Liarozole potentiates the cancer chemopreventive activity of and the up-regulation of gap junctional communication and connexin43 expression by retinoic acid and beta-carotene in 10T1/2 cells

Homology Models and Molecular Modeling of Human Retinoic Acid Metabolizing Enzymes Cytochrome P450 26A1 (CYP26A1) and P450 26B1 (CYP26B1)

Homology models of cytochrome P450 26A1 and cytochrome P450 26B1 were constructed using the crystal structures of human, CYP2C8, CYP2C9, and CYP3A4 as templates for the model building. The homology models generated were investigated for their docking capacities against the natural substrate all-trans-retinoic acid (atRA), five different tetralone-derived retinoic acid metabolizing blocking agents (RAMBAs), and R115866. Interaction energies (IE) and linear interaction energies (LIE) were calculated for all inhibitors in both homology models after molecular dynamics (MD) simulation of the enzyme-ligand complexes. The results revealed that the homologues had the capacity to distinguish between strong and weak inhibitors. Important residues in the active site were identified from the CYP26A1/B1-atRA complexes. Residues involved in hydrophobic interactions with atRA were Pro113, Phe222, Phe299, Val370, Pro371, and Phe374 in CYP26A1 and Leu88, Pro118, Phe222, Phe295, Ile368, and Tyr272 in CYP26B1. Hydrogen bonding interactions were observed between the atRA carboxylate group and Arg 90 in CYP26A1 and with Arg76, Arg95, and Ser369 in CYP26B1.

Therapeutic potential of the inhibition of the retinoic acid hydroxylases CYP26A1 and CYP26B1 by xenobiotics

Retinoic acid (RA), the active metabolite of vitamin A, is an important endogenous signaling molecule regulating cell cycle and maintenance of epithelia. RA isomers are also used as drugs to treat various cancers and dermatological diseases. However, the therapeutic uses of RA isomers are limited due to side effects such as teratogenicity and resistance to treatment emerging mainly from autoinduction of RA metabolism. To improve the therapeutic usefulness of retinoids, RA metabolism blocking agents (RAMBAs) have been developed. These inhibitors generally target the cytochrome P450 (CYP) enzymes because RA clearance is predominantly mediated by P450s. Since the initial identification of inhibitors of RA metabolism, CYP26 enzymes have been characterized as the main enzymes responsible for RA clearance. This makes CYP26 enzymes an attractive target for the development of novel therapeutics for cancer and dermatological conditions. The basic principle of development of CYP26 inhibitors is that endogenous RA concentrations will be increased in the presence of a CYP26 inhibitor, thus, potentiating the activity of endogenous RA in a cell-type specific manner. This will reduce side effects compared to administration of RA and allow for more targeted therapy. In clinical trials, inhibitors of RA metabolism have been effective in treatment of psoriasis and other dermatological conditions as well as in some cancers. However, no CYP26 inhibitor has yet been approved for clinical use. This review summarizes the history of development of RAMBAs, the clinical and preclinical studies with the various structural series and the available knowledge of structure activity relationships of CYP26 inhibitors.

Study on cytochrome p-450 dependent retinoic Acid metabolism and its inhibitors as potential agents for cancer therapy

The relative lack of clinical success with conventional anticancer agents may be due in part to the traditional concept of cancer being a biological state rather than a dynamic process. Redefining cancer as a dynamic disease commencing with carcinogenesis introduces the possibility of chemoprevention. Retinoids offer the promise of a therapeutic option based on differentiation of premalignant as well as malignant cells. Research to date has concentrated on the use of exogenous retinoids in cancer. Although this research continues with new retinoid derivatives, an alternative approach to overcoming the drawbacks associated with exogenous retinoids has been to increase the levels of endogenous retinoic acid (RA) by inhibiting the cytochrome P450- mediated catabolism of RA using a novel class of agents known as retinoic acid metabolism blocking agents (RAMBAs which increase the level of endogenous retinoic acid (RA) within the tumor cells by blocking their metabolism. This approach presents several theoretic advantages.

In the present study a wide range of established P-450 inhibitors has been screened to examine their inhibitory activity on all-trans-Retinoic acid (ATRA) metabolism. Forty-one known P450 inhibitors were tested for their inhibitory activity against RA metabolism. Most of them are nitrogen-containing compounds. The results showed that among these compounds only six compounds (N-benzyl-2-phenylethanamine, itraconazole, chlorpromazine, 5-chloro-1,3-benzoxazol-2-amine, proadifen and furazolidone) showed inhibition of RA metabolism which was > 50%. Ketoconazole and liarozole were also screened as standard potent inhibitors in the same system and gave 87.5% and 89% inhibition, respectively. The results indicate that mostly azoles with substituents in positions other than the 1-position on the ring are very weak inhibitors of RA metabolism. The most effective inhibitors (ketoconazole, itraconazole, bifonazole and clotrimazole) are 1-substituted and possess relatively large aromatic groups in the molecule. 1-Substituted imidazoles bind to cytochrome P-450 with a very high affinity but substitution in the other position of the imidazole decreases the binding affinity.

Regulation of gap junction function and Connexin 43 expression by cytochrome P450 oxidoreductase (CYPOR)

Cytochrome P450 oxidoreductase (CYPOR) is a microsomal electron-transferring enzyme containing both FAD and FMN as co-factors, which provides the reducing equivalents to various redox partners, such as cytochromes P450 (CYPs), heme oxygenase (HO), cytochrome b(5) and squalene monooxygenase. Human patients with severe forms of CYPOR mutation show bone defects such as cranio- and humeroradial synostoses and long bone fractures, known as Antley-Bixler-like Syndrome (ABS). To elucidate the role of CYPOR in bone, we knocked-down CYPOR in multiple osteoblast cell lines using RNAi technology. In this study, knock-down of CYPOR decreased the expression of Connexin 43 (Cx43), known to play a critical role in bone formation, modeling, and remodeling. Knock-down of CYPOR also decreased Gap Junction Intercellular Communication (GJIC) and hemichannel activity. Promoter luciferase assays revealed that the decrease in expression of Cx43 in CYPOR knock-down cells was due to transcriptional repression. Primary osteoblasts isolated from bone specific Por knock-down mice calvariae confirmed the findings in the cell lines. Taken together, our study provides novel insights into the regulation of gap junction function by CYPOR and suggests that Cx43 may play an important role(s) in CYPOR-mediated bone defects seen in patients.

Treatment of cutaneous T-cell lymphoma with retinoids

Retinoids are biologic regulators of differentiation, proliferation, apoptosis, and immune response. Retinoids (all-trans retinoic acid, 13-cis-retinoic acid, and the synthetic analogs isotretinoin, etretinate, and acitretin) have been used for years as monotherapy and/or in combination for treatment of cutaneous T-cell lymphomas (CTCL). Orally administered bexarotene, the first synthetic highly selective retinoid X receptor retinoid to be approved by the Food and Drug Administration for CTCL, was shown to be active against the cutaneous manifestations of all stages of CTCL. The topical gel formulation was also effective for early cutaneous manifestations of CTCL or as an adjunct to systemic or phototherapy. Use of retinoids in future long-term clinical trials and their eventual application in CTCL regiments will require strategies to decrease the side effects of existing retinoids, identify novel receptor subtype-selective retinoids with better therapeutic index, and explore biologically based synergistic combination therapies with other active agents.

Retinoic acid metabolism blocking agents (RAMBAs) for treatment of cancer and dermatological diseases

The naturally occurring retinoids and their synthetic analogs play a key role in differentiation, proliferation, and apoptosis, and their use/potential in oncology, dermatology and a variety of diseases are well documented. This review focuses on the role of all-trans-retinoic acid (ATRA), the principal endogenous metabolite of vitamin A (retinol) and its metabolism in oncology and dermatology. ATRA has been used successfully in differentiated therapy of acute promyelocytic leukemia, skin cancer, Kaposi's sarcoma, and cutaneous T-cell lymphoma, and also in the treatment of acne and psoriasis. However, its usefulness is limited by the rapid emergence of acquired ATRA resistance involving multifactoral mechanisms. A key mechanism of resistance involves ATRA-induced catabolism of ATRA. Thus, a novel strategy to overcome the limitation associated with exogenous ATRA therapy has been to modulate and/or increase the levels of endogenous ATRA by inhibiting the cytochrome P450-dependent ATRA-4-hydroxylase enzymes (particularly CYP26s) responsible for ATRA metabolism. These inhibitors are also referred to as retinoic acid metabolism blocking agents (RAMBAs). This review highlights development in the design, synthesis, and evaluation of RAMBAs. Major emphasis is given to liarozole, the most studied and only RAMBA in clinical use and also the new RAMBAs in development and with clinical potential.

Upregulation of connexin 43 by retinoids but not by non-provitamin A carotenoids requires RARs

Expression of connexin 43 (Cx43) is correlated with reduced indexes of neoplasia and is upregulated by cancer-preventive retinoids and carotenoids in nontransformed human and murine fibroblasts and keratinocytes. The molecular mechanism of upregulation, however, is poorly understood. Three retinoic acid receptor (RAR) antagonists (Ro 41-5253, BMS453, and BMS493) were capable of suppressing retinoid-induced Cx43 protein expression in 10T1/2 cells. However, Ro 41-5253 did not suppress protein expression by the non-provitamin A carotenoids astaxanthin or lycopene. In contrast, Cx43 induction by astaxanthin but not by a RAR-specific retinoid was inhibited by GW9662, an antagonist of peroxisome proliferator activated receptor-gamma activation. Simultaneous treatment with the maximally effective concentration of a retinoid and with beta-carotene or the non-provitamin A carotenoid astaxanthin resulted in supraadditive upregulation of Cx43 expression, again indicating separate mechanisms of gene regulation by these two cancer preventive agents.

Transcriptional regulation of connexin 43 expression by retinoids and carotenoids: similarities and differences

Gap junctions, connexons, are formed by assembly of trans-membrane connexin proteins and have multiple functions including the coordination of cell responses. Most human tumors are deficient in gap junctional communication (GJC) and restoration of GJC by forced expression of connexins reduces indices of neoplasia. Expression of connexin 43 (Cx43), the most widely-expressed connexin family member, is upregulated by cancer-preventive retinoids and carotenoids in normal and preneoplastic cells; an action considered of mechanistic significance. However, the molecular mechanism for upregulated expression is poorly understood. The retinoic acid receptor antagonist Ro 41-5253 was capable of suppressing retinoid-induction Cx43 luciferase reporter construct in F9 cells, but did not suppress reporter activity induced by the non-pro-vitamin A carotenoids astaxanthin or lycopene, indicating that retinoids have separate mechanisms of gene activation than non-pro-vitamin A carotenoids. Neither class of compound required protein synthesis for induction of Cx43 mRNA, nor was the 5.0 h half-life of Cx43 mRNA altered, indicating direct transcriptional activation. The responsive region was found within -158 bp and +209 bp of the transcription start site; this contains a Sp1/Sp3 GC-box to which Sp1 and Sp3 were bound, as revealed by electrophoretic mobility shift assays (EMSA), but no retinoic acid response element (RARE). Site directed mutagenesis of this GC-box resulted in increased basal levels of transcription and loss of responsiveness to a synthetic retinoid. In this construct astaxanthin and lycopene produced marginally, but not significantly higher, reporter activity than the control.

Cancer prevention by retinoids and carotenoids: Independent action on a common target

Virtually all human tumors are deficient in gap junctional communication (GJC) and the restoration of GJC by forced expression of connexins reduces indices of neoplasia. The expression of connexin 43 (Cx43) is upregulated by cancer-preventive retinoids and carotenoids which correlates with the suppression of carcinogen-induced transformation in 10T1/2 cells. However, the molecular mechanism for upregulated expression is poorly understood. The retinoic acid receptor antagonist, Ro 41-5253, suppressed retinoid-induced Cx43 protein expression in 10T1/2 cells and the induction of a Cx43 luciferase reporter construct in F9 cells, but did not suppress protein expression or reporter activity induced by the non-pro-vitamin A carotenoid astaxanthin. In contrast, Cx43 induction by astaxanthin, but not by a RAR-specific retinoid, was inhibited by GW9662, a PPAR-gamma antagonist. Neither compound required protein synthesis for the induction of Cx43 mRNA, nor was the 5.0 h half-life of Cx43 mRNA altered, indicating direct transcriptional activation. The responsive region was found within -158 bp and +209 bp of the transcription start site. Site directed mutagenesis of a GC-box in this region increased basal levels of transcription and loss of retinoid responsiveness. Simultaneous treatment with a retinoid and beta-carotene or astaxanthin resulted in supra-additive Cx43 expression, again indicating separate mechanisms of gene regulation.

Upregulation of connexin 43 protein expression and increased gap junctional communication by water soluble disodium disuccinate astaxanthin derivatives

Carotenoids are plant pigments whose consumption is associated with lower cancer rates in humans. Studies in experimental animal and cell systems have confirmed the cancer chemopreventive activity of these compounds. However, their extremely hydrophobic nature makes these compounds biologically unavailable unless delivered in organic solution to model systems. We have synthesized novel disodium salt disuccinate astaxanthin derivatives that possess high aqueous dispersibility. When delivered to mouse embryonic fibroblast C3H/10T1/2 cell cultures, either in aqueous or aqueous/ethanol solutions, these derivatives are biologically active. Biological activity was demonstrated by (1) upregulated expression of connexin 43 (Cx43) protein; (2) increased formation of Cx43 immunoreactive plaques in regions of the plasma membrane consistent with localization of gap junctions; (3) significantly upregulated gap junctional intercellular communication (GJIC) as demonstrated by Lucifer Yellow dye transfer after microinjection (P < 0.03; Fisher's Exact test). Enhanced expression of Cx43 and increased GJIC have been previously demonstrated to result in inhibition of in vitro neoplastic transformation of 10T1/2 cells as well as growth reduction of human tumors in xenografts. These novel derivatives possess increased utility as water soluble and water dispersible agents, allowing for aqueous delivery both in vitro and in vivo, properties that could enhance their potential clinical utility as potent cancer chemopreventive agents.

Retinoids: therapeutic applications and mechanisms of action in cutaneous T-cell lymphoma

Retinoids, natural and synthetic derivatives of vitamin A, are biological regulators of differentiation, proliferation, apoptosis, and immune response. Retinoic-acid-receptor-selective retinoids (all-trans retinoic acid, 13-cis-retinoic acid, and the synthetic analogs isotretinoin, etretinate and acitretin) have been used for years as monotherapy and/or in combination for treatment of cutaneous T-cell lymphoma (CTCL). Orally administered bexarotene, the first synthetic highly selective retinoid-X-receptor retinoid to be approved by the FDA for CTCL, was shown to be active against the cutaneous manifestations of all stages of CTCL. The topical gel formulation was also effective for early cutaneous manifestations of CTCL or as an adjunct to systemic or phototherapy. Bexarotene treatment induces apoptosis of CTCL cells with down-regulation of its receptors and of survivin, an inhibitor of apoptosis. Identification of new receptor subtype-selective retinoids, combination of various receptor-selective retinoids or other agents, and a new drug delivery system may improve the clinical efficacy of retinoids in the future.

Oral Cancer Cells Differ From Normal Oral Epithelial Cells in Tissuelike Organization and in Response to Lycopene Treatment: An Organotypic Cell Culture Study

We established distinctive monolayer and organotypic cell culture techniques to assess possible differences in cross-talk and spatial and structural organization of oral cancer cells compared with normal oral cells and also to evaluate possible differential responses of the cells to carotenoids. In monolayers, we investigated the effect of lycopene on the proliferation of an established oral cancer cell line, KB-1, and compared it with a primary cell line obtained from normal oral mucosa. Lycopene exerted a significant inhibitory effect on KB-1 cell proliferation inducing a dose-dependent downregulation of proliferating cell nuclear antigen (PCNA) associated with upregulation of connexin-43 (Cx-43) expression, whereas in the normal oral mucosal cells lycopene did not affect either PCNA expression, which was very low, or the expression of Cx-43, which was basically very high. Lycopene significantly inhibited the formation of colonies induced by the carcinogen 3-methylcholanthrene (MCA) on normal oral cells and almost completely abrogated the hyperplastic effect induced by MCA. KB-1 cells and normal oral epithelial cells in the organotypic cell culture method differed in their stratification and intercellular adhesion patterns as well as in the expression profile of cytokeratins, vimentin, and Cx-43. Lycopene induced Cx-43 expression in KB-1 cells grown by the organotypic raft method, similar to KB-1 cells grown as monolayers. We conclude that lycopene is a promising chemopreventive, pro-differentiating, and anticarcinogenic agent. No adverse effects of lycopene were detected in normal cells cultured in either monolayer or organotypic systems.

Lycopene inhibits proliferation and enhances gap-junction communication of KB-1 human oral tumor cells

Cell-cell interaction via gap junctions is considered to be a key factor in tissue homeostasis, and its alteration is associated with the neoplastic phenotype. Experimental and epidemiologic data suggest that carotenoids, particularly lycopene and beta-carotene, can reduce the risk of certain cancers. The aim of this study was to assess whether lycopene and beta-carotene interfere at some stage with the carcinogenic processes in human cancer cells derived from the oral cavity. KB-1 cells, originating from a human oral cavity tumor, were incubated with different concentrations of lycopene or beta-carotene delivered via the cell culture media from stock solutions in tetrahydrofuran. Lycopene strongly and dose dependently inhibited proliferation of KB-1 human oral tumor cells. beta-Carotene was a far less effective growth inhibitor. Lycopene (3 and 7 micro mol/L) significantly upregulated both the transcription (P < 0.005) and the expression (P < 0.05) of connexin 43, a key protein in the formation of gap-junctional communication. beta-Carotene (3 micro mol/L) tended to upregulate connexin 43 expression (P = 0.07) and significantly affected transcription of connexin 43 at 7 micro mol/L (P < 0.05). Gap-junctional communication measured by scrape-loading dye transfer and electron microscopy showed that lycopene enhanced gap-junctional communication between the cancer cells, whereas beta-carotene was less effective in this regard. The pattern of cellular uptake and incorporation into cancer KB-1 cells differed significantly between the carotenoids. beta-Carotene was avidly and rapidly incorporated into KB-1 cells, whereas lycopene uptake into the cells took place after longer incubation periods and only at the highest concentrations. The results of the present study further support the hypothesis that carotenoids in general, and lycopene in particular, may be effective anticarcinogenic agents in oral carcinogenesis.

Differential effects of TPA and retinoic acid on cell-cell communication in human bronchial epithelial cells

Understanding how normal and immortalized bronchial epithelial cells respond to modulators of gap junctional communication will increase our understanding of the process of tumor promotion. In the present study we compared to effects of retinoic acid (RA) and 12-O-tetradecanoylphorbol-13-acetate (TPA) on the rate of fluorescent dye transfer via gap junctions in primary human tracheo-bronchial epithelial cells (TBE) and SV40 large T-antigen immortalized, non-tumorigenic bronchial epithelial cells (BEAS-2B). RA in the physiological range (0.001-1 microM) inhibited cell proliferation (DNA synthesis, mitotic index) more in primary TBE cells than BEAS-2B cells. Also in RA-treated cells, decreased cell proliferation was coupled to decreased gap junctional communication (GJC) in TBE but not in BEAS-2B cells. TPA strongly suppressed GJC and proliferation in primary TBE cells, whereas BEAS-2B exhibited increased GJC and retained a significant fraction of cells undergoing DNA synthesis. Our studies show that an uncoupling of GJC and cell proliferation is associated with a differential response to the growth inhibitory effects of RA and phorbol esters in immortalized compared to primary human bronchial epithelial cells.

Retinoids and their receptors in cancer development and chemoprevention

Retinoids play an important role in regulating the growth and differentiation of normal, premalignant and malignant cell types, especially epithelial cells, mainly through interaction with two types of nuclear receptors: retinoic acid receptors (RARalpha, beta and gamma) and retinoid X receptors (RXRalpha, beta and gamma). Vitamin A deficiency in experimental animals has been associated with a higher incidence of cancer and with increased susceptibility to chemical carcinogens. This is in agreement with the epidemiological studies indicating that individuals with a lower dietary vitamin A intake are at a higher risk to develop cancer. At the molecular level, aberrant expression and function of nuclear retinoid receptors have been found in various types of cancer including premalignant lesions. Thus, aberrations in retinoid signaling are early events in carcinogenesis. Retinoids at pharmacological doses exhibit a variety of effects associated with cancer prevention. They suppress transformation of cells in vitro, inhibit carcinogenesis in various organs in animal models, reduce premalignant human epithelial lesions and prevent second primary tumors following curative therapy for epithelial malignancies such as head and neck, lung, liver, and breast cancer.

Effects of beta-carotene and vitamin A on oval cell proliferation and connexin 43 expression during hepatic differentiation in the rat(1)

The effects of beta-carotene and vitamin A administrations were evaluated in an in vivo model of hepatic cell differentiation. For this purpose, male Wistar rats received beta-carotene (70 mg/kg of body weight), vitamin A (10 mg/kg of body weight) or corn oil (control group), by gavage and at every other day during the entire experimental period. After 4 consecutive weeks of treatment, the animals were submitted to the AAF/PH model of hepatic cell differentiation (6 x 20 mg of AAF [2-acetylaminofluorene]/kg of body weight and partial hepatectomy) and killed on different days following the surgery (until day 16 after hepatectomy). Liver samples were collected for determination of beta-carotene, retinol and retinyl palmitate concentrations, for histopathological (hematoxilin-eosin) examination, for immunohistochemical detection of glutathione S-transferase, as well as for the evaluation of connexin 43 (a structural protein of gap junctions of oval cells) expression by northern blot analysis. Compared to controls, the oval cell proliferation peaks (observed by histopathological examination and immunohistochemistry) and connexin 43 expression peaks, were postponed to later days after hepatectomy, in a similar way in beta-carotene and vitamin A treated animals. Compared to the other experimental groups, the vitamin A treated group showed an increase in connexin 43 expression. It was concluded that beta-carotene and vitamin A modulated oval cell proliferation and connexin 43 expression, delaying both events. These findings suggest that beta-carotene and vitamin A can modulate the hepatic differentiation process in vivo.

Comparative effects of all-trans beta-carotene vs. 9-cis beta-carotene on carcinogen-induced neoplastic transformation and connexin 43 expression in murine 10T1/2 cells and on the differentiation of human keratinocytes

9-cis beta-Carotene was extracted from a commercial extract of the algae Dunaliella salina (Betatene), and its actions on proliferation and gene expression were examined in murine 10T1/2 cells and human HaCaT keratinocytes. The 9-cis isomer was less active than all-trans beta-carotene in reducing proliferation and in upregulating expression of connexin 43 in 10T1/2 cells. However, it had comparable ability to suppress carcinogen-induced neoplastic transformation. When tested in HaCaT cells in organotypic culture, it was less active in inducing connexin 43 expression and suppressing expression of keratin K1. In this assay the all-trans isomer was highly active at 10(-6) M, whereas 10(-5) M 9-cis beta-carotene was required to produce a comparable effect. Only small reductions in expression of the basal keratin 5 were seen. All-trans and 9-cis retinoic acids, potential metabolites of beta-carotene isomers, were studied in the same systems. In contrast to the carotenoids, the 9-cis isomer of retinoic acid was approximately 10-fold more active in suppressing neoplastic transformation and inducing connexin 43 expression in both cell types than the all-trans isomer. The retinoic acid isomers were about equipotent in suppressing K1 expression. Cellular levels of 9-cis beta-carotene were approximately 3.5-fold lower than levels of all-trans beta-carotene, suggesting that part, but not all, of this decreased activity of the 9-cis isomer was due to decreased cell uptake. Thus 9-cis beta-carotene is consistently less active than the all-trans isomer; that 9-cis retinoic acid is, in general, much more potent than the all-trans isomer suggests little or no conversion from the carotenoid to the retinoid under these culture conditions.

Chromatographic determination of carotenoids in foods

In recent years, there has been particular emphasis on obtaining more accurate data on the types and concentrations of carotenoids in foods for various health and nutrition activities. The analysis of carotenoids is complicated because of the diversity and the presence of cis-trans isomeric forms of this group of compounds. In addition, a wide variety of food products of vegetal and animal origin, vegetables and animal samples contain carotenoids, and a great range of carotenoids can be found in these samples. The characteristic conjugated double bond system of carotenoids produces the main problem associated with work and manipulation on carotenoids, that is their particular instability, especially towards light, heat, oxygen and acids. For this reason, several precautions are necessary when handling carotenoids. Another problem associated with analysis of carotenoids is the difficulty in obtaining standard compounds. High-performance liquid chromatographic methods for the determination of carotenoids in foods are reviewed. The sample extraction and treatment, carotenoid purification and standard manipulation are briefly commented on. We present a critical assessment of chromatographic methods developed for the determination of carotenoids in foods. Finally, some methods for carotenoid ester separation are reviewed.

Dynamic connexin43 expression and gap junctional communication during endoderm differentiation of F9 embryonal carcinoma cells

Gap junctional communication permits the direct intercellular exchange of small molecules and ions. In vertebrates, gap junctions are formed by the conjunction of two connexons, each consisting of a hexamer of connexin proteins, and are either established or degraded depending on the nature of the tissue formed. Gap junction function has been implicated in both directing developmental cell fate decisions and in tissue homeostasis/metabolite exchange. In mouse development, formation of the extra embryonal parietal endoderm from visceral endoderm is the first epithelial-mesenchyme transition to occur. This transition can be mimicked in vitro, by F9 embryonal carcinoma (EC) cells treated with retinoic acid, to form (epithelial) primitive or visceral endoderm, and then with parathyroid hormone-related peptide (PTHrP) to induce the transition to (mesenchymal) parietal endoderm. Here, we demonstrate that connexin43 mRNA and protein expression levels, protein phosphorylation and subcellular localization are dynamically regulated during F9 EC cell differentiation. Dye injection showed that this complex regulation of connexin43 is correlated with functional gap junctional communication. Similar patterns of connexin43 expression, localization and communication were found in visceral and parietal endoderm isolated ex vivo from mouse embryos at day 8.5 of gestation. However, in F9 cells this tightly regulated gap junctional communication does not appear to be required for the differentiation process as such.

Beta carotene: from biochemistry to clinical trials

Three large-scale clinical trials tested the effects of supplemental beta-carotene on the risk for chronic diseases such as cancer. The populations involved were Finnish male heavy smokers (the Alpha Tocopherol Beta Carotene [ATBC] trial), male asbestos workers and male and female heavy smokers (Beta-Carotene and Retinol Efficacy Trial [CARET]), and U.S. male physicians, 11% of whom were current smokers (Physician's Health Study). All three trials concluded that beta-carotene provided no protection against lung cancer; however, quite unexpectedly, two of the trials found a higher risk for lung cancer for those subjects given beta-carotene compared with those that were not. Several authors concluded from these beta-carotene trials that the protective effects of antioxidants against chronic disease are not as great as had been hoped. As reviewed here, however, beta-carotene may or may not be an antioxidant; it certainly differs in many respects from the prototypical antioxidant, vitamin E. In any case, the majority of beta-carotene's effects in vivo are probably not derived from any antioxidant properties that it may possess, but rather from its effect on a number of biochemical systems. Whether taking supplemental antioxidants can reduce the risk for chronic diseases remains to be established, although the case for vitamin E and heart disease appears strong. However, the association between eating a diet sufficient in fruits and vegetables and reduced risk for a number of diseases is consistent. There is no evidence at present that consuming small amounts of supplemental beta-carotene, i.e., amounts in foods or in a multivitamin tablet, is unwise for any population. The role of supplementation, however, particularly at high levels, with compounds that may be anti-oxidants but that are less well understood than vitamin E (e.g., carotenoids, plant polyphenols, and other phytochemicals), is less clear. The surprising results of the ATBC and CARET trials are a red flag, signaling the need for further research; a number of areas for future work are suggested here. Future research should lead to a clearer understanding of the effects of beta-carotene and other phytochemicals, as well as to more refined strategies for intervention, with important clinical and public health implications.

Carotenoids and gene regulation

Consumption of dietary carotenoids, plant pigments found in green, yellow, and orange fruits and vegetables, has been linked to decreased risk of cancer. Several intervention trials with beta-carotene, however, have failed to confirm this association. Indeed, in current smokers, beta-carotene appeared to increase risk. These disturbing results have not been explained. Laboratory studies with experimental animals and cells in culture have shown cancer preventive activity for a diverse range of carotenoids. Studies using human and animal cells have identified a gene, connexin 43, whose expression is upregulated by chemopreventive carotenoids and which allows direct intercellular gap junctional communication (GJC). GJC is deficient in many human tumors and its restoration or upregulation is associated with decreased proliferation. This review will focus on the growing body of evidence that carotenoids have unexpected biologic effects in experimental systems, some of which may contribute to their observed cancer preventive properties in models of carcinogenesis.

Carotenoids inhibit DNA synthesis in human aortic smooth muscle cells

Quiescent, serum-starved human aortic smooth muscle cells were restimulated with 20% foetal calf serum in Dulbecco's modified Eagle medium, in the presence and absence of beta-carotene, canthaxanthin, zeaxanthin, lycopene, lutein or beta-cryptoxanthin, at final concentrations up to 23 microM. Concentration-dependent inhibition of DNA synthesis, measured by [methyl-3H]thymidine incorporation, was observed for the carotenoids, except for canthaxanthin and lutein which had no effect. Lycopene was the most potent of the carotenoids tested. The results suggest that antiproliferative effects of dietary carotenoids might be of significance in vivo.

Beta - caroteno e câncer

Há mais de vinte anos tem-se pesquisado sobre a relação entre β-caroteno e câncer. Inúmeros trabalhos têm tentado esclarecer a hipótese, formulada a partir de achados epidemiológicos e experimentais, de que o carotenóide poderia agir como anticarcinogênico de origem alimentar. Estudos epidemiológicos observacionais, tipo caso-controle e coorte, sugerem que o consumo elevado de frutas e vegetais ricos em β-caroteno reduz o risco de câncer, especialmente de pulmão. Estudos de intervenção em humanos, administrando doses farmacológicas crônicas a milhares de indivíduos, têm demonstrado resultados contraditórios. Pesquisas em animais experimentais e em cultura de células apontam o carotenóide como quimiopreventivo, agindo sobretudo como retinóides, através de conversão metabólica extra-intestinal e como antioxidante de membrana. Conclui-se que o β-caroteno pode proteger contra o câncer, quando suplementado em doses fisiológicas, isto é, cerca de 4 a 6mg/dia. Doses farmacológicas crônicas não estão recomendadas para indivíduos saudáveis e particularmente para tabagistas.

The emerging role of retinoids and retinoic acid metabolism blocking agents in the treatment of cancer

BACKGROUND

Although significant advances have been made in the treatment of some malignancies, the prognosis of patients with metastatic tumors remains poor. Differentiating agents redirect cells toward their normal phenotype and therefore may reverse or suppress evolving malignant lesions or prevent cancer invasion. In addition, they offer a potential alternative to the classic cytostatic drugs.

METHODS

The purpose of this review was to examine the current and potential future roles of differentiating agents in the treatment of cancer.

RESULTS

Initial studies with differentiating agents focused on retinoid therapy. Although retinoids have shown some clinical success, their widespread use has been limited by resistance and, in the chemopreventive setting, toxicity. This has led to the synthesis of a number of new retinoids that currently are undergoing clinical investigation. A further approach to overcoming the drawbacks associated with exogenous retinoids has been to increase the levels of endogenous retinoic acid (RA) by inhibiting the cytochrome P450-mediated catabolism of RA using a novel class of agents known as retinoic acid metabolism blocking agents (RAMBAs). Liarozole, the first RAMBA to undergo clinical investigation, preferentially increases intratumor levels of endogenous RA resulting in antitumor activity.

CONCLUSIONS

Although studies using exogenous retinoids in this setting have not yet fulfilled their initial promise, studies with a growing set of synthetic retinoids are ongoing. Furthermore, modulation of endogenous retinoids may offer a significant new potential treatment for cancer.

Basic FGF increases communication between cells of the developing neocortex

We have found that basic fibroblast growth factor (bFGF), applied to cortical progenitor cells in vitro, produces an increase in the expression of the gap junction protein connexin (Cx) 43 and in the mRNA encoding Cx 43. This effect was evident in both proliferating and nonproliferating cells. The elevated levels of mRNA suggest that bFGF is likely to exert its effect by upregulating the rate of transcription of the Cx 43 gene. We have further shown that the increase in Cx 43 expression is mediated through the receptor tyrosine kinase pathway and is associated with enhanced intercellular dye-coupling mediated by gap junctions. These results suggest that gap junction channels provide a direct conduit for mitogens released in response to bFGF to effectively regulate proliferation during corticogenesis.

Phase II study of liarozole in advanced non-small cell lung cancer

The aim of this phase II study was to investigate the efficacy and tolerability of liarozole, a novel benzimidazole derivative, in non-small cell lung cancer (NSCLC). Liarozole 300 mg twice daily orally was evaluated in 14 patients with stage IIIB and IV NSCLC. 8 patients had received prior treatment with chemotherapy and/or radiotherapy. WHO toxicity grading and response criteria were used. Liarozole was well tolerated. Grade 2 toxicities included alopecia (1 patient), dermatological toxicity (5 patients), dry mouth (2 patients) and nausea and vomiting (2 patients). Leukocytosis was seen in 5 patients, including 2 cases with an elevated white cell count pretreatment. Liarozole was discontinued in 1 patient who developed intolerable progressive pruritus associated with an erythematous rash. No objective tumour response was seen, all 14 patients developing progressive disease within 4 months of commencing treatment. Liarozole was well tolerated but was ineffective as single agent therapy in the management of NSCLC. The side-effect profile was compatible with inhibition of retinoic acid degradation.

The antiproliferative activity of all-trans-retinoic acid catabolites and isomers is differentially modulated by liarozole-fumarate in MCF-7 human breast cancer cells

The clinical use of all-trans-retinoic acid (ATRA) in the treatment of cancer is significantly hampered by the prompt emergence of resistance, believed to be caused by increased ATRA catabolism. Inhibitors of ATRA catabolism may therefore prove valuable for cancer therapy. Liarozole-fumarate is an anti-tumour drug that inhibits the cytochrome P450-dependent catabolism of ATRA. ATRA, but also its naturally occurring catabolites, 4-oxo-ATRA and 5,6-epoxy-ATRA, as well as its stereoisomers, 9-cis-RA and 13-cis-RA, show significant antiproliferative activity in MCF-7 human breast cancer cells. To further elucidate its mechanism of action, we investigated whether liarozole-fumarate was able to enhance the antiproliferative activity of ATRA catabolites and isomers. Liarozole-fumarate alone up to a concentration of 10(-6) M had no effect on MCF-7 cell proliferation. However, in combination with ATRA or the ATRA catabolites, liarozole-fumarate (10(-6) M) significantly enhanced their antiproliferative activity. On the contrary, liarozole-fumarate (10(-6) M) was not able to potentiate the antiproliferative activity of the ATRA stereoisomers, most probably because of the absence of cytochrome P450-dependent catabolism. Together, these findings show that liarozole-fumarate acts as a versatile inhibitor of retinoid catabolism in that it not only blocks the breakdown of ATRA, but also inhibits the catabolic pathway of 4-oxo-ATRA and 5,6-epoxy-ATRA, thereby enhancing their antiproliferative activity.

beta-Carotene uptake and effects on intracellular levels of retinol in vitro

The purpose of the present study was to determine beta-carotene uptake and resultant effects on intracellular levels of retinol in cell lines of varied origin. Human skin fibroblasts, mouse embryonic fibroblasts, rabbit corneal epithelial cells, and rat liver cells were studied. Cells were cultured in medium supplemented with beta-carotene in a water-dispersible beadlet formulation. At selected intervals, cells and media were sampled and analyzed by high-performance liquid chromatography for beta-carotene and retinol content. beta-Carotene was taken up by all four cell lines. An increase in the intracellular levels of retinol was concomitant with beta-carotene uptake in all cell lines. The uptake of beta-carotene and the increase in intracellular retinol were highest in the two fibroblast cell lines. Incubation with media supplemented with crystalline beta-carotene, dissolved in tetrahydrofuran, resulted in significantly lower beta-carotene uptake and intracellular retinol levels. We view these results as a demonstration that a wide variety of cells, cultured in vitro, are able to convert beta-carotene to retinol. Therefore, beta-carotene's provitamin A activity should be carefully considered when the protective effects of beta-carotene in vitro are interpreted.

Differential dose-dependent effects of alpha-, beta-carotenes and lycopene on gap-junctional intercellular communication in rat liver in vivo

In order to examine the relevance of alteration of gap-junctional intercellular communication (GJIC) to chemopreventive activity against carcinogenesis, the effects of alpha- and beta-carotene as well as lycopene, typical chemopreventive carotenoids, on cell coupling via gap junctions in rat liver in vivo were studied using a direct functional dye-transfer technique. We found that all three test compounds given at a dose of 50 mg/kg-body weight (b.w.) daily, 5 times by gavage, inhibited GJIC, while similar treatment with 5 mg/kg b.w. caused enhancement, especially in the beta-carotene- and lycopene-treated groups. At the dose level of 0.5 mg/kg b.w., the three compounds had no effect. The findings show that all three agents differentially modulate GJIC depending on the dose, with beneficial effects on cell communication only detected at the one dose. The result suggests that determination of the dose of chemicals to be used is crucial for human intervention studies.

Induction of cytochrome P450 3A by retinoids in rat hepatocyte culture

1. Rat hepatocytes cultured on a Matrigel matrix were exposed for 48 h to all-trans-retinoic acid, 9-cis-retinoic acid, 13-cis-retinoic acid or fenretinide. 2. Cytochrome P450 3A (CYP3A) RNA levels were increased by approximately eightfold in hepatocytes treated with the retinoids compared to control cultures. 3. CYP1A1 and CYP1A2 RNA levels were only slightly affected or unaffected by the retinoids. 4. The induction of CYP3A by these therapeutically-useful retinoids suggests that they may share a common mechanism for accelerated drug catabolism and acquired clinical resistance.