The effects of amino acids on protein degradation and translocation of non-histone proteins to the nucleus in lymphocytes

The role of the intralysosomal pH in the control of autophagic proteolytic flux in rat hepatocytes

Current methods to estimate changes in intralysosomal pH in hepatocytes do not discriminate between lysosomes and other intracellular acidic compartments. To obtain selective information on the change in lysosomal function in response to a change in lysosomal pH we have used the liberation of fluorescent 4-methoxy-2-naphthylamide from low concentrations of lysyl-alanyl-4-methoxy-2-naphthylamide, a substrate of lysosomal dipeptidylpeptidase II. Using permeabilized and intact hepatocytes, the activity of this enzyme in response to manipulations meant to increase the intralysosomal pH was compared with intralysosomal protein degradation and with the accumulation of [14C]chloroquine as a pH indicator of intracellular acidic compartments. The data show that changes in intralysosomal pH are indicated by changes in dipeptidylpeptidase II activity and that these are mainly due to a pH-dependent change in substrate accumulation in the lysosomes. Subsequently, the method was applied to establishing the extent to which an increase in intralysosomal pH can contribute to the inhibition of autophagic proteolysis in intact hepatocytes caused by a decrease in intracellular ATP, by an increase in amino acid concentration and by hypo-osmotic cell swelling. The following observations were made. (a) Moderate changes in intracellular ATP do not affect the lysosomal pH. (b) Hypo-osmotic cell swelling, which promotes inhibition of proteolysis by amino acids in freshly isolated hepatocytes, does not affect the lysosomal pH. (c) In addition to their known inhibitory effect on autophagic sequestration, amino acids (leucine in particular) can increase the lysosomal pH and thus inhibit intralysosomal protein degradation directly. (d) Low concentrations of the acidotropic agent methylamine increase the lysosomal pH without having an effect on autophagic proteolytic flux. It is concluded that autophagic proteolysis is not controlled by changes in the lysosomal pH.

Effects of epidermal growth factor on protein degradation the translocation of non-histone proteins to the nucleus and DNA synthesis

Stimulation of resting Chang liver or monkey kidney cells, prelabeled with [3H]leucine, by epidermal growth factor (EGF), caused inhibition of cellular protein degradation and a parallel increase nuclear translocation of 3H-labeled non-histone proteins and DNA synthesis. Nuclear translocation of these proteins was independent of protein synthesis. Fractionation of the nuclear 3H-labeled non-histone proteins in a pH gradient of 2.5-6.5 showed that the protein fractions with a high degree of proteolysis in resting cells corresponded to the protein fractions with a high extent of translocation in stimulated cells, suggesting that degradation and translocation of these proteins may be related. EGF inhibited cellular uptake of [3H]chloroquine, suggesting that EGF inhibits non-histone protein degradation via the lysosomal pathway. These observations support the hypothesis that EGF induces non-histone protein translocation to the nucleus by inhibiting lysosomal degradation of these proteins.

Demonstration that some of the nonhistone proteins, inducible to translocate into the nucleus, are glycosylated

Con A, NaF, and eserine (lysosomotropic agents) induced marked translocation of acidic [3H] nonhistone proteins (NHP) from the cytoplasm to the nucleus in lymphocytes prelabeled with [3H]-2-mannose. The nuclear [3H] NHP contents were 38-120% higher in cells treated with these agents than in control cells. Tunicamycin, a strong inhibitor of N-glycosylation via the dolichol pathway, caused a concentration-dependent inhibition of [3H]-2-mannose incorporation into the nuclear [3H] NHP. Considerable amounts of nuclear [3H] NHP from lymphocytes labeled with either [3H]-2-mannose or [3H] leucine, bound specifically to Con A-Sepharose and could be eluted by alpha-methyl mannoside. Con A and NaF caused also nuclear translocation of acidic [3H] NHP in cells labeled with [3H] glucosamine, [3H] galactose, or [3H] fucose. Fractionation of the nuclear proteins by isoelectric focusing in a pH gradient of 2.5-6.5 showed that multiple species of acidic NHP were labeled with each of the four 3H-sugars. These results indicate that a fraction of the acidic nuclear NHP are N-glycosylated proteins and that gene activation and mitogenesis are associated with the translocation of these glycoproteins to the nucleus. Considering the known intracellular traffic of nascent glycoproteins our results suggest that at least some of the acidic NHP are synthesized and glycosylated in the endoplasmic reticulum and the Golgi (secretory pathway). It is likely that these proteins, after completion of synthesis and glycosylation, emerge from the trans-stack of the Golgi packaged in vesicles and accumulate in the cytoplasm. Induction of nuclear translocation of such NHP by various agents may be mediated by a vesicular transport mechanism.

Energy dependence of autophagic protein degradation in isolated rat hepatocytes

The effect of small changes in intracellular ATP on autophagic flux was studied in isolated rat hepatocytes by using inhibitors of ATP production or by varying the metabolic conditions. The following observations were made. There was a linear relationship between endogenous protein degradation and intracellular ATP, the rate of proteolysis declining with decreasing ATP concentrations. 15% of the maximal proteolysis is either independent of ATP or has a very high affinity for this metabolite. There was a linear relationship between the autophagic sequestration of cytosolic [14C]sucrose and intracellular ATP, the sequestration rate decreasing with decreasing ATP concentrations. ATP depletion did not cause release of [14C]sucrose previously sequestered in autophagosomes and lysosomes at high ATP levels. Intracellular accumulation of chloroquine, used as an indicator of the pH inside lysosomes and other acidic cell compartments, diminished with decreasing cellular ATP content. Amino acids inhibited proteolysis without affecting ATP levels or chloroquine accumulation. We conclude from the high sensitivity of autophagy towards relatively small changes in the concentration of intracellular ATP that, besides amino acids, ATP is a very important factor in controlling the rate of autophagy in rat hepatocytes.

Effects of serum and conditioned medium on protein degradation, migration of nonhistone proteins to the nucleus, and DNA synthesis in transformed cells

Stimulation of resting transformed cells (Chang liver cells), prelabeled with [3H] leucine, with fetal calf serum, caused increased nuclear translocation of [3H] nonhistone proteins ([3H] NHP) and DNA synthesis and a parallel inhibition of proteolysis of cellular proteins. [3H] NHP migration was independent of protein synthesis. Fractionation of the nuclear proteins in a pH gradient of 2.5-6.5, showed that [3H] NHP fractions with high degradation rates in resting cells corresponded to the [3H] NHP fractions with high migration rates in stimulated cells, suggesting that degradation and migration of [3H] NHP are linked. Conditioned medium (COM) produced by Chang cells had similar effects as serum, suggesting that factors produced by these transformed cells, control cell growth by a mechanism that is similar to serum. The lysosomotropic amine eserine had similar effects as serum and COM. Based on the similarity of the effects, it would appear that serum and COM inhibit lysosomal proteolysis. It is proposed that serum and COM induce NHP migration to the nucleus by inhibiting lysosomal degradation of these proteins. Serum and COM caused also migration of [3H] histones to the nucleus, however the mechanism is not clear.

Degradation of nuclear proteins: studies on transplanted B82 cell karyoplast proteins

Karyoplasts were prepared from B82 cells (thymidine kinase deficient mouse L cells) by cytochalasin B mediated enucleation. Morphological measurements show that the nucleus constitutes 89% of a karyoplast by volume. Homokaryons were obtained by Sendai virus mediated karyoplast-B82 cell fusion. Transplanted nuclei were not destroyed catastrophically but were maintained intracellularly for at least 140 h. Transplanted nuclear proteins were degraded with an average half-life of 84 +/- 7 h by processes partially sensitive to inhibition by NH4Cl (50%) and leupeptin (30%). The data therefore suggest that some nuclear proteins are translocated to the cytoplasm for lysosomal degradation.

Endocytosed non-histone proteins translocate in part to the nucleus in lymphocytes

[3H]Non-histone proteins ([3H]NHP), dissolved in the culture medium, are endocytosed by lymphocytes and equilibrate rapidly between the cytoplasm and the nucleus. During incubation, the proteins are gradually degraded in the lysosomes. The lysosomotropic agents conA, NaF, eserine and atropine have two parallel effects on resting lymphocytes, after they have endocytosed [3H]NHP: inhibition of degradation and increased translocation of [3H]NHP from the cytoplasm to the nucleus. This indicates that lysosomal degradation and translocation of [3H]NHP to the nucleus are linked and suggests that this translocation may be the result of inhibited lysosomal degradation of the [3H]NHP. The behaviour of endocytosed [3H]NHP appears similar to that of endogenous [3H]NHP in cells prelabeled with [3H]leucine, when subjected to the same lysosomotropic agents, reported previously (Polet, H, Exp cell res 148 (1983) 345). This observation may provide a model to study the mechanism(s) controlling nucleo-cytoplasmic traffic of NHP.