Degradation of short-lived proteins is decreased by centrifugation

Effects of polyamine levels on the degradation of short-lived and long-lived proteins in cultured L-132 human lung cells

Biogenic polyamines have important regulatory functions in various biological processes and it has also been suggested that they could modulate intracellular protein degradation. For an overall assessment of the role of polyamines in this process, we have investigated the effect that the decrease in intracellular polyamine levels caused by inhibitors of polyamine biosynthesis brings about on the degradation of the pools of short- and long-lived proteins in cultured L-132 human lung cells. Treatment of cells with 100 microM (2R,5R)-delta-methyl acetylenic putrescine (MAP), a potent enzyme-activated irreversible inhibitor of ornithine decarboxylase, or with 100 microM MAP plus 50 microM N-butyl 1,3-diaminopropane, a specific inhibitor of spermine synthase, caused a similar decrease (65-70% of control) in the total intracellular levels of polyamines, although they affected the concentrations of spermidine and spermine differently. The effect of the two treatments on protein degradation was essentially the same. In polyamine-depleted cells we observed an inhibition of degradation in long-lived proteins of 16% (P<0.05), with a significant increase in the half-life (t12) of this pool from 100.5 to 120.1 h. This was concomitant with an increase of 26% (P<0. 05) in degradation in short-lived proteins, with a significant decrease in the t12 of this pool from 0.85 to 0.67 h. Recovery of polyamine levels by the addition of 50 microM spermidine to polyamine-depleted cells resulted in a restoration of the degradation rates in both pools of proteins. The way(s) by which polyamines could modulate proteolysis are discussed.

Endocytosis of liposomes containing lysosomal proteins increases intracellular protein degradation in growing L-132 cells

We have used a new approach to test the possible participation of lysosomes in the degradation of long-lived proteins. Rat liver lysosomal proteins were introduced, via multilamellar liposomes, into L-132 cells. Viability and protein synthesis were not impaired by this treatment. The liposomal content was released into the lysosomes of the cultured cells, as revealed by ferritin uptake and electron microscopy. Degradation rates of long-lived proteins increased with the uptake of lysosomal proteases. However, the increased protein degradation of chloroquine and leupeptin, in contrast to the inhibition by these reagents of the increased protein degradation of cells 'starved' of serum (step-down conditions). This approach opens a new way of investigating the degradation of intracellular proteins in cultured cells.

Effects of centrifugation on the degradation of short-lived proteins in exponentially growing cultured cells

The degradation mechanisms of short-lived proteins in cultured cells are unknown, probably due to the lack of procedures which specifically affect the degradation of these proteins. We found that centrifugation of cultured cells, growing either in monolayer or in suspension, between 5000 and 25,000g for 30 min, inhibits (more than 50%) the degradation of short-lived proteins but not of long-lived proteins. Protein synthesis or cell viability is not affected. Centrifugation also disorganizes the Golgi apparatus, as checked by routine electron microscopy, and inhibits the degradation of endocytosed proteins (a lysosomal process which is controlled by the Golgi apparatus). Using different centrifugation speeds, a good correlation was found between alteration of the Golgi apparatus and inhibition of protein degradation.

Vanadate inhibits degradation of short-lived, but not of long-lived, proteins in L-132 human cells

Vanadate, at concentrations higher than 0.04 mM, inhibits the intracellular degradation of short-lived proteins in exponentially growing L-132 human cells. The inhibition is not due to a decrease in viability or in the ATP contents of the cells. Since vanadate decreases proteolysis in cell extracts, the inhibition appears to affect the proteinases which degrade these proteins. Under optimal nutritional conditions, the degradation of long-lived proteins is accelerated by vanadate, thus providing additional evidence that in exponentially growing cultured cells degradation of short- and long-lived proteins occurs by different processes. Vanadate also efficiently inhibits the lysosomal degradation of endocytosed proteins and of long-lived proteins under step-down conditions. However, this effect seems to be unrelated to the observed inhibition of degradation of short-lived proteins, because chloroquine and leupeptin, which inhibit degradation of proteins by lysosomes, do not modify the degradation of these proteins. Our results provide for the first time a probe which, owing to its opposite effects on the degradation of short- and long-lived proteins, could be useful to clarify the mechanisms involved in protein degradation in cultured cells.