Growth defects in intramitochondrial energy depleted cells: role of mitochondrial biogenesis

Examining the role of cyclin D1 in breast cancer

Cyclin D1 overexpression is found in more than 50% of human breast cancers and causes mammary cancer in transgenic mice. Dysregulation of cyclin D1 gene expression or function contributes to the loss of normal cell cycle control during tumorigenesis. Recent studies have demonstrated that cyclin D1 conducts additional specific functions to regulate gene expression in the context of local chromatin, promote cellular migration and inhibit mitochondrial metabolism. It is anticipated that these additional functions contribute to the pathology associated with dysregulated cyclin D1 abundance. This article discusses evidence that examines the significance of cyclin D1 in breast cancer with emphasis on its role in breast cancer stem cell expansion.

Mitochondrial DNA depletion induces radioresistance by suppressing G2 checkpoint activation in human pancreatic cancer cells

We hypothesized that mitochondrial function regulates cell cycle checkpoint activation and radiosensitivity. Human pancreatic tumor cells (MiaPaCa-2, rho(+)) were depleted of mitochondrial DNA (rho degrees ) by culturing cells in the presence of ethidium bromide. Depletion of mitochondrial DNA was verified by PCR amplification of total DNA using primer pairs specific for mitochondrial DNA. Loss of mitochondrial DNA decreased plating efficiency and the percentage of cells in S phase. Exponential cultures were irradiated with 2, 4 and 6 Gy (dose rate: 0.83 Gy/min) of ionizing radiation and harvested for determination of cell viability, growth and cell cycle phase distributions. Rho degrees cells were radioresistant compared to rho(+) cells, with a dose-modifying factor (DMF) of 1.6. Although cell growth was significantly inhibited in irradiated rho(+) cells compared to unirradiated control cells, the inhibition in Rho degrees cells was minimal. In addition, mitochondrial DNA depletion suppressed radiation-induced G(2) checkpoint activation, which was accompanied by increases in both cyclin B1 and CDK1. These results suggest that mitochondrial function may regulate cell cycle checkpoint activation and radiosensitivity in pancreatic cancer cells.

Regulation of the cell cycle in response to inhibition of mitochondrial generated energy

Cell cycle control is regulated through the temporal action of both cyclin-dependent kinases and cyclin binding partners. Previously, we have demonstrated that low doses of oligomycin result in a cell cycle arrest of HL-60 cells in G(1) [S. Sweet, G. Singh, Accumulation of human promyelocytic leukemic (HL-60) cells at two energetic cell cycle checkpoints, Cancer Res. 55 (1995) 5164-5167]. In this study, we provide the molecular mechanisms for the observed G(1) arrest following mitochondrial ATPase inhibition. Protein expression of cyclin E and CDK2, the kinase activity of complexed cyclin E/CDK2, and protein expression of p16, p21, and p27 were all unaffected by oligomycin administration. While CDK4 levels were unchanged following oligomycin treatment, a dramatic reduction in cyclin D(1) was observed. Moreover, increased amounts of hypo-phosphorylated retinoblastoma protein (Rbp) and Rbp bound E2F were observed following mitochondrial ATP synthase inhibition. These data provide further evidence that surveillance of available energy occurs during G(1) and ATP deprivation results in cell cycle arrest via a reduction in cyclin D.

Changes in mitochondrial mass, membrane potential, and cellular adenosine triphosphate content during the cell cycle of human leukemic (HL-60) cells

Oxidative phosphorylation within the inner mitochondrial membrane generates the majority of cellular adenosine triphosphate (ATP) required for normal physiological functions (including regulation of cell volume and solute concentration, maintenance of cellular architecture, and synthesis of essential macromolecules). Its efficient functioning depends on the maintenance of an electrochemical gradient and is tightly coupled to the energetic demands of the cell and/or tissue. Commitment to and completion of the cell division cycle are sensitive to changes in the availability of mitochondrially derived ATP, although the relationship between cell cycle and mitochondrial physiology is poorly understood. Using vital, mitochondrial-specific fluorochromes to differentiate between mitochondrial mass (10-N-nonyl acridine orange) and mitochondrial membrane potential (Rhodamine 123), together with a quantification of total cellular ATP levels, it was possible to generate profiles of these mitochondrial characteristics in HL-60 cells at different stages of their cell cycle. The data suggest that the availability of ATP changes in a cell cycle-specific manner and cannot be predicted by changes in mitochondrial mass or membrane potential. Furthermore, transition points in the cell cycle where ATP availability is low with respect to the amount of functional inner mitochondrial membrane have been observed. We suggest that these cell cycle phase transitions are sensitive to inhibition of mitochondrial activity because the basal levels of available ATP at these points are nearer to a theoretical "minimal threshold" below which cell cycle progression is inhibited.

Expression of the AAC2 gene encoding the major mitochondrial ADP/ATP carrier in Saccharomyces cerevisiae is controlled at the transcriptional level by oxygen, heme and HAP2 factor

Expression of the Saccharomyces cerevisiae AAC2 gene encoding the major mitochondrial ADP/ATP carrier was examined. The intracellular level of the carrier protein, as well as the level of the AAC2-gene-specific mRNA, is influenced by the presence or absence of oxygen or of heme, and it is subject to carbon-source control. In addition, the expression of AAC2 gene requires the products of the HAP2 and HAP3 genes, but not that of the HAP1 gene. The 5'-flanking region of the gene was isolated, sequenced and fused to the lacZ reporter gene in order to study the effect of carbon sources and of specific deletion mutations on expression of the gene in yeast transformants. The expression of the reporter gene reveals that the AAC2 gene possesses a strong inducible promoter. The promoter analysis, combined with expression studies in the wild-type as well as in various mutant strains, identified an upstream activation site (UAS) contained within a sequence between -393 bp and -268 bp, and several major initiation sites of AAC2 mRNA between -105bp and -95 bp. Deletion analysis also shows that the TATA boxes located 45 bp and 104 bp upstream of the 5'-ends of AAC2 mRNA are not essential for the transcription. The UAS of the AAC2 gene is required for activation by HAP2 and heme and for release from glucose repressin. A restriction fragment containing the UAS conferred oxygen and carbon source regulation when placed upstream of another yeast gene encoding ADP/ATP carrier (AAC3), deleted of its regulatory sequences. The UAS of the AAC2 gene contains at least two distinct motifs for DNA-binding transcriptional activators, including one which is identical with the core HAP2/3/4 binding motif, and a second one with the ABF1 consensus binding sequence. Our results indicate that these sequences mediate the effects of the respective transactivator on the oxygen- and carbon-source-dependent transcription of the AAC2 gene.

Specific changes in nucleotide and lipid synthesis are linked to growth defects in intramitochondrial energy-depleted yeast cells

Energy depletion of yeast mitochondria caused immediate arrest of cell growth which could be partially reversed by enrichment of the medium. The analysis of radiolabel incorporation into nucleotides and lipids revealed significant changes in substances with possible links to mitochondrial activity (UMP, phosphatidyl-ethanolamine, cardiolipin, phosphatidylglycerol). Changes in sterol pattern indicated inhibition of squalene epoxidase located in endoplasmic reticulum. Although the results demonstrated severe impairment of anabolic processes in intramitochondrial energy-depleted cells, all major nucleotide and lipid species were significantly labelled in long-term experiments.

ADP/ATP translocator is essential only for anaerobic growth of yeast Saccharomyces cerevisiae

All three genes (AAC1, AAC2 and AAC3) encoding the mitochondrial ADP/ATP translocator, were inactivated in a haploid yeast strain by a gene disruption technique. The triple mutant was still able to grow on fermentable carbon sources but only in the presence of oxygen. Under aerobic conditions neither translocator-protein nor carrier-mediated transport was detected in all mutants in which the AAC2 gene was disrupted. It was further shown that a functional AAC genes product is essential only for anaerobic growth of Saccharomyces cerevisiae but not for growth under derepressed conditions. Under anaerobic conditions a non-detectable amount of AAC3 gene product is sufficient to ensure the cell growth and multiplication.

Metabolic and contractile responses of rat fast-twitch muscle to 10-Hz stimulation

Contractile and metabolic responses of rat fast-twitch gastrocnemius-plantaris muscles were studied. Acute in situ 10-Hz stimulation (STIM) for two 60-min periods, separated by 60 min of recovery (REC), was used. Muscles were removed at 1, 3, 15, 60, 75, 120, 123, or 180 min for metabolite measurements. Twitch and tetanic tensions were reduced to 36 and 28% of initial during the first 60 min of STIM. During REC, these tensions returned only to 56-58% of initial by 120 min. These contractile responses did not parallel changes in metabolites in mixed muscle. pH was reduced from 7.0 to 6.4 by 1 min, but by 15 min of STIM had returned to resting levels. Free ADP and AMP increased 3- and 15-fold during STIM, then decreased to resting levels by 3 min of REC. The most sensitive indicator of metabolic stress during STIM and REC was the phosphorylation potential, which varied up to 40-fold. After initial phases of depletion, ATP and phosphocreatine levels were partially restored despite ongoing STIM. Approximately 75% of the change in ATP level could be accounted for by IMP. In red gastrocnemius [fast-twitch red (FTR)] muscle, IMP was increased by 3 min of STIM but returned to control values by 60 min. Thus reamination of IMP occurred during contractions of FTR muscle. Metabolic and contractile responses during the second STIM period (120-180 min) were similar to the first. This cycle of metabolic and contractile responses occurs in fast-twitch muscle which, with chronically repeated STIM and REC periods, undergoes large phenotypic changes as a result of use.