Obesity is a major public health problem for which current therapies are largely unsuccessful. Work in our laboratory has provided new insights into how fat accumulates in adipose tissue. These insights, in turn, suggest a potential new strategy for management of obesity. Specifically, we have shown that uptake of long chain free fatty acids (FFA) by selected cell types, including adipocytes, is not passive, as previously thought, but is a highly regulated transport process. Studies in adult homozygous (falfa) Zucker fatty rats and in the Zucker Diabetic Fatty (ZDF) strain have shown striking 9- to 13-fold increases in the capacity (Vmax) of adipocytes to take up FFA. By contrast, FFA uptake is unchanged in hepatocytes and only modestly increased in cardiac myocytes from these strains. The effect of these changes is to divert FFA progressively from tissues in which they are oxidized as fuel to adipose tissue where they are stored as fat. Studies in weanling animals reveal that the increase in adipocyte FFA uptake in Zucker pups procedes onset of obesity, increased plasma FFA, or increased adipocyte TNFalpha expression. While these conclusions are independent of the role of any particular FFA transporter, the increase in FFA uptake Vmax in Zucker adipocytes is highly correlated with mRNA levels for one of three cloned, putative FFA transporters, plasma membrane fatty acid binding protein. To determine if similar changes in FFA transport occur in diet-induced obesity, preliminary studies were conducted in three rat strains fed either a control or high fat (35 percent lard, 55 percent of calories) diet. There were no fat-induced changes in the FFA uptake Vmax in adipocytesof S 5B/Pl rats, but Vmax increased significantly in Sprague-Dawley and Osborne Mendel animals. Changes in adipocyte Vmax correlated with weight gain (r=0.91, p less than 0.01). Hepatocyte Vmax did not increase in any strain. The presence of tissue specific physiologic regulation of FFA uptake suggests that tissue-specific pharmacologic manipulatin might be feasible, reducing the propensity of adipocytes to store FFA as fat. To exploit this strategy requires that the physiological regulators of FFA uptake be determined. Preliminary data suggest that leptin may be a critical regulator of this process. We therefore have expressed rat leptin in the baculovirus system for use in future studies. Thus, the goals of this proposal are: to compare FFA uptake kinetics with other variables of interest in progressively younger pre-weaned falfa rats, to elucidate the evolution of the fatty phenotype; to better characterize the FFA transport and leptin responses to high fat intake in the rat models described; to define the effects of exogenous leptin on FFA uptake in different tissues and on specific FFA transporters in the rat; and to study FFA transport in normal and obese human adipocytes.
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