In the context of obesity, body fat distribution is an important predictor of disease risk, but what creates regional variations in body fat distribution in not known. We have largely ruled out the likelihood that regional imbalances in lipolysis or meal fatty acid uptake are primarily responsible for body fat distribution differences. We found that meal fatty acid disposal into upper and lower body subcutaneous and visceral fat does not follow patterns to indicate that LPL mediated fat storage largely determines fat distribution, even when challenged by a high fat meal. We did find new evidence that human adipocytes can take up and store FFA directly from the circulation. The unique aspect of this process, in contrast to meal fatty acid storage, is that it varies in the direction that could create or maintain sex and obesity phenotype differences in fat distribution. Specifically, the efficiency of direct FFA uptake/storage in subcutaneous fat is 70 % greater in women than men (women preferentially store fat in subcutaneous compartments). In addition, while men maintain a pattern of preferential direct FFA uptake/storage in abdominal vs. femoral fat, women do not; direct FFA uptake in femoral fat equals or exceeds abdominal fat. We believe this direct FFA uptake may be related to facilitated fatty acid transport into adipocytes or differences in intracellular fatty acid sequestration processes.
The specific aims of this proposal are to: 1) Assess whether regional differences in subcutaneous and visceral adipose tissue direct FFA uptake/storage in men vs. women and subcutaneous uptake in upper body obese men vs. upper body obese women vs. lower body obese women vary in a manner consistent with fat distribution differences, and whether fatty acid transport protein or intracellular sequestration pathways relate in a similar way. 2) Determine whether chronic vs. acute estrogen deficiency in women is associated with regional differences in subcutaneous adipose tissue direct FFA uptake/storage or LPL/meal fatty acid storage in a manner consistent with fat distribution differences, and whether fatty acid transport protein or intracellular sequestration pathways relate in a similar way. 3) Examine whether chronic vs. acute testosterone deficiency in men is associated with regional differences in subcutaneous adipose tissue direct FFA uptake/storage or LPL/meal fatty acid storage in a manner consistent with fat distribution differences, and whether fatty acid transport protein or intracellular sequestration pathways relate in a similar way. 4) Evaluate whether suppression of lipolysis with insulin vs. long-acting nicotinic acid increases the efficiency of direct FFA uptake/storage in subcutaneous adipose tissue and whether membrane bound fatty acid transport proteins or intracellular sequestration pathways are altered in ways that can explain greater direct uptake. Completing these studies should provide firm evidence as to what types of cellular/molecular processes account for the physiologic observations and lead directly to productive models to prove cause and effect.
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