Role of insulin in the regulation of the hepatic messenger RNA for α(2U)-globulin in diabetic rats

α(2u)-Globulin, the major urinary protein of mature male rats, is synthesized and secreted by the hepatic parenchymal cells. Insulin deficiency in diabetes is known to cause a drastic reduction in the urinary output of α(2u)-globulin. The role of insulin in the hepatic synthesis of α(2u)-globulin was investigated in mature male rats made diabetic with single injections of streptozotocin (4.5 mg/100 g of body weight). The above dose of streptozotocin produced a mild diabetic condition with blood sugar levels between 250 to 350 mg/dl and resulted in only minor changes in the hepatic polysomal profiles and ultrastructural characteristics of the rough endoplasmic reticulum. Even this mild degree of diabetes caused more than 80% reduction in the hepatic concentration of α(2u)-globulin. The depressed synthesis of α(2u)-globulin in diabetic rats can be increased 3-fold within 4 days after continuous insulin supplementation through osmotic minipumps. The decreased hepatic concentration of α(2u)-globulin in diabetes and its reversal with insulin were also reflected by similar changes in the in vivo incorporation of (³H)-leucine into α(2u)-globulin. Examination of the translational integrity of the poly(A)⁺ hepatic mRNA in the wheat germ cell-free system showed no major difference between diabetic and normal states. However, the relative proportion of α(2u)-globulin synthesized in the wheat germ translational system primed with the hepatic mRNA from normal and diabetic rats with and without insulin supplementation showed changes consistent with the hepatic levels of α(2u)-globulin in these animals. In order to explore the changes in the actual concentration of α(2u) mRNA in diabetes, the relative concentrations of α(2u) mRNA sequences within the total poly(A)⁺ hepatic mRNA was analyzed by RNA excess hybridization with a ³²P-labeled Hae III fragment of α(2u) cDNA. The results show that the decreased synthesis of α(2u)-globulin in diabetes and its reversal with insulin correlate with corresponding changes in the relative concentration of α(2u)-globulin mRNA sequences as indicated by their Rt(1/2) of hybridization.