Tyrosine phosphorylation of mitochondrial pyruvate dehydrogenase kinase 1 is important for cancer metabolism

p90 RSK2 mediates antianoikis signals by both transcription-dependent and -independent mechanisms

How invasive and metastatic tumor cells evade anoikis induction remains unclear. We found that knockdown of RSK2 sensitizes diverse cancer cells to anoikis induction, which is mediated through phosphorylation targets including apoptosis signal-regulating kinase 1 (ASK1) and cyclic AMP (cAMP) response element-binding protein (CREB). We provide evidence to show that RSK2 inhibits ASK1 by phosphorylating S83, T1109, and T1326 through a novel mechanism in which phospho-T1109/T1326 inhibits ATP binding to ASK1, while phospho-S83 attenuates ASK1 substrate MKK6 binding. Moreover, the RSK2→CREB signaling pathway provides antianoikis protection by regulating gene expression of protein effectors that are involved in cell death regulation, including the antiapoptotic factor protein tyrosine kinase 6 (PTK6) and the proapoptotic factor inhibitor-of-growth protein 3 (ING3). PTK6 overexpression or ING3 knockdown in addition to ASK1 knockdown further rescued the increased sensitivity to anoikis induction in RSK2 knockdown cells. These data together suggest that RSK2 functions as a signal integrator to provide antianoikis protection to cancer cells in both transcription-independent and -dependent manners, in part by signaling through ASK1 and CREB, and contributes to cancer cell invasion and tumor metastasis.

Proliferation-independent control of tumor glycolysis by PDGFR-mediated AKT activation

The differences in glucose metabolism that distinguish most malignant and normal tissues have called attention to the importance of understanding the molecular mechanisms by which tumor energy metabolism is regulated. Receptor tyrosine kinase (RTK) pathways that are implicated in proliferation and transformation have been linked to several aspects of tumor glucose metabolism. However, the regulation of glycolysis has invariably been examined under conditions in which proliferation is concomitantly altered. To determine whether RTKs directly regulate glycolysis without prerequisite growth modulation, we first identified a specific RTK signaling pathway, platelet-derived growth factor (PDGF)/PDGF receptor (PDGFR) that regulates glycolysis in glioma-derived tumor stem-like cells from a novel mouse model. We determined that PDGF-regulated glycolysis occurs independent of PDGF-regulated proliferation but requires the activation of AKT, a known metabolic regulator in tumor. Our findings identifying a key characteristic of brain tumors, aerobic glycolysis, mediated by a pathway with multiple therapeutic targets suggests the possibility of inhibiting tumor energy metabolism while also treating with agents that target other pathways of pathologic significance.

The Warburg effect: insights from the past decade

Several decades ago, Otto Warburg discovered that cancer cells produce energy predominantly by glycolysis; a phenomenon now termed "Warburg effect". Warburg linked mitochondrial respiratory defects in cancer cells to aerobic glycolysis; this theory of his gradually lost its importance with the lack of conclusive evidence confirming the presence of mitochondrial defects in cancer cells. Scientists began to believe that this altered mechanism of energy production in cancer cells was more of an effect than the cause. More than 50 years later, the clinical use of FDG-PET imaging in the diagnosis and monitoring of cancers rekindled the interest of the scientific community in Warburg's hypothesis. In the last ten years considerable progress in the field has advanced our understanding of the Warburg effect. However, it still remains unclear if the Warburg effect plays a causal role in cancers or it is an epiphenomenon in tumorigenesis. In this review we aim to discuss the molecular mechanisms associated with the Warburg effect with emphasis on recent advances in the field including the role of epigenetic changes, miRNAs and post-translational modification of proteins. In addition, we also discuss emerging therapeutic strategies that target the dependence of cancer cells on altered energy processing through aerobic glycolysis.

Screening of mutations affecting protein stability and dynamics of FGFR1-A simulation analysis

Single amino acid substitutions in Fibroblast Growth Factor Receptor 1 (FGFR1) destabilize protein and have been implicated in several genetic disorders like various forms of cancer, Kallamann syndrome, Pfeiffer syndrome, Jackson Weiss syndrome, etc. In order to gain functional insight into mutation caused by amino acid substitution to protein function and expression, special emphasis was laid on molecular dynamics simulation techniques in combination with in silico tools such as SIFT, PolyPhen 2.0, I-Mutant 3.0 and SNAP. It has been estimated that 68% nsSNPs were predicted to be deleterious by I-Mutant, slightly higher than SIFT (37%), PolyPhen 2.0 (61%) and SNAP (58%). From the observed results, P722S mutation was found to be most deleterious by comparing results of all in silico tools. By molecular dynamics approach, we have shown that P722S mutation leads to increase in flexibility, and deviated more from the native structure which was supported by the decrease in the number of hydrogen bonds. In addition, biophysical analysis revealed a clear insight of stability loss due to P722S mutation in FGFR1 protein. Majority of mutations predicted by these in silico tools were in good concordance with the experimental results.

Molecular biology of chronic myeloid leukemia

Detailed information on the crystal structure of the pharmacologically targeted domains of the BCR-ABL molecule and on its intracellular signaling, which are potentially involved in growth, anti-apoptosis, metabolism and stemness, has made the study of chronic myeloid leukemia the most successful field in tumor biology. However, we now face the issue of drug resistance due to deregulation in the quality control of both DNA and protein. BCR-ABL is basically a misfolded protein with intrinsically disordered regions, which not only produces endoplasmic reticulum stress followed by unfolded protein response in some settings, but also conformational plasticity that may affect the structure of the whole molecule. The intercellular signaling derived from the leukemic cell microenvironment may influence the intracellular responses that take place in a manner both dependent on and independent of BCR-ABL tyrosine kinase activity.

Chronic myeloid leukemia stem cells display alterations in expression of genes involved in oxidative phosphorylation

The mitochondrial respiratory chain (MRC) consists of protein complexes I, II, III, IV and V that support oxidative phosphorylation (OXPHOS), which depends on electron transport to generate adenosine triphosphate (ATP). Electron "leakage" from the MRC generates reactive oxygen species (ROS). Chronic myeloid leukemia in chronic phase (CML-CP) stem cells (LSCs) produce high levels of mitochondrial ROS, causing oxidative DNA damage, resulting in genomic instability, generating imatinib-resistant BCR-ABL1 kinase mutants and additional chromosomal aberrations. Using global mRNA microarray analysis combined with Ingenuity Pathway Analysis we found that LSCs display enhanced expression of genes encoding MRC complexes I, II, IV and V. However, expression of genes encoding complex III was deregulated. Treatment with imatinib did not correct the aberrant levels of MRC genes. Therefore we postulate that abnormal expression of MRC genes may facilitate electron "leakage" to promote the production of ROS and accumulation of genomic instability in LSCs in imatinib-naive and imatinib-treated patients.

Quantitative maps of protein phosphorylation sites across 14 different rat organs and tissues

Deregulated cellular signalling is a common hallmark of disease, and delineating tissue phosphoproteomes is key to unravelling the underlying mechanisms. Here we present the broadest tissue catalogue of phosphoproteins to date, covering 31,480 phosphorylation sites on 7,280 proteins quantified across 14 rat organs and tissues. We provide the data set as an easily accessible resource via a web-based database, the CPR PTM Resource. A major fraction of the presented phosphorylation sites are tissue-specific and modulate protein interaction networks that are essential for the function of individual organs. For skeletal muscle, we find that phosphotyrosines are over-represented, which is mainly due to proteins involved in glycogenolysis and muscle contraction, a finding we validate in human skeletal muscle biopsies. Tyrosine phosphorylation is involved in both skeletal and cardiac muscle contraction, whereas glycogenolytic enzymes are tyrosine phosphorylated in skeletal muscle but not in the liver. The presented phosphoproteomic method is simple and rapid, making it applicable for screening of diseased tissue samples.

The pyruvate dehydrogenase complex as a therapeutic target for age-related diseases

Considerable research has been conducted on mitochondrial biology as it pertains to aging. However, relatively little attention has been accorded the pyruvate dehydrogenase complex (PDC) relative to how we grow old and acquire age-related diseases. The purpose of this review is threefold: first, to describe the physiological chemistry of the PDC and define its place in normal cellular bioenergetics; second, to compare and contrast the pathogenesis and clinical features of congenital PDC deficiency with discrete examples of age-associated dysfunction of the complex; and third, to summarize recent findings in Caenorhabditis elegans that shed additional new light on the significance of the PDC to the aging process.

Metabolic consequences of NDUFS4 gene deletion in immortalized mouse embryonic fibroblasts

Human mitochondrial complex I (CI) deficiency is associated with progressive neurological disorders. To better understand the CI pathomechanism, we here studied how deletion of the CI gene NDUFS4 affects cell metabolism. To this end we compared immortalized mouse embryonic fibroblasts (MEFs) derived from wildtype (wt) and whole-body NDUFS4 knockout (KO) mice. Mitochondria from KO cells lacked the NDUFS4 protein and mitoplasts displayed virtually no CI activity, moderately reduced CII, CIII and CIV activities and normal citrate synthase and CV (F(o)F(1)-ATPase) activity. Native electrophoresis of KO cell mitochondrial fractions revealed two distinct CI subcomplexes of ~830kDa (enzymatically inactive) and ~200kDa (active). The level of fully-assembled CII-CV was not affected by NDUFS4 gene deletion. KO cells exhibited a moderately reduced maximal and routine O(2) consumption, which was fully inhibited by acute application of the CI inhibitor rotenone. The aberrant CI assembly and reduced O(2) consumption in KO cells were fully normalized by NDUFS4 gene complementation. Cellular [NAD(+)]/[NADH] ratio, lactate production and mitochondrial tetramethyl rhodamine methyl ester (TMRM) accumulation were slightly increased in KO cells. In contrast, NDUFS4 gene deletion did not detectably alter [NADP(+)]/[NADPH] ratio, cellular glucose consumption, the protein levels of hexokinases (I and II) and phosphorylated pyruvate dehydrogenase (P-PDH), total cellular adenosine triphosphate (ATP) level, free cytosolic [ATP], cell growth rate, and reactive oxygen species (ROS) levels. We conclude that the NDUFS4 subunit is of key importance in CI stabilization and that, due to the metabolic properties of the immortalized MEFs, NDUFS4 gene deletion has only modest effects at the live cell level. This article is part of a special issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Flicking the Warburg switch-tyrosine phosphorylation of pyruvate dehydrogenase kinase regulates mitochondrial activity in cancer cells

In this issue of Molecular Cell, Hitosugi et al. (2011) show that the switch from oxidative phosphorylation to glycolysis in cancer cells is regulated by tyrosine phosphorylation of PDHK1.