INDUCED POLYPLOIDY AND SORTING OF DAMAGED DNA BY MICRONUCLEATION IN RADIORESISTANT RAT LIVER EPITHELIAL STEM-LIKE CELLS EXPOSED TO X-RAYS

OBJECTIVE

Rat liver stem-like epithelial cells (WB-F344) that under certain conditions may differentiate into hepa- tocyte and biliary lineages were subjected to acute X-irradiation with the aim to examine cell cycle peculiarities dur- ing the course of survival.

MATERIALS AND METHODS

Suspensions of WB-F344 cells that grew as a monolayer and reached sub-confluence were irradiated with 1, 5, and 10 Gy of X-rays (2 Gy/min). As an intact control, sham-irradiated cells were used. After irra- diation, cells were plated into 25-cm2 tissue culture flasks to culture them for over several days without reaching contact inhibition. On days 1, 2, 3, and 5 post-irradiation, cells were harvested and examined for nuclear morpholo- gy and DNA ploidy by stoichiometric toluidine blue reaction and image cytometry. On days 7 and 9 post-irradiation, only heavily irradiated (10 Gy) cells were examined. Also, 10 Gy-irradiated cells were chosen for immunofluorescence staining to monitor persistence of DNA lesions (γ-H2AX), cell proliferation (Ki-67), and self-renewal factors charac- teristic for stem cells (OCT4 and NANOG).

RESULTS

Radioresistance of WB-F344 cells was evidenced by the findings that they do not undergo rapid and mas- sive cell death that in fact was weakly manifested as apoptotic even in heavily irradiated cells. Instead, there was cell cycle progression delay accompanied by polyploidization (via Ki-67-positive mitotic slippage or via impaired cytokinesis) and micronucleation in a dose-dependent manner, although micronucleation to some extent went ahead of polyploidization. Polyploid cells amenable for recovering from DNA damage can mitotically depolyploidize. Many micronuclei contained γ-H2AX clusters, suggesting isolation of severely damaged DNA fragments. Both factors, OCT4 and NANOG, were expressed in the intact control, but became enhanced after irradiation.

CONCLUSIONS

Although the fact of micronucleation is indicative of genotoxic effect, WB-F344 cells can probably escape cell death via sorting of damaged DNA by micronuclei. Induction of polyploidy in these cells can be adaptive to promote cell survival and tissue regeneration with possible involvement of self-renewal mechanism.

Opening of potassium channels protects mitochondrial function from calcium overload

Ischemic preconditioning (IPC) protects myocardium from ischemia reperfusion injury by activating mitochondrial K(ATP) channels. However, the mechanism underlying the protective effect of K(ATP) channel activation has not been elucidated. It has been suggested that activation of mitochondrial K(ATP) channels may prevent mitochondrial dysfunction associated with Ca(2+) overload during reperfusion. The purpose of this experiment was to study, in an isolated mitochondrial preparation, the effects of mitochondrial K(ATP) channel opening on mitochondrial function and to determine whether it protects mitochondria form Ca(2+) overload. Mitochondria (mito) were isolated from rat hearts by differential centrifugation (n = 5/group). Mito respiratory function was measured by polarography without (CONTROL) or with a potassium channel opener (PINACIDIL, 100 microM). Different Ca(2+) concentrations (0 to 5 x 10(-7) M) were used to simulate the effect of Ca(2+) overload; state 2, mito oxygen consumption with substrate only; state 3, oxygen consumption stimulated by ADP; state 4, oxygen consumption after cessation of ADP phosphorylation; respiratory control index (RCI: ratio of state 3 to state 4); rate of oxidative phosphorylation (ADP/Deltat); and ADP:O ratio were measured. PINACIDIL increased state 2 respiration and decreased RCI compared to CONTROL. Low Ca(2+) concentrations stimulated state 2 and state 4 respiration and decreased RCI and ADP:O ratios. High Ca(2+) concentrations increased state 2 and state 4 respiration and further decreased RCI, state 3, and ADP/Deltat. PINACIDIL improved state 3, ADP/Deltat, and RCI at high Ca(2+) concentrations compared to CONTROL. Pinacidil depolarized inner mitochondrial membrane, as evidenced by decreased RCI and increased state 2 at baseline. Depolarization may decrease Ca(2+) influx into mito, protecting mito from Ca(2+) overload, as evidenced by improved state 3 and RCI at high Ca(2+) concentrations. The myocardial protective effects resulting from activating K(ATP) channels either pharmacologically or by IPC may be the result of protecting mito from Ca(2+) overload.

Metabolic control of sodium transport in streptozotocin-induced diabetic rat hearts

Diabetic and control cardiomyocytes encapsulated in agarose beads and superfused with modified medium 199 were studied with 23Na- and 31P-NMR. Baseline intracellular Na+ was higher in diabetic (0.076 +/- 0.01 micromoles/mg protein) than in control (0.04 +/- 0.01 micromoles/mg protein) (p < 0.05). Baseline betaATP and phosphocreatine (PCr) (peak area divided by the peak area of the standard, methylene diphosphonate) were lower in diabetic than in control, e.g., betaATP control, 0.70 +/- 0.07; betaATP diabetic, 0. 49 +/- 0.04 (p < 0.027); PCr control, 1.20 +/- 0.13; PCr diabetic, 0. 83 +/- 0.11 (p < 0.03). This suggests that diabetic cardiomyocytes have depressed bioenergetic function, which may contribute to abnormal Na,K-ATPase function, and thus, an increase in intracellular Na+. In the experiments presented herein, three interventions (2-deoxyglucose, dinitrophenol, or ouabain infusions) were used to determine whether, and the extent to which, energy deficits or abnormalities in Na,K-ATPase function contribute to the increase in intracellular Na+. In diabetic cardiomyocytes, 2-deoxyglucose and ouabain had minimal effect on intracellular Na+, suggesting baseline depression of, or resetting of both glycolytic and Na,K-ATPase function, whereas in control both agents caused significant increases in intracellular Na+after 63 min exposure: 2-deoxyglucose control, 32.9 +/- 8.1%; 2-deoxyglucose diabetic, -4.6 +/- 6% (p < 0.05); ouabain control, 50.5 +/- 8.8%; ouabain diabetic, 21.2 +/- 9.2% (p < 0.05). In both animal models, dinitrophenol was associated with large increases in intracellular Na+: control, 119.0 +/- 26.9%; diabetic, 138.2 +/- 12.6%. Except for the dinitrophenol intervention, where betaATP and PCr decreased to levels below 31P-NMR detection, the energetic metabolites were not lowered to levels that would compromise sarcolemmal function (Na,K-ATPase) in either control or diabetic cardiomyocytes. In conclusion, in diabetic cardiomyocytes, even though abnormal glycolytic and Na, K-ATPase function was associated with increases in intracellular Na+, these increases were not directly related to global energy deficit.

Mitochondrial oxidative phosphorylation in heart from stressed cardiomyopathic hamsters

Stress alone is generally not sufficient to produce serious disease, but stress imposed upon pre-existing disease can contribute to disease progression. To explore this phenomenon, cold-immobilization stress was imposed on young 12.5 month, necrotic phase with small vessel coronary spasm) and older (5 month, quiescent phase, between necrosis and heart failure) cardiomyopathic hamsters. Our hypothesis was that changes in mitochondrial energy processes are involved in stress induced pathology. Polarographic and high performance liquid chromatography (HPLC) techniques were used to measure mitochondrial respiration and oxidative phosphorylation and concentrations of phosphocreatine and adenylates, respectively, in hearts from young and old cardiomyopathic hamsters (stressed and unstressed). No significant differences were found between the young (2.5 month) and old (5 month) age groups in unstressed and stressed healthy hamsters and between young (2.5 month) and old (5 month) unstressed cardiomyopathic hamsters with respect to different parameters of mitochondrial oxidative phosphorylation and with respect to concentration of bioenergetic metabolites, except that ADP concentration was higher in older cardiomyopathic hamsters. Application of stress uncovered differences between young and old cardiomyopathic hamsters: respiration control index was lower and State 4 respiration was higher in young compared to old cardiomyopathic hamsters; whereas the total concentration of ATP was decreased to the same level in both cardiomyopathic groups when compared to control. Mitochondrial oxidative phosphorylation in young cardiomyopathic hamsters was more sensitive to Ca2+, as evidenced by partial uncoupling of respiration and oxidative phosphorylation, than in older cardiomyopathic hamsters and controls. In conclusion, young cardiomyopathic hamsters, i.e. in the necrotic phase of disease, were more susceptible to stress induced changes in mitochondrial oxidative phosphorylation than older cardiomyopathic hamsters and controls.

Encapsulation and perfusion of mitochondria in agarose beads for functional studies with 31P-NMR spectroscopy

An NMR method to study on-line mitochondrial function was developed. Mitochondria were maintained in a stable physiologic state in agarose beads that were continuously superfused with oxygenated buffer at 28 degrees C. Oxidative function of both heart and liver mitochondria was evaluated with 31P NMR at 9.4 T using pyruvate plus malate as substrate. This method allows clear resolution of adenosine triphosphate-gamma (ATPgamma) and adenosine diphosphate-beta (ADPbeta) phosphate signals, whereas alpha signals of ATP and ADP overlap. ATP production by mitochondria was documented to be very sensitive to different interventions (hypoxia, ischemia, carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP)) and depended on the ADP concentration in superfusion medium. These data demonstrate that the new application of NMR to study mitochondrial function can discriminate, on-line, between several physiologic and biochemical processes in intact physiologically stable mitochondria.

Adenosine improves cardiomyocyte respiratory efficiency

The role of adenosine on the regulation of mitochondrial function has been studied. In order to evaluate this the following experiments were done in isolated rat cardiomyocites and mitochondria using polarographic techniques. Cardiomyocyte oxygen consumption (MVO2) and mitochondrial respiratory function (State 3 and State 4, respiratory control index, and ADP/O ratio) were evaluated after exposure to adenosine. Cardiomyocyte MVO2 was significantly lower in cells previously exposed to adenosine (10 microM, 15 min or 30 min cell incubation) than in cells not exposed to adenosine (control). Addition of dipyridamole (10 microM) or 8-(p-Sulfophenyl) theophylline (50 microM) to cardiomyocytes before adenosine incubation prevented the adenosine-induced changes in MVO2. Mitochondria obtained from isolated perfused beating heart previously perfused with adenosine (10 microM, 30 min heart perfusion) also resulted in significant increases in ADP/O and respiratory control index compared to matching control. Mitochondria isolated from cardiomyocytes previously exposed to adenosine (10 microM, 15 min or 30 min cell incubation) resulted in a significant increase in mitochondrial ADP/O ratio compared to control. Adenosine-induced decrease in cardiomyocyte MVO2 may be related to an increase in efficiency of mitochondrial oxidative phosphorylation, and more economical use of oxygen, which is necessary for survival under ischemic stress.