Oxidation and tyrosine phosphorylation: synergistic or antagonistic cues in protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) have been generally recognised as key modulators of cell proliferation, differentiation, adhesion and motility. During signalling, several PTPs undergo two posttranslational modifications that greatly affect their enzymatic activity: tyrosine phosphorylation and cysteine oxidation. Although these modifications share their reversibility depending on the intracellular environment, their effects on enzymatic activity are opposite, tyrosine phosphorylation being correlated to enzyme activation and thiol oxidation to complete inactivation. Several papers have suggested that both these modifications occur in response to the same stimuli i.e. cell proliferation induced by numerous growth factors and cytokines. Conversely, the possibility that these two regulation mechanisms act simultaneously on PTPs has not been established and very few reports investigated this dual regulation of PTPs. To underline the relevance of the question, we discuss several possibilities: (i) that tyrosine phosphorylation and cysteine oxidation of PTPs may share the same target molecules but with different kinetics; (ii) that PTP phosphorylation and oxidation may take place on different subcellular pools of the same protein and (iii) that these two modifications, although having divergent effects on enzyme activity, cooperate in the integrated and coordinated function of PTPs during receptor tyrosine kinase signalling. We believe that our perspective will open new perspectives on an ancient problem – the apparent contradiction of opposing enzymatic regulation of many PTPs – thus clarifying their role as positive or negative transducers (or both) of many extracellular stimuli.Received 11 October 2004; received after revision 26 January 2005; accepted 10 February 2005 Available online 29 March 2005     

Regulation of mitochondrial oxidative phosphorylation by second messenger-mediated signal transduction mechanisms

The mitochondrial oxidative phosphorylation system is responsible for providing the bulk of cellular ATP molecules. There is a growing body of information regarding the regulation of this process by a number of second messenger-mediated signal transduction mechanisms, although direct studies aimed at elucidating this regulation are limited. The main second messengers affecting mitochondrial signal transduction are cAMP and calcium. Other second messengers include ceramide and reactive oxygen species as well as nitric oxide and reactive nitrogen species. This review focuses on available data on the regulation of the mitochondrial oxidative phosphorylation system by signal transduction mechanisms and is organised according to the second messengers involved, because of their pivotal role in mitochondrial function. Future perspectives for further investigations regarding these mechanisms in the regulation of the oxidative phosphorylation system are formulated. Received 11 December 2005; received after revision 14 January 2006; accepted 6 February 2006     

Alteration of mitochondrial bioenergetics due to intravenous injection of a perfluorocarbon emulsion

Wistar albino rats were intravenously injected with 1 ml of an oxyphoretic emulsion of perfluorobutylfurane and killed 3, 7 or 30 days later. Mitochondria isolated from the liver and kidneys of treated rats showed a small decrease in the transmembrane electrical potential and a substantial depression of the rates of both ATP synthesis and ADP-stimulated respiration. These alterations in mitochondrial oxidative phosphorylation appear to be induced by perfluorocarbon and/or tensioactive molecules interacting with hydrophobic cell structures.     

Interaction of mitochondrial porin with cytosolic proteins

Summary Intracellular phosphorylation is an important step in active uptake and utilization of carbohydrates. For example glucose and glycerol enter the liver, cell along the extra intracellular gradient by facilitated diffusion through specific carriers and are concentrated inside the cell by phosphorylation via hexokinase or glycerol kinase. Depending on the function of the respective tissue the uptake of carbohydrates serves different metabolic purposes. In brain and kidney medulla cells which depend on carbohydrates, glucose and glycerol are taken up according to the energy demand. However, in tissues such as muscle which synthesize glycogen or like liver which additionally produce fat from glucose, the uptake of carbohydrates has to be regulated according to the availability of glucose and glycerol. How the reversible coupling of the kinases to the outer membrane pore and the mitochondrial ATP serves to fulfil these specific requirements will be explained as well as how this regulates the carbohydrate uptake in brain according to the activity of the oxidative phosphorylation and how this allows glucose uptake in liver, and muscle to persist in the presence of high glucose 6-phosphate without activating the rate of glycolysis.     

A new selective insecticidal uncoupler of oxidative phosphorylation

Summary A new aryl hydrazone structure with high insecticidal activity against the Australian sheep blowfly,Lucilia cuprina, was shown to have a higher activity as an uncoupler of oxidative phosphorylation in insect compared to mammalian mitochondrial preparations. This compound possesses the requirements of other uncouplers in its measured pKa and lipid solubility. However, when compared to a closely related structure with similar physicochemical properties, its insecticidal and insect mitochondrial uncoupling activities are greater and it exhibits decreased mammalian toxicity corresponding to this differential biochemical selectivity.Acknowledgment. We thank Mr K. Rihs for preparation of the hydrazones and Prof. Dr K.H. Büchel for supply of hydrazone III.     

The ATP synthase (F0−F1) complex in oxidative phosphorylation

The transmembrane electrochemical proton gradient generated by the redox systems of the respiratory chain in mitochondria and aerobic bacteria is utilized by proton translocating ATP synthases to catalyze the synthesis of ATP from ADP and P_i. The bacterial and mitochondrial H⁺-ATP synthases both consist of a membranous sector, F₀, which forms a H⁺-channel, and an extramembranous sector, F₁, which is responsible for catalysis. When detached from the membrane, the purified F₁ sector functions mainly as an ATPase. In chloroplasts, the synthesis of ATP is also driven by a proton motive force, and the enzyme complex responsible for this synthesis is similar to the mitochondrial and bacterial ATP synthases. The synthesis of ATP by H⁺-ATP synthases proceeds without the formation of a phosphorylated enzyme intermediate, and involves co-operative interactions between the catalytic subunits.     

The role of myosin phosphorylation in the contraction-relaxation cycle of smooth muscle

Summary Considerable evidence from a variety of experimental procedures indicates that the phosphorylation of myosin is involved in the regulation of contractile activity in smooth muscle. Phosphorylation of the 20,000-dalton myosin light chains is required to initiate crossbridge cycling and this is consistent with the observation that the actin-activated Mg²⁺-ATPase activity of myosin is phosphorylation-dependent. In the simplest interpretation of this process it may be proposed that phosphorylation acts as an on-off switch. Clearly this cannot explain the observed complexity of smooth muscle contractile behavior and such may imply either that additional mechanisms are involved or that the role of myosin phosphorylation is not fully appreciated. Recently it has been shown that monomeric smooth muscle myosin can exist in a folded and an extended conformation and that each form is characterized by distinct enzymatic properties. Under appropriate solvent conditions phosphorylation of myosin favors the extended conformation. It is tentatively suggest that this, or an analogous, transition might be involved in the regulation of the smooth muscle contractile apparatus, and this possibility is discussed.The authors are supported by grants HL 23615 and HL 20984 from the National Institutes of Health.     

Regulation of nicotinic acetylcholine receptors by tyrosine kinases in the peripheral and central nervous system: same players, different roles

Nicotinic acetylcholine receptors (nAChRs) exist in many subtypes and are found in the peripheral and central nervous system where they mediate or modulate synaptic transmission. We review how tyrosine phosphorylation and kinases regulate muscle and neuronal nAChRs. Interestingly, although some of the same kinase players interact with the various receptor subtypes, the functional consequences are different. While concerted action of MuSK, Abl- and Src-family kinases (SFKs) regulates the synaptic distribution of nAChRs at the neuromuscular junction, SFKs activate heteromeric neuronal nAChRs in adrenal chromaffin cells, thereby enhancing catecholamine secretion. In contrast, the activity of homomeric neuronal nAChRs, as found in the hippocampus, is negatively regulated by tyrosine phosphorylation and SFKs. It appears that tyrosine kinases provide the means to regulate all nAChRs; but the functional consequences, even those caused by the same kinase family, are specific for each receptor subtype and location. Received 21 February 2006; received after revision 24 July 2006; accepted 30 August 2006     

Expression of the Stp1 LMW-PTP and inhibition of protein CK2 display a cooperative effect on immunophilin Fpr3 tyrosine phosphorylation and <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> growth

Although the yeast genome does not encode bona fide protein tyrosine kinases, tyrosine-phosphorylated proteins are numerous, suggesting that besides dual-specificity kinases, some Ser/Thr kinases are also committed to tyrosine phosphorylation in Saccharomyces cerevisiae. Here we show that blockage of the highly pleiotropic Ser/Thr kinase CK2 with a specific inhibitor synergizes with the overexpression of Stp1 low-molecular-weight protein tyrosine phosphatase (PTP) in inducing a severe growth-defective phenotype, consistent with a prominent role for CK2 in tyrosine phosphorylation in yeast. We also present in vivo evidence that immunophilin Fpr3, the only tyrosine-phosphorylated CK2 substrate recognized so far, interacts with and is dephosphorylated by Spt1. These data disclose a functional correlation between CK2 and LMW-PTPs, and suggest that reversible phosphorylation of Fpr3 plays a role in the regulation of growth rate and budding in S. cerevisiae.Received 15 January 2004; received after revision 20 February 2004; accepted 4 March 2004     

Towards an understanding of the role of glutamate in neurodegenerative disorders: energy metabolism and neuropathology

It is thought that impairment, of energy metabolism that results in deterioration of membrane function, leading to loss of the Mg²⁺ block on NMDA receptors, and allowing persistent activation of these receptors by glutamate, might be a cause of neuronal death in neurodegenerative disorders. Studies in rodents using mitochondrial respiratory chain toxins, such as aminooxyacetic acid, 1-methyl-4-phenylpyridinium, malonic acid and 3-nitropropionic acid, suggest that such processes may indeed be involved in neurotoxicity. Striatal and nigral degeneration induced by mitochondrial toxins in rodents resembles the neuropathology seen in humans suffering from Huntington's or Parkinson's disease, and can be prevented either by decortication or by NMDA receptor antagonists. Such experimental observations suggest that glutamate may be involved in neuronal death leading to neurodegenerative disorders in humans. If so, glutamate antagonists may offer a therapeutic approach for retarding the progression of these disabling disorders.