B. Dean Nelson

Pore protein and the hexokinase-binding protein from the outer membrane of rat liver mitochondria are identical.

Hexoklnase of tumor cells [l] and certain normal tissues [2] are bound to the outer membrane of the mitochondrion. This binding is, at least in part, responsible for increased aerobic glycolysis, which characterizes rapidly growing tumor cells [3]. The rat liver outer membrane protein responsible for binding of hexokinase has been isolated and partially characterized [4]. The app. Mr reported for this peptide (31 000) is nearly identical to that of the outer membrane pore protein [5], purified and characterized in [6]. Since SDS electrophoresis of outer membranes reveals a single major band in the 3 1 000 Mr region, we have investigated the possibility that pore protein and the hexokinase-binding protein are identical. These results indicate that this is the case. [6,6’(n)-3H]Sucrose (l-5 Ci/mmol) was purchased from The Radiochemical Center (Amersham). [carboxyl-‘4C]Dextran (M, 70 000) with spec. act. 1 .l mCi/g was from New England Nuclear. Staphylococcus aureus V8 protease was purchased from Pharmacia (Uppsala). Chymotrypsin, soy bean lecithin and octyl-fl-D-glucopyranoside were all purchased from Sigma. Ampholines were from LKB (Stockholm).

Potent suppression of the adaptive immune response in mice upon dietary exposure to the potent peroxisome proliferator, perfluorooctanoic acid

In a previous investigation, we demonstrated that severe thymus and spleen atrophy occurs in mice upon dietary exposure to several potent peroxisome proliferators (PPs). In the present investigation, the effects of the potent PP perfluorooctanoic acid (PFOA) on the adaptive immunity of mice was evaluated both in vivo and ex vivo. The in vivo immune response examined involved immunization of mice with horse red blood cells (HRBCs), displaying T-cell-dependent antigens after pre-treatment with a PFOA-containing diet for 10 days. Subsequent quantitation of the primary humoral response was performed employing both the plaque-forming cell (PFC) assay and determination of the antibody titer by ELISA. The results clearly demonstrate that oral administration of PFOA prevents both the increases in plaque formations by anti-IgM and -IgG and in serum levels of IgM and IgG normally evoked by such immunization. Ex vivo spleen cells proliferation (assayed as incorporation of 3H-thymidine) in response to both T- and B-cell activators was attenuated by dietary treatment with PFOA, although the analogous in vitro treatment of mouse spleen cells with this same compound had no such effects. Thus, the relatively metabolically inert PP PFOA may exert adaptive immunosuppression in mice by an indirect mechanism. The possible relevance of this immunosuppression to the alterations in plasma lipids caused by PPs is discussed.

Involvement of the peroxisome proliferator-activated receptor alpha in the immunomodulation caused by peroxisome proliferators in mice.

Peroxisome proliferators (PPs) are a large class of structurally diverse chemicals, which includes drugs designed to improve the metabolic abnormalities linking hypertriglyceridemia to diabetes, hyperglycemia, insulin-resistance and atherosclerosis. We have recently demonstrated that exposure of rodents to potent PPs indirectly causes a number of immunomodulating effects, resulting in severe adaptive immunosuppression. Since the peroxisome proliferator-activated receptor alpha (PPARalpha) plays a central role in mediating the pleiotropic responses exerted by PPs, we have compared here the immunomodulating effects of the PPs perfluorooctanoic acid (PFOA) and Wy-14,643 in wild-type and PPARalpha-null mice. The reductions in spleen weight and in the number of splenocytes caused by PP treatment in wild-type mice was not observed in PPARalpha-null mice. Furthermore, the reductions in thymus weight and in the number of thymocytes were potently attenuated in the latter animals. Similarly, the dramatic decreases in the size of the CD4(+)CD8(+) population of cells in the thymus and in the number of thymocytes in the S and G2/M phases of the cell cycle observed in wild-type mice administered PPs were much less extensive in PPARalpha-null mice. Finally, in contrast to the case of wild-type animals, the response of splenocytes isolated from the spleen of PP-treated PPARalpha-null mice to appropriate T- or B-cell activators in vitro was not reduced. Altogether, these data indicate that PPARalpha plays a major role in the immunomodulation caused by PPs. The possible relevance of these changes to the alterations in plasma lipids also caused by PPs is discussed.

Further evidence for the involvement of inhibition of cell proliferation and development in thymic and splenic atrophy induced by the peroxisome proliferator perfluoroctanoic acid in mice

We recently demonstrated that severe thymic and splenic atrophy occur upon dietary treatment of mice with potent peroxisome proliferators (PPs), e.g. perfluorooctanoic acid (PFOA), WY-14,643, nafenopin, and di(2-ethylhexyl)phthalate (DEHP). In the present study, we investigated this phenomenon further employing a relative inert PP, PFOA. Comparison of the dose-dependencies and time-courses indicated that the peroxisome proliferative effect occurred prior to atrophy of both the thymus and spleen. However, following withdrawal of PFOA from the diet, the weight of the thymus and spleen rapidly returned to normal within 10 and 5 days, respectively, in contrast to the more persistent peroxisome proliferation. Furthermore, the changes in thymus and spleen weight upon PFOA treatment and the following withdrawal from diet paralleled the changes in total thymocyte and splenocyte counts, respectively. It was found previously that the decreases in the thymocyte populations present in the S and G2/M phases, as well as in the number of CD4+CD8+ cells upon PFOA treatment, were the most dramatic, perhaps reflecting inhibition of thymocyte proliferation in connection with thymocyte development. Here, the recovery of thymocytes began with increases in the populations in these same phases of the cell cycle, with CD4+CD8+ cells recovering most rapidly, lending further support to our previous hypothesis. The possible relationship of these immunotoxic effects of PPs to the changes they cause in fatty acid metabolism is discussed.

ON THE ROLE OF THE GENERAL TRANSCRIPTION FACTOR SP1 IN THE ACTIVATION AND REPRESSION OF DIVERSE MAMMALIAN OXIDATIVE PHOSPHORYLATION GENES

To gain insight into the role of the general transcription factor,Sp1, in the expression of nuclear genes involved in mitochondrial biogenesis,we investigated Sp1 activation of the adenine nucleotide translocator 2,cytochrome c1, F1-ATPase β subunit, and themitochondria transcription factor (mtTFA) promoters transfected intoDrosophila cell lines. The numbers and organization of GC elementsvary in the four promoters, but the magnitude of activation by coexpressedhuman Sp1 was similar. A feature common to the four promoters is the presenceof multiple, proximal Sp1-activating elements that account for 50% ormore of the transcription activation by Sp1. The distribution and function ofindividual distal Sp1 elements is less defined and appear to be morepromoter-specific. Finally, data from transfected Drosophila cellsprovide the first direct proof for the involvement of Sp1 in the negativeregulation of the ANT2 promoter and as a possible participant in repressionof the β-subunit promoter. The role of Sp1 in both the positive andnegative regulation of OXPHOS promoters is unique.

Brown adipocytes differentiated in vitro can express the gene for the uncoupling protein thermogenin: effects of hypothyroidism and norepinephrine.

Expression of the gene for the brown-fat specific uncoupling protein thermogenin was investigated in cell cultures by hybridization of isolated RNA with a cDNA clone corresponding to mouse thermogenin. The RNA was isolated 3-4 days after confluence from cells differentiated in culture from precursors isolated from the interscapular brown adipose tissue of 5-week-old mice. Very low thermogenin mRNA levels were found in cells derived from untreated mice, and there was only little effect of added norepinephrine on thermogenin gene expression in these cells. However, in cells derived from hypothyroid (methimazole-treated) mice there was a higher expression of thermogenin, and norepinephrine had a marked augmenting effect on the thermogenin mRNA level in these cells. These effects of thermogenin mRNA levels were specific, in that they contrasted with the effects of hypothyroidism and norepinephrine on the level of other mRNA species in these cells (coding for beta-actin, lipoprotein lipase, cytochrome-c oxidase, and glycerol-3-phosphate dehydrogenase). It was concluded that brown-fat cells in culture can reach a differentiated state, sufficiently advanced that the unique properties of these cells can be expressed, and that thermogenin gene expression (i.e., the level of thermogenin mRNA) is under direct control of norepinephrine.

The role of thyroid hormone and promoter diversity in the regulation of nuclear encoded mitochondrial proteins

Thyroid hormone regulates the in vivo expression of a selected set of rat nuclear genes encoding mitochondrial inner membrane proteins. Certain mRNAs, such as that for cytochrome c1, are increased as much as 20-50-fold, while others, such as core protein 1 of Complex III and the F1-ATPase beta-subunit do not respond. The promoter region of human cytochrome c1 also supports thyroid hormone induction of a reporter gene in transient transfection experiments. Thus, thyroid hormone regulates only selected genes, even for subunits within the same complex and in widely varying species. By contrast, growth activation of quiescent NIH3T3 cells, a second paradigm used for stimulating mitochondrial biogenesis, does not increase cytochrome c1 mRNA but does increase F1-ATPase beta-subunit mRNA. These findings suggest that nuclear OXPHOS genes are not necessarily expressed in a coordinated manner, and that multiple regulatory circuits might exist which are linked to different physiological stimuli. Analysis of the promoters of several OXPHOS genes reveals a great diversity and heterogeneity of transfactor binding elements. No single regulatory feature exists which could account for a coordinated expression of all OXPHOS genes. The potential diversity for regulating expression of nuclear OXPHOS genes raises the possibility for the existence of disease states linked to regulatory defects.

Thyroid hormone regulation of mitochondrial function. Comments on the mechanism of signal transduction.

Thyroid hormone exerts two types of effect on mitochondria. The first of these is a rapid activation of respiration which takes place within minutes after hormone injection, and is preserved in isolated mitochondria. The second response occurs after 1 to several days of injection and leads to mitochondrial biogenesis and increases in mitochondria mass. The hormone signal for these two responses involves either triiodothyronine (T3)-responsive nuclear genes or a direct action of T3 at mitochondria binding sites.

Studies on the characteristics of a proton pump in phospholipid vesicles inlayed with purified complex III from beef heart mitochondria.

It is well documented that the respiratory chain functions as a proton pump [l] . Two views have been put forward to describe the details of this pumping action. Mitchell’s original hypothesis described the pump as a vectorial transmembrane movement of the hydrogen and electron carriers [ 11. More recently, Papa et al. [2-41 proposed that the redox-proton pump is generated by a linkage between the oxidoreduction of the metal centers of electron carriers and the protonic equilibrium of acidic groups of the apoprotein. Consistent with this suggestion is the demonstration of a proton carrier in the Complex III region of the respiratory chain [3-51 . In this region, no classical hydrogen carriers are known [6-81. More recently, the presence of a proton pump has been demonstrated in phospholipid liposomes containing purified Complex III [9]. In the present paper we -eport a study on some properties of this proton lump, demonstrating its electrogenic nature and pH dependency.

Subfractionation of the outer membrane of rat brain mitochondria: evidence for the existence of a domain containing the porin-hexokinase complex

Isolated and well-characterized rat brain nonsynaptic mitochondria were subfractionated by digitonin. Antibodies to a uniquely outer membrane protein, porin, have allowed us to use this protein for the first time as an outer membrane marker in brain. Hexokinase, which binds to porin, was also measured. Based upon the sequential release of these and other marker enzymes with increasing concentrations of digitonin, three outer membrane domains have been identified. Two populations of porin were found by this treatment. The most plausible interpretation of our results is that the two porin populations exist in different membrane environments with regard to cholesterol. One of these populations binds most of the hexokinase and appears to be associated with the inner membrane. It is proposed that the porin-hexokinase complex in brain mitochondria is located in a cholesterol-free membrane domain together with inner membrane components. This domain has the features of contact points which have been visualized by electron microscopy.