Overexpression of apolipoprotein A-IV enhances lipid secretion in IPEC-1 cells by increasing chylomicron size

Intestinal apolipoprotein A- IV expression is highly regulated by dietary lipid in newborn swine, suggesting a role in lipid absorption. Constitutive overexpression of apoA- IV in newborn swine enterocytes enhances basolateral secretion of triacylglycerol ( TG) in TG-rich lipoproteins 4.9- fold ( Lu, S., Yao, Y., Meng, S., Cheng, X., and Black, D. D. ( 2002) J. Biol. Chem. 277, 31929 - 31937). To investigate the mechanism of this enhancement, IPEC- 1 cells were transfected with a tetracycline- regulatable expression system ( Tet- On). In cells incubated with oleic acid, a dose response relationship was observed between medium doxycycline concentration and basolateral apoA- IV and TG secretion. Similarly regulated expression of apoA- I did not enhance lipid secretion. The mean diameter of TG-rich lipoproteins secreted from doxycycline- treated cells was larger than from untreated cells ( 87.0 nm versus 53.4 nm). Basolateral apoB secretion decreased. Using the same expression system, full-length human apoA-IV ( 376 amino acids); a pig- like human apoA-IV, lacking the C- terminal EQQQ repeats ( 361 amino acids); and a chicken- like apoA- IV, further truncated to 343 amino acids, were expressed in IPEC- 1 cells. With increasing protein secretion, cells expressing the full- length human apoA- IV displayed a 2- fold increase in TG secretion; in sharp contrast, cells expressing the piglike human apoA- IV displayed a 25- fold increase in TG secretion and a 27- fold increase in lipoprotein diameter. When human apoA- IV was further truncated to yield a chicken- like protein, TG secretion was inhibited. We conclude that overexpression of swine apoA- IV enhances basolateral TG secretion in a dose- dependent manner by increasing the size of secreted lipoproteins. These data suggest that the region in the human apoA- IV protein from residues 344 to 354 is critical to its ability to enhance lipid secretion, perhaps by enabling the packaging of additional core TG into chylomicron particles. The EQQQ- rich region may play an inhibitory or modulatory role in chylomicron packaging in humans.