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PubMed2003

Retinoids: pleiotropic agents of therapy for vascular diseases?

Retinoids, the natural and synthetic derivatives of vitamin A, exert broad biological effects and have been used clinically to treat a variety of dermatological and neoplastic diseases. The principal mode of action of many retinoids is through the binding and activation of a family of nuclear receptors that modulate gene transcription. Recent evidence demonstrates that retinoids effectively attenuate experimental vessel wall narrowing due to atherosclerosis, post-balloon injury stenosis, and bypass graft failure. Moreover, retinoids promote a differentiated phenotype in smooth muscle cells (SMC) which, unlike other muscle types, is not fixed and is subject to considerable modulation in disease states. A growing number of in vitro studies have reported desirable effects of retinoids on cell migration, proliferation, apoptosis, matrix remodeling, fibrinolysis, coagulation, and inflammation, all of which impinge on vascular disease. Since vascular SMC and endothelial cells (EC) express most retinoid receptors, the mechanisms underlying retinoid-mediated events in these cells and the vessel wall likely relate to an altered transcriptome. In fact, there is a growing list of retinoid-response genes encoding proteins that likely mediate the actions of retinoids. Retinoid-response genes, therefore, represent promising targets of therapy for the refined treatment of vascular diseases. The purpose of this review is to summarize the emerging importance of retinoids in the control of vascular cell responses with special emphasis on potential mechanisms underlying retinoid-induced changes in the vessel wall following injury. Given the similarities in the pathogenesis of neoplasia and vascular disease, it is reasonable to consider testing the efficacy of retinoids for the treatment of human vascular disease.

Amino Acid Sequence↗

PubMed1997

Evidence of a retinoid signaling alteration involving the activator protein 1 complex in tumorigenic human bronchial epithelial cells and non-small cell lung cancer cells.

Retinoids, including retinol and retinoic acid derivatives, inhibit the growth of normal human bronchial epithelial (HBE) cells. Using a lung carcinogenesis model consisting of normal, immortalized, and tumorigenic HBE cells, we showed previously that, compared to normal HBE cells, the tumorigenic HBE cell line 11701 is resistant to the growth-inhibitory effects of all-trans-retinoic acid (t-RA). Retinoid receptor function is preserved in tumorigenic 11701 cells, suggesting that other retinoid signaling components are altered. The activator protein 1 (AP-1) complex is a component of the retinoid signaling pathway and has demonstrated importance in cellular growth and differentiation. Therefore, we investigated whether AP-1 is involved in a retinoid signaling defect in tumorigenic 11701 cells and in retinoid-resistant non-small cell lung cancer (NSCLC) cell lines. We found that t-RA treatment inhibited AP-1 transcriptional activity in normal HBE cells but not in tumorigenic 11701 cells nor in the NSCLC cell lines Calu-1, Calu-6, SKMES-1, and ChaGo K1. We sought mechanisms for this retinoid signaling alteration involving AP-1 in tumorigenic 11701 cells. Basal AP-1 transcriptional activity; AP-1 DNA-binding activity; and the mRNA levels of c-fos, the AP-1 coactivator Jun activation domain-binding protein 1, and the retinoid receptor corepressor, the silencing mediator for retinoid and thyroid hormone receptors (SMRT), were lower in tumorigenic 11701 cells than in normal HBE cells. Transient transfection of tumorigenic 11701 cells with c-fos or CREB binding protein, which is a coactivator of AP-1 and retinoid receptors, enhanced basal AP-1 transcriptional activity but did not alter the effects of t-RA on AP-1 transcriptional activity. These findings provide evidence of a retinoid signaling alteration involving AP-1 in tumorigenic 11701 and NSCLC cells. Furthermore, the inhibitory effect of t-RA on AP-1 transcriptional activity was not restored in tumorigenic 11701 cells by transfection of c-fos, silencing mediator for retinoid and thyroid hormone receptors, Jun activation domain-binding protein 1, or CREB-binding protein, suggesting the involvement of other transcriptional coregulators in this retinoid signaling defect.

Blotting, Northern↗

PubMed2005

Role of retinoid signalling in the adult brain.

Vitamin A (all-trans-retinol) is the parent compound of a family of natural and synthetic compounds, the retinoids. Retinoids regulate gene transcription in numerous cells and tissues by binding to nuclear retinoid receptor proteins, which act as transcription factors. Much of the research conducted on retinoid signalling in the nervous system has focussed on developmental effects in the embryonic or early postnatal brain. Here, we review the increasing body of evidence indicating that retinoid signalling plays an important role in the function of the mature brain. Components of the metabolic pathway for retinoids have been identified in adult brain tissues, suggesting that all-trans-retinoic acid (ATRA) can be synthesized in discrete regions of the brain. The distribution of retinoid receptor proteins in the adult nervous system is different from that seen during development; and suggests that retinoid signalling is likely to have a physiological role in adult cortex, amygdala, hypothalamus, hippocampus, striatum and associated brain regions. A number of neuronal specific genes contain recognition sequences for the retinoid receptor proteins and can be directly regulated by retinoids. Disruption of retinoid signalling pathways in rodent models indicates their involvement in regulating synaptic plasticity and associated learning and memory behaviours. Retinoid signalling pathways have also been implicated in the pathophysiology of Alzheimer's disease, schizophrenia and depression. Overall, the data underscore the likely importance of adequate nutritional Vitamin A status for adult brain function and highlight retinoid signalling pathways as potential novel therapeutic targets for neurological diseases.

Adult↗

PubMed2002

Relaxation of beta-structure in tear lipocalin and enhancement of retinoid binding.

PURPOSE: To study binding of retinoids to human tear lipocalin (TL) to assess factors influencing ligand affinity and delivery. Mechanistic features of retinoid interactions with TL were investigated, including the influence of the retinoid functional group on ligand affinity, the relative affinity of retinol versus fatty acids, the influence of relaxation of secondary structure in TL on ligand binding, the role of specific conserved hydrophobic residues in maintaining the rigidity of the secondary structure, and the potential release of retinol in a low-pH environment that promotes structural relaxation at lipid interfaces. METHODS: The binding and displacement of retinoids were monitored by quenching of protein fluorescence. Circular dichroic spectra were used to evaluate structural and conformational changes in TL-retinoid complexes. Site-directed mutagenesis was performed to determine the influence of the residues Trp17, Ile98, Gly15, and Leu19 in retinoid binding to TL and to correlate these effects with changes in secondary structure. RESULTS: Retinal and retinol bound TL with similar affinity. Fatty acids competed with retinoids for the same binding site on TL. Optical activity associated with retinal binding to TL was reduced in the presence of palmitic acid. In comparison with TL, the mutants W17C and I98C displayed relaxation of secondary structure, manifested as diminution of beta-sheet content in conjunction with a destabilization in urea, reduced aromatic asymmetry, and greater binding affinity for retinoids. Unlike fatty acids, retinol is not released from TL at low pH. CONCLUSIONS: The unique spectral properties of retinoids permit the simultaneous study of structural changes in TL and ligand binding. Retinoid binding is enhanced by specific mutations that induce relaxation of TL structure but is altered minimally by the functional group in retinoids. Two key hydrophobic residues, Trp17 (A strand) and Ile98 (G strand), contribute to backbone rigidity and influence retinoid binding through their participation in an internal hydrophobic cluster and external hydrophobic patch, respectively. The contributions of these sites to ligand binding may explain their conserved nature in the lipocalin family. Information regarding the binding and release of retinoids compared with fatty acids favors a role for TL in the delivery of lipids other than retinol to the tear film interfaces.

Binding Sites↗

PubMed2003

The renal retinoid system: time-dependent activation in experimental glomerulonephritis.

Retinoids reduce renal damage in rat experimental glomerulonephritis. It is unknown, however, how local and systemic retinoid pathways respond to renal injury. We used a rat model of artificially induced acute anti-Thy1.1-nephritis (THY-GN). We examined the extrarenal and glomerular expression of the retinol (RoDH) and retinal (RalDH) dehydrogenases 1 and 2 as well as the expression of the retinoic acid (RAR) and retinoid X (RXR) receptor subtypes alpha, beta, and gamma. Furthermore, we investigated serum and glomerular retinoid concentration patterns. On days 3, 7, and 14, we compared nonnephritic rats (control group; CON) to THY-GN rats with respect to systolic blood pressure and glomerular cell count per cross section. Systolic blood pressure and glomerular cell count were significantly higher in THY-GN rats on days 7 and 14 (P < 0.001). We found a 60% reduction in expression levels for retinoid receptors and dehydrogenases in nephritic glomeruli on day 3, but a threefold increase on day 7 (P < 0.001 vs. CON). The same applies to RAR alpha protein. Hepatic expression of retinoid receptors was not influenced. On day 14, glomerular expression levels for retinoid receptors and retinoid-metabolizing enzymes had returned to a normal level, glomerular cell count being still increased. Administering 13-cis retinoic acid (isotretinoin) lowered blood pressure and glomerular cell count in nephritic rats but failed to influence the glomerular expression of retinoid receptors or retinoid-metabolizing enzymes. Our data document a stimulation of glomerular retinoid-synthesizing enzymes and expression of retinoid receptors in the early repair phase of THY-GN, suggesting activation of this system in acute renal disease.

Alcohol Oxidoreductases↗

PubMed2004

Retinoids in cancer chemoprevention.

We review the therapeutic and preventive applications of a retinoid analog (vitamin A and its derivatives) for human cancers. Chemoprevention of cancer is an intervention in the carcinogenic process by chemical agents that block or reverse the malignant transformation of cells. Retinoids are prime candidates for cancer chemoprevention since cancer is characterized by abnormal growth with a lack of differentiation, which could be modified by retinoids. Retinoids exert their biological functions through nuclear receptors, retinoic acid receptor (RAR) and retinoid X receptor (RXR). A number of experimental and clinical studies have been performed in the past two decades with retinoids showing that they inhibit or reverse the carcinogenic process in some organs, including hematological malignancy as well as premalignant and malignant lesions in the oral cavity, head and neck, breast, skin and liver. We particularly focus upon the therapeutic application of all-trans RA (atRA) to acute promyelocytic leukemia (APL) and on the preventive approach to hepatocellular carcinoma (HCC) by a synthetic retinoid analog, acyclic retinoid. In both malignancies, malfunction of retinoid nuclear receptors is closely related to their carcinogenic process. In APL, a chromosomal translocation produces a chimeric protein between RAR alpha and a protein called promyelocyte leukemia protein (PML). PML-RAR alpha works as a dominant negative receptor in the leukemic cells, interfering with the normal function of RAR alpha and/or PML, which in turn results in the arrest of cell maturation at the stage of promyelocytes. Oral administration of atRA induces differentiation of promyelocytic leukemic cells to mature neutrophils, and leads to a high rates (over 90%) of complete remission. AtRA therapy has become standard in the treatment of APL. In the case of HCC, post-translational modification of RXR by phosphorylation impairs its function, which leads to uncontrolled cell growth. Acyclic retinoid suppresses the phosphorylation of RXR alpha, restores its function in the presence of its endogenous ligand, 9-cis RA, and thereby induces apoptosis of the cancer cells. Acyclic retinoid given orally successfully suppresses the development of second primary tumors in cirrhotic patients who undergo curative removal of preceding HCC. Eradication of (pre)malignant clones ('clonal deletion') from the liver is suggested as a mechanism of the chemopreventive effect. Further development of more effective retinoids as well as their use in combination with other classes of anticancer agents including immunopreventive drugs like interferons may provide strategies for cancer prevention.

Animals↗

PubMed2000

Identification of receptor-selective retinoids that are potent inhibitors of the growth of human head and neck squamous cell carcinoma cells.

Retinoids modulate the growth and differentiation of cancer cells presumably by activating gene transcription via the nuclear retinoic acid receptor (RAR) alpha, beta, and gamma and retinoid X receptor (RXR) alpha, beta, and gamma. We analyzed the effects of 38 RAR-selective and RXR-selective retinoids on the proliferation of 10 human head and neck squamous cell carcinoma (HNSCC) cell lines. All of these cell lines expressed constitutively all of the receptor subtypes except RARbeta, which was detected in only two of them. Most of the RAR-selective retinoids inhibited the growth of HNSCC cells to varying degrees, whereas the RXR-selective retinoids showed very weak or no inhibitory effects. Three RAR antagonists suppressed growth inhibition by RAR-selective agonists, as well as by RAR/RXR panagonists such as 9-cis-retinoic acid. Combinations of RXR-selective and RAR-selective retinoids exhibited additive growth-inhibitory effects. Furthermore, we found that CD437, the most potent growth-inhibitory retinoid induced apoptosis and up-regulated the expression of several apoptosis-related genes in HNSCC cells. These results indicate that: (a) retinoid receptors are involved in the growth-inhibitory effects of retinoids; (b) RXR-RAR heterodimers rather than RXR-RXR homodimer are the major mediators of growth inhibition by retinoids in HNSCC cells; and (c) induction of apoptosis can account for one mechanism by which retinoids such as CD437 inhibit the growth of HNSCC cells. Finally, these studies identified several synthetic retinoids, which are much more effective than the natural RAs and can be good candidates for chemoprevention and therapy of head and neck cancers.

Apoptosis↗

PubMed1993

Retinoids inhibit the oxidative modification of protein kinase C induced by oxidant tumor promoters.

Recently we reported that oxidant tumor promoters can induce the oxidative modification of protein kinase C (PKC) resulting in either activation or inactivation of the kinase (R. Gopalakrishna and W. B. Anderson, Arch. Biochem. Biophys. 285, 382-387, 1991). Since retinoids previously have been shown to antagonize the actions of tumor promoters, studies were carried out to determine if retinoids can inhibit the oxidative modification of PKC induced by tumor promoters. Prior treatment of B16 melanoma cells or C6 glioma cells with all-trans-retinoic acid (0.1 microM) for a short time period (15 to 60 min) followed by subsequent treatment with oxidants such as hydrogen peroxide resulted in a 30 to 70% decrease in the oxidative modification of PKC. This resulted in a decrease in oxidant-induced conversion of PKC from a Ca2+/lipid-dependent form (peak A) to a Ca2+/lipid-independent form (peak B). This retinoid-mediated protection also was observed with the reversible oxidative modification of PKC induced by m-periodate treatment of intact cells. To understand whether this protection offered by retinoids was caused by a direct influence of retinoids on PKC, experiments were carried out using the purified enzyme. The results of experiments using isolated PKC suggested that retinoids can act directly to protect the regulatory domain of PKC from oxidative modification induced by oxidants. However, high (1-10 microM) concentrations of retinoids are necessary to elicit this protection of isolated PKC. In contrast, in experiments with intact cells, only low (submicromolar) concentrations of retinoids are required to protect PKC from oxidation. The differences noted in the retinoid concentrations required to protect PKC from oxidant modification in the test tube versus in the intact cell may be due to increased retention of retinoids in the cell membrane by partitioning, or to other indirect actions of retinoids in the intact cells to decrease cellular oxidations. These results suggest that some of the anti-tumor promoter actions of retinoids may be mediated, in part, by inhibiting the oxidative modification of protein kinase C induced by oxidant tumor promoters.

Animals↗

PubMed2003

Retinoids in cancer therapy and chemoprevention: promise meets resistance.

Retinoids (natural and synthetic derivatives of vitamin A) signal potent differentiation and growth-suppressive effects in diverse normal, premalignant, and malignant cells. A strong rationale exists for the use of retinoids in cancer treatment and chemoprevention based on preclinical, epidemiological, and early clinical findings. Despite the success of all-trans-retinoic acid (RA)-based differentiation therapy in acute promyelocytic leukemia (APL), the broad promise of retinoids in the clinic has not yet been realized. In addition to the expected limited activity of any single therapeutic agent, translation of retinoid activities from the laboratory to the clinic has met with intrinsic or acquired retinoid resistance. Evidence suggests that solid tumors develop intrinsic resistance to retinoids during carcinogenesis. In contrast, relapse of APL is often associated with acquired resistance to retinoid maturation induction. This review discusses what is known about retinoid resistance mechanisms in cancer therapy and chemoprevention. Strategies to overcome this resistance will be discussed, including combination therapy with other differentiation-inducing, cytotoxic or chromatin-remodeling agents, as well as the use of receptor-selective and nonclassical retinoids. Opportunities exist in the post-genomic era to bypass resistance to classical retinoids by identifying target genes and associated pathways that directly mediate the antineoplastic effects of retinoids. In this regard, the retinoids are useful pharmacological tools to reveal important pathways targeted in cancer therapy and chemoprevention.

Drug Resistance, Neoplasm↗

PubMed2000

The role of specific retinoid receptors in sebocyte growth and differentiation in culture.

Retinoic acid derivatives (retinoids) exert their pleiotropic effects on cell development through specific nuclear receptors, the retinoic acid receptors and retinoid X receptors. Despite recent progress in understanding the cellular and molecular mechanisms of retinoid activity, it is unknown which of the retinoid receptor pathways are involved in the specific processes of sebocyte growth and development. In this study, we investigated the roles of specific retinoid receptors in sebocyte growth and differentiation, by testing the effects of selective retinoic acid receptor and retinoid X receptor ligands at concentrations between 10-10 M and 10-6 M in a primary rat preputial cell monolayer culture system. Cell growth was determined by number of cells and colonies, and cell differentiation by analysis of lipid-forming colonies. All-trans retinoic acid and selective retinoic acid receptor agonists (CD271 = adapalene, an RAR-beta,gamma agonist; CD2043 = retinoic acid receptor pan-agonist; and CD336 = Am580, an RAR-alpha agonist) caused significant decreases in numbers of cells, colonies, and lipid-forming colonies, but with an exception at high doses of all-trans retinoic acid (10-6 M), with which only a small number of colonies grew but they became twice as differentiated as controls (42.2 +/- 4.0% vs 22.6 +/- 2.7%, mean +/- SEM, lipid-forming colonies, p < 0.01). Furthermore, the RAR-beta,gamma antagonist CD2665 antagonized the suppressive effects of all-trans retinoic acid, adapalene, and CD2043 on both cell growth and differentiation. In contrast, the retinoid X receptor agonist CD2809 increased cell growth slightly and lipid-forming colonies dramatically in a clear dose-related manner to a maximum of 73.7% +/- 6.7% at 10-6 M (p < 0. 001). Our data suggest that retinoic acid receptors and retinoid X receptors differ in their roles in sebocyte growth and differentiation: (i) retinoic acid receptors, especially the beta and/or gamma subtypes, mediate both the antiproliferative and antidifferentiative effects of retinoids; (ii) retinoid X receptors mediate prominent differentiative and weak proliferative effects; (iii) the antiproliferative and antidifferentiative effects of all-trans retinoic acid are probably mediated by retinoic acid receptors, whereas its differentiative effect at high dose may be mediated by retinoid X receptors via all-trans retinoic acid metabolism to 9-cis retinoic acid, the natural ligand of retinoid X receptors.

Adapalene↗

PubMed2000

Advances in the development of retinoids as chemopreventive agents.

With the inclusion of brief discussions of retinoid drug development in animal carcinogenesis models (e.g., skin, breast, oral cavity, lung, prostate or bladder) and clinical trials (e.g., head and neck or cervix), this review will focus on recent advances in retinoid molecular targeting studies designed primarily to develop retinoids with reduced toxicity, while maintaining or enhancing activity in the context of chemoprevention. Major current retinoid molecular targets include the six known nuclear retinoid receptors (RAR and RXR). Receptor numbers, distinct functions, tissue-expression patterns, ligand specificities, functional redundancy and regulation of multiple pathways make retinoid signaling highly complex. Development of receptor-selective synthetic retinoids is a major focus of molecular retinoid development. RAR heterodimerize with RXR and mediate classic retinoid activity/toxicity. RXR are more promiscuous, heterodimerizing with several other members of the steroid receptor superfamily [e.g., peroxisome proliferator-activated receptors (PPAR) or vitamin D receptors]. RXR-selective ligands are less toxic and more active in animal breast cancer prevention studies and less toxic than RAR ligands in clinical trials. Other new avenues of retinoid molecular drug development include newly identified retinoid-regulated genes, orphan-receptor ligands/functions, novel retinoid mechanisms involving potent receptor-independent apoptosis-inducing activity (e.g., 4-HPR or anhydroretinol), synergistic combinations [e.g., RXR agonists plus selective estrogen receptor modulators (SERM)], activity in other diseases and novel delivery systems.

Chemoprevention↗

PubMed2004

Retinoids in nephrology: promises and pitfalls.

BACKGROUND: Retinoids, a family of vitamin A metabolites or analogs, play an important role in regulating cell proliferation, differentiation, and apoptosis. METHODS: The biological importance of retinoids in the kidney and the potential of retinoids in the treatment of renal diseases are reviewed. RESULTS: Vitamin A deficiency and mutations of retinoid nuclear receptors cause abnormalities in fetal kidneys, which might predispose to adult diseases such as hypertension. Further, the therapeutic value of retinoids in animal models of kidney diseases, such as lupus nephritis, puromycin aminonucleoside nephrosis, anti-glomerular basement membrane nephritis, mesangioproliferative nephritis, and acute renal allograft rejection has been unveiled recently. Retinoids target mesangial cells, podocytes, tubular epithelial cells, interstitial fibroblasts, as well as lymphocytes and macrophages. The anti-inflammation, anti-coagulation effects, and the proliferation- and immunity-modulating actions of retinoids, have been widely appreciated. Our recent in vitro data revealed a direct antifibrotic effect and a cytoprotective effect of retinoids in various renal cell types. In animal studies, the adverse effects of retinoids are generally minimal; however, the clinical use of retinoids in other diseases points to some major side effects. In addition, in vitro, retinoids can induce lipid accumulation in smooth muscle cells and macrophages and increase expression of some proinflammatory molecules, indicating that their clinical toxicity profile in the setting of renal diseases needs to be better understood. CONCLUSION: Retinoids not only are important in renal development, but also show promise as a new generation of renal medication and deserve to be tested in clinical trials to clarify their full potential.

Animals↗

PubMed1999

Induction of retinoic acid receptor beta mediates growth inhibition in retinoid resistant human colon carcinoma cells.

BACKGROUND: The molecular mechanisms underlying the differential sensitivity of human colon carcinoma cells to retinoid mediated growth inhibition are poorly understood. AIM: To identify the intracellular mechanisms responsible for resistance against retinoid mediated growth inhibition in human colon carcinoma cells. METHODS: Anchorage independent growth of the human colon carcinoma cell lines HT29 and LoVo was determined by a human tumour clonogenic assay. Retinoid receptor expression was evaluated by reverse transcription polymerase chain reaction and northern blotting. Retinoid mediated transactivation was assessed by transient transfection of a pTK::betaREx2-luc reporter construct. Retinoid receptor overexpression was achieved by selecting stably transfected cell clones. RESULTS: Retinoid treatment resulted in profound dose dependent growth inhibition in HT29 cells, while LoVo cells were unaffected. The two cell lines express identical patterns of nuclear retinoid receptor mRNA transcripts. However, on retinoid treatment, retinoic acid receptor beta gene expression was upregulated only in retinoid sensitive HT29 cells, but not in retinoid resistant LoVo cells. In accordance, stable overexpression of retinoic acid receptor beta but not alpha or gamma conferred retinoid mediated growth inhibition on LoVo cells. CONCLUSION: Induction of retinoic acid receptor beta expression is required and sufficent to confer retinoid mediated growth inhibition on human colon carcinoma cells.

Blotting, Southern↗

PubMedDate not supplied

Penetration and metabolism of topical retinoids in ex vivo organ-cultured full-thickness human skin explants.

The human epidermis contains endogenous retinoids [retinol (vitamin A) and retinyl esters] and carotenoids (mostly beta-carotene). Previous studies in the mouse have shown that the enzymes involved in retinoid metabolism are present in the epidermis. In this study, we wanted to assess the skin penetration and metabolism of topical retinoids in the human. To do this, fresh surgically excised human abdominal skin was mounted on Franz perfusion cells. Topical retinoic acid, retinal, retinol and retinyl palmitate were applied at 2.5 mg/cm(2) in oil-in-water creams containing 0.05% retinoids on the donor compartment, while the receptor compartment was filled with culture medium. The skin was incubated for 24 h at 37 degrees C, then epidermal retinoid concentrations were determined by HPLC. The same experiment was performed with mouse back skin mounted on Franz cells. Finally, topical retinoids were applied on the back of hairless mice for 24 h; then the mice were sacrificed and retinoid concentrations were assayed in the epidermis. In all three models, retinol and its esters were found to be endogenous, as was the case in previous studies in the mouse in vivo. The four applied retinoids penetrated well into the epidermis. Topical retinoic acid did not increase endogenous retinoids, whereas the latter were greatly increased following topical retinal in the mouse. Retinal was also metabolized into retinoic acid, unlike topical retinol and retinyl palmitate, which only increased endogenous retinoids. Topical retinal and retinol did undergo a higher metabolism in both mouse models than in human skin. In summary, the penetration and metabolism patterns of topical retinoids were quite similar in the two mouse models used, indicating that the Franz cells appear to be a good model to predict in vivo metabolism of topical retinoids. When applying this concept to our results obtained in Franz cells with human skin, we conclude that topical retinol and retinal load human skin with both storage and functional vitamin A.

Administration, Cutaneous↗

PubMedDate not supplied

Retinoids in chemoprevention of cancer.

Retinoic acid (RA), a natural metabolite of circulating Vitamin A (retinol) and an irreversible oxidation product of retinol, is essential in maintaining the normal pathway of differentiation of epithelial tissues. RA and a number of its analogs, both natural and synthetic (retinoids), have been shown to be effective in the prevention of a variety of cancers in experimental animals, and in reversing preneoplastic lesions in humans. The retinoids exhibit a high degree of specificity in cancer chemoprevention. Diverse effects of retinoids are mediated by retinoid nuclear receptors, the ligand-inducible trans-acting transcription factors. The receptor-selective retinoids may be more effective and less toxic in cancer prevention. Our chemoprevention study with retinoids using the mouse skin carcinogenesis model indicated that retinoids are anti-tumor promoters. One of the mechanisms by which retinoids inhibit promotion of mouse skin tumor formation involves their property to inhibit the induction of ornithine decarboxylase by the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate. RARalpha and RARgamma, but not RXRs, may mediate mouse skin anti-tumor promotion activity of retinoids. Retinoids are highly selective chemopreventive agents and are toxic at high pharmacological doses. Clinical trials with retinoids should be conducted with a carefully evaluated, appropriate patient population and perhaps at low doses in combination with other chemopreventive agents with mechanisms of action different from retinoids.

Animals↗

PubMed1990

[Retinoid-induced changes of the bones and ligaments].

Bone and ligament lesions induced by systemic retinoids (premature epiphyseal closure, osteophytes, calcification of ligaments, osteoporosis, etc.) are radiologically nonspecific. We assessed the incidence of "possibly retinoid-induced bone and ligament lesions" (MRKBV) in 46 patients (aged 24-82 years) who had been treated with various systemic retinoids for a variety of chronic dermatoses for an average of 4.5 years and compared the data with observations in a similarly structured group of control patients. All types of MRKBV were found more frequently in the retinoid group (41.3% vs 30.2%) and were more severe. Differences were only slight, however, being statistically significant only for moderate and severe MRKBV and for calcification of ligaments. Within the retinoid group, MRKBV were strikingly correlated with age, whereas no correlation was found with duration of treatment, mean daily and cumulative retinoid dose, underlying dermatosis, type of retinoid used or presence of other retinoid side-effects. Within the control group, MRKBV were also correlated with age, although to a lesser degree. We conclude that retinoids amplify and accelerate physiological and pathophysiological remodelling of the bones, thereby producing a varied range of lesions, which are characteristic for the age and the individual constitution of the patient treated. Retinoid-induced bone and ligament lesions, as a rule, do not cause subjective symptoms and are not associated with predictive or accompanying laboratory values. Also, MRKBV are not paralleled by other retinoid-induced side-effects (hyperlipidaemia, elevation of liver transaminases). In 16 cases in which bone X-ray had been performed prior to retinoid treatment, no indication of reversibility of MRKBV was found.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1987

Retinoids as preventive and therapeutic anticancer agents (Part I).

Retinoids, the synthetic and natural analogs of vitamin A, frequently block the phenotypic expression of cancer in vitro; they also inhibit growth and induce differentiation in many animal and human malignant cell types. Only recently has it become possible to propose a unifying mechanism of retinoid action, which involves the protein kinase-C cascade system. This system may mediate retinoids' many diverse actions, including their effects on enzyme synthesis, membrane properties, growth factors, binding proteins, genomic and postgenomic expression, the extracellular matrix, and immunologic responses. Ongoing in vitro studies of retinoid structure-activity relationships, effects on oncogene expression, reversal of drug-resistance, and, especially, the protein kinase-C cascade system should help clarify the precise mechanism of their anticancer action. Many in vitro and in vivo assay systems are available for testing the 2000 + synthetic retinoids. These assays indicate specific drug sensitivities, which may help focus future clinical trials. In human cancer prevention, retinoids have been most effective for skin diseases, including actinic keratosis, keratoacanthoma, and basal cell carcinoma; however, nondermatologic premalignancies, such as oral leukoplakia, bronchial metaplasia, laryngeal papillomatosis, cervical dysplasia, myelodysplastic syndromes, and the urinary bladder, also respond to retinoid therapy. Significant therapeutic advances are also occurring with this class of drugs in refractory malignancies, including advanced cutaneous squamous and basal cell cancer, mycosis fungoides, and acute promyelocytic leukemia. Newer third-generation retinoids, such as the highly potent retinoidal benzoic acid derivatives, are demonstrating therapeutic indexes far higher than earlier-generation retinoids. Current in vitro testing is also demonstrating that retinoids have synergistic activity in combination with other agents (eg, biologic modifiers, hormones, and DNA synthesis inhibitors) and treatment modalities (eg, irradiation). Notwithstanding the progress already made with retinoids in human cancer, many in vitro questions remain, and clinical work is just beginning.

Animals↗

PubMed1998

Current research and future developments in retinoids: oral and topical agents.

The retinoids are a class of small molecules capable of affecting biological systems in a fundamental way by interacting with a variety of nuclear receptors and affecting gene transcription. As such, they have powerful therapeutic potential in the treatment of a variety of diseases. However, non-selective retinoid drugs that indiscriminately activate many or all of the different retinoid receptors and their signaling pathways will invariably produce both undesirable and beneficial effects. Therefore, current and future retinoid research must employ a rational approach to drug discovery, the goal of which is to design retinoid drugs with sufficient receptor or function selectivity that only those biological systems relevant to a specific disease are affected. Tazarotene is a retinoid with properties that are a result of this rational approach to drug discovery. Its molecular structure was engineered in such a way as to make it selective for the retinoic acid receptor (RAR) class of nuclear receptors over retinoid X receptors (RXRs), with preferential activation of RAR beta and RAR gamma over RAR alpha. It was further engineered to possess clinically favorable pharmacokinetic properties. To date, all retinoid drugs available for therapeutic use are retinoid agonists; they bind to retinoid receptors and facilitate gene transcription. However, ongoing research has developed retinoid antagonists (that bind to receptors without activating gene transcription) and inverse agonists (that bind to receptors and inhibit gene transcription). These new drugs and other retinoids currently under development represent new opportunities for the treatment of a variety of disorders in dermatology, oncology, and metabolic disease.

Administration, Oral↗