Immunostimulation by retinoic acid

New Insights in Cushing Disease Treatment With Focus on a Derivative of Vitamin A

Cushing's disease (CD) is an endocrine disorder originated by a corticotroph tumor. It is linked with high mortality and morbidity due to chronic hypercortisolism. Treatment goals are to control cortisol excess and achieve long-term remission, therefore, reducing both complications and patient's mortality. First-line of treatment for CD is pituitary's surgery. However, 30% of patients who undergo surgery experience recurrence in long-term follow-up. Persistent or recurrent CD demands second-line treatments, such as pituitary radiotherapy, adrenal surgery, and/or pharmacological therapy. The latter plays a key role in cortisol excess control. Its targets are inhibition of adrenocorticotropic hormone (ACTH) production, inhibition of adrenal steroidogenesis, or antagonism of cortisol action at its peripheral receptor. Retinoic acid (RA) is a metabolic product of vitamin A (retinol) and has been studied for its antiproliferative effects on corticotroph tumor cells. It has been shown that this drug regulates the expression of pro-opiomelanocortin (POMC), ACTH secretion, and tumor growth in corticotroph tumor mouse cell lines and in the nude mice experimental model, via inhibition of POMC transcription. It has been shown to result in tumor reduction, normalization of cortisol levels and clinical improvement in dogs treated with RA for 6 months. The orphan nuclear receptor COUP-TFI is expressed in normal corticotroph cells, but not in corticotroph tumoral cells, and inhibits RA pathways. A first clinical human study demonstrated clinical and biochemical effectiveness in 5/7 patients treated with RA for a period of up to 12 months. In a recent second clinical trial, 25% of 16 patients achieved eucortisolemia, and all achieved a cortisol reduction after 6- to 12-month treatment. The goal of this review is to discuss in the context of the available and future pharmacological treatments of CD, RA mechanisms of action on corticotroph tumor cells, and future perspectives, focusing on potential clinical implementation.

Host and microbial factors in regulation of T cells in the intestine

The intestine is divided into specialized tissue areas that provide distinct microenvironments for T cells. Regulation of T-cell responses in the gut has been a major focus of recent research activities in the field. T cells in the intestine are regulated by the interplay between host and microbial factors. In the small intestine, retinoic acid (RA) is a major tissue factor that plays important roles in regulation of immune responses. In the large intestine, the influence of RA diminishes, but that of commensal bacterial products increases. RA, gut microbiota, and inflammatory mediators co-regulate differentiation, distribution, and/or effector functions of T cells. Coordinated regulation of immune responses by these factors promotes well-balanced immunity and immune tolerance. Dysregulation of this process can increase infection and inflammatory diseases.

Retinoic acid inhibits in vivo interleukin-2 gene expression and T-cell activation in mice

Interleukin-2 (IL-2) is an essential cytokine for T-lymphocyte homeostasis. We have previously reported that all-trans retinoic acid (atRA) enhances the secretion of IL-2 from human peripheral blood T cells in vitro, followed by increased proliferation and inhibition of spontaneous cell death. In this study we used a transgenic IL-2 gene luciferase reporter model to examine the effects of atRA in vivo. In contrast to the observations in human T cells, we found an overall reduction in luciferase-reported IL-2 gene expression in mice treated with atRA. Whole-body luminescence of anti-CD3-treated and non-treated mice was reduced in mice receiving atRA. Accordingly, after 7 hr, IL-2 gene expression was on average 55% lower in the atRA-treated mice compared with the control mice. Furthermore, mice fed a vitamin A-deficient diet had a significantly higher basal level of luciferase activity compared with control mice, demonstrating that vitamin A modulates IL-2 gene expression in vivo. Importantly, the atRA-mediated inhibition of IL-2 gene expression was accompanied by decreased DNA synthesis in murine T cells, suggesting a physiological relevance of the reduced IL-2 gene expression observed in transgenic reporter mice.

Retinoic acid prevents experimental Cushing syndrome

Cushing syndrome is caused by an excess of adrenocorticotropic hormone (ACTH) production by neuroendocrine tumors, which subsequently results in chronic glucocorticoid excess. We found that retinoic acid inhibits the transcriptional activity of AP-1 and the orphan receptors Nur77 and Nurr1 in ACTH-secreting tumor cells. Retinoic acid treatment resulted in reduced pro-opiomelanocortin transcription and ACTH production. ACTH inhibition was also observed in human pituitary ACTH-secreting tumor cells and a small-cell lung cancer cell line, but not in normal cells. This correlated with the expression of the orphan receptor COUP-TFI, which was found in normal corticotrophs but not in pituitary Cushing tumors. COUP-TFI expression in ACTH-secreting tumor cells blocked retinoic acid action. Retinoic acid also inhibited cell proliferation and, after prolonged treatment, increased caspase-3 activity and induced cell death in ACTH-secreting cells. In adrenal cortex cells, retinoic acid inhibited corticosterone production and cell proliferation. The antiproliferative action and the inhibition of ACTH and corticosterone produced by retinoic acid were confirmed in vivo in experimental ACTH-secreting tumors in nude mice. Thus, we conclude that the effects of retinoic acid combine in vivo to reverse the endocrine alterations and symptoms observed in experimental Cushing syndrome.

Application of chemical immunomodulators to the treatment of cancer and AIDS

A number of potential advantages, development of promising new agents, and the discovery of synergy with cytokines or cell products continue to spur research into the application of chemical immunomodulators for the treatment of cancer and AIDS. In preclinical in vitro and in vivo systems, chemical immunomodulators definitely modulate the immune system and have therapeutic efficacy. Although clinical trials have shown the ability of these agents to modulate the human immune system, thus far chemical immunomodulators have generally not fulfilled the therapeutic promise generated in animal models for the treatment of human diseases. While the discrepancy in results between animal models and human trials is obvious, the basis is not apparent. Species differences in elimination kinetics, presentation of active drug at the site of action, and the development of tachyphylaxis have been postulated as reasons for the minimal activity of these agents in humans. In addition, the use of investigational techniques established for cytotoxic agents may not be appropriate for immunomodulators. As with any immunomodulator, determining an optimal immunostimulatory dose and schedule and applying the therapy to patients with minimal tumor burden would perhaps be more appropriate than use of a maximally tolerated dose in patients with advanced disease. A dose-immunological effect relationship has recently been demonstrated for levamisole at doses higher than those used for many years in levamisole trials (99). While research and clinical investigation have identified several potentially useful chemical immunomodulators, the elementary understanding of the biochemical mechanisms involved in immunoregulation remains basic. Future research must elucidate these mechanisms, particularly in humans, to maximize the benefits of chemical immunomodulators as single agents or combined with cytotoxic chemotherapeutic agents, surgery, radiation therapy, other immunomodulators, and antiviral agents.

The effects of retinol on in vitro immunoglobulin synthesis by cord blood and adult peripheral blood mononuclear cells

In this study we examined the effects of retinol (ROH), a metabolic precursor of retinoic acid (RA), on Staphylococcus aureus Cowan I (SAC)-induced immunoglobulin synthesis of cord blood mononuclear cells (CBMC) and adult peripheral blood mononuclear cells (PBMC). ROH augmented SAC-induced IgM synthesis of CBMC by 5.9 +/- 1.5-fold (n = 7, mean +/- s.d.), and IgG synthesis of adult PBMC by 16.3 +/- 5.1-fold (n = 3) at optimal concentrations of 10(-6) M and 10(-11) M, respectively. No augmenting effects could be demonstrated for the other immunoglobulin isotypes. Time-course studies showed that the synthesis of IgM by CBMC was accelerated with detectable immunoglobulin in supernatant fluids starting on day 3. ROH augmented immunoglobulin synthesis of CBMC stimulated by Epstein-Barr virus (EBV), a T cell-independent polyclonal activator, and of EBV-transformed B cell clones (2.5 +/- 0.2 and 4.1 +/- 1.5-fold increase, respectively), which suggests that ROH can act directly on B cells to enhance immunoglobulin synthesis. In contrast, when ROH was preincubated with cord blood T cells, washed and added to the B cell-enriched fraction with SAC, no increase (0.9-1.8-fold) in IgM synthesis was obtained. Thus, the principal mechanism(s) by which ROH augments immunoglobulin synthesis is by acting on B cells. This is in contrast to the immunoglobulin-enhancing effects of RA which is mediated by T cells, or T cell products, e.g. cytokine. Our studies suggest that RA and ROH may have different pathways of immunoglobulin-enhancing effects, perhaps mediated by different retinoid binding proteins resulting in gene activation and immunoglobulin synthesis.

Physiological and clinical aspects of vitamin A and its metabolites

Retinoids, including retinol and retinoic acid (RA), are a group of naturally occurring and synthetic compounds that exhibit vitamin A-like biological activity. They achieve their effects by binding to intracellular proteins. Important sites of action are the nuclear retinoic acid receptors (RAR). These receptors, namely, RAR alpha, RAR beta, and RAR gamma, function as transcription factors by binding to RA-responsive elements (RARE) of multiple genes. Retinoids play a role in vision, embryogenesis, immune modulation, growth and differentiation of normal, premalignant and malignant tissues, the suppression of carcinogenesis, and the inhibition of tumor growth in experimental systems and humans. Reports of the significant antitumor effect of all-trans-RA in acute promyelocytic leukemia and the synthesis of new, less toxic, and more potent retinoids has generated renewed interest in these compounds. Retinoids may have an important role to play in the chemoprevention and therapy of cancer.