Amino Acid Starvation Has Opposite Effects on Mitochondrial and Cytosolic Protein Synthesis

Link: Amino Acid Starvation Has Opposite Effects on Mitochondrial and Cytosolic Protein Synthesis (PLoS One)

Proteins form a substantial component of cellular mass, and their synthesis from amino acids is a one of the cell's principal energy expenditures. Amino acids have a second purpose, however - they can be catabolised and used as a a metabolite in the cell's mitochondria. Conversely, they can also be synthesised (at a significant expense of ATP) from precursor molecules, also derived partly from mitochondrial pathways. This puts the mitochondria at an important control point in protein and energy homeostasis, especially in conditions of amino acid scarcity.

In this paper, researchers from the MRC Mitochondrial Biology unit and the NIMR investigated the response of human cells to conditions of amino acid scarcity. They find that when amino acids are scarce, mitochondrial respiration and membrane potential increases, as does mitochondrial protein synthesis, but with no concomitant mitochondrial biogenesis. Cytosolic protein synthesis is downregulated, and cell cycle arrest was observed after 30 hours of amino acid starvation, 24 hours after the increase in mitochondrial protein synthesis.

Paradoxically, components of amino acid catabolic pathways were observed to be upregulated, with a decrease in amino acid anabolism, as measured by increased expression of glycine catabolism proteins and downregulation of asparagine synthetase. The authors suggest that the downregulation of cytosolic proteins is mediated by TORC1 and serves to reduce cell proliferation during amino acid scarcity, rather than as a way of reducing energy consumption. It is also suggested that this evidence shows that mitochondrial components can be replaced, rather than having to synthesise new mitochondria constantly.

Additionally, the findings in this paper indicate the citrate synthase may not be as reliable a measure of mitochondrial mass as previously thought, and that TFAM levels may not always correlate strictly with mtDNA copy number.

Oncogene ablation-resistant pancreatic cancer cells depend on mitochondrial function

Oncogene ablation-resistant pancreatic cancer cells depend on mitochondrial function

KRAS is an an oncogene which is found to be upregulated in many cancers. In pancreatic ductile carcinoma, KRAS mutations are known to be a driver mutation in tumorigenesis. The authors use an inducible mouse model of mutated KRAS, where the mutation is a loss of function. They show that, after KRAS ablation, a small set of surviving cancer cells persist. These are metabolically different to prototypical cancer cells, as they are addicted to OXPHOS whereas tumour cells tend to be addicted to glycolysis. The authors hypothesize that the mitochondrial hyperpolarization of these cells causes them to have a higher threshold for activating the mitochondrial permeability transition pore, which induces cell death. They find that inhibiting OXPHOS can eliminate the survivor cells.

mTORC1 Controls Mitochondrial Activity and Biogenesis through 4E-BP-Dependent Translational Regulation (Cell Metabolism)

Paper link: mTORC1 Controls Mitochondrial Activity and Biogenesis through 4E-BP-Dependent Translational Regulation (Cell Metabolism)

Mammalian target of rapamycin (mTOR) is a protein responsible for integrating cellular signals related to energy generation and consumption - in essence it is integral to balancing the cellular energy budget. It forms two complexes, known as mTORC1 and mTORC2, and the authors find that mTORC1 stimulates both mitochondrial biogenesis and activity by phosphorylation of 4E-BP proteins. These function by binding to mRNAs and bringing them to ribosomes, and are essential for the translation of most proteins.

The authors find that when mTOR is inhibited the expression of a subset of mitochondrial proteins is repressed and the level of mitochondrial respiration decreases, lowering the cellular ATP concentration. Reduced levels of Raptor (a protein necessary for mTORC1 formation) led to decreased mitochondrial activity and copy number, whereas a reduction in Rictor (which is necessary for mTORC2) did not have these effects, and led to an increase in cellular ATP turnover, indicating that mTORC1 exerts most of the control on mitochondrial biogenesis.

This indicates that mTORC1 plays an important role in regulating nuclear-encoded mitochondrial protein synthesis to allow mitochondrial activity to match the energy demands of the cell.

Predicting selective drug targets in cancer through metabolic networks

Predicting selective drug targets in cancer through metabolic networks

The authors generated a general metabolic model of cancer, by looking for metabolic genes which are highly expressed across many cancer cell lines. Using a Model Building Algorithm from a previous publication, they created a minimal set of reactions which are needed to activate these highly expressed genes. From this, they investigated the effects of in silico knockdown for each gene, on the viability of the network relative to a model of healthy cells. They found significant agreement with shRNA gene silencing data in the literature. They also explored synthetic lethality, by comparing the synergistic effect of knocking down pairs of genes relative to knockdown of each individually. They identified important metabolic pathways for anticancer activity, as well as potential genetic targets for drug therapy.

Oxidative stress-mediated activation of extracellular signal-regulated kinase contributes to mild cognitive impairment-related mitochondrial dysfunction

 http://www.sciencedirect.com/science/article/pii/S0891584914003384

They find that mitochondria of people with Mild Cognitive Impairment (MCI) (which is what you experience before Alzeimer's disease kicks in) have more MFN2. The expression levels of MFN2 are 1.8 fold higher than in control cells, meaning that the mitochondria of patients with MCI are more elongated.

There were decreased activities of complex I, III and IV in MCI cells, and ATP levels were lower in MCI cells. No difference in mitochondrial mass was observed.

Intensity of TMRM was decreased in MCI cells.

Deletion of the Mitochondrial Chaperone TRAP-1 Uncovers Global Reprogramming of Metabolic Networks.

Deletion of the Mitochondrial Chaperone TRAP-1 Uncovers Global Reprogramming of Metabolic Networks.

Chaperones are protein complexes which are used in the transport & folding of other cellular proteins. The authors found that deletion of the mitochondrial chaperone TRAP-1 led to a viable mouse model with reduced signs of ageing and age-associated pathologies such as obesity, dysplasia and tumour formation.

On a cellular level, TRAP-1 knockout reduced mitochondrial respiration, possibly by affecting complex II formation, leading to an upregulation in glyolytic activity. Somewhat paradoxically, the oxidative phosphorylation transcriptome was significantly upregulated, with a larger increase in complexes III and IV transcripts. Hsp90 (a similar chaperone) was upregulated and transported to the mitochondria in what may be a compensatory response.

Cell cultures showed reduced proliferation consistent with cell-cycle arrest, and showed a small but significant increase in ROS production.

The authors suggest that TRAP-1 knockout impairs successful folding of the SDHB subunit of complex II, which is partially rescued by dramatically upregulating all elements of oxidative phosphorylation and partially compensated for by reduced glycolysis.

It is unclear whether the cell's ability to generate energy from its mitochondria is impaired, or if the cell simply has to create more oxphos complexes to do so, however a small increase in ROS is observed, which may have a protective effect via mitohormesis and thus play a role in protecting the mice from age-associated pathologies.

Dynamics of nucleoid structure regulated by mitochondrial fission contributes to cristae reformation and release of cytochrome c

http://www.ncbi.nlm.nih.gov/pubmed/23821750

 In this paper they suggest that fission is needed to distribute mtDNA through the mitochondrial network, because if Drp1 is knocked out (and the mitochondria start to fuse) the nucleoids cluster together and large areas in the mitochondrial networks do not contain any mtDNA. This clustering starts 96 hours after Drp1 knockout (and later also 24 hours after Drp1 knockdown) and the total mtDNA content remains constant which means it really appears as though nucleoids diffuse through the mitochondrial network towards each other. If Drp1 is re-expressed, the effect is undone and the nucleoids distribute themselves again.

Knocking down Mff (mitochondrial fission factor), a protein that is also involved in mitochondrial fission, also causes clustering of nucleoids (nucleoid enlarging actually, that is what they really see). No enlarging is seen when OPA1 or MFN1,2 are knocked down and also not if both OPA1 and Drp1 are knocked down. Overexpression of MFN1 does lead to enlarged nucleoids.

When the enlarged nucleoids were observed at with higher resolution, it was clear that they consisted of several nucleoids clustered together.

They also observe that fission events tend to occur in the vicinity of nucleoids, this happened in 70% of the fission events seen (the number of which is 'several'). They then suggest that the distribution of Drp1 and Mff is responsible for nucleoid distribution.

Cyclin-dependent kinases regulate lysosomal degradation of hypoxia-inducible factor 1α to promote cell-cycle progression

Cyclin-dependent kinases regulate lysosomal degradation of hypoxia-inducible factor 1α to promote cell-cycle progression

HIF-1α is known to be important for adaptations to hypoxia, which is of interest for solid tumours which often have hypoxic regions due to their poorly formed vasculature. In addition, HIF-1α is a negative regulator of DNA replication but somehow some cells are able to replicate under hypoxic conditions. Here, the authors report that HIF-1α levels are coupled to the cell cycle through lysosomal degradation of HIF-1α. Blocking lysosomal degradation of HIF-1α through manipulation of Cdk proteins led to cell-cycle arrest in cancer cells.

The regulation of mitochondrial DNA copy number in glioblastoma cells

The regulation of mitochondrial DNA copy number in glioblastoma cells

Tumour cells and stem cells share some common traits, they are both proliferative and also appear to be more glycolytic than normal cells. In this paper, Dickinson et al. elucidate the role of mtDNA copy number in glioblastoma cells, by comparison with human neuronal stem cells. They found that depletion of mtDNA in glioblastoma enhanced their differentiation, and prolonged depletion reduced their proliferation. They observed that depleted glioblastoma cells recovered their mtDNA copy number as part of tumorigenesis, supporting the idea of a mtDNA set point.