APOLIPOPROTEIN SYNTHESIS IN NEWBORN PIGLET INTESTINAL EXPLANTS

To determine the effects of hormones and epidermal growth factor (EGF) on the small intestinal synthesis of apolipoproteins B-48, A-I, and A-IV in the neonatal mammal, apolipoprotein synthesis by proximal jejunal explants from 2-d-old female piglets was studied in tissue culture. Initial comparison studies with various media showed optimal total protein and apo A-I synthesis with Williams' medium E without fetal bovine serum. Sets of explants were prepared containing EGF and various hormones in the medium. After S-35-methionine radiolabeling, explants were homogenized. and specific apolipoprotein synthesis was quantitated by immunoprecipitation as the percentage of total protein synthesis. Apo B-48 synthesis was not affected by any additives except the combination of EGF and hydrocortisone, which slightly decreased synthesis. Apo A-I synthesis was significantly increased by EGF. This EGF-induced increase in apo A-I synthesis was blunted by concomitant treatment with hydrocortisone. In contrast, the combination of insulin and hydrocortisone induced a significant increase in apo A-I synthesis. Although EGF and insulin modestly increased apo A-IV synthesis, the combination of insulin and hydrocortisone treatment up-regulated apo A-IV synthesis by 2.6-fold. Thyroid hormone lacked effect on synthesis of any of the apolipoproteins. EGF, glucocorticoids, and insulin may play regulatory roles in the developmental expression of apolipoprotein synthesis in the neonatal small intestine.