Synthesis of the acid-labile subunit of the growth-hormone-dependent insulin-like-growth-factor-binding protein complex by rat hepatocytes in culture.
Insulin-like growth factors (IGFs) circulate predominantly in a growth-hormone-dependent ternary complex of 125-150 kDa. This study investigates the production of the alpha-subunit of this complex, an acid-labile glycoprotein without intrinsic IGF-binding activity, which binds to the IGF-binding protein IGFBP-3 in the presence of IGFs. Medium conditioned by primary cultures of rat hepatocytes produced alpha-subunit with similar complex-forming activity to purified rat serum alpha-subunit. Bovine growth hormone stimulated hepatocyte production of both IGF-I and alpha-subunit. IGF-I tracer bound to pure rat IGFBP-3 was converted from approx. 60 kDa to 150 kDa by serum alpha-subunit, whole rat serum or rat hepatocyte culture medium; this converting activity was destroyed by transient acidification. In contrast, IGF-I bound to hepatocyte-medium IGF-binding proteins could not be converted into a high-molecular-mass from by purified rat serum alpha-subunit. Rat serum and hepatocyte-medium alpha-subunit appeared identical by electrophoretic analysis, since reaction of either with cross-linked IGF-I.IGFBP-3 tracer resulted in bands of molecular mass 130 kDa and 160 kDa, probably representing intact and partially deglycosylated complexes. However, IGF-binding proteins in rat serum and hepatocyte medium were different, in that affinity labelling of medium binding proteins, depleted of endogenous IGFs, showed no evidence of the 50-60 kDa cluster of bands characteristic of rat serum IGFBP-3. We conclude that rat hepatocytes in primary culture produce alpha-subunit similar to that in rat serum; however, alpha-subunit is unable to form ternary complexes with hepatocyte IGF-binding proteins, since cultured hepatocytes do not secrete IGFBP-3.
Scott
C D
CD
Department of Endocrinology, Royal Prince Alfred Hospital, Camperdown, N.S.W., Australia.
Baxter
R C
RC
D000818:Animals; D002352:Carrier Proteins / Q000494:pharmacology*; D002478:Cells, Cultured; D005260:Female; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D046911:Macromolecular Substances; D008970:Molecular Weight; D051381:Rats; D011920:Rats, Inbred WF; D011956:Receptors, Cell Surface / Q000302:isolation & purification / Q000378:metabolism*; D017451:Receptors, Somatomedin
D002352:Carrier Proteins; D018969:Insulin-Like Growth Factor Binding Proteins; D046911:Macromolecular Substances; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D007334:Insulin-Like Growth Factor I