Evidence for a novel ATP-dependent protease from the rat liver mitochondrial intermembrane space: purification and characterisation

II - Insulin processing in mitochondria

Our objective was to know how insulin is processing in mitochondria; if IDE is the only participant in mitochondrial insulin degradation and the role of insulin degradation on IDE accumulation in mitoplasts. Mitochondria and its fractions were isolated as described by Greenwalt. IDE was purified and detected in immunoblot with specific antibodies. High insulin degradation was obtained through addition to rat's diet of 25 g/rat of apple and 10 g/rat of hard-boiled eggs, 3 days a week. Mitochondrial insulin degradation was assayed with 5 % TCA, insulin antibody or Sephadex G50 chromatography. Degradation was also assayed 60 min at 37 °C in mitochondrial fractions (IMS and Mx) with diet or not and without IDE. Degradation in fractions precipitated with ammonium sulfates (60-80 %) were studied after mitochondrial insulin incubation (1 ng. insulin during 15 min, at 30 °C) or with addition of 2.5 mM ATP. Supplementary diet increased insulin degradation. High insulin did not increase mitoplasts accumulation and did not decrease mitochondrial degradation. High insulin and inhibition of degradation evidence insulin competition for a putative transport system. Mitochondrial incubation with insulin increased IDE in matrix as observed in immunoblot. ATP decreased degradation in Mx and increased it in IMS. Chromatography of IMS demonstrated an ATP-dependent protease that degraded insulin, similar to described by Sitte et al. Mitochondria participate in insulin degradation and the diet increased it. High insulin did not accomplish mitochondrial decrease of degradation or its accumulation in mitoplasts. Mitochondrial incubation with insulin increased IDE in matrix. ATP suggested being a regulator of mitochondrial insulin degradation.

Protein degradation in mitochondria: implications for oxidative stress, aging and disease: a novel etiological classification of mitochondrial proteolytic disorders

The mitochondrial genome encodes just a small number of subunits of the respiratory chain. All the other mitochondrial proteins are encoded in the nucleus and produced in the cytosol. Various enzymes participate in the activation and intramitochondrial transport of imported proteins. To finally take their place in the various mitochondrial compartments, the targeting signals of imported proteins have to be cleaved by mitochondrial processing peptidases. Mitochondria must also be able to eliminate peptides that are internally synthesized in excess, as well as those that are improperly assembled, and those with abnormal conformation caused by mutation or oxidative damage. Damaged mitochondrial proteins can be removed in two ways: either through lysosomal autophagy, that can account for at most 25-30% of the biochemically estimated rates of average mitochondrial catabolism; or through an intramitochondrial proteinolytic pathway. Mitochondrial proteases have been extensively studied in yeast, but evidence in recent years has demonstrated the existence of similar systems in mammalian cells, and has pointed to the possible importance of mitochondrial proteolytic enzymes in human diseases and ageing. A number of mitochondrial diseases have been identified whose mechanisms involve proteolytic dysfunction. Similar mechanisms probably play a role in diminished resistance to oxidative stress, and in the aging process. In this paper we review current knowledge of mammalian mitochondrial proteolysis, under normal conditions and in several disease states, and we propose an etiological classification of human diseases characterized by a decline or loss of function of mitochondrial proteolytic enzymes.

Sue1p is required for degradation of labile forms of altered cytochromes C in yeast mitochondria

Previous studies on certain altered holo-isocytochromes c revealed a rho(-)-dependent degradation (RDD) phenotype, in which certain altered holo-iso-1-cytochromes c are at normal or nearly normal levels in rho+ strains, but are at low levels or absent in rho- strains, although wild-type holo-iso-1-cytochrome c is present at normal levels in both rho+ and related rho- strains. The diminished levels of altered holo-iso-1-cytochrome c are due to the rapid degradation that is carried out by a novel proteolytic pathway in the IMS of mitochondria. SUE1, a nuclear gene that encodes a mitochondrial protein, was identified with a genetic screen for mutants that diminish RDD. The levels of RDD and certain other types of altered holo-iso-1-cytochrome c were elevated in rho- sue1 strains. Also, rho+ sue1 strains containing certain altered holo-iso-1-cytochromes c grew better on non-fermentable carbon sources than the corresponding rho+ SUE1 strains. These results indicate that Sue1p may play an important role in the degradation of abnormal holo-iso-1-cytochrome c in the mitochondria.

Turnover of matrix proteins in mammalian mitochondria

In cultured hepatocytes the turnover of several mitochondrial matrix proteins (e.g. acetyl-CoA acetyltransferase) appears to be initiated by CoA-mediated, sequential transformation into CoA-modified forms. This modification favours the notion that intramitochondrial degradation by a matrix-resident ATP-dependent protease may be preceded by a specific modification by CoA. In a mitochondrial matrix fraction the MgATP-dependent decrease in anti-CoA immunoreactivity coincided with both a decrease in the anti-protein immunoreactivity of acetyl-CoA acetyltransferase and/or of 3-ketoacyl-CoA thiolase, and with the appearance of proteolytic fragments. A closer analysis of the degradation pattern revealed, however, a breakdown of the unmodified acetyl-CoA acetyltransferase and of its CoA-modified form, A1, whereas the form that is more highly modified by CoA, A2, proved to be inaccessible towards an ATP-dependent protease. In mammalian mitochondrial matrix, proteins can be degraded selectively by a matrix-resident ATP-dependent protease. The process of CoA modification results finally in the protection of matrix proteins from degradation. In cultured hepatocytes, leupeptin, an inhibitor of lysosomal proteases, did not affect the steady-state level of the mitochondrial matrix protein acetyl-CoA acetyltransferase. However, leupeptin mediated a specific accumulation of mitochondrial matrix proteins in the cytosolic fractions of hepatocytes cultured over a 24 h period. The levels of acetyl-CoA acetyltransferase, 3-ketoacyl-CoA thiolase and glutamate dehydrogenase proteins increased 1.9-, 2.0- and 2.2-fold respectively. Their status as mature, oligomeric, but enzymically inactive enzymes strongly suggests that they originate from a leakage of autophagosomes, a constituent of the non-selective autophagic/lysosomal pathway for degradation of whole mitochondria.

Activation of mitochondrial ATP-dependent protease by peptides and proteins

We examined the effect of peptides or protein on the proteolytic and ATPase activities of mitochondrial ATP-dependent LON protease purified from bovine adrenal cortex. Peptides/proteins including angiotensin I which stimulated ATPase activity without hydrolysis of any peptide bonds stimulated proteolysis of 125I-labeled substrates at low concentrations; whereas at high concentrations they competitively inhibited proteolysis, thus displaying a biphasic activity profile. All peptides and proteins thus examined stimulated degradation of 125I-labeled substrates. When an ATP analog was substituted for ATP, only inhibition; i.e., no stimulation, of proteolysis by unlabeled peptides was observed. Without activator peptides, degradation of [125I] peptides was higher in the presence of an ATP analog than that in the presence of ATP. ADP, a product of the ATPase reaction, inhibited the proteolytic activity in the absence of an activator peptide but not in its presence. From analogy to E. coli ATP-dependent protease La (LON), we suggest that the activator peptides stimulated the proteolysis by releasing enzyme-bound ADP.

Ganglioside GD3 and its mimetics induce cytochrome c release from mitochondria

Ganglioside GD3 induced the release of cytochrome c from isolated rat liver mitochondria. This process was completely prevented by cyclosporin A and partially prevented by a cysteine protease inhibitor, n-acetyl-leu-leu-norleucinal. Cyclosporin A is a potent inhibitor of the permeability transition pore, whereas n-acetyl-leu-leu-norleucinal has no effect on this pore. These results indicate that the release of cytochrome c from mitochondria requires both the opening of the permeability transition pore and a cysteine protease inhibitor-sensitive mechanism. Gangliosides GD1a, GD1b, GT1b, and GQ1b along with the synthetic GD3 mimetics TMS-42 and CI-22, which are glycerophospholipids carrying a disialo residue, also induced cytochrome c release. In contrast, gangliosides GM1, GM2, and GM3 did not induce cytochrome c release. These results indicate that two sialo residues must play an important role in the induction of cytochrome c release by gangliosides.

Decreased levels of proteasome activity and proteasome expression in aging spinal cord

Neuron death and neuron degeneration occur in the CNS during the course of aging. Although multiple cellular alterations transpire during the aging process, those that mediate age-associated neuron death have not been identified. Recent evidence implicates oxidative stress as a possible means of neuron death and neuron degeneration during aging. In the present study, we demonstrate a marked decrease in multicatalytic proteasome activity in the spinal cord of Fisher 344 rats at 12, 24 and 28 months, compared with spinal cord tissue from 3-week- and 3-month-old animals. Application of oxidative injury (FeSO(4)) or the lipid peroxidation product 4-hydroxynonenal decreases multicatalytic proteasome activity in a time- and dose-dependent manner in a motor neuron cell line. Loss of multicatalytic proteasome activity occurs before the loss of multicatalytic proteasome immunoreactivity, with FeSO(4)- and 4-hydroxynonenal-mediated decreases ameliorated by the application of a cell permeable form of the antioxidant glutathione. Application of multicatalytic proteasome inhibitors, but not inhibitors of lysosomal proteases, induced neuron death that was attenuated by the caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp-(O-methyl) fluoromethyl ketone or N-acetyl-Asp-Glu-Val-Asp-Cho (aldehyde). Together, these data suggest that multicatalytic proteasome inhibition occurs during aging of the spinal cord, possibly as the result of oxidative stress, and that multicatalytic proteasome inhibition may be causally related to neuron death.

Protease activation during surgical stress in the rat small intestine

BACKGROUND

Surgical stress affects intestinal permeability and our earlier study using a rat model indicated that oxidative stress plays an important role in this process. Proteases are important mediators of cellular damage and are known to be activated in oxidative stress. This study looked at protease activity in enterocytes after surgical stress.

METHODS

Surgical stress was induced by opening the abdominal wall and handling the intestine as done during laparotomy, in normal and xanthine oxidase-deficient rats. Enterocytes at various stages of differentiation were isolated and protease activity and protection offered by xanthine oxidase inhibitors were determined. Mitochondria and cytosol were prepared from total isolated enterocytes at different periods after surgical stress and protease activation was studied.

RESULTS

Surgical stress induced activation of proteases in both the villus and crypt cells. Protease activation is seen in both mitochondria and cytosol, and similar to the other alterations in mucosal cells, protease activation was maximum 60 min after stress, returning to normal by 24 h. Thiol compounds modulate protease activity in both mitochondria and cytosol and the activation is not seen in xanthine oxidase-deficient animals.

CONCLUSIONS

Surgical stress induces activation of proteases in villus and crypt cells of the small intestine. Both mitochondrial and cytosolic proteases are activated and free radicals generated by xanthine oxidase may mediate protease activation after surgical stress in the intestine.

Protein degradation in mitochondria

The biogenesis of mitochondria and the maintenance of mitochondrial functions depends on an autonomous proteolytic system in the organelle which is highly conserved throughout evolution. Components of this system include processing peptidases and ATP-dependent proteases, as well as molecular chaperone proteins and protein complexes with apparently regulatory functions. While processing peptidases mediate maturation of nuclear-encoded mitochondrial preproteins, quality control within various subcompartments of mitochondria is ensured by ATP-dependent proteases which selectively remove non-assembled or misfolded polypeptides. Moreover; these proteases appear to control the activity- or steady-state levels of specific regulatory proteins and thereby ensure mitochondrial genome integrity, gene expression and protein assembly.

Enhanced mitochondrial biogenesis is associated with increased expression of the mitochondrial ATP-dependent Lon protease

Rats bearing the Zajdela hepatoma tumor and T3-treated hypothyroid rats were used to study the role of protein degradation in the process of mitochondrial biogenesis. It was shown that the activity, protein and mRNA levels of the ATP-dependent Lon protease increased in rapidly growing Zajdela hepatoma cells. The increase in the rate of mitochondrial biogenesis by thyroid hormone was similarly accompanied by enhanced expression of the Lon protease. The results imply that mitochondrial biogenesis in mammalian cells is, at least partially, regulated by the matrix Lon protease.

Irreversible conversion of xanthine dehydrogenase into xanthine oxidase by a mitochondrial protease

Irreversible conversion of xanthine dehydrogenase (XDH) to its oxygen free radical producing oxidase (XO) form occurs through an uncharacterized proteolytic process, which was studied in human liver. Upon incubation of fresh unfrozen liver cytosol, XDH remained intact. When recombinant human XDH was coincubated with subcellular fractions of human liver, the mitochondrial intermembrane space was shown to contain a heat-labile activity that converted XDH irreversibly to XO. This activity is resistant to inhibitors of all major groups of proteases. We postulate that this novel type of proteolytic enzyme is released into the cytosol upon mitochondrial damage.