Current Drug Targets - Inflammation & Allergy - Volume 3, Issue 1, 2004

The balance between polymorphonuclear leukocytes (PMNL) apoptosis and necrosis in inflamed tissues is an important determinant of the degree of tissue injury. To prevent senescent PMNL from releasing their toxic contents into surrounding tissues, these cells become apoptotic and are then internalized by tissue macrophages. PMNL apoptosis and subsequent ingestion by macrophages are the major mechanisms for clearing PMNL that have been recruited to the inflamed sites and thus for promoting resolution of the inflammation. PMNL have a short half-life that is extended at the inflamed site by pro-inflammatory cytokines including Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF), Interleukin-8 (IL-8), Gro- α, and they contact with the bacterial cell walls containing lipopolysaccharides (LPS). Conversely, antiinflammatory cytokines, such as IL-10, accelerate the apoptosis of LPS-activated PMNL. Spontaneous PMNL apoptosis does not require Fas ligation but involves proteolytic cascades -caspases (particularly caspases 3 and 8), calpains and the proteasome-that activate kinases, e.g. caspase 3-mediated activation of protein kinase C-δ, dissociate actin-binding proteins from filamentous actin, and participate in cell surface as well as nuclear morphological transformations. Members of the Bcl-2 protein family, Mcl-1 and A1, are involved in the regulation of PMNL apoptosis. Cell surface receptors and protein kinases, particularly mitogen-activated protein kinases (MAPK), also play critical roles in transducing the signals that result in PMNL apoptosis or extended survival. A growing understanding of the mechanisms regulating leukocyte apoptosis and of the molecules mediating safe phagocytic clearance of dying cells may yield new insights into the pathogenesis of inflammatory diseases. In this regard, therapeutic strategies to resolve chronic inflammation could usefully target PMNL. This review summarises current knowledge on the molecular mechanisms and components of PMNL apoptosis.

In the last decades several preclinical models for sepsis have been used to study the pathophysiologic processes during sepsis. Although these studies revealed promising immunomodulating agents for the treatment of sepsis, clinical trials evaluating the efficacy of these new agents in septic patients were disappointing. It should be realized that most of the preclinical models for sepsis lack a localized infectious source from which the infection disseminates. Studies on the effects of several immunomodulating strategies have demonstrated strikingly opposite results when using models for sepsis with a more natural route of infection, such as pneumonia, and when using models for sepsis lacking an infectious focus. In this review we will compare models for sepsis and models for pneumonia. We advise to use a combination of models, including models for sepsis and models for localized infections, to test new immunomodulating strategies before starting any clinical trial evaluating a new immunomodulating therapy.

Considerable amount of work has been done in the area of enzymatic and nonenzymatic oxidation of arachidonic acid. This effort resulted in understanding of the functions of lipid mediators - eicosanoids in various aspects of health and disease. A mechanism by which aspirin exerts therapeutic effects puzzled pharmacologists for a long time until John Vane, in 1971, discovered that aspirin and its congeners block formation of prostaglandins, a class of lipids that originate from oxidation of arachidonic acid by cyclooxygenase. Since that discovery the pharmacology of eicosanoids has substantially progressed, which resulted in new drugs available in clinics. In addition to many new inhibitors of cyclooxygenase, two isoforms of which are known, much effort has been given to find inhibitors of synthesis and function of leukotrienes, a class of lipids that are derived from 5-lipoxygenase. These lipids are generated in asthma and their uncontrolled biosynthesis aggravates the symptoms of asthma. A new class of drugs called lukasts, inhibitors of 5-LOX products, has been developed and entered clinics as the first new therapy to treat asthma in nearly 20 years. New discoveries in the field of lipoxygenase show great opportunities for drug development for cancer prevention and treatment as it has been established that lipoxygenases and their products are required for cancer growth. Intense research in this field is likely to produce new drugs in the near future.

In the last decade, the understanding of the molecular mechanisms of regulation of the inflammatory process in chronic inflammatory diseases has moved remarkably forward. Recent evidence in various fields has consistently indicated that Tcells play a key role in initiating and perpetuating inflammation, not only via the production of soluble mediators but also via cell / cell contact interactions with a variety of cell types through membrane receptors and their ligands. Signalling through CD40 and CD40 ligand is a versatile pathway that is potently involved in all these processes. In this article, we review how T-cells become activated by dendritic cells or inflammatory cytokines, and how these T-cells activate, in turn, monocytes / macrophages, endothelial cells, smooth muscle cells and fibroblasts to produce pro-inflammatory cytokines (tumour necrosis factor α, interleukin-6), chemokines (interleukin-8, monocyte chemotactic protein-1), tissue factor, the main initiator of the coagulation cascade in vivo, and finally matrix metalloproteinases, responsible for tissue destruction. Moreover, we discuss how CD40 ligand at inflammatory sites stimulates fibroblasts and tissue monocyte / macrophage production of VEGF, leading to angiogenesis, which promotes and maintains the chronic inflammatory process. This cascade of events is discussed in the context of disease initiation / progression, with particular reference to atherosclerosis and rheumatoid arthritis, and to potential novel therapeutic targets for their treatment.

The use of non-steroidal anti-inflammatory drugs (NSAIDs) is frequently associated with serious adverse effects related to the inhibition of cyclooxygenase (COX) in tissues where prostanoids exert physiological effects, such as gastric mucosal defence, renal homeostasis and platelet aggregation. The discovery of a second COX isoform (COX-2) specifically induced in pathological tissues led to the development of selective COX-2 inhibitors, believed to have an improved safety profile compared to traditional NSAIDs. Animal studies, however, have revealed a protective role for the COX-2 enzyme in the stomach, kidney, heart, vasculature and reproductive system, and therefore, the safety of COX-2 selective inhibitors needs to be reassessed. On the other hand, new therapeutic indications have emerged as a result of the role played by COX-2 overexpression in cancer or Alzheimer's disease. A second approach aimed at obtaining safer NSAIDs is based on the gastroprotective effects of nitric oxide (NO). Traditional NSAIDs chemically linked to NO-releasing moieties retain the therapeutic efficacy, but not the adverse effects, of the parent NSAIDs. Moreover, additional therapeutic applications in cardiovascular diseases, Alzheimer's disease and cancer have been suggested. Animal data, however, need to be confirmed in large clinical trials. Finally, the increase in endogenous NO via a selective increase in inducible NO synthase in the gastric mucosa is the mechanism underlying the good gastric tolerability and the gastroprotective effects of the non-selective NSAID amtolmetin guacyl, documented to date in the rat.

Chronic hepatitis B virus (HBV) infection is a well-recognized risk factor for the development of hepatocellular carcinoma (HCC), which is becoming a more prevalent clinical problem, especially in HBV-endemic areas. It is estimated that 1.25 million people in the United States and more than 300 million people worldwide are chronically infected with HBV. Despite the introduction of universal vaccination against hepatitis B in over 100 countries, persistent HBV infection is still a serious problem worldwide, causing an estimated annual death rate of one million. It may take several decades until the effect of vaccination will be translated into reduced transmission and morbidity. Meanwhile, patients with persistent HBV infection require better antiviral therapeutic modalities than are currently available. It is well accepted that antiviral therapy for chronic hepatitis B is effective to improve prognosis of patients with HBV by preventing development of hepatitis state and HCC. The therapeutic endpoints for hepatitis B treatment are: 1) sustained suppression of HBV replication, as indicated by HBsAg and HBeAg loss, 2) decrease of serum HBV DNA of an undetectable level by a non-PCR method, 3) remission of disease, as shown by normalization of ALT, 4) improvement in liver histology, and 5) reduction of the acute exacerbation, cirrhosis, and HCC. In the present, the antiviral treatment of hepatitis B consists of either interferon alpha or oral lamivudine alone or in combination with existing therapy. Each major antiviral drug of interferon alpha and lamivudine has pros and cons, and effect of combination therapy of both drugs is also still limited. More powerful and safe new antiviral therapies are required to achieve final goal of these therapeutic endpoints. Management of chronic hepatitis B requires significant knowledge of approved pharmacotherapeutic agents and their limitations. Therapeutic options for managing hepatitis infection after liver transplantation (LT) are also evolving. In this article, we would like to adjust the focus of current clinical issue of antiviral therapy for hepatitis B and also indicate the issue of LT for patients with hepatitis B.

Non-steroid anti-inflammatory drugs (NSAIDs) are major drugs used in the treatment of inflammation and pain in a wide variety of disorders. NSAIDs constitute a diverse group of chemicals, categorized according to their chemical structures that share the same therapeutic properties. Among the main compounds are aspirin and salicylate, diclofenac and flurbiprofen. The best-known mechanism of action of NSAIDs is the inhibition of prostaglandin synthesis secondary to their action on cyclooxygenases (COXs). However, data have been accumulating through the years indicating that NSAIDs also act on other targets to counteract pain. Their analgesic effects are not necessarily the consequence of their anti-inflammatory action. Administration of NSAIDs reduces cutaneous and corneal pain induced by acidic pH in the absence of inflammation. Tissue acidosis, which is a dominant factor in inflammation, tumors and ischemia, has an important contribution in pain and hyperalgesia. This is due to direct excitation of the nociceptive sensory neurons by protons-gated depolarizing currents. Actually, these neurons bear a major category of ion channels that are sensitive to extracellular pH changes, the acid-sensing ion channels (ASICs). ASIC channels are able to induce action potential triggering on sensory neurons after a moderate extracellular pH decrease. They undergo transcriptional induction and post-translational regulation during inflammation and thus participate in the hypersensitization of the nociceptive system in this physiopathological condition. One specific ASIC isoform is also thought to mediate cardiac ischemic pain. COX-independent direct inhibition of their activity by different NSAIDs has been shown to occur at therapeutic doses of these compounds, on native ASIC currents on sensory neurons, as well as on ASIC channels expressed in heterologous systems. Moreover, NSAIDs also prevent the large inflammation-induced increase of their expression. These two effects are thus proposed to play an important role in the analgesic effects of NSAIDs in addition to their well-known action through COXs, and particularly in case of inflammation.

Human innate immunity can respond to diverse microbial products, as well as other substances such as heat shock proteins, taxol, and unsaturated fatty acids. Mediated largely by a family of Toll-like-receptors (TLR) and associated intracellular downstream signaling molecules, human innate immune response serves multiple functions ranging from providing the first line of defense to coordinating cellular growth as well as other cellular functions. To date, about 10 distinct human TLR receptors have been identified in the human genome. Biochemical studies and genetic analyses using transgenic mice have revealed specific ligands for several TLR receptors. TLR intracellular domains could then specifically recruit several adaptor proteins including MyD88, TIRAP / MAL, TRIF, and TOLLIP. These adaptor proteins subsequently associate with a family of interleukin-1 receptor-associated kinases (IRAK1, 2, M, and 4). Recruitments of numerous downstream signaling proteins lead to activation of a range of transcription factors such as NFκB, AP-1, and IRFs, which are responsible for specific gene transcriptions. Human innate immunity is manifested in diverse cells and tissues. Well-coordinated innate immunity signaling enables human cells and tissues to properly respond to various substances. Improper regulations of such event have been shown to cause various diseases including asthma, atherosclerosis, and cancer. TLR receptors as well as other intracellular signaling proteins can potentially serve as therapeutic targets for numerous human diseases. This review will discuss at the molecular level, regulation of innate immunity signaling as well as its intricate connection with human diseases.

The complement system is a key component of innate immunity, acting to protect the host from micro-organisms such as bacteria and other “foreign” threats, including tumor cells. However, excessive activation of complement can injure the host and can even be life threatening. These toxic effects are caused primarily by the excessive production of the anaphylatoxins C3a and C5a during complement activation and excessive formation of membrane attack complex on the host cell membrane. Many inflammatory diseases, including rheumatoid arthritis and glomerulonephritis, are thought to involve excessive activation of complement, both for their development and perpetuation. Uncontrolled complement activation is also implicated in post-ischemic inflammation and tissue damage and in sepsis. Therefore, it is important to regulate the complement system to treat disease. There are still no broadly applicable agents for the therapeutic regulation of excessive complement activation. However, there are now some agents in the development that might provide useful anti-complement therapies in the near future. Current strategies include the use of neutralizing antibodies, small synthetic antagonists, soluble recombinant forms of the natural complement regulators, and gene therapies to control excessive complement activation. Here we describe these new agents, their strengths and weaknesses and progress in testing the agents in relevant animal models.

Estrogen appears to play a central role in the immune response and immunemediated diseases. Estrogen receptors are expressed in a variety of immunocompetent cells, including CD4+ and CD8+ T cells and macrophages. Clinical observations indicate that some autoimmune diseases, such as rheumatoid arthritis and multiple sclerosis, frequently remit during pregnancy but exacerbate, or have their onset during the postpartum period. Pharmacological levels of estrogen also appear to ameliorate certain autoimmune diseases. In addition, estrogen is known to suppress certain infectious diseases, as well as T cell-mediated responses toward oxazolone, keyhol lympet hemocyanin, Listeria soluble protein and purified protein derivatives. The immune basis for these phenomena is poorly understood. Based on a distinctive profile of cytokine production, data accumulated thus far have revealed modulatory effects for estrogen on the TH1-type and TH2-type cells, which represent two polarized forms of the effector specific immune response. Recent evidence indicates that estrogens inhibit the production of TH1 proinflammatory cytokines, such as IL-12, TNF-α and IFN-γ, whereas they stimulate the production of TH2 anti-inflammatory cytokines, such as IL-10, IL-4, and TGF-β. This can explain why estrogen suppresses and potentiates TH1- and TH2-mediated diseases, respectively. We hypothesize that exacerbation or suppression of inflammatory diseases by estrogen is mediated by skewing TH1-type to TH2-type response. This view represents a novel mechanism for the modulatory effect of estrogen on certain inflammatory diseases that can lead to beneficial or detrimental impacts depending on the type of immune involved. Such a concept is valuable when considering the application of combination therapies that include estrogen.

Thrombin is well known in its function as the ultimate serine protease in the coagulation cascade. Emerging evidence indicates that thrombin also functions as a potent signaling molecule that regulates physiologic and pathogenic responses alike in a large variety of cell populations and tissues. Accompanying CNS injury and other cerebral vascular damages, prothrombin activation and leakage of active thrombin into CNS parenchyma has been documented. Due to the irreplaceable feature of neurons, overreactive inflammatory reactions in the CNS often cause irreversible neuronal damage. Therefore, particular attention is required to develop strategies that restrict CNS inflammatory responses to beneficial, in contrast to neurotoxic ones. In this regard, thrombin not only activates endothelial cells and induces leukocyte infiltration and edema but also activates astrocytes, and particularly microglia, as recently demonstrated, to propagate the focal inflammation and produce potential neurotoxic effects. Recently revealed molecular mechanisms underlying these thrombin effects appear to involve proteolytic activation of two different thrombin-responsive, protease-activated receptors (PARs), PAR1 and PAR4, possibly in concert. Potential therapeutic strategies based on appreciation of the current understanding of molecular mechanisms underlying thrombin-induced CNS inflammation are also discussed.