Study on ATP-generating system and related hexokinase activity in mitochondria isolated from undifferentiated or differentiated HT29 adenocarcinoma cells

Molecular layer interneurons in the cerebellum encode for valence in associative learning

The cerebellum plays a crucial role in sensorimotor and associative learning. However, the contribution of molecular layer interneurons (MLIs) to these processes is not well understood. We used two-photon microscopy to study the role of ensembles of cerebellar MLIs in a go-no go task where mice obtain a sugar water reward if they lick a spout in the presence of the rewarded odorant and avoid a timeout when they refrain from licking for the unrewarded odorant. In naive animals the MLI responses did not differ between the odorants. With learning, the rewarded odorant elicited a large increase in MLI calcium responses, and the identity of the odorant could be decoded from the differential response. Importantly, MLIs switched odorant responses when the valence of the stimuli was reversed. Finally, mice took a longer time to refrain from licking in the presence of the unrewarded odorant and had difficulty becoming proficient when MLIs were inhibited by chemogenetic intervention. Our findings support a role for MLIs in learning valence in the cerebellum.

Characterization of porin isoforms expressed in tumor cells

Mitochondria from malignant tumor cell lines show a higher capability for hexokinase binding than those from normal liver. To explore possible differences in hexokinase binding sites of mitochondria between tumor cells and normal liver, we characterized porin isoforms expressed in tumor cells. Cloning experiments on the three porin isoforms, VDAC1, VDAC2 and VDAC3 from malignant tumor cell line AH130 clearly showed that their primary structures were completely identical to those of the corresponding VDACs of normal liver cells. Possible expression of the fourth porin isoform in AH130 cells was excluded by degenerate primer-based RT-PCR. However, the transcript levels of the three VDAC isoforms in AH130 cells were significantly higher than those in normal liver. These results suggest that the high hexokinase-binding capability of malignant tumor cell mitochondria was not due to any structural difference, but due to a quantitative difference in binding sites.

Further studies on the coupling of mitochondrially bound hexokinase to intramitochondrially compartmented ATP, generated by oxidative phosphorylation

Hexokinase, bound to nonphosphorylating rat brain mitochondria, exhibits Michaelis-Menten kinetic behavior, with an apparent K(m) for ATP of 0.44 +/- 0.08 mM. After initiation of oxidative phosphorylation, a steady-state rate of Glc phosphorylation is maintained despite the fact that extramitochondrial [ATP] continues to increase but remains well below saturating levels (i.e., < 0.4 mM). This independence from extramitochondrial [ATP] is taken to indicate that hexokinase is not utilizing extramitochondrial ATP as substrate, but rather draws substrate ATP from an intramitochondrial compartment supplied by oxidative phosphorylation. The steady-state rate of Glc phosphorylation by hexokinase bound to phosphorylating mitochondria is not altered by increase in total rate of ATP production resulting from addition of hexokinase-depleted mitochondria to the system. In contrast, the steady-state rate of Glc phosphorylation by yeast hexokinase, which does not bind to mitochondria, is directly related to the total rate of ATP production in the system. These results are also consistent with the view that, during oxidative phosphorylation, mitochondrially bound hexokinase is selectively using intramitochondrially compartmented ATP; such substrate selectivity would be expected to require physical association of hexokinase with the mitochondria and be dependent solely on the oxidative phosphorylation activity of the hexokinase-bearing organelles. The K(m) for Glc is only modestly affected by the binding of hexokinase to mitochondria and not further altered upon induction of active oxidative phosphorylation, suggesting that neither binding nor oxidative phosphorylation greatly affects the conformation of the Glc binding site. The reliance on intramitochondrial ATP is suggested to result from oxidative phosphorylation-dependent changes in the interaction between the mitochondrial surface and the regions of the hexokinase molecule involved in binding ATP.

Source of ATP for hexokinase-catalyzed glucose phosphorylation in tumor cells: dependence on the rate of oxidative phosphorylation relative to that of extramitochondrial ATP generation

We isolated highly intact and tightly coupled mitochondria from the rat ascites hepatoma cell line AH130 by disruption of the cell membrane by nitrogen cavitation. These isolated mitochondria were found to have essentially the same functional properties as rat liver mitochondria, but unlike the latter, hexokinase (HK) was bound to their membrane. Using the tumor mitochondrial preparation, we examined the source of ATP for phosphorylation of glucose by HK under conditions in which intra- and extramitochondrial ATP-generation systems operated separately or together. Results showed that the membrane-bound HK utilized ATP derived from the most efficiently operating ATP generation system, i.e., oxidative phosphorylation. However, when the rate of extramitochondrial ATP generation was much greater than that of oxidative phosphorylation, HK used ATP from the extramitochondrial ATP-generation system.

Relationship between the content of [14C]glucose-derived monosaccharides in glycoprotein oligosaccharide chains and the state of enterocytic differentiation of HT-29 cells

The HT-29 cell line derived from a human colon adenocarcinoma has a glucose-dependent state of differentiation which is negatively correlated with the presence of D-glucose in the culture medium. The contribution of glucose to the biosynthesis of N-glycan chains, as a function of the differentiation state of HT-29 cells, was shown by: (a) [14C]glucose incorporation by undifferentiated HT-29 cells being lower after 2 h and higher after 19 h of metabolic labeling than that by differentiated cells; (b) a lack of glucose in the culture medium of undifferentiated HT-29 cells diminishing [14C]glucose incorporation into glycan chains, but not changing the glucose distribution between lipid- and protein-linked saccharides; (c) glucose behavior in undifferentiated HT-29 cells being not related to mannose-glycan metabolism, as the high-mannose compounds labeled with glucose and observed by HPLC showing a different distribution associated with the duration of glucose labeling; and (d) glucose being interconverted into other monosaccharide-glycan constituents in proportions different in differentiated and undifferentiated cell populations.

Insulin, growth factors, and cancer cell energy metabolism: an hypothesis on oncogene action

The hexokinase-mitochondrial acceptor theory provides a model of insulin action which unifies the metabolic effects of this hormone and suggests that these result from insulin's stimulatory effect on mitochondrial ATP synthesis. There are similarities between these changes in cells exposed to insulin and in mitochondria of transformed cell lines and cancer cells, where an increased binding of hexokinase to mitochondria is observed. This phenomenon may play a key role in the high rates of glycolysis sustained by cancer cells under aerobic conditions. Structural and functional evidence support the hypothesis that certain growth factors and oncogenes act through stimulation of oxidative phosphorylation via promoting HK binding to mitochondria.

Uptake of 2-fluoro-2-deoxy-D-[U-14C]-glucose during chemotherapy in murine Lewis lung tumor

Mice bearing intramuscular Lewis lung tumor were treated with BCNU and doxorubicin (ADM) to study chemotherapy-induced changes in the uptake of 2-fluoro-2-deoxy-[U-14C]glucose (FDG). A decreased FDG uptake, tumor regression and a diminished proportion of aneuploid versus diploid cells as evaluated by DNA flow cytometry were seen after treatment with BCNU but not with ADM; HPLC indicated that most of the 14C activity in tumors was from FDG6-phosphate. The results suggest that changes in FDG uptake reflect the effectiveness of antitumor therapy. FDG may be valuable in follow-up studies of cancer treatment.

Porin interaction with hexokinase and glycerol kinase: metabolic microcompartmentation at the outer mitochondrial membrane

Porin is the pore-forming protein involved in the movement of adenine nucleotides across the outer mitochondrial membrane (OMM). Hexokinase and glycerol kinase interact with porin on the outer surface of the OMM in a manner which provides these enzymes with preferred access to the ATP generated in the mitochondrion. We review recent evidence which permits refinement of our knowledge of these proteins and their interactions at the OMM. The involvement of this system in metabolic microcompartmentation is discussed, as well as possible pathological consequences of its disruption in malignancy and genetic deficiencies of hexokinase, glycerol kinase, and porin.

Hexokinase of rat brain mitochondria: relative importance of adenylate kinase and oxidative phosphorylation as sources of substrate ATP, and interaction with intramitochondrial compartments of ATP and ADP

Interactions between intramitochondrial ATP-generating, ADP-requiring processes and ATP-requiring, ADP-generating phosphorylation of glucose by mitochondrially bound hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) have been investigated using well-coupled mitochondria isolated from rat brain. ADP generated by mitochondrially bound hexokinase was more effective at stimulating respiration than was ADP generated by hexokinase dissociated from the mitochondria, and pyruvate kinase was less effective as a scavenger of ADP generated by the mitochondrially bound hexokinase than was the case with ADP generated by the dissociated enzyme. These results indicate that ADP generated by the mitochondrially bound enzyme is at least partially sequestered and directed toward the mitochondrial oxidative phosphorylation apparatus. Under the conditions of these experiments, the maximum rate of ATP production by oxidative phosphorylation was approximately 10-fold greater than the maximum rate of ATP generation by the adenylate kinase reaction. Moreover, during periods of active oxidative phosphorylation, adenylate kinase made no detectable contribution to ATP production. Thus, adenylate kinase does not represent a major source of ATP for hexokinase bound to actively phosphorylating brain mitochondria. With adenylate kinase as the sole source of ATP, a steady state was attained in which ATP formation was balanced by utilization in the hexokinase reaction. In contrast, when oxidative phosphorylation was the source of ATP, a steady state rate of Glc phosphorylation was attained, but it was equivalent to only about 40-50% of the rate of ATP production and thus there was a continued net increase in ATP concentration in the system. Rates of Glc phosphorylation with ATP generated by oxidative phosphorylation exceeded those seen with equivalent levels of exogenously added ATP. Moreover, at total ATP concentrations greater than approximately 0.2 mM, hexokinase bound to actively phosphorylating mitochondria was unresponsive to continued slow increases in ATP levels; acute increase in ATP (by addition of exogenous nucleotide) did, however, result in increased hexokinase activity. The relative insensitivity of mitochondrially bound hexokinase to extramitochondrial ATP suggested dependence on an intramitochondrial pool (or pools) of ATP during active oxidative phosphorylation. Two intramitochondrial compartments of ATP were identified based on their selective release by inhibitors of electron transport or oxidative phosphorylation. These compartments were distinguished by their sensitivity to inhibitors and the kinetics with which they were filled with ATP generated by oxidative phosphorylation. Exogenous glycerol kinase competed effectively with mitochondrially bound hexokinase for extramitochondrial ATP, with relatively low levels of glycerol kinase completely inhibiting phosphorylation of Glc.(ABSTRACT TRUNCATED AT 400 WORDS)

Hexokinase bound to rat brain mitochondria uses externally added ATP more efficiently than internally generated ATP

The rates of glucose phosphorylation by bound hexokinase were investigated in mitochondria isolated from rat brain. Initial rates obtained either with ATP generated from oxidative phosphorylation or with ATP added externally were compared. Our results show that the external ATP supports a 2-3-fold higher hexokinase activity than does ATP generated by oxidative phosphorylation under Stage 3 conditions. ATP formed by mitochondrial creatine kinase in the presence of creatine phosphate also supports higher initial rates of glucose phosphorylation than does oxidative phosphorylation. The data suggest that concentrations of ATP present in the cytosol of normal tissue will probably maintain higher rates of glucose phosphorylation than ATP being exported directly from the mitochondrial matrix at maximal State 3 rates.

Respiration of mitochondria isolated from differentiated and undifferentiated HT29 colon cancer cells in the presence of various substrates and ADP generating systems

1. Oxygen consumption was investigated in two cultured subpopulations of either undifferentiated (Glc+ cells) or differentiated (Glc- cells) HT29 colon cancer cells and in the corresponding isolated mitochondria. In Glc+ cells, a decrease of the respiration is induced by the presence of glucose (Crabtree effect), whereas it is not the case in Glc- cells. 2. The oxidative phosphorylation rate of Glc- mitochondria is found to be much higher than that of Glc+ mitochondria, due to a higher efficiency to oxidize glutamine, glutamate, 2-oxoglutarate, succinate or malate. 3. In both types of mitochondria, respiration can be supported by the ADP formed by adenylate kinase or nucleotide diphosphate kinase, and, although to a lesser extent in Glc- mitochondria, by hexokinase. 4. Glc+ cells are characterized by a low respiration capacity and a high glycolytic flux leading to the Crabtree effect. Glc- cells are characterized by a better correlation between a moderate glycolytic flux and a high respiratory capacity.

Mitochondrial hexokinase from differentiated and undifferentiated HT29 colon cancer cells: effect of some metabolites on the bound/soluble equilibrium

1. Solubilization of mitochondrial bound hexokinase (HK), which represents 75-80% of the total enzyme activity in the cells, was investigated in freshly isolated mitochondria from undifferentiated (Glc+) or differentiated (Glc-) HT29 adenocarcinoma cells. In both models, the bound HK is almost completely released in vitro by 100 microM glucose 6-P (G 6-P). 2. Free ATP (5 mM) or palmitate (800 microM) produce a partial solubilization of bound HK, more markedly in the case of Glc- mitochondria. 3. Glucose or glucose 1-P are found unable to solubilize bound HK. Glucose 1,6-P2, 2-deoxyglucose 6-P or glucosamine 6-P can solubilize the enzyme but are less efficient than G 6-P. 4. Mg2+ and Pi are found to counteract the glucose 6-P induced solubilization of HK in both types of mitochondria. Taking into account the intracellular concentrations of these ions, this could in part explain why, in HT29 cells, HK is predominantly bound to the mitochondria.