Mechanism of free fatty acid re-esterification

Within adipose tissue, free fatty acids liberated by lipolysis may be re-esterified into newly synthesized triacylglycerol. We hypothesized that re-esterification may occur via an extracellular route, such that free fatty acids arising from lipolysis must leave the adipocyte and be taken up again before they can be re-esterified. We simultaneously measured rates of lipolysis, acylglycerol synthesis, and free fatty acid re-esterification in human adipose tissue and isolated adipocytes in vitro, utilizing a dual-isotopic technique. We manipulated incubations to increase mixing of released free fatty acids with the incubation medium. Such manipulations should decrease the probability that released free fatty acids would be taken up and re-esterified. We found that re-esterification was decreased in isolated adipocytes compared to fragments of tissue, in shaken compared to unshaken incubations, and in low adipocyte concentrations compared to high adipocyte concentrations. Rates of acylglycerol synthesis and lipolysis were unaltered by these manipulations, indicating that changes in free fatty acid re-esterification are not secondary to effects on these processes. The results are consistent with an extracellular route for free fatty acid reesterification. Such a mechanism suggests that adipose tissue blood flow may play an important role in the regulation of free fatty acid release from adipose tissue.-Edens, N. K., R. L. Leibel, and J. Hirsch. Mechanism of free fatty acid reesterification in human adipocytes in vitro. J. Lipid Res. 1990. 31: 1423-1431. Supplementary key words terenol adenosine lipid metabolism lipolysis adipose tissue triacylglycerol isoproadipose tissue blood flow In adipose tissue, stored triacylglycerol (E) undergoes continuous, simultaneous synthesis and breakdown. Glycerol arising from lipolysis cannot be reutilized, since human adipose tissue has negligible glycerokinase activity (1). However, newly hydrolyzed free fatty acids (FFA) can be activated and re-esterified into newly-synthesized TG. In this report, "FFA re-esterification'' refers specifically to this cycle of TG hydrolysis and resynthesis within adipose tissue. Previous investigations into the regulation of FFA reesterification in vitro have used the balance technique (2), in which re-esterification is calculated from the difference between glycerol and FFA release (3-6). This method does not distinguish between FFA re-esterification and TG synthesis. Studies that measured both processes simultaneously did not provide FFA in the incubation medium (5, 7), and so do not accurately mimic the situation in vivo, in which FFA substrate may be derived from plasma, either from hydrolysis of circulating lipoproteins or from albumin-bound FFA. Another study in which both processes were measured simultaneously did not indicate how much FFA was provided by the human serum albumin used in the incubation medium (8) so that FFA re-esterification cannot be distinguished from total TG synthesis. Whether FFA re-esterification may be regulated independently from TG synthesis is unknown but can be addressed by the recently developed dual-isotopic technique for measuring FFA re-esterification in adipose tissue (9). This method allows TG synthesis and re-esterification to be measured independently and simultaneously. Studies using this technique have suggested that the rate of FFA re-esterification is not regulated at the cellular level, but rather by changes in the extracellular environment (10). A model of FFA re-esterification consistent with this idea is that FFA are not re-esterified primarily by an intracellular pathway, but must leave the adipocyte and be taken up again before being re-esterified. To test this hypothesis, we examined the influence of degree of mixing of released FFA with incubation medium on the rate of re-esterification. Such mixing should specifically affect re-esterification only if FFA arising from lipolysis must leave the adipocyte and re-enter before being available for reesterification. The results of three different types of experiments designed to test this hypothesis indicated that I ) the rate of re-esterification was higher in intact fragments of adipose tissue compared to isolated adipocytes, 2) the rate of Abbreviations: TG, triacylglycerol; FFA, free fatty acid; ADA, adenosine deaminase; PIA, N6-phenylisopropyl adenosine; BSA, bovine serum albumin. 'To whom correspondence should be addressed. Journal of Lipid Research Volume 31, 199