFβ by 80%. 60 min after intrafemoral injection, 63% of the recovered label was present in liver and/or bile; by 90 min, most of the label removed by the liver (83%) had been slowly excreted into bile. Nearly all the label in bile (96%) was soluble in trichloracetic acid and not immunoprecipitable by specific antiserum. Colchicine and vinblastine inhibited cumulative biliary excretion of label by 28 and 37%, respectively; chloroquine and leupeptin each increased the amount of label in bile that was precipitable by trichloracetic acid and that coeluted with authentic 125I-TGFβ on molecular sieve chromatography. There was efficient first-pass hepatic extraction of 125I-TGFβ (36%) in the isolated perfused rat liver, which was inhibited by unlabeled TGFβ (but not by epidermal growth factor, EGF) and by lectins in a dose-dependent manner; prolonged fasting also decreased clearance (26%). After fractionation of liver by differential or isopycnic centrifugation, radiolabel codistributed with marker enzymes for lysosomes. The results indicate rapid, extensive, inhibitable, and organ-selective extraction of TGFβ by the liver. After extraction, TGFβ undergoes efficient transhepatic transport, extensive intracellular metabolism, and slow but complete biliary excretion of its metabolites. Liver fractionation studies and pharmacologic manipulations suggest that these processes are associated with organelles that include microtubules and lysosomes. The data suggest that the liver is a major target tissue or site of metabolism for biologically active TGFβ.