MECHANISM OF LIPID MOBILIZATION ASSOCIATED WITH CANCER CACHEXIA - INTERACTION BETWEEN THE POLYUNSATURATED FATTY-ACID, EICOSAPENTAENOIC ACID, AND INHIBITORY GUANINE NUCLEOTIDE-REGULATORY PROTEIN

During a study of the mechanism of cancer cachexia, a debilitating condition in which catabolism of host muscle and adipose tissue occurs, it has been observed that the process can be effectively reversed in vivo by the polyunsaturated fatty acid, eicosapentaenoic acid (EPA), but not by other PUFA of either the n-3 or n-6 series. In vitro studies showed that EPA blocked the action of a tumour-produced catabolic factor at the level of the adipocyte, and that the effect of EPA also extended to beta-adrenergic stimuli and polypeptide hormones. Again the effect was specific to EPA and appeared to arise from an inhibition of the elevation of cyclic AMP levels in adipocytes in response to varied stimuli. Using isoprenaline stimulated lipolysis as a model system we have shown that EPA has a direct inhibitory effect on isoprenaline-stimulated adenylate cyclase in isolated plasma membrane fractions with half maximal inhibition at a concentration of 165 muM. The inhibitory effect was specific for EPA and was not shown by docosahexaenoic or arachidonic acids. The inhibitory effect of EPA on adenylate cyclase showed properties similar to hormonal inhibition of the enzyme in that it was (i) GTP-dependent, (ii) non-competitive with isoprenaline, (iii) eliminated following treatment of either adipocytes or plasma membrane fractions with pertussis toxin, which is known to ADP-ribosylate the alpha-subunit of an inhibitory guanine nucleotide-regulatory protein (Gi), thus leading to its inactivation. This suggests that inhibition of cyclic AMP formation by EPA was due, at least in part, to a Gi-mediated inhibition of adenylate cyclase activity. Reduction in lipolysis by EPA in response to beta-adrenergic stimuli, could lower the output of free fatty acids and thus reduce hepatic triglyceride synthesis.