Microsomal triglyceride transfer protein activity remains unchanged in rat livers under conditions of altered very-low-density lipoprotein secretion

Microsomal triglyceride transfer protein (MTP) is a heterodimeric protein consisting of a unique 97 kDa subunit and protein disulphide isomerase, that mediates the transfer of lipid between membranes and nascent lipoproteins. Mutations in the gene encoding the 97 kDa subunit of the protein cause the rare autosomal recessive disorder abetalipoproteinaemia. Recent findings in cultured cells indicate that the 5′ promoter region contains an insulin-responsive element, suggesting that the gene is responsive to insulin regulation. In this study we examined two cases where very-low-density lipoprotein (VLDL) secretion is markedly reduced: the streptozotocin-diabetic rat and 10-day-old suckling rats. In both cases MTP activity was unaltered compared with that in control animals. Equal levels of MTP were also apparent in the livers of all groups of animals, as measured by immunoblotting with an anti-MTP antiserum. These findings indicate that factors other than MTP activity are responsible for the reduction in hepatic VLDL triglyceride secretion observed in the suckling and diabetic animals.