Absence of lipoprotein lipase in cultured human adipose stromal cells

To develop a system for studying the regulation of adipose tissue lipoprotein lipase (LPL) in human cells, we isolated cells from the stomalvascular fraction of human adipose tissue and propagated them in tissue culture. These cells were cultured in medium containing fetal bovine, horse, or human sera in the absence or presence of aminoglycoside antibiotics. After the cells had been at confluence for at least 1 week, they were exposed to medium containing insulin, 3-isobutyl-l-methylxanthine (IBMX), and dexamethasone, in an attempt to induce differentiation into cells which produce LPL. Stromal cells in 62 cultures from 20 subjects were monitored for any LPL activity secreted into the culture medium. In addition, cultures were assayed for activity releasable with heparin, and extractable from cell digests in nonionic detergent. Upon reaching confluence, the human adipose tissue stromal cells began to accumulate lipid droplets. However, no consistent LPL activity was measured in the culture medium or after exposure to heparin. Five cultures contained a minimal amount of detergent extractable hydrolytic activity. In contrast, cultures of rat stromal cells not exposed to insulin, IBMX, and dexamethasone, contained assayable LPL in the culture medium (3.3 ± 1.6 nEq/min/ml) and in the heparin-releasable fraction (1.7 ± 0.5 nEq/min/10⁶ cells). Thus, human adipose tissue stromal cells do not represent a useful system for studying the regulation of LPL. Although these cells accumulate some lipid, the absence of measurable LPL suggests that complete differentiation has not yet occurred.