Similarities between the insulin-like stimulatory effect of human IgG and NSILP

Since the insulin-like stimulatory effect of human IgG on adipocyte lipogenesis exerted through its Fc moiety, is not neutralized by anti-insulin antisera, IgG may contribute significantly to the non-suppressible insulin-like activity (NSILA) of plasma. 91% of NSILA has been shown previously to be associated with a high mol. wt. protein (NSILP). The purpose of this investigation was to assess (a) whether IgG and NSILP have similar stimulatory effects on adipocyte lipogenesis and whether this effect can be neutralized by preincubation with γ-chain specific anti-IgG antiserum; (b) whether IgG stimulates ³⁵S-sulphate uptake by porcine cartilage, known to be stimulated by insulin-like growth factors but not NSILP; and (c) whether γ-chain specific anti-IgG anti-sera precipitate IgG in a fashion similar to that with IgG preparations. Our investigations show that (a) both IgG and NSILP have similar dose response relationships with respect to the stimulation of adipocyte lipogenesis and that both lose their adipocyte stimulating effect following preincubation with anti-IgG antiserum; (b) neither IgG nor NSILP stimulate ³⁵S-sulphate uptake by porcine cartilage, unlike serum somatomedin and crude NSILA-s preparations; and that (c) γ-chain specific anti-IgG antisera form precipitin lines with NSILP. Therefore, NSILP and IgG molecules have immunological and biological similarities; there may occur a homology between the Fc fragment of IgG and the NSILP molecule.