Effects of neuraminidase treatment on solubilized insulin receptor

Role of sialic acid in insulin action and the insulin resistance of diabetes mellitus

Adipocytes treated with neuraminidase show markedly reduced responsiveness to insulin without any alteration in insulin binding. In addition, several studies have separately demonstrated both insulin resistance and decreases in membrane sialic acid content and associated biosynthetic enzymes in diabetes mellitus. In the present study, we investigated the role that sialic acid residues may play in insulin action and in the hepatic insulin resistance associated with nonketotic diabetes. Primary cultures of hepatocytes from normal rats treated with neuraminidase demonstrated a dose-dependent decrease in insulin-stimulated lipogenesis. At a concentration of neuraminidase that decreases insulin action by 50%, 23% of total cellular sialic acid content was released. Neuraminidase-releasable sialic acid was significantly decreased in hepatocytes from diabetic rats and this was associated with significant insulin resistance. Treatment of hepatocytes from diabetic rats with cytidine 5'-monophospho-N-acetylneuraminic acid (CMP-NANA) enhanced insulin responsiveness 39%. The enhanced insulin responsiveness induced by CMP-NANA was blocked by cytidine 5'-monophosphate (CMP) suggesting that the CMP-NANA effect was catalyzed by a cell surface sialyltransferase. CMP reduced neuraminidase-releasable [14C]sialic acid incorporation into hepatocytes by 43%. The data demonstrate a role for cell surface sialic acid residues in hepatic insulin action and support a role for decreased cell surface sialic acid residues in the insulin resistance of diabetes mellitus.

Removal of sialic acids from the purified insulin receptor results in enhanced insulin-binding and kinase activities

Neuraminidase treatment of the purified insulin receptor resulted in an increase in both insulin-binding and kinase activities. Neuraminidase-treated alpha and beta subunits moved further than native subunits on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. The enhancement of insulin-binding and kinase activities and increased mobility of the subunits on SDS-PAGE were not observed when the receptor was treated with neuraminidase in the presence of neuraminidase inhibitor. These results suggest that terminal sialic acid residues have a significant role in insulin-binding and kinase activities. The involvement of sialic acid residues in the activities of the receptor has not been detected by previous studies.

Different characteristics of solubilized lactogen receptors from livers of pregnant and non-pregnant female rats

Receptors specific for lactogenic hormones were solubilized by 1% (v/v) Triton X-100 from the crude particulate membrane fraction of livers of pregnant and non-pregnant female rats and the characteristics of both preparations were compared. Human (125)I-labelled somatotropin was used for binding studies of lactogenic hormone. The solubilized receptor retained most of the characteristics noted in the particulate fraction. The binding of human (125)I-labelled somatotropin to the solubilized receptor is a saturable process, depending on temperature and time. Scatchard analysis of displacement curves revealed similar affinity constants ranging from 1.02 x 10(9) to 1.20 x 10(9) 1/mol, while the binding capacity was 4.5 times greater in the pregnant rat livers than in the non-pregnant female rat livers. The receptors for human (125)I-labelled somatotropin from livers of non-pregnant and pregnant female rats were equally adsorbed onto a concanavalin-A-Sepharose column and were dissociated from the column with alpha-methyl-d-glucoside or alpha-methyl-d-mannoside in the same manner. By gel filtration on Sepharose 6B, however, the molecular sizes of the hepatic receptors were found to be different. The apparent M(r) value was approx. 270000 with a Stokes' radius of 5.5nm in the non-pregnant female rats and approx. 330000 with a Stokes' radius of 5.5nm in the pregnant rats. Furthermore, isoelectric-focusing experiments showed that a major part of the receptor from the non-pregnant female rat livers had a neutral pI (7.0-8.5), whereas that from pregnant-rat livers had an acidic pI (4.2-4.7). These data suggest that the increase in the lactogenic binding capacity in rat liver membranes during pregnancy may be associated with marked changes of the physicochemical properties of the receptors.