Vitamin A in epithelial differentiation and skin carcinogenesis

The immunomodulatory effects of all-trans retinoic acid and docosahexaenoic acid combination treatment on the expression of IL-2, IL-4, T-bet, and GATA3 genes in PBMCs of multiple sclerosis patients

OBJECTIVES

Multiple sclerosis (MS) is a potentially disabling autoimmune disease of the central nervous system. Neither the pathogenesis nor the effectiveness of treatment of MS has been fully understood. This in vitro trial evaluated the beneficial immunomodulatory effects of single and combined treatments of all-trans retinoic acid (ATRA) and docosahexaenoic acid (DHA) on the peripheral blood mononuclear cells (PBMCs) of relapsing-remitting MS (RRMS) patients who were receiving interferon beta (IFN-β).

METHODS

The PBMCs of 15 RRMS patients were isolated, cultured, and treated with single and combined treatments of ATRA and DHA. The expressions of IL-2, IL-4, T-bet, and GATA3 genes were evaluated using real-time PCR.

RESULTS

The results showed that a single treatment of ATRA could significantly suppress the gene expression of the pro-inflammatory cytokine, IL-2 (P < 0.05), and related transcription factor, T-bet (P < 0.001). The gene expression level of the anti-inflammatory cytokine, IL-4, and its transcription factor, GATA3, were not significantly changed. The expression of IL-2 and T-bet genes was significantly decreased in combination treatments of ATRA and DHA (P < 0.001). Significant suppression of IL-2 and T-bet (P < 0.001) was observed in ATRA and DHA combination therapy with half doses of their single treatment, which suggested a synergistic effect of these components.

DISCUSSION

Co-administration of vitamin A and DHA, an omega-3 fatty acid derivative, may exert a synergistic effect in modulating the immune system in MS patients; however, more studies are needed to evaluate the exact effects and mechanism of their actions on the immune cells.

New Insights into Molecular Mechanism behind Anti-Cancer Activities of Lycopene

Lycopene is a well-known compound found commonly in tomatoes which brings wide range of health benefits against cardiovascular diseases and cancers. From an anti-cancer perspective, lycopene is often associated with reduced risk of prostate cancer and people often look for it as a dietary supplement which may help to prevent cancer. Previous scientific evidence exhibited that the anti-cancer activity of lycopene relies on its ability to suppress oncogene expressions and induce proapoptotic pathways. To further explore the real potential of lycopene in cancer prevention, this review discusses the new insights and perspectives on the anti-cancer activities of lycopene which could help to drive new direction for research. The relationship between inflammation and cancer is being highlighted, whereby lycopene suppresses cancer via resolution of inflammation are also discussed herein. The immune system was found to be a part of the anti-cancer system of lycopene as it modulates immune cells to suppress tumor growth and progression. Lycopene, which is under the family of carotenoids, was found to play special role in suppressing lung cancer.

Identification of biomarkers regulated by rexinoids (LGD1069, LG100268 and Ro25-7386) in human breast cells using Affymetrix microarray

Retinoids possess anti-proliferative properties, which suggests that they possess chemopreventive and therapeutic potential against cancer. In the current study, genes modulated by rexinoids (retinoid X receptor (RXR)-pan agonists, LGD1069 and LG100268; and the RXRα agonist, Ro25-7386) were identified using an Affymetrix microarray in normal and malignant breast cells. It was observed that LGD1069, LG100268 and Ro25-7386 suppressed the growth of breast cells. Secondly, several rexinoid-regulated genes were identified, which are involved in cell death, cell growth/maintenance, signal transduction and response to stimulus. These genes may be associated with the growth-suppressive activity of rexinoids. Therefore, the identified genes may serve as biomarkers and novel molecular targets for the prevention and treatment of breast cancer.

Role of dietary factors in the development of basal cell cancer and squamous cell cancer of the skin

The role of dietary factors in the development of skin cancer has been investigated for many years; however, the results of epidemiologic studies have not been systematically reviewed. This article reviews human studies of basal cell cancer (BCC) and squamous cell cancer (SCC) and includes all studies identified in the published scientific literature investigating dietary exposure to fats, retinol, carotenoids, vitamin E, vitamin C, and selenium. A total of 26 studies were critically reviewed according to study design and quality of the epidemiologic evidence. Overall, the evidence suggests a positive relationship between fat intake and BCC and SCC, an inconsistent association for retinol, and little relation between beta-carotene and BCC or SCC development. There is insufficient evidence on which to make a judgment about an association of other carotenoids with skin cancer. The evidence for associations between vitamin E, vitamin C, and selenium and both BCC and SCC is weak. Many of the existing studies contain limitations, however, and further well-designed and implemented studies are required to clarify the role of diet in skin cancer. Additionally, the role of other dietary factors, such as flavonoids and other polyphenols, which have been implicated in skin cancer development in animal models, needs to be investigated.

Emerging role of rexinoids in non-small cell lung cancer: focus on bexarotene

Although the introduction of third-generation antineoplastic agents in the treatment of non-small cell lung cancer has led to modest improvements in overall patient survival, lung cancer continues to be the leading cause of cancer-related death worldwide, and improved therapies are needed. Retinoids play a critical role in the regulation of cell division, growth, differentiation, and proliferation, and they represent an exciting new avenue for targeted therapy. Several synthetic retinoids that bind to retinoic acid receptors are currently being investigated in a variety of tumor types. However, many of these agents have been associated with cheilitis, skin reactions, severe headache, and hypertriglyceridemia. Synthetic agents that bind specifically to retinoid X receptors are called rexinoids. Bexarotene (Targretin; Ligand Pharmaceuticals; San Diego, CA; http://www.ligand.com) is a novel, multitargeted synthetic rexinoid that is currently being investigated in the treatment of non-small cell lung cancer. Phase I and II studies have demonstrated that bexarotene is safe and well tolerated in this patient population either alone or in combination with chemotherapeutic agents. Patients treated with bexarotene experience manageable adverse events at reduced levels compared with retinoic acid receptor-specific retinoids. Bexarotene in combination with chemotherapeutic agents has demonstrated an encouraging median survival for patients with advanced non-small cell lung cancer compared with historical results with combination chemotherapy alone. Two phase III trials are currently under way to fully characterize the role of bexarotene in the treatment of this disease. The purpose of this review is to explore the rationale for rexinoids in the treatment of malignancies and to discuss the clinical profile of bexarotene in the treatment of non-small cell lung cancer.

The tumor suppressor adenomatous polyposis coli and caudal related homeodomain protein regulate expression of retinol dehydrogenase L

Development of normal colon epithelial cells proceeds through a systematic differentiation of cells that emerge from stem cells within the base of colon crypts. Genetic mutations in the adenomatous polyposis coli (APC) gene are thought to cause colon adenoma and carcinoma formation by enhancing colonocyte proliferation and impairing differentiation. We currently have a limited understanding of the cellular mechanisms that promote colonocyte differentiation. Herein, we present evidence supporting a lack of retinoic acid biosynthesis as a mechanism contributing to the development of colon adenomas and carcinomas. Microarray and reverse transcriptase-PCR analyses revealed reduced expression of two retinoid biosynthesis genes: retinol dehydrogenase 5 (RDH5) and retinol dehydrogenase L (RDHL) in colon adenomas and carcinomas as compared with normal colon. Consistent with the adenoma and carcinomas samples, seven colon carcinoma cell lines also lacked expression of RDH5 and RDHL. Assessment of RDH enzymatic activity within these seven cell lines showed poor conversion of retinol into retinoic acid when compared with normal cells such as normal human mammary epithelial cells. Reintroduction of wild type APC into an APC-deficient colon carcinoma cell line (HT29) resulted in increased expression of RDHL without affecting RDH5. APC-mediated induction of RDHL was paralleled by increased production of retinoic acid. Investigations into the mechanism responsible for APC induction of RDHL indicated that beta-catenin fails to repress RDHL. The colon-specific transcription factor CDX2, however, activated an RDHL promoter construct and induced endogenous RDHL. Finally, the induction of RDHL by APC appears dependent on the presence of CDX2. We propose a novel role for APC and CDX2 in controlling retinoic acid biosynthesis and in promoting a retinoid-induced program of colonocyte differentiation.

Inhibitory effects of beta-carotene and vitamin a during the progression phase of hepatocarcinogenesis involve inhibition of cell proliferation but not alterations in DNA methylation

The inhibitory effects of Beta-carotene and vitamin A administered to rats in the progression phase of the resistant hepatocyte model of hepatocarcinogenesis were investigated. Beta-Carotene- and vitamin A-treated animals tended to present with a lower incidence of hepatic cancers than controls at sacrifice. Vitamin A, but not Beta-carotene, administration also tended to reduce the total number of persistent hepatocyte nodules. Histological examination of sections stained with hematoxylin and eosin confirmed these results. This suggests that both compounds exhibit inhibitory effects during conversion of persistent nodules to cancers, whereas only the retinoid is also capable of inhibiting the evolution of persistent nodules or causing them to regress. Moreover, Beta-carotene- and vitamin A-treated animals showed lower hepatic bromodeoxyuridine labeling indexes in neoplastic lesions as well as in adjacent normal tissues than controls, suggesting an inhibitory action of these substances on cell proliferation. However, neither Beta-carotene nor vitamin A administration resulted in substantial alterations in the CCGG sequence methylation pattern of hydroxymethylglutaryl coenzyme A reductase, c-myc, and c-Ha-ras genes, the products of which are related to cell proliferation and carcinogenesis. Therefore, these inhibitory effects of Beta-carotene and vitamin A on progression of hepatocarcinogenesis do not seem to be related to DNA methylation.

All-trans-retinoic acid binds to and inhibits adenine nucleotide translocase and induces mitochondrial permeability transition

We investigated the effects of retinoic acids on mitochondrial permeability transition (MPT) measured as changes in rhodamine 123 fluorescence from both isolated heart mitochondria and HeLa cells. We report that all-trans-retinoic acid (atRA), 9-cis-retinoic acid, and 13-cis-retinoic acid induce a drop in mitochondrial membrane potential in isolated mitochondria. The atRA effect was done through the induction of MPT because it was dependent on Ca(2+), in a synergic mechanism, and inhibited by cyclosporin A (CsA). Furthermore, atRA also opened MPT in vivo, because treatment of HeLa cells with atRA results in a CsA-sensitive drop of mitochondrial membrane potential. We demonstrated for the first time that retinoic acids inhibit adenine nucleotide translocase (ANT) activity in heart and liver mitochondria. Kinetic studies revealed atRA as an uncompetitive inhibitor of ANT. Photoaffinity labeling of mitochondrial proteins with [3H]atRA demonstrated the binding of a 31-kDa protein to atRA. This protein was identified as ANT because the presence of carboxyatractyloside, a specific ANT inhibitor, prevented labeling. The specific photolabeling of ANT was also prevented in a concentration-dependent manner by nonlabeled atRA, whereas palmitic acid was ineffective. This study indicates that specific interaction between atRA and ANT takes place regulating MPT opening and adenylate transport. These observations establish a novel mechanism for atRA action, which could control both energetic and apoptotic mitochondrial processes in situations such as retinoic acid treatment.

Vitamin and carotenoid intake and risk of squamous cell carcinoma of the skin

Our objective was to examine prospectively the intake of vitamins A (including retinol and total vitamin A), C and E; folate; total carotene; and several individual carotenoids (alpha-carotene, beta-carotene, beta-cryptoxanthin and lutein/zeaxanthin) in relation to incidence of SCC of the skin in 2 large cohorts of men and women. We used a prospective cohort study design with up to 14 years of follow-up in women and 10 years in men. Diet was measured with FFQs every 2-4 years; cases of SCC of the skin were ascertained on biennial questionnaires and confirmed by medical records. Participants were female nurses and male health professionals, from the Nurses' Healthy Study and the Health Professionals Follow-up Study in the United States, without a history of any cancer in 1982 (n = 85,944 women) and 1986 (n = 43,867 men). Follow-up response was achieved for over 90% of potential person-years. Relative risks and 95% confidence intervals for development of SCC of the skin are reported. We recorded 369 cases of SCC in women and 305 cases in men. After multivariate adjustment for various known behavioral, sun-exposure and sun-sensitivity risk factors for SCC, there were no significant inverse associations between these dietary factors and SCC incidence. No evidence was found that vitamins A, C and E; folate; or carotenoids play an important protective role against incident SCC.

Alterations in cellular retinol metabolism contribute to differential retinoid responsiveness in normal human mammary epithelial cells versus breast cancer cells

The present study was undertaken to compare ROH growth responsiveness between normal human mammary epithelial cells (HMECs), estrogen receptor positive (MCF-7) and negative (MDA-MB-231) breast cancer cells, and assess whether this responsiveness is correlated with differences in ROH metabolism, particularly RA synthesis. HMECs were markedly more growth sensitive to a physiological dose of ROH than breast cancer cells, exhibiting a significant decrease in cell number by 48h and >70% decrease by 144h. In comparison, numbers of MCF-7s were only decreased 32% by 144h. MDA-MB-231 cells were not affected. However, HMECs and MCF-7 cells displayed similar growth responsiveness to 1 microM RA, while MDA-MB-231 cells were minimally affected. Although the initial rates and extent of ROH uptake were comparable among cell types, ROH levels in HMECs progressively decreased to 20% of the peak by 24h and < or = 10% by 72h. In contrast, ROH levels in the cancer cells remained relatively constant through 48 h. The decrease in HMEC ROH was attributable to greater metabolism as evidenced by rapid and predominant retinyl ester formation. HMECs also produced approximately 5 times more RA from ROH than MCF-7s and approximately 10 times more than MDA MB-231 cells. Our results demonstrate that normal HMECs are markedly more responsive to the growth inhibitory effects of ROH than breast cancer cells, and that this responsiveness is associated with greater ROH metabolism including greater RA synthesis. These data suggest that altered ROH metabolism may be a factor in breast cancer progression.

Metabolic conversion of retinol to retinoic acid mediates the biological responsiveness of human mammary epithelial cells to retinol

The biological effects of vitamin A are mediated in part by retinoic acid (RA) modulation of gene transcription. In this study, we examined whether normal human mammary epithelial cells (HMECs) are biologically responsive to retinol (ROH), the metabolic precursor of RA. While both ROH and tRA resulted in time- and dose-dependent decreases in total cell number, tRA was markedly more potent. Metabolically, treatment of HMECs with physiological doses of ROH resulted in rapid uptake and subsequent production of both retinyl esters and tRA. Although a comparatively minor metabolite, tRA levels peaked at 6 h and remained above endogenous levels for up to 72 h in proportion to cellular ROH concentrations. In HMECs transfected with an RA-responsive luciferase reporter gene, treatment with 3 microM ROH resulted in an increase in luciferase activity to a level intermediate between that observed with 0.001 and 0.01 microM tRA. Citral, an RA-synthesis inhibitor, was also used to examine the biological activity of ROH. Compared to ROH alone, ROH plus citral treatment resulted in three-fold less tRA synthesis and a > 65% attentuation of RA-responsive reporter gene activity which persisted through 72 h. Citral also significantly attenuated the extent of ROH-mediated reductions in total HMEC number. Thus, treatment with physiological concentrations of ROH results in fewer total numbers of HMECs and this response is a consequence of cellular tRA synthesis which can induce RA-responsive gene expression.

Direct interaction of all-trans-retinoic acid with protein kinase C (PKC). Implications for PKC signaling and cancer therapy

Protein kinase C (PKC) regulates fundamental cellular functions including proliferation, differentiation, tumorigenesis, and apoptosis. All-trans-retinoic acid (atRA) modulates PKC activity, but the mechanism of this regulation is unknown. Amino acid alignments and crystal structure analysis of retinoic acid (RA)-binding proteins revealed a putative atRA-binding motif in PKC, suggesting existence of an atRA binding site on the PKC molecule. This was supported by photolabeling studies showing concentration- and UV-dependent photoincorporation of [(3)H]atRA into PKCalpha, which was effectively protected by 4-OH-atRA, 9-cis-RA, and atRA glucuronide, but not by retinol. Photoaffinity labeling demonstrated strong competition between atRA and phosphatidylserine (PS) for binding to PKCalpha, a slight competition with phorbol-12-myristate-13-acetate, and none with diacylglycerol, fatty acids, or Ca(2+). At pharmacological concentrations (10 micrometer), atRA decreased PKCalpha activity through the competition with PS but not phorbol-12-myristate-13-acetate, diacylglycerol, or Ca(2+). These results let us hypothesize that in vivo, pharmacological concentrations of atRA may hamper binding of PS to PKCalpha and prevent PKCalpha activation. Thus, this study provides the first evidence for direct binding of atRA to PKC isozymes and suggests the existence of a general mechanism for regulation of PKC activity during exposure to retinoids, as in retinoid-based cancer therapy.

Characterization of retinoic acid receptor-deficient keratinocytes

Retinoids are essential for normal epidermal growth and differentiation and show potential for the prevention or treatment of various epithelial neoplasms. The retinoic acid receptors (RARalpha, -beta, and -gamma) are transducers of the retinoid signal. The epidermis expresses RARgamma and RARalpha, both of which are potential mediators of the effects of retinoids in the epidermis. To further investigate the role(s) of these receptors, we derived transformed keratinocyte lines from wild-type, RARalpha, RARgamma, and RARalphagamma null mice and investigated their response to retinoids, including growth inhibition, markers of growth and differentiation, and AP-1 activity. Our results indicate that RARgamma is the principle receptor contributing to all-trans-retinoic acid (RA)-mediated growth arrest in this system. This effect partially correlated with inhibition of AP-1 activity. In the absence of RARs, the synthetic retinoid N-(4-hydroxyphenyl)-retinamide inhibited growth; this was not observed with RA, 9-cis RA, or the synthetic retinoid (E)-4-[2-(5, 5, 8, 8 tetramethyl-5,6,7,8-tetrahydro-2-naphthalenyl)-1-propenyl] benzoic acid. Finally, both RARalpha and RARgamma differently affected the expression of some genes, suggesting both specific and overlapping roles for the RARs in keratinocytes.

Patterns of CYP26 expression in human prenatal cephalic and hepatic tissues indicate an important role during early brain development

CYP26 (P450RAI) catalyzes catabolic retinoic acid (RA) hydroxylation and thereby appears to play a critical role in retinoid signaling pathways during development. In this study, a quantitative competitive reverse transcriptase-polymerase chain reaction (RT-PCR) assay was developed for evaluation of CYP26 message levels in human prenatal tissues. Statistical analyses of transcription levels in 12 prenatal human brains and six prenatal human livers demonstrated good sensitivity and reproducibility. Quantitative profiles of CYP26 gene expression in early (gestational days 57-110) prenatal cephalic and hepatic tissues and comparisons with adult counterparts are reported for the first time. Prenatal cephalic tissues at days 57-67 exhibited values of 1950+/-420 (CYP26 molecules/10(6) GAPDH molecules) whereas prenatal cephalic tissues at days 105-110 exhibited values of 22300+/-4450 (CYP26 molecules/10(6) GAPDH molecules), indicating a sharp developmental increase (approximately 11-fold). Levels in human adult cephalic tissues were slightly less than the prenatal cephalic levels measured during the earliest stages of gestation and were approximately 3-fold lower than those measured in adult human hepatic tissues. Levels in human prenatal hepatic tissues at days 63-110 gestation were less than 800 (CYP26 molecules/10(6) GAPDH molecules) and did not exhibit developmental increases. Considered together, the data have strong implications for the importance of CYP26 in early development of the human brain.

Retinoic acid hydroxylase (CYP26) is a key enzyme in neuronal differentiation of embryonal carcinoma cells

Besides nuclear retinoid receptors and cellular retinoid binding proteins also retinoic acid (RA)-synthesizing enzymes (using all-trans-retinal as substrate) and RA-catabolizing enzymes (producing hydroxylated products) may explain the specific effects of retinoids. In the past we have established an active role for 4-hydroxy-RA and 4-oxo-RA, which originally were considered to be inactive retinoids, but in fact are highly active modulators of positional specification in Xenopus development. Here we present evidence for a specific role of hydroxylated RA metabolites in the onset of neuronal differentiation. 4-Hydroxy- and 18-hydroxy-RA are products of the hydroxylation of RA by a novel cytochrome P450 (CYP)-type of enzyme, CYP26, expression of which is rapidly induced by RA. P19 embryonal carcinoma (EC) cell lines stably expressing hCYP26 undergo extensive and rapid neuronal differentiation in monolayer at already low concentrations of RA, while normally P19 cells under these conditions differentiate only in endoderm-like cells. Our results indicate that the effects on growth inhibition and RARbeta transactivation of P19 EC cells are mediated directly by RA, while the onset of neuronal differentiation and the subsequent expression of neuronal markers is mediated by hCYP26 via the conversion of RA to its hydroxylated products.

Embryonal carcinoma cell lines stably transfected with mRARbeta2-lacZ: sensitive system for measuring levels of active retinoids

Embryonal carcinoma cell lines (F9 EC and P19 EC) were stably transfected with 1.8 kb promoter sequence of RARbeta2 coupled to the lacZ gene as a system for measuring active retinoids. These stable transfectants, designated F9-1.8 and P19-1.8, were used as reporter cell lines to investigate different retinoids for their ability to activate the reporter gene. F9-1.8 cells showed similar EC(50) values for the acidic retinoids all-trans retinoic acid (RA), 4-oxo RA, 9-cis RA, and 13-cis RA, in the range of 1-7 nM, while P19-1.8 cells were less sensitive. Retinal showed decreased activity compared to the RA isomers in both lines. However, P19-1.8 cells hardly showed beta-gal activity after treatment with retinol, while the lacZ reporter in F9-1.8 cells was still inducible by this retinoid. In addition, the reporter system was used to investigate RA metabolism and its inhibition by P450 inhibitors. A combination of RA and liarozole showed a 10 times greater induction of the RARbeta2-lacZ reporter in P19-1.8 cells, but not in F9-1.8 cells. The EC(50) value for 4-oxo RA, however, was not altered, indicating that metabolic conversion of RA to 4-oxo RA is the target for inhibition by liarozole in P19-1.8 cells. HPLC analysis revealed nearly complete inhibition of RA metabolism after liarozole treatment in P19-1.8 cells, resulting in higher levels of RA. Finally, the F9-1.8 cells were used to detect active retinoids during different stages of chick limb bud development, demonstrating that it is the limb bud mesenchyme which generates RA and not the epidermis, with a twofold higher level of RA in the posterior half than in the anterior half.

Retinoids: present role and future potential

Vitamin A and its biologically active derivatives, retinal and retinoic acid (RA), together with a large repertoire of synthetic analogues are collectively referred to as retinoids. Naturally occurring retinoids regulate the growth and differentiation of a wide variety of cell types and play a crucial role in the physiology of vision and as morphogenic agents during embryonic development. Retinoids and their analogues have been evaluated as chemoprevention agents, and also in the management of acute promyelocytic leukaemia. Retinoids exert most of their effects by binding to specific receptors and modulating gene expression. The development of new active retinoids and the identification of two distinct families of retinoid receptors has led to an increased understanding of the cellular effects of activation of these receptors. In this article we review the use of retinoids in chemoprevention strategies, discuss the cellular consequences of activated retinoid receptors, and speculate on how our increasing understanding of retinoid-induced signalling pathways may contribute to future therapeutic strategies in the management of malignant disease.

Ultraviolet irradiation of human skin causes functional vitamin A deficiency, preventable by all-trans retinoic acid pre-treatment

We report here that ultraviolet irradiation substantially reduced the mRNA and protein of the two major nuclear retinoid receptors, RAR-gamma and RXR-alpha, in human skin in vivo. Pre-treatment with retinoic acid mitigated this loss of nuclear retinoid receptors. Ultraviolet irradiation caused a near-total loss of retinoic acid induction of two RAR/RXR target genes, cellular retinoic acid binding protein-II and RA 4-hydroxylase, but did not affect 1,25-dihydroxyvitamin D3 induction of the vitamin D receptor/RXR-regulated gene vitamin D 24-hydroxylase. In effect, ultraviolet irradiation causes a functional vitamin A deficiency that may have deleterious effects on skin function, contributing to skin photo-aging and carcinogenesis.

CYP2C7 expression in rat liver and hepatocytes: regulation by retinoids

Rats deficient in vitamin A express low levels of P4502C7 mRNA in the liver. Administration of all-trans retinoic acid (at-RA) or growth hormone (GH) to deficient animals only partially restored the expression whereas the combined treatment returned the P4502C7 mRNA levels to that observed in normal rats. That a retinoid is the predominant inducer of P4502C7 at the cellular level is evident from studies performed with primary hepatocytes, but it became clear that GH is a prerequisite for the vitamin A effect in vivo. The at-RA induction of P4502C7 mRNA in primary rat hepatocytes was inhibited by ketoconazole, an inhibitor of P450 activity, and by cycloheximide, blocking ongoing protein synthesis. In contrast, the at-RA induction of RAR-beta2 mRNA was not affected by any of these compounds. This could indicate previously not recognized mechanisms of at-RA action. Interestingly, at-4-oxo-RA, an at-RA metabolite formed by a P450 catalyzed reaction, also induced P4502C7 mRNA. Induction of P4502C7 mRNA by the retinoic acid receptor (RAR) selective agonist TTNPB indicated that this pathway is preferred over the retinoid X receptor (RXR) pathway. In addition, analysis of RA metabolites in liver cell extracts revealed the formation of several as yet unidentified metabolites. The formation of some of these metabolites was inhibited by ketoconazole and they could therefore constitute potential inducers of CYP2C7. We suggest that metabolism of at-RA, possibly by a P450 enzyme, is an important step in the at-RA induction of P4502C7.

Protection against malignant conversion in SENCAR mouse skin by all trans retinoic acid: inhibition of the ras p21-processing enzyme farnesyltransferase and Ha-ras p21 membrane localization

Many studies have shown that all trans retinoic acid (RA) exhibits significant protective effects against mouse skin tumor promotion and spontaneous as well as enhanced malignant conversion. In a recently completed study, we showed that under treatments in which papillomas on SENCAR mouse skin are induced at low and high probabilities to convert to malignant carcinomas, RA affords significant protection against both tumor promotion and subsequent malignant conversion. More than 95% of these mouse skin papillomas and carcinomas have been shown to contain point mutation at the 61 codon of Ha-ras oncogene. The ras oncogene encodes a p21 protein that, in its mutated form, transforms mammalian cells only when p21 is at the inner surface of the plasma membrane, by a series of enzymatic reactions in which the initial step is catalyzed by farnesyltransferase (FTase). In this study, we assessed whether the protective effect of RA against malignant conversion involves the inhibition of ras p21 processing in those tumors that contain the activated ras oncogene. The FTase activity and the levels of cytosolic and membrane-bound Ha-ras p21 were determined in all papillomas and carcinomas obtained from acetone- or RA-treated animals. No matter how the data were analyzed and what comparisons were considered, in all the protocols used, compared with controls, papillomas and carcinomas obtained from RA-treated groups showed significantly decreased (P < 0.01-0.001) FTase activity. Furthermore, the tissue samples from RA-treated groups in different protocols also showed significantly diminished membrane localization of Ha-ras p21, with a concomitant increase in cytosolic Ha-ras p21 levels. The analysis of these data also showed that in all the protocols used, the increased FTase activity and membrane localization of Ha-ras p21 were associated with the induction of papillomas and their subsequent malignant conversion to squamous cell carcinomas. Taken together, these results indicate a strong correlation between the inhibition of ras p21 farnesylation because of a decrease in FTase activity by RA and its protective effect against malignant conversion of papillomas to carcinomas. Based on the results of this study, it is tempting to suggest that clinical trials evaluating the preventive or therapeutic potential of retinoids may be directed more toward those clinical malignancies that are known to contain the activated ras oncogene.

Suppression of the tumorigenicity of human hepatoma hep3B cells by long-term retinoic acid treatment

We cultured human hepatoma Hep3B cells in the presence of RA (10(-5) M) for 30 days; the expression of both alpha-fetoprotein and hepatitis B virus surface antigen were suppressed over 70% at the transcriptional level by RA treatment. The doubling time of RA treated Hep3B cells was slightly different from the control cells when they were cultured in 5% fetal calf serum/DMEM medium. However, cultured under serum-free conditions, the control Hep3B cells still grow, but the RA treated cells could not attach to the substratum of the culture plate and stopped growing. In vivo assay indicated that RA treatment completely suppressed the tumorigenicity of Hep3B cells in nude mice.