Hormone-Sensitive Lipase in Human Adipose Tissue, Isolated Adipocytes, and Cultured Adipocytes

Abstract

Summary: Employing a perifusion technique, the activity of hormone-sensitive lipase (HSL) in human adipose tissue and isolated adipocytes was found to be significantly stimulated by 5 μM epinephrine (5-fold, P < 0.001) or 0.5 mM theophylline (2-fold, P < 0.03). The stimulatory effect of both, however, was not cumulative; instead, theophyilline appeared to blunt the epinephrine effect. The two effectors together produced a 3-fold increase in activity over basal (P < 0.01). Basal lipolysis was highly and significantly correlated with epinephrine and/or theophylline stimulated lipolysis in the tissue as well as in the isolated cells. The activity of the enzyme was examined in cultured human adipocytes using a cell-free system. The basal activity of HSL in the 20,000 x g supernatant (S₂₀) fraction of cultured cells grown in fat-enriched medium was significantly higher than the value in the same fraction of cells grown in regular medium (6.06 ± 1.49 versus 2.78 ± 0.89 nmole glycerol/min/mg protein, mean ± S.D., P < 0.01), and was similar in the S₂₀ fractions of cells grown in the enriched medium and the original tissue (6.06 ± 1.49 versus 5.44 ± 2.73 nmole glycerol/min/mg protein, mean ± S.D., P > 0.83). When the tissue and cells were stimulated in vitro before fractionation, the HSL activity in the S₂₀ fraction of the original tissue increased 4-fold over basal (P < 0.001), whereas that in the cultured cells increased 2-fold (P < 0.01) regardless of the culture medium employed. In the S₂₀ fractions derived from basally incuhated samples, but not in those derived from the epinephrine-stimulated samples, the HSL enzyme was slightly but significantly (P < 0.002) activated by exogenous addition of ATP, cyclic AMP, and protein kinase. The data suggest that the activity of HSL is retained in cultured human adipocytes and could be enhanced under conditions of culture which favor lipid accumulation and adipose conversion. Speculation: Although the hormone-sensitive lipase enzyme protein in adipose tissue of the rat and the chicken has been extensively purified, the human enzyme remains difficult to purify. The finding in this study, that human adipose tissue hormone-sensitive lipase activity is retained in culture, should facilitate the purification and characterization of the enzyme in man, promote the study of the cellular mechnisms involved in the synthesis, transport and degradation of the enzyme and enhance the search for possible genetic heterogeneity of the enzyme among individuals suspected to have genetic obesity.