CLONING AND REGULATION OF HAMSTER MICROSOMAL TRIGLYCERIDE TRANSFER PROTEIN - THE REGULATION IS INDEPENDENT FROM THAT OF OTHER HEPATIC AND INTESTINAL PROTEINS WHICH PARTICIPATE IN THE TRANSPORT OF FATTY-ACIDS AND TRIGLYCERIDES

Microsomal triglyceride transfer protein (MTP) is a heterodimer consis ting of protein disulfide isomerase and a unique large subunit. Recent studies showing that an absence of MTP is a cause of abetalipoprotein emia indicate that MTP is required for the assembly of very low densit y lipoproteins in the liver and chylomicrons in the intestine. In this study, complementary DNA encoding the large subunit of hamster MTP wa s cloned. The cDNA sequence was used to design a 50-base pair oligonuc leotide probe for a solution hybridization assay to quantitate MTP lar ge subunit mRNA levels in a study of MTP regulation in male Syrian Gol den hamsters. In animals fed a low fat diet, MTP exhibited a proximal to distal gradient of expression in the intestine. MTP activity and la rge subunit mRNA levels in the liver were about 25 and 10% that found in the proximal intestine, respectively. To investigate the effect of diet on MTP, hamsters were maintained for 31 days on one of four diets : 1) control low fat, 2) high fat, 3) low fat, high sucrose, or 4) die t 1 followed by a 48-h fast. The high fat diet increased MTP large sub unit mRNA levels in the liver and throughout the small and large intes tine. A 55 and 126% increase was observed in the liver and intestine ( duodenum and jejunum), respectively. A 40% increase of intestinal MTP protein mass was also observed. The high sucrose diet caused a signifi cant 55% increase in hepatic MTP mRNA levels but did not significantly affect the intestinal mRNA levels. MTP mRNA levels were unchanged in response to fasting. A short term dietary study showed that intestinal MTP mRNA was up-regulated within 24 h after initiating a high fat die t. An acute hepatic response was not observed. The regulation of MTP m RNA levels by high fat diets was compared to that of the liver fatty a cid binding protein (L-FABP) and apolipoprotein B (apoB). ApoB mRNA le vels were not significantly affected by a high fat diet. Although L-FA BP mRNA levels were increased in the Liver and intestine, the onset of the changes did not parallel that of MTP. These results suggest that L-FABP, apoB, and MTP, three proteins which play important roles in th e transport of fatty acids and triglyceride in the liver and intestine , are not coordinately regulated by diet in hamsters.