The effect of nutritional state on the lipoprotein lipase activity of isolated fat cells.

The lipoprotein lipase activity of rat epididymal adipose tissue falls on starvation and increases on refeeding. Studies with fat cells isolated from this tissue have shown that increases in the activity of the enzyme occur under appropriate incubation conditions in vitro. The present study compares the responses of cells isolated from the adipose tissue of fed and 48-h starved rats. Fat cells from rats starved for 48 h display a lower initial lipoprotein lipase activity than cells from fed rats. When cells from rats in both nutritional states are incubated in a suitable medium at 25 degrees C, there is a progressive increase in the medium lipoprotein lipase activity. The absolute increase in the total activity of the incubation system during incubations of cells from 48-h starved rats is significantly less than during incubations of cells from fed rats. However, when expressed as a percentage of the initial cell activity, the rises in total activity are similar in the two nutritional states. Cycloheximide has no significant effect on the increase in activity of lipoprotein lipase that occurs with cells from 48-h starved rats. However, it does partially block the increase in activity seen with cells from fed rats and in a manner similar to that previously reported for cells from 24-h starved rats. The significance of the results is discussed in relation to previous studies with both intact adipose tissue and isolated fat cells.

Post-translational regulation of lipoprotein lipase activity in adipose tissue.

Changes in adipose-tissue lipoprotein lipase activity that are independent of protein synthesis were investigated in an incubation system in vitro. Under appropriate conditions at 25 degrees C a progressive increase in the enzyme activity occurs that is energy-dependent. Part of the enzyme is rapidly inactivated when the tissue is incubated with adrenaline or adrenaline plus theophylline. The mechanism of this inactivation appears to be distinct from, and to follow, the activation of the enzyme. A hypothesis is presented to account for the results in terms of an activation of the enzyme during obligatory post-translational processing and a catecholamine-regulated inactivation of the enzyme as an alternative to secretion from the adipocyte.