Intracellular protein degradation in growing, in density-inhibited, and in serum-restricted fibroblast cultures

Protein synthesis and degradation during expression of the temperature-sensitive defect in ts A1S9 mouse L-cells

The involvement of altered protein metabolism in the expression of the temperature-sensitive (ts) pleiotropic phenotype of ts A1S9 cells was investigated. Cells are ts in growth and DNA replication. They undergo decondensation of their heterochromatin, interruptions of chromatin synthesis, and changes in cell size and morphology at the non-permissive temperature (npt) of 38.5 degrees C. Whereas the rates of incorporation of 3H-leucine, 35S-methionine, and 3H-fucose into proteins were unaffected at 38.5 degrees C, net protein accumulation was greatly reduced. This imbalance resulted from a rapid increase in the rate of protein degradation at the npt. Enhancement of protein degradation was detected within 2-4 hours after temperature upshift and constitutes the earliest metabolic alteration thus far observed during expression of the temperature-sensitive phenotype. The average half-life of proteins performed in ts A1S9 cells at 34 degrees C was decreased four-fold at the npt, and all major cytoplasmic proteins were affected equally. Enhanced protein degradation at the npt was shown to be sensitive to cycloheximide, ammonia, chloroquine, and vinblastine at concentrations that did not affect the basal protein degradation of normally cycling cells. Increased protein degradation at 38.5 degrees C did not involve an equivalent increase in total cellular protease activity. The data obtained are compatible with a model that suggests that temperature inactivation of the ts A1S9 gene product results in activation of a lysosome-mediated mechanism for the rapid degradation of cytoplasmic proteins.

Protein degradation in skin fibroblasts from patients with Duchenne muscular dystrophy

The rates of degradation of [3H]leucine-labelled proteins have been measured in cultures of skin fibroblasts obtained from normal controls (five subjects) and patients with Duchenne muscular dystrophy (six subjects). Cultures were incubated with [3H]leucine (10 microCi/ml) for 60 min to label "short-lived" proteins, and with [3H]leucine (5 microCi/ml) for 60 h to label "long-lived" proteins. Optimal wash procedures were devised for removal of [3H]leucine from the extracellular space and from cell pools before beginning degradation measurements. Re-utilization of [3H]leucine released from degraded labelled proteins was prevented by supplementing the medium with 4mM-leucine. Rates of degradation did not depend on the growth state of the cells or on cell age over the range used (passages eight-20). Degradation of long-lived proteins was approximately linear over a 24h period, at a rate of 1.0% per h. 30% of short-lived protein was degraded within 6h. No differences were observed between protein degradation in normal fibroblasts and in those from patients with Duchenne muscular dystrophy.

Amino acid inhibition of the autophagic/lysosomal pathway of protein degradation in isolated rat hepatocytes

Protein degradation in isolated rat hepatocytes, as measured by the release of [14C]valine from pre-labelled protein, is partly inhibited by a physiologically balanced mixture of amino acids. The inhibition is largely due to the seven amino acids leucine, phenylalanine, tyrosine, tryptophan, histidine, asparagine and glutamine. When the amino acids are tested individually at different concentrations, asparagine and glutamine are the strongest inhibitors. However, when various combinations are tested, a mixture of the first five amino acids as well as a combination of leucine and asparagine inhibit protein degradation particularly strongly. The inhibition brought about by asparagine plus leucine is not additive to the inhibition by propylamine, a lysosomotropic inhibitor; thus indicating that the amino acids act exclusively upon the lysosomal pathway of protein degradation. Following a lag of about 15 min the effect of asparagine plus leucine is maximal and equal to the effect of propylamine, suggesting that their inhibition of the lysosomal pathway is complete as well as specific. Degradation of endocytosed 125I-labelled asialofetuin is not affected by asparagine plus leucine, indicating that the amino acids do not affect lysosomes directly, but rather inhibit autophagy at a step prior to the fusion of autophagic vacuoles with lysosomes. The aminotransferase inhibitor, aminooxyacetate, does not prevent the inhibitory effect of any of the amino acids, i.e. amino acid metabolites are apparently not involved.

The effect of protease inhibitors and decreased temperature on the degradation of different classes of proteins in cultured hepatocytes

Leupeptin, chymostatin and antipain inhibited the degradation of long-lived proteins in cultured rat hepatocytes by 20-30%, probably by inhibiting lysosomal proteases: (1) Leupeptin and chymostatin decreased to a similar extent the degradation of an exogenous protein 125I-asialo fetuin, a process known to occur within lysosomes. (2) In extracts of cells treated with leupeptin, cathepsin B activity was inhibited by 35-50%. (3) Leupeptin, chymostatin and antipain inhibited proteolysis by homogenates of liver lysosomes but not by the supernatant fraction. These agents, however, do not appear to rapidly permeate the membrane of isolated lysosomes. Leupeptin, chymostatin and antipain did not inhibit the breakdown of short-lived normal cell proteins, and ones containing amino acid analogs. Even when the amount of abnormal proteins was increased, such that it comprised a large fraction of cell protein, the degradation of these polypeptides was still very rapid and not affected by these inhibitors. The pathway for the degradation of short-lived cell proteins thus appears distinct from that responsible for degradation of long-lived cell proteins. In accord with this conclusion, reduction of the temperature of cultures inhibited the breakdown of long-lived proteins to a much greater extent than it affected the breakdown of short-lived ones. Treatment of cultured hepatocytes with glucagon, or deprivation for serum or amino acids stimulated the degradation of the more stable cell proteins but did not affect the breakdown of 125I-asialo-fetuin. Under these conditions leupeptin and chymostatin inhibited the breakdown of long-lived cell proteins to the same extent as in control cultures. Thus, lysosomal enzymes seem to play an important role in protein breakdown both in fed hepatocytes and in cells where proteolysis is accelerated.

Degradation of proteins microinjected into cultured mammalian cells

Iodinated proteins were degraded after injection into HeLa cells at first-order rates with half-lives varying from three hours for the trout monhistone chromosomal protein, HMG-T, -to 60 hours for whale myoglobin. Fluoresceinated-bovine serum albumin (fl-BSA) was degraded almost twice as fast as unmodified BSA. The rate of degradation of 125I-BSA was very similar in eight cell lines of mouse, human, monkey and rat origin. Microinjected proteins were analyzed on SDS-acrylamide gels after injection, and for BSA and immunoglobin G, all remaining intracellular 125I migrated at the molecular weight of the injected proteins. By contrasting, more than 80% of the extracellular 125I chromatographed as iodotyrosine. With the exception of fl-BSA, which exhibited perinuclear accumulation in approximately one-half of the injected cells, autoradiography showed that throughout the period of study the injected proteins remained dispersed in the cytoplasm.

The effect of protein degradation on cellular growth characteristics

The role of protein degradation in cellular proliferation was investigated by measurements of the rates of degradation of labile and stable proteins for a number of cell types under various growth conditions. The rate of protein degradation was found to be a relatively invariant parameter in that it did not change after strong inhibition of protein synthesis with cychloheximide or histidinol, it was the same in both exponential and stationary phase, and it did not correlate with the presence or absence of malignant transformation. Using three different cell types with widely differing division rates, the rate of cell division and DNA synthesis (in %/hr) was found to be precisely equal to the rate of protein accumulation (in %/hr) , i.e., to the rate of protein synthesis minus the rate of protein degradation. Division rates between the different cell types appeared to be determined chiefly by the rate of protein synthesis though, especially at low division rates, the rate of protein degradation could represent a large component of the protein accumulation rate.