REGULATION OF INTESTINAL APO A-IV MESSENGER-RNA ABUNDANCE IN RAT PUPS DURING FASTING AND REFEEDING

The amount of intestinal apolipoprotein (apo) A-IV mRNA was examined in rat pups during fasting and refeeding. When 14-day old pups were fasted for 15 h, apo A-IV mRNA levels in the whole intestine decreased to 20% of the prefasting level. Refeeding casein and lactose, and the artificial milk composed of Intralipid, casein and lactose, caused an elevation of the apo A-IV mRNA after 3 h, without accompanying an elevation of serum triacylglycerols and apo A-IV (fat-independent elevation of apo A-IV mRNA). Refeeding Intralipid alone simultaneously elevated the apo A-IV mRNA, and serum triacylglycerols and apo A-IV after 3 h (fat-dependent elevation of apo A-IV mRNA). Administration of physiological saline during fasting partly suppressed the reduction of the apo A-IV mRNA (40% of the prefasting level), and the dietary fat-independent elevation of the message disappeared. Refeeding dam's milk to the pups, fasted without water administration, increased the apo A-IV mRNA after 3 and 15 h, although the elevation of serum triacylglycerols and apo A-IV occurred only after 15 h. Refeeding the milk increased the apo A-IV mRNA after 3 h and 15 h. Refeeding dam's milk to the pups fasted with saline administration accelerated the fat-dependent elevation of the apo A-IV mRNA. Simultaneously refeeding Intralipid and Pluronic L-81, an inhibitor of lymphatic fat transport, delayed the elevation of the apo A-IV mRNA and serum triacylglycerols and apo A-IV. Transcription rates of the apo A-IV mRNA, determined by nuclear run/on assay, were similar before and after fasting and refeeding Intralipid. During fasting, administration of puromycin, as compared with actinomycin D, enhanced the disappearance rate of the apo A-IV message. Intestinal mRNA for apo B, but not for apo A-I and beta-actin, similarly changed to the apo A-IV message. Thus, it can be concluded that: (1) dietary fat-dependent and -independent factors are involved in the elevation of the intestinal apo A-IV message; (2) the elevation of the message is not mediated by lipid uptake in the enterocytes but rather stimulated by the events leading to secretion of chylomicrons; and, (3) dietary fat-dependent elevation of the message appears to be due to the stabilization of the message.