Retinoids as chemopreventive agents

Retinoids are promising agents for cancer chemoprevention. The myriad effects of retinoids on biological processes including development, differentiation, homeostasis, carcinogenesis and apoptosis are mediated through their molecular targets, the retinoid and rexinoid receptors. Tissue specific expression patterns, ligand specificities, receptor numbers, their distinct functions and functional redundancy make retinoid signaling highly complex. The cross-talks of these receptors with cell surface receptors signaling pathways, as well as their interactions with multiple co-activators and co-repressors further add to the complexity of the pleiotropic effects of retinoids. Elucidation of retinoid signaling pathways and indepth understanding of the mechanisms that underlie the anti-proliferative and apoptotic action of retinoids has paved the way for designing synthetic retinoids for effective chemoprevention and therapy of cancer. Development of receptor selective synthetic retinoids is a major focus of molecular retinoid development. Other new avenues encompass identification of novel retinoid regulated genes, orphan-receptor ligands/functions, novel retinoid mechanisms involving receptor-independent apoptosis inducing activity and synergistic combinations with other agents for cancer prevention and therapy. This review focuses on recent advances in the understanding of molecular mechanisms underlying the action of retinoids and retinoid molecular targeting studies designed primarily to develop retinoids with reduced toxicity, while maintaining or enhancing activity in context of chemoprevention. The clinical efficacy of retinoid based chemoprevention trials is discussed.     

A new class of RAR subtype selective retinoids: correlation of pharmacological effects with receptor activity

The synthesis and biological activity of a series of structurally related retinoids with different RAR subtype selectivities are described. These retinoids bind to all three RAR subtypes but in functional transactivation assays, they show RARbeta or RARbeta,gamma selectivity with weak RARalpha activity. The subtype selectivity of these retinoids was found to correlate with their efficacy (ODC inhibition) and toxicity (topical irritation and teratogenicity) profiles. The degree of RARgamma transactivation activity correlates with their topical toxicity and teratogenicity as measured by the inhibition of chondrogenesis. Of the RARbeta selective retinoids reported here, retinoid 12 is the most promising, as it is completely devoid of two common retinoid related toxicities, namely topical irritation and teratogenesis.     

Delivery of retinoid-based therapies to target tissues

Through its various metabolites, vitamin A controls essential physiological functions. Both naturally occurring metabolites and novel retinoid analogues have shown effectiveness in many clinical settings that include skin diseases and cancer, and in animal models of human conditions affecting vision. In this review, we analyze several potential retinoid-based therapies from the point of view of drug metabolism and transport to target tissues. We focus on the endogenous factors that affect the absorption, transport, and metabolism of retinoids by taking into account data obtained from the analysis of animal models that lack the enzymes or proteins involved in the storage and absorption of retinoids. We also discuss findings of toxicity associated with retinoids in an effort to improve the outcome of retinoid-based therapies. In this context, we review evidence that esterification of retinol and retinol-based drugs within target tissues provides one of the most efficient means to improve the absorption and to reduce the toxicity associated with pharmacological doses of retinoids. Future retinoid-based therapeutic strategies could involve targeted delivery mechanisms leading to lower toxicity and improved effectiveness of retinoids.     

肿瘤鞘脂代谢异常与肿瘤细胞对维甲酸类药物耐药性

维甲酸类药物对多种癌症有效,其作用包括诱导凋亡、抑制生长、促进分化等,这主要通过调节维甲酸受体包括维甲酸受体(retinoic acid receptor,RAR)和维甲酸X受体(rexinoid X receptor,RXR)的表达实现。目前发现,一些患者癌细胞的RAR、RXR或RAR/RXR表达缺乏,可能导致癌细胞对维甲酸产生耐药性。鞘脂代谢异常和维甲酸类受体表达缺失密切相关,在癌细胞对维甲酸产生耐药性中发挥着重要作用。本文就鞘脂代谢异常与维甲酸受体表达异常及维甲酸类药物耐药的相关性做一简要综述。     

Differential and Isomer-Specific Modulation of Vitamin A Transport and the Catalytic Activities of the RBP Receptor by Retinoids

Retinoids are vitamin A derivatives with diverse biological functions. Both natural and artificial retinoids have been used as therapeutic reagents to treat human diseases, but not all retinoid actions are understood mechanistically. Plasma retinol binding protein (RBP) is the principal and specific carrier of vitamin A in the blood. STRA6 is the membrane receptor for RBP that mediates cellular vitamin A uptake. The effects of retinoids or related compounds on the receptor’s vitamin A uptake activity and its catalytic activities are not well understood. In this study, we dissected the membrane receptor-mediated vitamin A uptake mechanism using various retinoids. We show that a subset of retinoids strongly stimulates STRA6-mediated vitamin A release from holo-RBP. STRA6 also catalyzes the exchange of retinol in RBP with certain retinoids. The effect of retinoids on STRA6 is highly isomer-specific. This study provides unique insights into the RBP receptor’s mechanism and reveals that the vitamin A transport machinery can be a target of retinoid-based drugs.     

Receptor-independent induction of apoptosis by synthetic retinoids

Retinoids modulate cell proliferation, differentiation and apoptosis. Many of these effects are mediated by nuclear retinoid receptors. However, studies with certain synthetic retinoids, including some that can activate retinoid receptors, revealed that they affect cell growth and especially apoptosis by mechanisms that are independent of nuclear receptors. This chapter describes the pro-apoptotic effects of the synthetic retinoid CD437 [6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid], and structurally-related retinoids and summarizes the mechanisms by which they induce apoptosis.     

Retinoids and ovarian cancer

Each year, an estimated 26,000 women in the United States are diagnosed with ovarian cancer. During any given year, approximately 14,500 women die from this disease. Ovarian cancer is the seventh most common cancer in women worldwide, after breast, cervix, colon/rectum, stomach, corpus uteri, and lung cancers. In the U.S., ovarian cancer is the second most common gynecologic cancer, and is the fourth leading cause of solid tumor cancer deaths among women. Currently, postoperative chemotherapy of ovarian cancer is still suboptimal. Drug resistance is a common problem resulting in only 20 approximately 30% overall 5-year survival rates. Clearly, continued development of alternative therapeutic strategies is essential for the management of this fatal disease. A number of recent studies have suggested that retinoids may play a potential role as an ovarian cancer chemotherapeutic agent. Retinoids, the natural and synthetic derivatives of vitamin A, have been shown to inhibit the growth of human ovarian cancer cells both in vivo and in culture. This review will initially summarize what is known about the pathological and molecular characteristics of ovarian carcinoma. It will then describe retinoid metabolism and the role of the cellular and nuclear retinoid binding proteins in mediating retinoid action. Following this general review of retinoids and their function, data supporting the role of retinoic acid as a suppresser of ovarian carcinoma cell growth will be presented. Particular attention will be paid to studies suggesting that members of the RB family of proteins and RB2/p130, in particular, are the molecular targets responsible for retinoid mediated inhibition of ovarian carcinoma cell growth. This review will then conclude with a brief discussion of two synthetic retinoids, 4 HPR R(fenretinide) and AHPN/CD437, which have been shown to induce apoptosis in ovarian tumor cells. It will be clear from the studies summarized in this review that retinoids represent a potentially powerful alternative to present chemotherapeutic approaches to the treatment of late stage ovarian cancer.     

Structure and function of cytoplasmic retinoid binding proteins

We examined the ligand protein interactions of two highly homologous cellular retinol binding proteins, CRBP and CRBP-II, and two highly homologous cellular retinoic acid binding proteins, CRABP-I and CRABP-II. While the crystal structures of all four have been determined, nuclear magnetic resonance studies provide a means for observing dynamic aspects of ligand protein interactions of these proteins in solution. The cellular functions of these proteins are less well understood. We have modeled retinoid flux between cytoplasmic retinoid proteins and model membranes and with nuclear receptors. Based on our in vitro studies, we propose that certain retinoids may indirectly influence retinoid signaling by displacing endogenous retinoids from the cytoplasmic proteins to the nuclear receptors.     

BMP-2 mediates retinoid-induced apoptosis in medulloblastoma cells through a paracrine effect

The mechanisms of retinoid activity in tumors remain largely unknown. Here we establish that retinoids cause extensive apoptosis of medulloblastoma cells. In a xenograft model, retinoids largely abrogated tumor growth. Using receptor-specific retinoid agonists, we defined a subset of mRNAs that were induced by all active retinoids in retinoid-sensitive cell lines. We also identified bone morphogenetic protein-2 (BMP-2) as a candidate mediator of retinoid activity. BMP-2 protein induced medulloblastoma cell apoptosis, whereas the BMP-2 antagonist noggin blocked both retinoid and BMP-2-induced apoptosis. BMP-2 also induced p38 mitogen-activated protein kinase (MAPK), which is necessary for BMP-2- and retinoid-induced apoptosis. Retinoid-resistant medulloblastoma cells underwent apoptosis when treated with BMP-2 or when cultured with retinoid-sensitive medulloblastoma cells. Retinoid-induced expression of BMP-2 is thus necessary and sufficient for apoptosis of retinoid-responsive cells, and expression of BMP-2 by retinoid-sensitive cells is sufficient to induce apoptosis in surrounding retinoid-resistant cells.     

Cellular retinoic acid-binding protein and its relationship to the biological activity of four synthetic retinoids in hamster tracheal organ culture

Summary The mechanism of action of retinoid in reversing keratinization in hamster trachea is yet unknown. The purpose of this study was to determine if cellular retinoic acid binding protein (CRABP) is present in tracheal epithelium following incubation in serum-free, vitamin A-deficient culture medium for 10 days, and if the effectiveness of a retinoid in reversing keratinization in organ culture is correlated with its ability to compete for CRABP sites. The cytosol prepared from tracheal cultures contained CRABP at a concentration of 2.61 pmoles per mg protein. Of the four retinoids with carboxyl end group selected for the study, two of the biological active retinoids competed for the CRABP sites. However, no correlation was observed between the biological activity of the inactive retinoids and their ability to associate with the CRABP sites. These results indicate that even though the action of retinoid may be mediated by retinoid binding protein, it cannot be used as a sole predicator of retinoid response in hamster trachea. This investigation was supported by Contract N01-CP-31012 and U. S. P. H. Grants CA30512 and CA32428, which were awarded by the Division of Cancer Etiology, National Cancer Institute, DHHS. Editor's Statement Tracheal organ cultures provide a useful model for the study of epithelial differentiation and carcinogenesis. Much attention has been given to the action of retinoids in this process. Mehta et al. demonstrate a lack of correlation between biological activity and specific cytosolic binding of members of this class of compounds, pointing out the need for a more complete biochemical understanding of the mechanism of action and active forms of retinoids in this and other systems in vivo and in vitro. David W. Barnes     

The effects of local application of retinoids to different positions along the proximo-distal axis of embryonic chick wings

Summary Two retinoids, all-trans-retinoic acid and a synthetic analog, TTNPB, were locally applied to different positions along the proximo-distal axis of embryonic chick wing buds using controlled release carriers. Truncations or limbs with duplicated structures across the antero-posterior axis develop after retinoid application to distal positions in buds from stage 20–24 embryos. Phocomelic limbs develop when the retinoids are applied more proximally to buds of stage 23–24 embryos. Duplications of the pattern of structures along the proximo-distal axis never occur.Using TTNPB that is relatively stable, the amount of retinoid in the wing tissue when phocomelia is induced was measured. There is twice as much retinoid per cell in the proximal half of the bud as in the distal half of the bud. The concentration of TTNPB in proximal tissue is estimated to be three times higher than in distal tissue in which pattern formation and cartilage morphogenesis are relatively normal.At early stages in the development of phocomelia, the shape of the bud changes and the indentation that marks the elbow does not arise. Neither retinoid-induced cell killing nor effects on the pattern of programmed cell death were detected.The induction of phocomelia by retinoids appears to be based on effects on proximal cells, whereas retinoids produce pattern changes by acting on distal cells. Furthermore, compared with pattern changes, higher concentrations of retinoid in the bud tissue are required to produce phocomelia.