Human immunoglobulin M stimulates adipocyte lipogenesis

In view of our recent demonstration that human immunoglobulin G (IgG) and its Fc moiety exert a stimulatory effect on lipogenesis by adipocytes, we have examined the possibility that human immunoglobulin M (IgM) may induce a similar effect upon adipocytes. Three IgM preparations, obtained from patients with Waldenstrom's macroglobulinemia, and a commercial preparation produced consistent dose-related stimulation of lipogenesis by adipocytes. This stimulation was additive to, and greater than, the maximal stimulation produced by insulin; it was also neutralized completely by preincubation with μ-chain specific anti-IgM antiserum but not by an excess of anti-insulin antiserum. Similar stimulation of adipocyte lipogenesis was observed with (Fc)₅ fragments prepared from IgM fractions. Therefore, IgM exerts a stimulatory effect upon adipocyte lipogenesis similar to that of IgG; this effect is probably mediated through a putative Fc receptor on the adipocyte membrane.