Endocrine, Metabolic & Immune Disorders-Drug Targets (Formerly Current Drug Targets - Immune, Endocrine & Metabolic Disorders) - Volume 9, Issue 2, 2009

Retinoids are vitamin A derivatives that critically regulate several physiological and pathological processes, including immune functions and cancer development. These biological response modifiers exert their pleiotropic effects through the interaction with nuclear receptors, defined as retinoic acid receptors (RARs) and retinoid X receptors (RXRs). These ligand-activated nuclear receptors induce the transcription of target genes by binding to responsive elements in the promoter regions. RARs and RXRs are also capable to interact with other nuclear receptors, thus expanding their spectrum of action on gene expression. Evidence has been accumulated indicating that retinoids may exert beneficial effects in both immune-mediated disorders and tumors. With regard to cancer, retinoids directly target neoplastic cells by inducing differentiation, inhibiting cell growth or promoting survival. However, the efficacy of these compounds in cancer treatment probably resides in their ability to modulate also the function of immune effectors. Vitamin A derivatives are currently used in the therapy of acute promyelocytic leukemia and of cutaneous T cell lymphomas, but they could be effective also on B-cell malignancies. Clinical trials are ongoing to test their efficacy in solid tumors. In this review, we give a broad depiction of how retinoids influence the function of immune effectors and affect growth and survival of hematological malignancies. This with the aim to better understand the clinical effects of retinoid-based therapies and provide the rationale to combine retinoids with other active compounds in new synergistic treatment strategies.

The trace element zinc is a crucial cofactor for many proteins involved in cellular processes like differentiation, proliferation and apoptosis. Zinc homeostasis is tightly regulated and disturbance of this homeostasis due to genetic defects, zinc deficiency, or supplementation influences the development and the progression of various infectious and autoimmune diseases. The immune system is strongly impaired during zinc deficiency, predominantly the cell-mediated response by T-lymphocytes. During zinc deprivation T-lymphocyte development, polarization into effector cells, and function are impaired. This leads to reduced T-cell numbers, a decreased ratio of type 1 to type 2 T-helper cells with reduced production of T-helper type 1 cytokines like interferon-gamma, and compromised T-cell mediated immune defense. Accordingly, disturbed zinc homeostasis increases the risk for infections, and zinc supplementation restores normal immune function. Furthermore, several disorders, like mycobacterial infections, asthma, diabetes, and rheumatoid arthritis are accompanied by decreased zinc levels and in some cases disease progression can be affected by zinc supplementation. On the molecular level, apoptosis of T-cell precursors is influenced by zinc via the Bcl-2/Bax ratio, and zinc ions inhibit caspases-3, -6, -7, and -8. In mature T-cells, zinc interacts with kinases involved in T-cell activation, like protein kinase C and the lymphocyte protein tyrosine kinase (Lck), while higher zinc concentrations are inhibitory, reducing the activities of the interleukin-1 receptor-associated kinase (IRAK) and calcineurin. Taken together, zinc homeostasis influences Tlymphocytes via several molecular targets, leading to a modulation of T-cell-dependent immune responses.

Autoimmunity affects a substantial fraction of our population. In patients with autoimmune disease, the immune system recognizes self-tissues as foreign. Common autoimmune diseases include rheumatoid arthritis, diabetes mellitus, lupus and multiple sclerosis. Though different target organs may be affected in different autoimmune diseases, aberrations in adaptive or innate immunity underlie all of these diseases. Abnormal functioning, differentiation and/or activation of T-cells, B-cells and myeloid cells have been documented in various autoimmune diseases. More recent studies have also detailed anomalous activation of various signaling axes including various MAPK, AKT, NF-κB, Bcl-2 family members, and JAK/STAT molecules in these cells, in the context of systemic autoimmunity. Among these, one molecular pathway that appears to be particularly attractive for therapeutic targeting is the PI3K/AKT/mTOR axis. In this review, we summarize how the AKT axis affects multiple molecular processes in autoimmune diseases and discuss the potential of targeting this axis in these diseases.

The enzyme indoleamine 2,3-dioxygenase (IDO) regulates immune responses through the capacity to degrade the essential amino acid tryptophan into kynurenine and other downstream metabolites that suppress effector T-cell function and favour the differentiation of regulatory T cells. Considerable experimental evidence indicates that IDO can be expressed by dendritic cells, by tumour cells or by surrounding stromal cells, either within proximity of the tumour or at distal sites. Recent advances in the biochemistry of IDO and in our understanding of the biological relevance of IDOmediated tryptophan consumption to the establishment of dominant immune tolerance to cancer will be summarised and discussed. Within the wider context of cancer immunotherapy, this Review also delineates how IDO could be exploited as a molecular target for therapeutic intervention in order to boost anti-cancer immunity.

Discovered in 1993, IL-17 has been the focus of intensive research during the last decade, in particular because of its neutrophil-accumulating capacity in several mammalian organs. We now know that the IL-17 family includes as a minimum 6 members, of whom at least IL-17A and IL-17F can be produced by T cells. Thus, IL-17 is positioned at the interface of acquired and innate immunity and constitutes a link between T cell activity and the accumulation of neutrophils locally in organs. Interestingly, there is now accumulating evidence that IL-17 has effects on myeloid cells other than neutrophils as well, namely on cells of the monocyte lineage. This review article scrutinizes the evidence that IL-17 exerts a functional impact on the cytokine production and functional activity in cells of the monocyte lineage in health and disease. Notably, this evidence includes data suggesting that there are conditions in which cells of the monocyte lineage are likely to play a significant pathogenic role and where IL-17 is directly controlling the activity of these key effector cells.

Life expectancy in the developed world is increasing, but this comes with a simultaneous explosion in ‘agerelated’ as well as ‘lifestyle-related’ diseases, resulting in a decline in quality of life. Three such diseases are Type 2 diabetes mellitus (T2DM), Polycystic Ovarian Syndrome (PCOS) and non-alcoholic fatty liver disease (NAFLD), which all share a common reduced cellular response to the hormone insulin (termed insulin resistance). In T2DM, insulin resistance is clearly a contributing factor to disease progression, and is associated with obesity, the single greatest risk factor for all three conditions. Current research is focused on identifying the initial molecular lesion that results in reduced sensitivity to insulin, as improving insulin sensitivity would be beneficial to the prognosis of these conditions. However, the bulk of evidence suggests that more than one molecular defect in the insulin signalling pathway can lead to an insulin resistant phenotype. This raises the possibility that individuals with the same clinical phenotype may have distinct molecular reasons for the presence of the syndrome, and that the specific lesion influences the rate and direction of progression to the associated disease. Clearly the same insulin sensitiser could be of equal benefit in each disorder, if it reversed multiple signalling problems, however we suggest that appropriate molecular diagnosis of the defect may lead to a more targeted and effective therapeutic approach. This review discusses the molecular pathology of insulin resistance in relation to T2DM, PCOS and NASH. We highlight the shortcomings of current therapies, and suggest potential novel drug targets for each disorder.

Inflammatory bowel disease, which covers Crohn's disease and ulcerative colitis, and celiac disease are both inflammatory diseases of the intestinal tract. In both diseases an antigen activates several inflammatory pathways, which cause extensive damage to the intestinal mucosa and lead to increased permeability of the intestinal epithelium. The causative antigen in inflammatory bowel disease is the microflora in the intestinal lumen, facilitated by an impaired innate immune system that is unable to halt the invasion of microbes into the lamina propria. These provoke T helper 1 and T helper 17 responses in Crohn's disease and a T helper 2 response in ulcerative colitis. Pro-inflammatory cytokines and interleukins produced in these processes lead to impairment of tight junctions and increased permeability of the intestinal epithelial lining. In celiac disease, inflammation is caused by dietary gluten, a peptide present in wheat, barley and rye. In genetically predisposed people, gliadin peptides (derivatives of gluten) are presented on the Human Leukocyte Antigen DQ2 or DQ-8 molecules of antigen-presenting cells to T helper cells. This provokes a T helper 1 response, which leads to the production of pro-inflammatory cytokines and subsequent damage to, and increased permeability of the intestinal epithelium. We describe the details and overlaps in the pathomechanism and genetics of inflammatory bowel disease and celiac disease, and discuss potential drug targets for intervention.