Current Medicinal Chemistry - Volume 10, Issue 23, 2003

Mitochondria are responsible for over 80% of ATP production in normal cells. Without mitochondrial ATP production, individuals could not maintain their normal functions or survive [1]. Although mitochondria have long been considered solely to be a powerhouse for production of cellular energy, recently mitochondria have been clearly recognized to play a central role in execution of apoptosis or programmed cell death; many mitochondrially localizing proteins have been identified as pro- or anti-apoptotic factors. Thus, mitochondria are a kind of an arsenal. In addition, mitochondria are involved in cell proliferation and differentiation [2]. Mitochondria may also be an operating tower. Thus mitochondria regulate three essential parts of life: living, dying, and growing. Considering all these critical features of mitochondria, it is natural that mitochondria can be a very effective target of medicinal drugs. In this special issue, we focus on mitochondria from therapeutic aspects. For better understanding, Ohta overviewed the mitochondrial diseases with mitochondrial DNA mutations and the apoptotic signaltransduction pathways with special reference with mitochondria [3]. Mitochondria account for ∼90% of cellular O2 consumption due to ATP production by oxidative phosphorylation, i.e. the aerobic ATP production. The O2 consumption for the aerobic ATP production inevitably accompanies production of reactive oxygen species (ROS). It is considered that 1∼5% of the O2 consumed is converted to ROS. This huge amount of the ROS production by mitochondria is involved in a variety of pathological conditions and furthermore contributes to the cumulative oxidative damages of organs and tissues with age. The proper control of the mitochondrial ROS is crucial for coping with many pathological situations and maintaining physiological states. Inoue et al. described critical importance of the mitochondrial ROS management [4]. Mitochondrial dysfunction in neurodegeneration, e.g. Parkinson disease and Alzheimer disease, is one of keen interests in aging processes. 1-Methyl-4-phenylpyridinium ion (MPP+) causes Parkinson disease-like symptoms in human. This drug accumulates selectively in mitochondria of dopaminergic cells and impairs mitochondrial functions directly and indirectly, giving many insights into the mitochondrial roles in neurodegenrative diseases and neuronal aging per se. Kotake and Ohta extensively delineated the mechanisms of the drug-induced cell damage using derivatives of MPP+ [5]. Induction of apoptosis in damaged cells is an essential part of a physiological cancer prevention system. Artificial or forced induction of apoptosis in cancer cells, in turn, becomes a valuable anti-cancer therapy. Mitochondria occupy a central position of apoptotic pathways. Morisaki and Katano proposed theoretically and experimentally that mitochondria-targeting drugs are useful alternatives as anti-cancer drugs [6]. Mitochondria harbor their own genome. This mitochondrial genome is indispensable for the normal construction of the respiratory chain responsible for the aerobic ATP production. The mitochondrial genome is more fragile than nuclear genome in part due to its location in the ROS-producing organelle [7]. Patients with mutations of the mitochondrial genome mainly suffer from encephalomyopathy partly because both of brain and muscle are highly energy-demanding organs. Currently we do not have effective and practical therapies for those patients. Schon and DiMauro reported their unique therapeutic approaches to increase the ATP production by using medicinal drugs or by introduction of mitochondrial genes into nuclear genome [8]. ATP is essential not only for human individuals but also for parasites living on the hosts. Therefore if we take advantage of the differential properties of the respiratory chain between hosts and parasites, the mitochondrial machineries of parasites can be selectively attacked. Kita et al. beautifully demonstrated this example [9]. All articles in this special issue unambiguously illustrate a variety of aspects of mitochondria as a therapeutic target. I believe that these articles help us understand mitochondria from a point of view of medicinal chemistry. References [1] Kang, D.; Takeshige, K.; Sekiguchi, M.; Singh, K. K. (1998) in Mitochondrial DNA Mutations in Aging, Disease and Cancer (Singh, K. K., ed), pp. 1, Springer-Verlag and R.G. Landes Company, Austin. [2] Rochard, P.; Rodier, A.; Casas, F.; Cassar-Malek, I.; Marchal-Victorion, S.; Daury, L.; Wrutniak, C.; Cabello, G. J. Biol. Chem. 2000 275, 2733. [3] Ohta, S. this issue. [4] Inoue, M.; Sato, E.; Nishikawa, M.; Park, A.-M.; Kira, K.; Imada, I.; Utsumi, K. this issue. [5] Kotake, Y.; Ohta, S. this issue. [6] Morisaki, T; Katano, M. this issue. [7] Kang, D.; Hamasaki, N. Curr. Genet. 2002 41, 311. [8] Schon, E.; DiMauro, S. this issue. [9] Kita, K.; Nihei, C; Tomitsuka, E. this issue.

The mitochondrion, long considered an organelle specific to energy metabolism, is in fact multi-functional and involved in many diseases. Mitochondrial DNA accumulates somatic mutations during aging, the progression of cancer and diabetes. Most cancer cells contain homoplasmic mutations in the mitochondrial genome. Although little is known about the contributions of mutations to carcinogenesis, some mutations in the nuclear genes encoding mitochondrial proteins have been identified as responsible for certain familial cancers. Mitochondria play an essential role in generating the germ line by releasing mitochondrial ribosomal RNAs, by which the germ line transfers the genetic information necessary for life to the next generation. Collaboration between mitochondria and the cytosol occurs in several metabolic pathways. Many enzymes involved in synthesizing uridine, heme and steroids and in the urea cycle are located inside mitochondria. Notably, a reaction involved in the synthesis of UMP is coupled with the energized state of mitochondria. Thus, the synthesis of DNA and RNA should be indirectly coupled with the energized state of mitochondria. Additionally, storing calcium is an important role of mitochondria. Calcium functions as a second messenger in signal transduction, however, it also activates several proteinases or lipases to induce damage. The mitochondrion plays a significant role in necrosis and is a center for apoptosis, determining its initiation, regulation and execution. Thus, the mitochondrion is widely involved in cell proliferation, cell death and disease.

Mitochondria are the major site for the generation of ATP at the expense of molecular oxygen. Significant fractions (∼2%) of oxygen are converted to the superoxide radical and its reactive metabolites (ROS) in and around mitochondria. Although ROS have been known to impair a wide variety of biological molecules including lipids, proteins and DNA, thereby causing various diseases, they also play critical roles in the maintenance of aerobic life. Because mitochondria are the major site of free radical generation, they are highly enriched with antioxidants including GSH and enzymes, such as superoxide dismutase (SOD) and glutathione peroxidase, on both sides of their membranes to minimize oxidative stress in and around this organelle. The present work reviews the sites and mechanism of ROS generation by mitochondria, mitochondrial localization of Mn-SOD and Cu,Zn-SOD which has been postulated for a long time to be a cytosolic enzyme. The present work also describes that a cross-talk of molecular oxygen, nitric oxide (NO) and superoxide radicals regulates the circulation, energy metabolism, apoptosis, and functions as a major defense system against pathogens. Pathophysiological significance of ROS generation by mitochondria in the etiology of aging, cancer and degenerative neuronal diseases is also described.

This review focuses on the mechanisms of action and the injurious effect of complex I inhibitors, of which 1-methyl-4-phenylpyridinium ion (MPP+) is a well studied example. These compounds can be divided into two groups, i.e. competitive inhibitors with respect to ubiquinone, such as piericidine A, and noncompetitive inhibitors such as rotenone. Complex I inhibitors such as MPP+ have been reported to induce anatomical, behavioral, and biochemical changes similar to those seen in Parkinson's disease, which is characterized by nigrostriatal dopaminergic neuro-degeneration. Spectroscopic analyses and structure-activity relationship studies have indicated that the V-shaped structure of the rotenone molecule is critical for binding to the rotenone binding site on complex I. Many isoquinoline derivatives, some of them endogenous, are also complex I inhibitors. Many lines of evidence show that complex I inhibitors elicit neuronal cell death. Recently, it was reported that chronic and systemic exposure to low-dose rotenone reproduces the features of Parkinson's disease. This work further focused attention on compounds acting on mitochondria, such as MPP+. In Guadeloupe, the French West Indies, patients with atypical parkinsonism or progressive supranuclear palsy are frequently encountered. These diseases seem to be associated with ingestion of tropical herbal teas or tropical fruits of the Annonaceae family, which contain complex I inhibitors such as benzylisoquinoline derivatives and acetogenins. Complex I inhibitors may not simply result in reactive oxygen species generation or ATP exhaustion, but may influence complex downstream signal transduction processes. An understanding of these changes would throw light on the ways in which complex I inhibitors induce a wide range of abnormalities.

The cytotoxic effects of many anticancer drugs are mediated via the apoptotic pathways. Chemoresistant tumor cells have acquired the ability to evade the action of multiple classes of anti-cancer drugs. One mechanism by which tumor cells survive in the presence of chemotherapy is by increasing antiapoptotic activities. Since mitochondria are critical 'gatekeepers' to the apoptosis process, development of cytotoxic drugs that target mitochondria may provide a new strategy to induce apoptosis in tumor cells. Mitochondrial permeability transition pore complex (PTPC) controls mitochondrial membrane permeabilization, which is a critical event in the process leading to chemotherapy-induced apoptosis. Therefore, targeting of PTPC components may overcome chemoresistance in tumor cells. Moreover, alterations in mitochondrial DNA such as mutation and the subsequent dysfunction of mitochondrial respiratory enzyme have been reported in various types of cancer, and their functional consequences are associated cancer development, chemoresistance, and therapeutic implications. In this mini-review, we aim to provide a brief review on several mitochondria-targeting strategies to overcome chemoresistance in cancer.

Although great progress has been made in our understanding of the molecular bases of mitochondrial disorders due to defects in the respiratory chain, little exists in the way of rational therapy. Possible therapeutic approaches include: palliative therapy; removal of noxious metabolites; administration of artificial electron acceptors, metabolites, and free radical scavengers; genetic counseling; and gene therapy. There has been progress with each of these approaches, although much work remains to be done. Finally, a novel approach to treating a specific mitochondrial disorder, MELAS, is presented.

Parasites have developed a wide variety of physiological functions to survive within the specialized environments of the host. Regarding energy metabolism, which represents an essential factor for survival, parasites adapt low oxygen tension in host mammals using metabolic systems that differ substantially from those of the host. Most parasites do not use free oxygen available within the host, but employ systems other than oxidative phosphorylation for ATP synthesis. Furthermore, parasites display marked changes in mitochondrial morphology and components during the life cycle, and these represent very interesting elements of biological processes such as developmental control and environmental adaptation. The enzymes in parasite-specific pathways offer potential targets for chemotherapy. Cyanide-insensitive trypanosome alternative oxidase (TAO) is the terminal oxidase of the respiratory chain of long slender bloodstream forms of the African trypanosome, which causes sleeping sickness. Recently, the most potent inhibitor of TAO to date, ascofuranone, was isolated from the phytopathogenic fungus, Ascochyta visiae. The inhibitory mechanisms of ascofuranone have been revealed using recombinant enzyme. Parasite-specific respiratory systems are also found in helminths. The NADH-fumarate reductase system in mitochondria form a final step in the phosphoenolpyruvate carboxykinase (PEPCK)-succinate pathway, which plays an important role in anaerobic energy metabolism for the Ascaris suum adult. Enzymes in this system, such as NADH-rhodoquinone reductase (complex I) and rhodoquinol-fumarate reductase (complex II), form promising targets for chemotherapy. In fact, a specific inhibitor of nematode complex I, nafuredin, has been found in mass-screening using parasite mitochondria.

Over the past few decades, remarkable advances have been achieved in cancer therapy, including chemotherapeutic agents, their mode of application and more broader therapeutic strategies. Promising new therapeutic targets have emerged in the past ten years as a result of recent advances in our understanding of the pathobiology of malignant cells, in particular, regarding functions of suppressor oncogene products. Among them, the agents that alter the cell cycle have recently been of particular interest, since cell cycle regulation is basic mechanism underlying cell fate, i.e., proliferation, differentiation or death. Furthermore, the human genome project has made possible the future development of so-called “tailor-made medicine”, i.e. the design of appropriate drugs for specific genetic profiles and application of drugs that are tailored to each tumor and patient. In this article, we will introduce and discuss recent progress in the development of agents that influence the cell cycle and their future potential in cancer therapy from three standpoints with our own experimental works; i) the inhibition of cell proliferation and / or induction of differentiation by cyclindependent kinase (cdk)-inhibitor, e.g. olomoucin, butyrolactone-I, ii) induction of apoptosis by directing “abortive cell cycle”, or the transient upregulation of cdk activity, e.g. flavopiridol, and iii) countering the development of drug resistance by adjunctive administration of cdk-inhibitors with conventional anti-cancer drug, e.g., p21-gene transfer with cisplatin. Conclusively none of these three approaches by itself is satisfactory, and that the effective cancer therapies will require the administration of several agents and / or methods under the design of their synergistic effects.

Abrupt cessation of long-term alcohol consumption produces well-defined symptoms called alcohol withdrawal (AW). The exact pathophysiological mechanisms involved in the appearance of AW symptoms and particularly those related to the precipitation of delirium tremens (DT), still await clarification in spite of the fact that the prediction of complicated AW is essential to guarantee that appropriate therapies may be planned in advance. Changes in central nervous system (CNS) glutamate- and GABA-transmission and a role of voltageoperated calcium channels are equally important elements of neuroadaptation to the chronic presence of alcohol. In addition to the CNS regulation, however, changes in peripheral fluid and electrolyte homeostasis may accompany, and are expected to modify the clinical symptoms of AW. In an early phase of acute withdrawal, plasma levels of atrial natriuretic peptide (ANP), plasma renin activity and aldosterone are high. In patients with DT, elevated levels of ANP were observed days before the actual onset of DT. It is concluded that the altered plasma ANP secretion might be associated with, and therefore used as an indicator of the onset of DT. However, ANP is present in and produced by the brain and thus it can be regarded as a neuropeptide. The role of CNS ANP was studied in mice, rendered tolerant to and physically dependent on alcohol. Intracerebroventricular injections of ANP attenuated, whereas those of an antiserum against ANP intensified hyperexcitability during AW. ANP in the brain - the content of which undergoes sensitive changes in the hippocampus during AW - appears to interact primarily with glutamate transmission through the NMDA-receptors. This brain structure is of utmost importance for the generation of withdrawal-related hyperexcitability. It is concluded that peripheral secretion of ANP might be a diagnostics indicator, whereas ANP in the CNS might be a modulator of AW.

Polyglutamine diseases are hereditary neurodegenerative disorders caused by the expansion of a CAG repeat in the disease gene. A dominant gain of function is associated with these expanded alleles. The resulting elongated polyglutamine repeats are thought to cause structural changes in the affected proteins, leading to aberrant interactions such as those that allow formation of extra- and intranuclear aggregates. However, self-association is not the only interaction the polyglutamine domain is capable of mediating. Many cellular proteins can be sequestered into inclusions or bound by more soluble forms of the mutant proteins. One group of proteins that binds to and whose activity may be altered by polyglutamines is Histone Acetyltransferases (HATs). HATs are responsible for the acetylation of histones and several other important proteins and this modification results in altered function of the target protein. HATs regulate cellular processes at levels as different as modifying transcriptional competence of chromosomes, temporal regulation of promoter activity and protein activation / inactivation. Recent studies show that the altered balance between protein acetylation and deacetylation may be a key process contributing to expanded polyglutamine-induced pathogenesis. The restoration of this balance is possible by the genetic or pharmacological reduction of the opposing enzyme group, i.e. the Histone Deacetylases (HDACs). Recent progress in HDAC research has made the development of inhibitors of specific HDAC family proteins possible and these compounds could be effective candidates for treatment of these devastating diseases.

The ability of ribozymes (i.e. RNA enzymes) to specifically recognize and subsequently catalyze the cleavage of an RNA substrate makes them attractive for the development of therapeutic tools for the inactivation of both viral RNAs and mRNAs associated with various diseases. Several applicable ribozyme models have been tested both in vitro and in a cellular environment, and have shown significant promise. However, several hurdles remain to be surpassed before we generate a useful geneinactivation system based on a ribozyme. Among the most important requirements for further progress are a better understanding of the features that contribute to defining the substrate specificity for cleavage by a ribozyme, and the identification of the potential cleavage sites in a given target RNA. The goal of this review is to illustrate the importance of both of these factors at the RNA level in the development of any type of ribozyme based gene-therapy. This is achieved by reviewing the recent progress in both the structure-function relationships and the development of a gene-inactivation system of a model ribozyme, specifically delta ribozyme.

Oxidative stress is defined as the consequence of overpowering of the immune system's reaction, which causes increased production of the reactive oxidative species (ROS) greater than the antioxidant protection. Tissue injury and oxidation of the circulating molecules may be the consequences. Moreover, the sulphur-containing amino acids (SAA) fate is perturbed during stress. The altered biochemical rules during inflammation weaken the anti-oxidant mechanism, and the extra-supply of SAA under inflammatory conditions can help to restore homeostasis. In brief, the main biochemical steps during inflammation are: - The production of Cytokines, Acute Phase Protein, and Glutathione (GSH) pool are strongly modified during inflammation. - The GSH participates in many important physiological processes controlling the homeostasis of the cells. - A higher demand of Cysteine (Cys) supply causes difficulties in maintaining a constant GSH level. - The role of GSH as a key regulator of thiol redox intracellular balance is established. This reveals that GSH is essential in regulating the cell's life cycle and that the reduction of intracellular GSH contributes to chronic inflammation. The fact that Cys availability is generally a limiting factor for the GSH synthesis stimulated the development of a pharmacologically useful Cys pro-drug. The simplest derivative is N-acetylcysteine (NAC), which appears to be the prototype of all Cys suppliers. Different approaches are presented here.

Acetylcholine, the first identified neurotransmitter acts on both types of cholinergic receptors. Both rigid and flexible derivatives of acetylcholine could either be selective muscarinic or selective nicotinic agonists while some compounds show activity at both receptor subclasses. Earlier structure-activity considerations are revisited. Ligand and receptor based calculations have been applied in the hope to identify characteristic geometrical and steric requirements for the activity on the receptor subtypes. Results are treated critically and applied cautiously for predicting selective structural requirements by the cholinergic receptor subclasses.

Aurones [2-benzylidenebenzofuran-3(2H)-ones] are the secondary metabolites natural compounds belong to the flavonoids family, and structurally are the isomers of flavones, widely present in fruits and flowers where they play significant role in the pigmentation of the part of plant in which they occur. Literature survey clearly indicates that flavones, chalcones, flavonols and isoflavones have been studied largely for their therapeutical potential. Somehow, aurones still are less studied and it is only recently that these compounds have begun to be investigated. In this review, we report the recent advances made on the therapeutical potential of aurones in different biological areas. Their synthesis, structure-activity relationships, the importance of the substitution pattern will also be discussed. Finally, some aspects regarding the possible development of aurones will be highlighted briefly.