Inhibition of adipose S-100 protein release by insulin

The release of S-100 protein brought about in rat epididymal fat pads by 10 microM epinephrine was inhibited by about 50% in the presence of more than 8 nM insulin. The inhibitory effect of insulin was also observed in the release of S-100 protein induced by isoproterenol or adrenocorticotropin (ACTH), but not in the release induced by a high concentration (5 mM) of dibutyryl cyclic AMP. Since insulin suppressed (to about 50%) the increase in cyclic AMP content induced by epinephrine under the same conditions, it is suggested that the inhibitory mechanism is mediated by the cyclic AMP levels in adipocytes. The S-100 protein release induced by catecholamine was significantly decreased (to about 50%) in the fat pads obtained from insulin-injected rats. In contrast, in the fat pads obtained from diabetic or long-term starved rats, the S-100 protein release was greatly enhanced, showing several-fold higher levels of basal release in the absence of hormones, and S-100 protein contents in the epididymal adipose tissues of these rats were significantly lower than those of the control rats. These results suggest that the S-100 protein content in adipocytes is regulated by insulin as well as the lipolytic hormones.