Proteolytic degradation and impaired secretion of an apolipoprotein A-I mutant associated with dominantly inherited hypoalphalipoproteinemia

We have devised a combined in vivo, ex vivo, and in vitro approach to eluci date the mechanism(s) responsible for the hypoalphalipoproteinemia in heter ozygous carriers of a naturally occurring apolipoprotein A-I (apoA-I) varia nt (Leu(159) to Arg) known as apoA-I Finland (apoA-I-FIN). Adenovirus-media ted expression of apoA-I-FIN decreased apoA-I and high density lipoprotein cholesterol concentrations in both wild-type C57BL/6J mice and in apoA-I-de ficient mice expressing native human apoA-I (hapoA-I), Interestingly, apoA- I-FIN was degraded in the plasma, and the extent of proteolysis correlated with the most significant reductions in murine apoA-I concentrations. ApoA- I-FIN had impaired activation of lecithin:cholesterol acyltransferase in vi tro compared with hapoA-I, but in a mixed lipoprotein preparation consistin g of both hapoA-I and apoA-I-FIN there was only a moderate reduction in the activation of this enzyme. Importantly, secretion of apoA-I was also decre ased from primary apoA-I-deficient hepatocytes when hapoA-I was co-expresse d with apoA-I-FIN following infection with recombinant adenoviruses, a cond ition that mimics secretion in heterozygotes. Thus, this is the first demon stration of an apoA-I point mutation that decreases LCAT activation, impair s hepatocyte secretion of apoA-I, and makes apoA-I susceptible to proteolys is leading to dominantly inherited hypoalphalipoproteinemia.