Retinoic acid causes cell growth arrest and an increase in p27 in F9 wild type but not in F9 retinoic acid receptor β2 knockout cells

DNA methylation patterns of RAR-β2 and RASSF1A gene promoters in FNAB samples from Greek population with benign or malignant breast lesions

BACKGROUND

Promoter hypermethylation is common in Breast Cancer (BC) with studies mainly in histological specimens showing frequent methylation of tumor suppressor genes (TSGs) compared with normal tissues. The aim of this study was to estimate the frequency of promoter methylation of RAR-β2 and RASSF1A genes in breast FNAB material aiming to evaluate the methylation status of these two genes as biomarker for detecting BC in Greek population.

METHODS

FNAB material from 104 patients was collected for cytological evaluation and epigenetic analysis. DNA was extracted and subjected to bisulfite conversion. A methylation-specific PCR was carried out and the final products were separated with electrophoresis in 2% agarose gels.

RESULTS

From 104 samples, RASSF1A hypermethylation was observed in 78 (75%) and RAR-β2 hypermethylation in 64 (61.6%). 84% and 78% of the cases diagnosed with breast malignancy (n = 50) were methylated for RASSF1A and RAR-β2, respectively. Methylated RASSF1A and RAR-β2 were also detected in 88.3% and 76.5% in samples diagnosed as suspicious for malignancy (n = 17) and in 57.2% of samples diagnosed with atypia (n = 14). The Odds Ratio for breast malignancy was 4.545 in patients with RASSF1A hypermethylation and 9.167 in patients with RAR-β2 hypermethylation underlying their promoter's methylation positive correlation with breast malignancy.

CONCLUSION

To optimize the sensitivity and specificity of this epigenetic setting, more TSGs related to BC should be gradually imported in our evaluated methylation panel and be validated in a larger study sample with the aim that the obtained epigenetic profiles will provide clinicians with valuable tools for management of BC patients in Greece.

Combinatorial knockout of RARα, RARβ, and RARγ completely abrogates transcriptional responses to retinoic acid in murine embryonic stem cells

All-trans-retinoic acid (RA), a potent inducer of cellular differentiation, functions as a ligand for retinoic acid receptors (RARα, β, and γ). RARs are activated by ligand binding, which induces transcription of direct genomic targets. However, whether embryonic stem cells respond to RA through routes that do not involve RARs is unknown. Here, we used CRISPR technology to introduce biallelic frameshift mutations in RARα, RARβ, and RARγ, thereby abrogating all RAR functions in murine embryonic stem cells. We then evaluated RA-responsiveness of the RAR-null cells using RNA-Seq transcriptome analysis. We found that the RAR-null cells display no changes in transcripts in response to RA, demonstrating that the RARs are essential for the regulation of all transcripts in murine embryonic stem cells in response to RA. Our key finding, that in embryonic stem cells the transcriptional effects of RA all depend on RARs, addresses a long-standing topic of discussion in the field of retinoic acid signaling.

Human papillomavirus type 16 E7 oncoprotein upregulates the retinoic acid receptor-beta expression in cervical cancer cell lines and K14E7 transgenic mice

Persistent infection with high-risk human papillomaviruses is the main etiological factor in cervical cancer (CC). The human papillomavirus type 16 (HPV16) E7 oncoprotein alters several cellular processes, regulating the expression of many genes in order to avoid cell cycle control. Retinoic acid receptor beta (RARB) blocks cell growth, inducing differentiation and apoptosis. This tumor suppressor gene is gradually silenced in late passages of foreskin keratinocytes immortalized with HPV16 and in various tumors, including CC, mainly by epigenetic modifications. We investigated the effect of E7 oncoprotein on RARB gene expression. We found that HPV16 E7 increases RARB mRNA and RAR-beta protein expression both in vitro and in the cervix of young K14E7 transgenic mice. In E7-expressing cells, RARB overexpression is further increased in the presence of the tumor suppressor p53 (TP53) R273C mutant. This effect does not change when either C33-A or E7-expressing C33-A cell line is treated with Trichostatin A, suggesting that E7 enhances RARB expression independently of histone deacetylases inhibition. These findings indicate that RARB overexpression is part of the early molecular events induced by the E7 oncoprotein.

Polycomb recruitment attenuates retinoic acid-induced transcription of the bivalent NR2F1 gene

Polycomb proteins play key roles in mediating epigenetic modifications that occur during cell differentiation. The Polycomb repressive complex 2 (PRC2) mediates the tri-methylation of histone H3 lysine 27 (H3K27me3). In this study, we identify a distinguishing feature of two classes of PRC2 target genes, represented by the Nr2F1 (Coup-TF1) and the Hoxa5 gene, respectively. Both genes are transcriptionally activated by all-trans retinoic acid (RA) and display increased levels of the permissive H3K9/K14ac and tri-methylated histone H3 lysine 4 epigenetic marks in response to RA. However, while in response to RA the PRC2 and H3K27me3 marks are greatly decreased at the Hoxa5 promoter, these marks are initially increased at the Nr2F1 promoter. Functional depletion of the essential PRC2 protein Suz12 by short hairpin RNA (shRNA) technology enhanced the RA-associated transcription of Nr2F1, Nr2F2, Meis1, Sox9 and BMP2, but had no effect on the Hoxa5, Hoxa1, Cyp26a1, Cyp26b1 and RARβ2 transcript levels in wild-type embryonic stem cells. We propose that PRC2 recruitment attenuates the RA-associated transcriptional activation of a subset of genes. Such a mechanism would permit the fine-tuning of transcriptional networks during differentiation.

CDK1 interacts with RARγ and plays an important role in treatment response of acute myeloid leukemia

Alterations in cell cycle pathways and retinoic acid signaling are implicated in leukemogenesis. However, little is known about the roles of cyclin-dependent kinases (CDKs) in treatment response of leukemia. In this study, we observed that CDK1 expression was significantly higher in bone marrow from 42 patients with acute myeloid leukemia (AML) at recurrence than that at first diagnosis (p = 0.04). AML patients had higher level of nuclear CDK1 in their leukemic blasts tended to have poorer clinical outcome compared with those with lower levels. We showed that CDK1 function is required for all-trans retinoic acid (ATRA) to achieve the optimal effect in U-937 human leukemic cells. CDK1 modulates the levels of P27(kip) and AKT phosphorylation in response to ATRA treatment. Further, we show, for the first time, that RARγ in concert with ATRA regulates protein levels of CDK1 and its subcellular localization. The regulation of the subcellular content of CDK1 and RARγ by ATRA is an important process for achieving an effective response in treatment of leukemia. RARγ and CDK1 form a reciprocal regulatory circuit in the nucleus and influence the function and protein stability of each other and the level of P27(kip) protein. In addition, expression of wee1 kinase and Cdc25A/C phosphatases also coincide with CDK1 expression and its subcellular localization in response to ATRA treatment. Our study reveals a novel mechanism by which CDK1 and RARγ coordinate with ATRA to influence cell cycle progression and cellular differentiation.

A clinically relevant bi-cellular murine mammary tumor model as a useful tool for evaluating the effect of retinoic acid signaling on tumor progression

BACKGROUND

The effect of retinoic acid (RA) on breast cancer progression is controversial. Our objective was to obtain information about breast cancer progression, taking advantage of the ER-negative murine mammary adenocarcinoma model LM38 (LM38-LP constituted by luminal (LEP) and myoepithelial-like cells (MEP), LM38-HP mainly composed of spindle-shaped epithelial cells, and LM38-D2 containing only large myoepithelial cells), and to validate the role of the retinoic acid receptors (RARs) in each cell-type compartment.

MATERIALS AND METHODS

We studied the expression and functionality of the RARs in LM38 cell lines. We analyzed cell growth and cell cycle distribution, apoptosis, the activity of proteases, motility properties, and expression of the molecules involved in these pathways. We also evaluated tumor growth and dissemination in vivo under retinoid treatment.

RESULTS

LM38 cell lines expressed most retinoic receptor isotypes that were functional. However, only the bi-cellular LM38-LP cells responded to retinoids by increasing RARβ2 and CRBP1 expression. The growth of LM38 cell sublines was inhibited by retinoids, first by inducing arrest in MEP cells, then apoptosis in LEP cells. Retinoids induced inhibitory effects on motility, invasiveness, and activity of proteolytic enzymes, mainly in the LM38-LP cell line. In in-vivo assays with the LM38-LP cell line, RA treatment impaired both primary tumor growth and lung metastases dissemination.

CONCLUSION

These in-vivo and in-vitro results show that to achieve maximum effects of RA on tumor progression both the LEP and MEP cell compartments have to be present, suggesting that the interaction between the LEP and MEP cells is crucial to full activation of the RARs.

Dissecting the retinoid-induced differentiation of F9 embryonal stem cells by integrative genomics

Retinoic acid (RA) triggers physiological processes by activating heterodimeric transcription factors (TFs) comprising retinoic acid receptor (RARα, β, γ) and retinoid X receptor (RXRα, β, γ). How a single signal induces highly complex temporally controlled networks that ultimately orchestrate physiological processes is unclear. Using an RA-inducible differentiation model, we defined the temporal changes in the genome-wide binding patterns of RARγ and RXRα and correlated them with transcription regulation. Unexpectedly, both receptors displayed a highly dynamic binding, with different RXRα heterodimers targeting identical loci. Comparison of RARγ and RXRα co-binding at RA-regulated genes identified putative RXRα-RARγ target genes that were validated with subtype-selective agonists. Gene-regulatory decisions during differentiation were inferred from TF-target gene information and temporal gene expression. This analysis revealed six distinct co-expression paths of which RXRα-RARγ is associated with transcription activation, while Sox2 and Egr1 were predicted to regulate repression. Finally, RXRα-RARγ regulatory networks were reconstructed through integration of functional co-citations. Our analysis provides a dynamic view of RA signalling during cell differentiation, reveals RAR heterodimer dynamics and promiscuity, and predicts decisions that diversify the RA signal into distinct gene-regulatory programs.

Retinoids, retinoic acid receptors, and cancer

Retinoids (i.e., vitamin A, all-trans retinoic acid, and related signaling molecules) induce the differentiation of various types of stem cells. Nuclear retinoic acid receptors mediate most but not all of the effects of retinoids. Retinoid signaling is often compromised early in carcinogenesis, which suggests that a reduction in retinoid signaling may be required for tumor development. Retinoids interact with other signaling pathways, including estrogen signaling in breast cancer. Retinoids are used to treat cancer, in part because of their ability to induce differentiation and arrest proliferation. Delivery of retinoids to patients is challenging because of the rapid metabolism of some retinoids and because epigenetic changes can render cells retinoid resistant. Successful cancer therapy with retinoids is likely to require combination therapy with drugs that regulate the epigenome, such as DNA methyltransferase and histone deacetylase inhibitors, as well as classical chemotherapeutic agents. Thus, retinoid research benefits both cancer prevention and cancer treatment.

Metabolism and regulation of gene expression by 4-oxoretinol versus all-trans retinoic acid in normal human mammary epithelial cells

We previously demonstrated that 4-oxoretinol (4-oxo-ROL) activated retinoic acid receptors (RARs) in F9 stem cells. We showed that 4-oxo-ROL inhibited the proliferation of normal human mammary epithelial cells (HMECs). To understand the mechanisms by which 4-oxo-ROL regulates HMEC growth we examined gene expression profiles following 4-oxo-ROL or all-trans retinoic acid (tRA). We also compared growth inhibition by tRA, 4-oxo-ROL, or 4-oxo-RA. All three retinoids inhibited HMEC proliferation. Gene expression analyses indicated that 4-oxo-ROL and tRA modulated gene expression in closely related pathways. The expression of many genes, e.g. ATP-binding cassette G1 (ABCG1); adrenergic receptorbeta2 (ADRB2); ras-related C3 botulinum toxin substrate (RAC2); and short-chain dehydrogenase/reductase 1 gene (SDR1) was changed after 4-oxo-ROL or tRA. Metabolism of these retinoids was analyzed by high-performance liquid chromatography (HPLC). In 1 microM tRA treated HMECs all of the tRA was found intracellularly, and tRA was the predominant intracellular retinoid. In 1 microM 4-oxo-ROL treated HMECs most 4-oxo-ROL was esterified to 4-oxoretinyl esters, no tRA was detected, and 4-oxo-ROL and 4-oxo-RA were observed intracellularly. In 1 microM 4-oxoretinoic acid (4-oxo-RA) treated HMECs little intracellular 4-oxo-RA was detected; most 4-oxo-RA was in the medium. Our results indicate that: (a) 4-oxo-ROL regulates gene expression and inhibits proliferation of HMECs; (b) 4-oxo-ROL and tRA regulate some of the same genes; (c) more tRA is found in cells, as compared to 4-oxoretinoic acid, when each drug is added at the same concentration in the medium; and (d) the mechanism by which 4-oxo-ROL exerts its biological activity does not involve intracellular tRA production.

Retinoids in biological control and cancer

More than 80 years ago, Wolbach and Howe provided the first evidence suggesting a link between alterations within human cells that lead to malignancies and vitamin A deficiencies (Wolbach and Howe 1925 Nutr. Rev. 36: 16-19). Since that time, epidemiological, preclinical and clinical studies have established a causative relationship between vitamin A deficiency and cancer. Laboratory research has provided insight into the intracellular targets, various signaling cascades and physiological effects of the biologically-active natural and synthetic derivatives of vitamin A, known as retinoids. Collectively, this body of research supports the concept of retinoids as chemopreventive and chemotherapeutic agents that can prevent epithelial cell tumorigenesis by directing the cells to either differentiate, growth arrest, or undergo apoptosis, thus preventing or reversing neoplasia. Continued refinement of the retinoid signaling pathway is essential to establishing their use as effective therapeutics for tumor subtypes whose oncogenic intracellular signaling pathways can be blocked or reversed by treatment with retinoids.

LIF removal increases CRABPI and CRABPII transcripts in embryonic stem cells cultured in retinol or 4-oxoretinol

Murine embryonic stem (ES) cells cultured without leukemia inhibitory factor (LIF) or with retinoids differentiate and concomitantly metabolize retinol (vitamin A) to 4-oxoretinol. Our objective was to examine the effects of retinol or 4-oxoretinol on cellular retinoic acid binding protein (CRABP) I and II mRNA levels and retinol metabolism. ES cells were cultured with or without LIF, and with various doses of all-trans-retinol, all-trans-4-oxoretinol, or all-trans-retinoic acid (RA). In ES cells treated with retinol or 4-oxoretinol in the absence of LIF the CRABP-I (Crabp1, NM_013496; GI:7304974) and CRABP-II (Crabp2, NM_007759; GI:33469074) mRNA levels at 72h were 66+/-4 and 413+/-6 fold higher, respectively, than the levels in control ES cells cultured without retinoids and in the presence of LIF. The increase in CRABPI mRNA occurred through an increase in CRABPI gene transcription. CRABPI protein was also increased by >50-fold in cells treated with retinol in the absence of LIF. However [(3)H]4-oxoretinol does not bind to murine CRABPI or CRABPII. CYP26A1 mRNA levels and [(3)H]4-oxoretinol production from [(3)H]retinol increased in cells cultured without LIF and with exogenous retinoids. The enormous increases in CRABPI and II transcripts ( approximately 60 and 400-fold, respectively) in the absence of LIF may regulate aspects of the ES cell differentiation program in response to retinol.

CYP26A1 knockout embryonic stem cells exhibit reduced differentiation and growth arrest in response to retinoic acid

CYP26A1, a cytochrome P450 enzyme, metabolizes all-trans-retinoic acid (RA) into polar metabolites, e.g. 4-oxo-RA and 4-OH-RA. To determine if altering RA metabolism affects embryonic stem (ES) cell differentiation, we disrupted both alleles of Cyp26a1 by homologous recombination. CYP26a1(-/-) ES cells had a 11.0+/-3.2-fold higher intracellular RA concentration than Wt ES cells after RA treatment for 48 h. RA-treated CYP26A1(-/-) ES cells exhibited 2-3 fold higher mRNA levels of Hoxa1, a primary RA target gene, than Wt ES cells. Despite increased intracellular RA levels, CYP26a1(-/-) ES cells were more resistant than Wt ES cells to RA-induced proliferation arrest. Transcripts for parietal endodermal differentiation markers, including laminin, J6(Hsp 47), and J31(SPARC, osteonectin) were expressed at lower levels in RA-treated CYP26a1(-/-) ES cells, indicating that the lack of CYP26A1 activity inhibits RA-associated differentiation. Microarray analyses revealed that RA-treated CYP26A1(-/-) ES cells exhibited lower mRNA levels than Wt ES cells for genes involved in differentiation, particularly in neural (Epha4, Pmp22, Nrp1, Gap43, Ndn) and smooth muscle differentiation (Madh3, Nrp1, Tagln Calponin, Caldesmon1). In contrast, genes involved in the stress response (e.g. Tlr2, Stk2, Fcgr2b, Bnip3, Pdk1) were expressed at higher levels in CYP26A1(-/-) than in Wt ES cells without RA. Collectively, our results show that CYP26A1 activity regulates intracellular RA levels, cell proliferation, transcriptional regulation of primary RA target genes, and ES cell differentiation to parietal endoderm.

Retinoic acid-mediated differentiation protects against nitrofen-induced apoptosis

BACKGROUND

Nitrofen is a diphenyl ether that induces a spectrum of birth defects subsequent to administration to pregnant rodents, in which the molecular etiology of these defects are poorly characterized. Because previous reports showed that nitrofen induced apoptosis in undifferentiated P19 teratocarcinoma cells, we hypothesized that undifferentiated fetal cells have greater susceptibility to nitrofen-induced apoptosis than their differentiated derivatives.

METHODS

To investigate this hypothesis, cell lines including P19 and F9 were differentiated with retinoic acid into neuronal and endodermal derivatives respectively. Apoptosis was characterized by caspase-3 cleavage and Terminal transferase dUTP nick end labeling (TUNEL) assays.

RESULTS

Both differentiated cell-types had reduced nitrofen-induced caspase-3 cleavage and DNA fragmentation compared with the naive controls, strongly suggesting that differentiation of these cells protects against nitrofen-induced apoptosis. In addition, resistance to apoptotic induction was proportional to the expression levels of the differentiation marker, p27 (kip1) while direct proportionality was not observed for the antiapoptotic protein Bcl-2.

CONCLUSIONS

These studies show that nitrofen may induce its associated birth defects via a mechanism involving apoptosis of undifferentiated fetal cells.

Overexpression of CRABPI in suprabasal keratinocytes enhances the proliferation of epidermal basal keratinocytes in mouse skin topically treated with all-trans retinoic acid

We investigated whether ectopic expression of CRABPI, a cellular retinoic acid binding protein, influenced the actions of all-trans retinoic acid (ATRA) in transgenic (TG) mice. We targeted CRABPI to the basal vs. suprabasal layers of mouse epidermis by using the keratin 14 (K14) and keratin 10 (K10) promoters, respectively. Greater CRABPI protein levels were detected in the epidermis of adult transgenic(+) mice than in transgenic(-) mice for both transgenes. In adult mouse skin CRABPI overexpression in the basal or suprabasal keratinocytes did not cause morphological abnormalities, but did result in decreased CRABPII mRNA levels. Ectopically overexpressed CRABPI in suprabasal keratinocytes, but not in basal keratinocytes, enhanced the thickening of the epidermis induced by topical ATRA treatments (10 microM, 400 microl for 4 days) by 1.59+/-0.2-fold (p<0.05). ATRA treatment (10 microM) resulted in a 59.9+/-9.8% increase (p<0.05) in the BrdU labeling index in K10/FLAG-CRABPI TG(+) mice vs. TG(-) mice. Retinoid topical treatments reduced p27 and CYP26A1 mRNA levels in TG(+) and TG(-) mouse skin in K14 and K10/FLAG-CRABPI transgenic mice. As epidermal basal keratinocyte proliferation is stimulated by paracrine growth factors secreted by ATRA activated suprabasal keratinocytes, our results indicate that CRABPI overexpression in suprabasal keratinocytes enhances the physiological functions of ATRA.

Diverse actions of retinoid receptors in cancer prevention and treatment

Retinoids (retinol [vitamin A] and its biologically active metabolites) are essential signaling molecules that control various developmental pathways and influence the proliferation and differentiation of a variety of cell types. The physiological actions of retinoids are mediated primarily by the retinoic acid receptors alpha, beta, and gamma (RARs) and rexinoid receptors alpha, beta, and gamma. Although mutations in RARalpha, via the PML-RARalpha fusion proteins, result in acute promyelocytic leukemia, RARs have generally not been reported to be mutated or part of fusion proteins in carcinomas. However, the retinoid signaling pathway is often compromised in carcinomas. Altered retinol metabolism, including low levels of lecithin:retinol acyl trasferase and retinaldehyde dehydrogenase 2, and higher levels of CYP26A1, has been observed in various tumors. RARbeta(2) expression is also reduced or is absent in many types of cancer. A greater understanding of the molecular mechanisms by which retinoids induce cell differentiation, and in particular stem cell differentiation, is required in order to solve the issue of retinoid resistance in tumors, and thereby to utilize RA and synthetic retinoids more effectively in combination therapies for human cancer.

Apo-10'-lycopenoic acid inhibits lung cancer cell growth in vitro, and suppresses lung tumorigenesis in the A/J mouse model in vivo

High intake of lycopene has been associated with a lower risk of a variety of cancers including lung cancer. We recently showed that lycopene can be converted to apo-10'-lycopenoids [Hu et al. (2006). J. Biol. Chem., 281, 19327-19338] in mammalian tissues both in vitro and in vivo, raising the question of whether apo-10'-lycopenoids have biological activities against lung carcinogenesis. In the present study, we report that apo-10'-lycopenoic acid inhibited the growth of NHBE normal human bronchial epithelial cells, BEAS-2B-immortalized normal bronchial epithelial cells and A549 non-small cell lung cancer cells. This inhibitory effect of apo-10'-lycopenoic acid was associated with decreased cyclin E, inhibition of cell cycle progression from G(1) to S phase and increased cell cycle regulators p21 and p27 protein levels. In addition, apo-10'-lycopenoic acid transactivated the retinoic acid receptor beta (RARbeta) promoter and induced the expression of RARbeta. We further examined the effect of apo-10'-lycopenoic acid treatment on 4-(N-methyl-N-nitrosamino)-1-(3-pyridal)-1-butanone (NNK)-induced lung tumorigenesis in the A/J mouse model. We found that the lung tumor multiplicity was decreased dose dependently from an average of 16 tumors per mouse in the NNK injection alone group, to an average of 10, 7 and 5 tumors per mouse in groups injected with NNK and supplemented with 10, 40 and 120 mg/kg diet of apo-10'-lycopenoic acid, respectively. These observations demonstrate that apo-10'-lycopenoic acid is a biological active metabolite of lycopene and suggest that apo-10'-lycopenoic acid is a potential chemopreventive agent against lung tumorigenesis.

Role of retinoic acid in the differentiation of embryonal carcinoma and embryonic stem cells

Retinoic acid (RA), the most potent natural form of vitamin A, plays an important role in many diverse biological processes such as embryogenesis and cellular differentiation. This chapter is a review of the mechanism of action of RA and the role of specific RA-regulated genes during the cellular differentiation of embryonal carcinoma (EC) and embryonic stem (ES) cells. RA acts by binding to its nuclear receptors and inducing transcription of specific target genes. The most studied mouse EC cell lines include F9 cells, which can be induced by RA to differentiate into primitive, parietal, and visceral endodermal cells; and P19 cells, which can differentiate to endodermal and neuronal cells upon RA treatment. ES cells can be induced to differentiate into a number of different cell types; many of which require RA treatment. Over the years, many RA-regulated genes have been discovered in EC and ES cells using a diverse set of techniques. Current research focuses on the elucidation how these genes affect differentiation in EC and ES cells using a variety of molecular biology approaches. However, the exact molecule events that lead from a pluripotent stem cell to a fully differentiated cell following RA treatment are yet to be determined.

Antitumor activity of the retinoid-related molecules (E)-3-(4'-hydroxy-3'-adamantylbiphenyl-4-yl)acrylic acid (ST1926) and 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) in F9 teratocarcinoma: Role of retinoic acid receptor gamma and retinoid-independent pathways

The retinoid-related molecules (RRMs) ST1926 [(E)-3-(4'-hydroxy-3'-adamantylbiphenyl-4-yl)acrylic acid] and CD437 (6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid) are promising anticancer agents. We compared the retinoic acid receptor (RAR) trans-activating properties of the two RRMs and all-trans-retinoic acid (ATRA). ST1926 and CD437 are better RARgamma agonists than ATRA. We used three teratocarcinoma cell lines to evaluate the significance of RARgamma in the activity of RRMs: F9-wild type (WT); F9gamma-/-, lacking the RARgamma gene; F9gamma51, aF9gamma-/-derivative, complemented for the RARgamma deficit. Similar to ATRA, ST1926 and CD437 activate cytodifferentiation only in F9-WT cells. Unlike ATRA, ST1926 and CD437 arrest cells in the G2/M phase of the cell cycle and induce apoptosis in all F9 cell lines. Our data indicate that RARgamma and the classic retinoid pathway are not relevant for the antiproliferative and apoptotic activities of RRMs in vitro. Increases in cytosolic calcium are fundamental for apoptosis, in that intracellular calcium chelators abrogate the process. Comparison of the gene expression profiles associated with ST1926 and ATRA in F9-WT and F9gamma-/-indicates that the RRM activates a conspicuous nonretinoid response in addition to the classic and RAR-dependent pathway. The pattern of genes regulated by ST1926 selectively, in a RARgamma-independent manner, provides novel insights into the possible molecular determinants underlying the activity of RRMs in vitro. Furthermore, it suggests that RARgamma-dependent responses are relevant to the activity of RRMs in vivo. Indeed, the receptor hinders the antitumor activity in vivo, in that both syngeneic and immunosuppressed SCID mice bearing F9gamma-/- tumors have increased life spans after treatment with ST1926 and CD437 relative to their F9-WT counterparts.

Disruption of the lecithin:retinol acyltransferase gene makes mice more susceptible to vitamin A deficiency

Lecithin:retinol acyltransferase (LRAT) catalyzes the esterification of retinol (vitamin A) in the liver and in some extrahepatic tissues, including the lung. We produced an LRAT gene knock-out mouse strain and assessed whether LRAT-/- mice were more susceptible to vitamin A deficiency than wild type (WT) mice. After maintenance on a vitamin A-deficient diet for 6 weeks, the serum retinol level was 1.34 +/- 0.32 microM in WT mice versus 0.13 +/- 0.06 microM in LRAT-/- mice (p < 0.05). In liver, lung, eye, kidney, brain, tongue, adipose tissue, skeletal muscle, and pancreas, the retinol levels ranged from 0.05 pmol/mg (muscle and tongue) to 17.35 +/- 2.66 pmol/mg (liver) in WT mice. In contrast, retinol was not detectable (<0.007 pmol/mg) in most tissues from LRAT-/- mice after maintenance on a vitamin A-deficient diet for 6 weeks. Cyp26A1 mRNA was not detected in hepatic tissue samples from LRAT-/- mice but was detected in WT mice fed the vitamin A-deficient diet. These data indicate that LRAT-/- mice are much more susceptible to vitamin A deficiency and should be an excellent animal model of vitamin A deficiency. In addition, the retinol levels in serum rapidly increased in the LRAT-/- mice upon re-addition of vitamin A to the diet, indicating that serum retinol levels in LRAT-/- mice can be conveniently modulated by the quantitative manipulation of dietary retinol.

Valproic acid, in combination with all-trans retinoic acid and 5-aza-2'-deoxycytidine, restores expression of silenced RARbeta2 in breast cancer cells

Epigenetic silencing of tumor suppressor genes has been established as an important process of carcinogenesis. The retinoic acid (RA) receptor beta2 (RARbeta2) gene is one such tumor suppressor gene often silenced during carcinogenesis. The combined use of histone deacetylase and DNA methyltransferase inhibitors has been shown to reverse the epigenetic silencing of numerous growth regulatory genes. Valproic acid (VPA), which has long been used in the treatment of epilepsy, was shown recently to be an effective histone deacetylase inhibitor that can induce differentiation of neoplastically transformed cells. In this study, we show for the first time that VPA, in combination with RA and the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine (Aza-dC), can overcome the epigenetic barriers to transcription of a prototypical silenced tumor suppressor gene, RARbeta2, in human breast cancer cells. Chromatin immunoprecipitation assays show that the combination of VPA, RA, and Aza-dC increases histone acetylation at the silenced RARbeta2 promoter of MCF-7 breast cancer cells. Furthermore, reverse transcription-PCR analyses reveal cell type-specific effects in the actions of VPA on RARbeta2 expression in cultured human breast cancer cells. Finally, we show that VPA, in combination with RA and Aza-dC, inhibits the proliferation of both estrogen receptor alpha-positive (MCF-7) and estrogen receptor alpha-negative (MDA-MB-231) breast cancer cell lines. These data suggest that VPA may ultimately be useful in combination therapies in the treatment of human breast cancers.

In vivo imaging of retinoic acid receptor beta2 transcriptional activation by the histone deacetylase inhibitor MS-275 in retinoid-resistant prostate cancer cells

BACKGROUND

In retinoid resistant epithelial tumors, the lack of retinoic acid receptor beta2 (RARbeta2) expression due to epigenetic silencing impairs the activation of retinoid target genes including RARbeta2, and has been associated with the development of cancer. In this study we developed a strategy to monitor the re-activation of RARbeta2 by chromatin remodeling agents combined with retinoids in real time, and to correlate the RARbeta2 re-activation with anti-tumor activity.

METHODS

We selected the RARbeta2-negative retinoid resistant human prostate carcinoma cell line PC3 and stably transfected it with a luciferase expression vector under the control of a functional segment of RARbeta2 promoter (pGL2-RARbeta2-PC3). Then, we used the bioluminescence technology to monitor the reporter gene expression in real time both in vitro and in vivo following combination treatment with the histone deacetylase inhibitor MS-275 and 13-cis retinoic acid (CRA). Based on the effective dose for the RARbeta2 re-activation, we tested the anti-tumor activity of this drug combination.

RESULTS

Following combination treatment with MS-275 and CRA, we observed endogenous RARbeta2 re-expression, acetylation at the RARbeta2 promoter level, and synergistic activation of the luciferase reporter gene by real time imaging both in vitro and in vivo. Combination treatment with MS-275 and CRA restored retinoid sensitivity in human prostate carcinoma cell lines, and had a greater inhibitory effect on tumor cell growth than single agents in vitro and in vivo.

CONCLUSIONS

This study provides evidence that HDAC inhibitors restore retinoid sensitivity in prostate cancer cells, and in vivo real time imaging of RARbeta2 activation may represent a useful tool to study the pharmacodynamics of combination therapy with HDAC inhibitors and retinoids.

A role for p21 (WAF1) in the cAMP-dependent differentiation of F9 teratocarcinoma cells into parietal endoderm

Combined treatment of teratocarcinoma F9 cells with retinoic acid and dibutyryl-cAMP induces the differentiation into cells with a phenotype resembling parietal endoderm. We show that the levels of cyclin-dependent kinase inhibitor p21/WAF1/Cip1 (p21) protein and mRNA are dramatically elevated at the end of this differentiation, concomitantly with the appearance of p21 in the immunoprecipitated CDK2-cyclin E complex. The induction of differentiation markers could not be achieved by expression of ectopic p21 alone and still required treatment with differentiation agents. Clones of F9 cells transfected with sense or antisense p21 cDNA constructs revealed, upon differentiation, upregulated levels of mRNA for thrombomodulin, a parietal endoderm-specific marker, or increased fraction of cells in sub-G1 phase of the cell cycle, respectively. Consistent with this observation, whereas p21 was strictly nuclear in undifferentiated cells, a large proportion of differentiated cells had p21 localized also in the cytoplasm, a site associated with the antiapoptotic function of p21. Furthermore, p21 activated the thrombomodulin promoter in transient reporter assays and the p21 mutant defective in binding to cyclin E was equally efficient in activation. The promoter activity in differentiated cells was reduced by cotransfection of p21-specific siRNA or antisense cDNA. Coexpression of p21 increased the activity of the GAL-p300(1-1303) fusion protein on the GAL sites-containing TM promoter. This implies that p21 might act through a derepression of the p300 N-terminal-residing repression domain, thereby enhancing the p300 coactivator function. As differentiation of F9 cells into parietal endoderm-like cells requires the cAMP signaling, the results together suggest that the cyclin-dependent kinase inhibitor p21 may promote specifically this pathway in F9 cells.