October 4, 2006 (Los Angeles, CA) – New experimental data has shown that apolipoprotein A-1 (apoA-1) Milano provides superior atheroprotective effects compared with wild-type apoA-1 [ 1]. In a unique head-to-head gene-transfer study in mice, researchers showed that apoA-1 Milano, a naturally occurring mutant of apoA-1, is significantly more effective than wild-type apoA-1 for reducing atherosclerosis and plaque inflammation.
"There have never been any comparative studies between wild-type apoA-1 and apoA-1 Milano, even through indirect evidence seemed to suggest that apoA-1 Milano was more effective," senior investigator
Dr Prediman Shah (University of California, Los Angeles [UCLA]) told
heartwire . "Some of the scientists in the field felt that in the absence of any clear-cut evidence maybe there wasn't that much of a difference between the two. So we wanted to investigate this in a head-to-head comparison. Although we had an intuitive feeling it was better, this is the first evidence to suggest that the Milano mutant is more effective, almost two-and-a-half times more effective, for reducing atherosclerosis when using the gene-transfer strategy."
The results of the study are published in the October 3, 2006 issue of the
Journal of American College of Cardiology.
Assessing the relative merits of apoA-1 and apoA-1 Milano
Previous studies have shown that intravenous recombinant apoA-1 Milano inhibits the progression and induces the regression and remodeling of atherosclerotic plaque in animal models, but it was a phase 2 trial in acute coronary syndrome patients that generated a great deal of excitement, both in the media and cardiology circles. In that study, conducted by
Dr Steven Nissen (Cleveland Clinic, OH), once-weekly injections of recombinant apoA-1 Milano induced rapid coronary atheroma regression after five weeks [
heartwire , Shah, who pioneered the early animal studies using apoA-1 Milano, noted that while the study by Nissen and colleagues showed that the intravenous infusions could induce atherosclerosis regression and remodeling, gene transfer, as a standalone or complementary approach, might be an option to remove some logistical issues associated with injections and large-scale production of recombinant apoA-1 Milano.
In this study, led by
Dr Lai Wang (UCLA), investigators performed bone-marrow transplantation in female double-knockout mice lacking both the apoE and apoA-1 genes using male donor mice-derived bone marrow. The bone-marrow cells were transduced with a retrovirus expressing apoA-1 or apoA-1 Milano, and then the female mice were injected with the transduced bone-marrow cells. A macrophage-specific promoter was used to target the expression of the genes to the macrophages, and testing indicated that the mice expressing apoA-1 or apoA-1 Milano showed strong immunoreactivity for the corresponding proteins, mostly at the base of lesions and around the lipid cores.
Regarding the effect on atherosclerosis, compared with control mice, apoA-1 Milano gene therapy significantly reduced aortic atherosclerosis by 65% and plaque macrophage immunoreactivity--a measure of inflammation--by 58%. In contrast, wild-type apoA-1 reduced atherosclerosis by 25% (p=0.1) and plaque macrophage immunoreactivity by 23% (p<0.05). In a direct head-to-dead comparison, apoA-1 Milano was significantly more effective in reducing atherosclerosis and inflammation compared with wild-type apoA-1.
The circulating levels of cholesterol, lipoprotein levels, and apoA-1 Milano or wild-type apoA-1 were similar in the two treatment groups, the group reports, adding that the antiatherogenic activity of apoA-1 Milano and apoA-1 was observed despite low levels of systemic apoA-1 and apoA-1 Milano, both being approximately 100 ng/dL.
"The beauty of this approach is that we have very little circulating levels of the
," said Shah. "The circulating levels of the wild-type and Milano are miniscule and barely measurable. Yet there is a profound effect because it is being produced in the macrophage itself. The gene is being translated within the macrophages in the lesion, supporting the concept that lesion-specific expression of even low levels can have a very profound antiatherogenic effect."
Reverse cholesterol transport appears key, but other factors might be involved
Investigators also examined the ability of both apoA-1 and apoA-1 Milano to move cholesterol from macrophages within the arterial lesions. They found that while both promoted cholesterol efflux, the removal of cholesterol from the macrophages was significantly greater in cells expressing apoA-1 Milano. Shah noted the while reverse cholesterol transport from foam cells is the most commonly known biological action of
, other biological actions, such as anti-inflammatory and antioxidative effects, have not been explored fully, although previous data have shown that apoA-1 Milano is more effective as an antioxidant than wild-type apoA-1.
Shah said the study shows that gene transfer is a feasible and efficacious approach for exploiting the benefits of apoA-1 Milano, although bone-marrow transplantation might not necessarily be the way gene therapy will be used in every case. Other studies with other vectors, such intramuscular and intravenous injections of apoA-1 Milano given with an adeno-associated virus, can also reduce atherosclerosis in animal models. These techniques are still being refined for future studies in humans, said Shah.
Commenting on the results of the study for
heartwire , Nissen reiterated that there has been debate over whether apoA-1 Milano is better than ordinary HDL cholesterol, but this study suggests that the mutant Milano provides superior antiatherogenic effects.
"I have always thought that there was something special about Milano protein, but a lot of people didn't believe it," said Nissen. There had been speculation, he added, that atherosclerotic regression could have been achieved in his group's 2003 study with ordinary HDL cholesterol, but this study suggests that that is not the case. This study suggests that there is something special about apoA-1 Milano, although just what makes apoA-1 Milano so special has not yet been determined, said Nissen.
Wang L, Sharifi BG, Pan T, et al. Bone marrow transplantation shows superior atheroprotective effects of gene therapy with apolipoprotein A-1 Milano compared with wild-type apolipoprotein A-1 in hyperlipidemic mice. J Am Coll Cardiol 2006; 48:1459-68.
Nissen SE, Tsunoda T, Tuzcu EM, et al. Effect of recombinant ApoA-I Milano on coronary atherosclerosis in patients with acute coronary syndromes: a randomized controlled trial. JAMA 2003; 290:2292-300.
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