Effect of in-vitro lifespan of 3T3-L1 cells on hormonal responsiveness of lipoprotein lipase activity.

The synthesis and secretion of lipoprotein lipase activity is one marker of the adipocyte state in cultured 3T3-L1 cells. Lipoprotein lipase in these cells is responsive to hormones (insulin and GIP); however, this has not been a constant feature. To determine if in-vitro lifespan or passage number contributes to this responsiveness, cells over an in-vitro lifespan of nine to 22 passages were examined for insulin- and GIP-mediated increases in enzyme synthesis and secretion. Differences in GIP responsiveness between the parent line and 3T3-L1 subclones were also tested. Insulin-mediated release of lipase into the culture medium (P less than 0.02) and the stimulation of heparin-releasable activity from cells by insulin (P less than 0.05) were inversely related to passage number. The responsiveness to GIP also decreased with increasing in-vitro lifespan. Differences between the parent cell and subclones were also seen for GIP. The loss of responsiveness in these cells appears to be associated with an increase in confluent density. There was no relationship between basal lipoprotein lipase activity and passage number. Therefore, the effect of insulin and GIP on lipoprotein lipase activity in 3T3-L1 cells is not a stable property and some caution is required before applying data obtained from these cells to the in-vivo situation.