Pharmacological Reviews current issue

Answering the Burning Question of How Transient Receptor Potential Vanilloid-1 Channel Antagonists Cause Unwanted Hyperthermia [Perspective]

Ayoub, S. S., Hunter, J. C., Simmons, D. L. — Mon, 05 Oct 2009 05:29:35 PDT

The treatment of chronic pain with new therapies that lack the side effects of existing analgesics is one of medicine's great unmet needs. Toward this goal, antagonists of the transient receptor potential vanilloid-1 (TRPV1) channel have shown some promise. However, the development of these compounds has been hindered by an unpleasant on-target hyperthermic side effect. With compelling evidence, the accompanying critical review by Romanovsky et al. (p. 228) regarding TRPV1 takes a position on the sites of action of TRPV1 agonists and antagonists on the thermoregulatory system that controls this side effect. From this comes insight on potential ways to overcome the unwanted hyperthermic action of TRPV1 antagonists.

The Transient Receptor Potential Vanilloid-1 Channel in Thermoregulation: A Thermosensor It Is Not [Review Articles]

Mon, 05 Oct 2009 05:29:35 PDT

The development of antagonists of the transient receptor potential vanilloid-1 (TRPV1) channel as pain therapeutics has revealed that these compounds cause hyperthermia in humans. This undesirable on-target side effect has triggered a surge of interest in the role of TRPV1 in thermoregulation and revived the hypothesis that TRPV1 channels serve as thermosensors. We review literature data on the distribution of TRPV1 channels in the body and on thermoregulatory responses to TRPV1 agonists and antagonists. We propose that two principal populations of TRPV1-expressing cells have connections with efferent thermoeffector pathways: 1) first-order sensory (polymodal), glutamatergic dorsal-root (and possibly nodose) ganglia neurons that innervate the abdominal viscera and 2) higher-order sensory, glutamatergic neurons presumably located in the median preoptic hypothalamic nucleus. We further hypothesize that all thermoregulatory responses to TRPV1 agonists and antagonists and thermoregulatory manifestations of TRPV1 desensitization stem from primary actions on these two neuronal populations. Agonists act primarily centrally on population 2; antagonists act primarily peripherally on population 1. We analyze what roles TRPV1 might play in thermoregulation and conclude that this channel does not serve as a thermosensor, at least not under physiological conditions. In the hypothalamus, TRPV1 channels are inactive at common brain temperatures. In the abdomen, TRPV1 channels are tonically activated, but not by temperature. However, tonic activation of visceral TRPV1 by nonthermal factors suppresses autonomic cold-defense effectors and, consequently, body temperature. Blockade of this activation by TRPV1 antagonists disinhibits thermoeffectors and causes hyperthermia. Strategies for creating hyperthermia-free TRPV1 antagonists are outlined. The potential physiological and pathological significance of TRPV1-mediated thermoregulatory effects is discussed.

Inflammatory Stress and Idiosyncratic Hepatotoxicity: Hints from Animal Models [Review Articles]

Deng, X., Luyendyk, J. P., Ganey, P. E., Roth, R. A. — Mon, 05 Oct 2009 05:29:35 PDT

Adverse drug reactions (ADRs) present a serious human health problem. They are major contributors to hospitalization and mortality throughout the world (Lazarou et al., 1998; Pirmohamed et al., 2004). A small fraction (less than 5%) of ADRs can be classified as "idiosyncratic." Idiosyncratic ADRs (IADRs) are caused by drugs with diverse pharmacological effects and occur at various times during drug therapy. Although IADRs affect a number of organs, liver toxicity occurs frequently and is the primary focus of this review. Because of the inconsistency of clinical data and the lack of experimental animal models, how IADRs arise is largely undefined. Generation of toxic drug metabolites and induction of specific immunity are frequently cited as causes of IADRs, but definitive evidence supporting either mechanism is lacking for most drugs. Among the more recent hypotheses for causation of IADRs is that inflammatory stress induced by exogenous or endogenous inflammagens is a susceptibility factor. In this review, we give a brief overview of idiosyncratic hepatotoxicity and the inflammatory response induced by bacterial lipopolysaccharide. We discuss the inflammatory stress hypothesis and use as examples two drugs that have caused IADRs in human patients: ranitidine and diclofenac. The review focuses on experimental animal models that support the inflammatory stress hypothesis and on the mechanisms of hepatotoxic response in these models. The need for design of epidemiological studies and the potential for implementation of inflammation interaction studies in preclinical toxicity screening are also discussed briefly.

Pituitary Adenylate Cyclase-Activating Polypeptide and Its Receptors: 20 Years after the Discovery [Review Articles]

Mon, 05 Oct 2009 05:29:35 PDT

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 38-amino acid C-terminally -amidated peptide that was first isolated 20 years ago from an ovine hypothalamic extract on the basis of its ability to stimulate cAMP formation in anterior pituitary cells (Miyata et al., 1989. PACAP belongs to the vasoactive intestinal polypeptide (VIP)-secretin-growth hormone-releasing hormone-glucagon superfamily. The sequence of PACAP has been remarkably well conserved during evolution from protochordates to mammals, suggesting that PACAP is involved in the regulation of important biological functions. PACAP is widely distributed in the brain and peripheral organs, notably in the endocrine pancreas, gonads, respiratory and urogenital tracts. Characterization of the PACAP precursor has revealed the existence of a PACAP-related peptide, the activity of which remains unknown. Two types of PACAP binding sites have been characterized: type I binding sites exhibit a high affinity for PACAP and a much lower affinity for VIP, whereas type II binding sites have similar affinity for PACAP and VIP. Molecular cloning of PACAP receptors has shown the existence of three distinct receptor subtypes: the PACAP-specific PAC1-R, which is coupled to several transduction systems, and the PACAP/VIP-indifferent VPAC1-R and VPAC2-R, which are primarily coupled to adenylyl cyclase. PAC1-Rs are particularly abundant in the brain, the pituitary and the adrenal gland, whereas VPAC receptors are expressed mainly in lung, liver, and testis. The development of transgenic animal models and specific PACAP receptor ligands has strongly contributed to deciphering the various actions of PACAP. Consistent with the wide distribution of PACAP and its receptors, the peptide has now been shown to exert a large array of pharmacological effects and biological functions. The present report reviews the current knowledge concerning the pleiotropic actions of PACAP and discusses its possible use for future therapeutic applications.

Therapeutic Targeting of Nuclear Protein Import in Pathological Cell Conditions [Review Articles]

Chahine, M. N., Pierce, G. N. — Mon, 05 Oct 2009 05:29:35 PDT

Proteins enter the nucleus through the nuclear pore complex. Once in the nucleus, some proteins, such as transcriptional regulators, can turn genes on or off, and change the composition of the cell and its function to meet the demands of its environment. This process of protein import into the nucleus is highly controlled and regulated by the expression or function of single cargoes, transport receptors, or the transport channels themselves. Thus, these components of the import process have an impact on transport capacity, which subsequently affects gene expression, signal transduction, and cell growth and development. With such a key position in the process of cell growth, it is reasonable to hypothesize that alterations in nuclear protein transport may play an important role in pathological cell conditions that have abnormal cell growth as a central feature. Indeed, there are now sufficient data to demonstrate that alterations in nuclear protein transport participate in alterations in cell proliferation and hypertrophy. Further study is needed to provide definitive proof that changes in nuclear protein import directly participate in the pathogenesis of diseases such as hypertension, atherosclerosis, cancer, viral infection, and diabetes. However, the data to date have, on select occasions, led to a clear association of alterations in nuclear transport with disease states. Furthermore,this research has led to the important identification of new targets within the process of nuclear protein import that hold therapeutic promise to inhibit viral replication, to improve drug delivery during cancer therapy, and, in general, to modify cell growth and viability during disease conditions.

Regulation of Skeletal Muscle Physiology and Metabolism by Peroxisome Proliferator-Activated Receptor {delta} [Review Articles]

Ehrenborg, E., Krook, A. — Mon, 05 Oct 2009 05:29:35 PDT

Agonists directed against the and isoforms of the peroxisome proliferator-activated receptors (PPARs) have become important for the respective treatment of hypertriglyceridemia and insulin resistance associated with metabolic disease. PPAR is the least well characterized of the three PPAR isoforms. Skeletal muscle insulin resistance is a primary risk factor for the development of type 2 diabetes. There is increasing evidence that PPAR is an important regulator of skeletal muscle metabolism, in particular, muscle lipid oxidation, highlighting the potential utility of this isoform as a drug target. In addition, PPAR seems to be a key regulator of skeletal muscle fiber type and a possible mediator of the adaptations noted in skeletal muscle in response to exercise. In this review we summarize the current status regarding the regulation, and the metabolic effects, of PPAR in skeletal muscle.