Esterification of all-trans-retinol in normal human epithelial cell strains and carcinoma lines from oral cavity, skin and breast: reduced expression of lecithin:retinol acyltransferase in carcinoma lines

Transcriptomic and epigenetic landscape of nimorazole-enhanced radiochemotherapy in head and neck cancer

BACKGROUND

Hypoxia remains a challenge for the therapeutic management of head and neck squamous cell carcinoma (HNSCC). The combination of radiotherapy with nimorazole has shown treatment benefit in HNSCC, but the precise underlying molecular mechanisms remain unclear.

PURPOSE

To assess and to characterize the transcriptomic/epigenetic landscape of HNSCC tumor models showing differential therapeutic response to fractionated radiochemotherapy (RCTx) combined with nimorazole.

MATERIALS/METHODS

Bulk RNA-sequencing and DNA methylation experiments were conducted using untreated and treated HNSCC xenografts after 10 fractions of RCTx with and without nimorazole. These tumor models (FaDu, SAS, Cal33, SAT and UT-SCC-45) previously showed a heterogeneous response to RCTx with nimorazole. The prognostic impact of candidate genes was assessed using clinical and gene expression data from HNSCC patients treated with primary RCTx within the DKTK-ROG.

RESULTS

Nimorazole responder and non-responder tumor models showed no differences in hypoxia gene signatures However, non-responder models showed upregulation of metabolic pathways. From that, a subset of 15 differentially expressed genes stratified HNSCC patients into low and high-risk groups with distinct outcome.

CONCLUSION

In the present study, we found that nimorazole non-responder models were characterized by upregulation of genes involved in Retinol metabolism and xenobiotic metabolic process pathways, which might contribute to identify mechanisms of resistance to nitroimidazole compounds and potentially expand the repertoire of therapeutic options to treat HNSCC.

Impact of RARα and miR-138 on retinoblastoma etoposide resistance

BACKGROUND

Retinoblastoma (RB) is the most common childhood eye cancer. Chemotherapeutic drugs such as etoposide used in RB treatment often cause massive side effects and acquired drug resistances. Dysregulated genes and miRNAs have a large impact on cancer progression and development of chemotherapy resistances.

OBJECTIVE

This study was designed to investigate the involvement of retinoic acid receptor alpha (RARα) in RB progression and chemoresistance as well as the impact of miR-138, a potential RARα regulating miRNA.

METHODS

RARα and miR-138 expression in etoposide resistant RB cell lines and chemotherapy treated patient tumors compared to non-treated tumors was revealed by Real-Time PCR. Overexpression approaches were performed to analyze the effects of RARα on RB cell viability, apoptosis, proliferation and tumorigenesis. Besides, we addressed the effect of miR-138 overexpression on RB cell chemotherapy resistance.

RESULTS

A binding between miR-138 and RARα was shown by dual luciferase reporter gene assay. The study presented revealed that RARα is downregulated in etoposide resistant RB cells, while miR-138 is endogenously upregulated. Opposing RARα and miR-138 expression levels were detectable in chemotherapy pre-treated compared to non-treated RB tumor specimen. Overexpression of RARα increases apoptosis levels and reduces tumor cell growth of aggressive etoposide resistant RB cells in vitro and in vivo. Overexpression of miR-138 in chemo-sensitive RB cell lines partly enhances cell viability after etoposide treatment.

CONCLUSIONS

Our findings show that RARα acts as a tumor suppressor in retinoblastoma and is downregulated upon etoposide resistance in RB cells. Thus, RARα may contribute to the development and progression of RB chemo-resistance.

Retinoids in Cutaneous Squamous Cell Carcinoma

Animal studies as early as the 1920s suggested that vitamin A deficiency leads to squamous cell metaplasia in numerous epithelial tissues including the skin. However, humans usually die from vitamin A deficiency before cancers have time to develop. A recent long-term cohort study found that high dietary vitamin A reduced the risk of cutaneous squamous cell carcinoma (cSCC). cSCC is a form of nonmelanoma skin cancer that primarily occurs from excess exposure to ultraviolet light B (UVB). These cancers are expensive to treat and can lead to metastasis and death. Oral synthetic retinoids prevent the reoccurrence of cSCC, but side effects limit their use in chemoprevention. Several proteins involved in vitamin A metabolism and signaling are altered in cSCC, which may lead to retinoid resistance. The expression of vitamin A metabolism proteins may also have prognostic value. This article reviews what is known about natural and synthetic retinoids and their metabolism in cSCC.

Vitamin A Rich Diet Diminishes Early Urothelial Carcinogenesis by Altering Retinoic Acid Signaling

Urinary bladder cancer is one of the leading malignancies worldwide, with the highest recurrence rates. A diet rich in vitamin A has proven to lower the risk of cancer, yet the molecular mechanisms underlying this effect are unknown. We found that vitamin A decreased urothelial atypia and apoptosis during early bladder carcinogenesis induced by N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN). Vitamin A did not alter urothelial cell desquamation, differentiation, or proliferation rate. Genes like Wnt5a, involved in retinoic acid signaling, and transcription factors Pparg, Ppara, Rxra, and Hoxa5 were downregulated, while Sox9 and Stra6 were upregulated in early urothelial carcinogenesis. When a vitamin A rich diet was provided during BBN treatment, none of these genes was up- or downregulated; only Lrat and Neurod1 were upregulated. The lecithin retinol acyltransferase (LRAT) enzyme that produces all-trans retinyl esters was translocated from the cytoplasm to the nuclei in urothelial cells as a consequence of BBN treatment regardless of vitamin A rich diet. A vitamin A-rich diet altered retinoic acid signaling, decreased atypia and apoptosis of urothelial cells, and consequently diminished early urothelial carcinogenesis.

Retinoic Acid Receptors in Acute Myeloid Leukemia Therapy

Retinoic acid (RA) signaling pathways regulate fundamental biological processes, such as cell proliferation, development, differentiation, and apoptosis. Retinoid receptors (RARs and RXRs) are ligand-dependent transcription factors. All-trans retinoic acid (ATRA) is the principal endogenous ligand for the retinoic acid receptor alpha (RARA) and is produced by the enzymatic oxidation of dietary vitamin A, whose deficiency is associated with several pathological conditions. Differentiation therapy using ATRA revolutionized the outcome of acute promyelocytic leukemia (APL), although attempts to replicate these results in other cancer types have been met with more modest results. A better knowledge of RA signaling in different leukemia contexts is required to improve initial designs. Here, we will review the RA signaling pathway in normal and malignant hematopoiesis, and will discuss the advantages and the limitations related to retinoid therapy in acute myeloid leukemia.

Retinoic acid signaling balances adult distal lung epithelial progenitor cell growth and differentiation

Background

Despite compelling data describing pro-regenerative effects of all-trans retinoic acid (ATRA) in pre-clinical models of chronic obstructive pulmonary disease (COPD), clinical trials using retinoids for emphysema patients have failed. Crucial information about the specific role of RA signaling in adult rodent and human lung epithelial progenitor cells is largely missing.

Methods

Adult lung organoid cultures were generated from isolated primary mouse and human lung epithelial cells, and incubated with pharmacological pathway modulators and recombinant proteins. Organoid number and size were measured, and differentiation was assessed with quantitative immunofluorescence and gene expression analyses.

Findings

We unexpectedly found that ATRA decreased lung organoid size, whereas RA pathway inhibition increased mouse and human lung organoid size. RA pathway inhibition stimulated mouse lung epithelial proliferation via YAP pathway activation and epithelial-mesenchymal FGF signaling, while concomitantly suppressing alveolar and airway differentiation. HDAC inhibition rescued differentiation in growth-augmented lung organoids.

Interpretation

In contrast to prevailing notions, our study suggests that regenerative pharmacology using transient RA pathway inhibition followed by HDAC inhibition might hold promise to promote lung epithelial regeneration in diseased adult lung tissue.

Fund

This project is funded by the Lung Foundation Netherlands (Longfonds) grant 6.1.14.009 (RG, MK, JS, PSH) and W2/W3 Professorship Award by the Helmholtz Association, Berlin, Germany (MK).

Retinoid X Receptor Agonists Upregulate Genes Responsible for the Biosynthesis of All-Trans-Retinoic Acid in Human Epidermis

UAB30 is an RXR selective agonist that has been shown to have potential cancer chemopreventive properties. Due to high efficacy and low toxicity, it is currently being evaluated in human Phase I clinical trials by the National Cancer Institute. While UAB30 shows promise as a low toxicity chemopreventive drug, the mechanism of its action is not well understood. In this study, we investigated the effects of UAB30 on gene expression in human organotypic skin raft cultures and mouse epidermis. The results of this study indicate that treatment with UAB30 results in upregulation of genes responsible for the uptake and metabolism of all-trans-retinol to all-trans-retinoic acid (ATRA), the natural agonist of RAR nuclear receptors. Consistent with the increased expression of these genes, the steady-state levels of ATRA are elevated in human skin rafts. In ultraviolet B (UVB) irradiated mouse skin, the expression of ATRA target genes is found to be reduced. A reduced expression of ATRA sensitive genes is also observed in epidermis of mouse models of UVB-induced squamous cell carcinoma and basal cell carcinomas. However, treatment of mouse skin with UAB30 prior to UVB irradiation prevents the UVB-induced decrease in expression of some of the ATRA-responsive genes. Considering its positive effects on ATRA signaling in the epidermis and its low toxicity, UAB30 could be used as a chemoprophylactic agent in the treatment of non-melanoma skin cancer, particularly in organ transplant recipients and other high risk populations.

Retinoic acid metabolism proteins are altered in trichoblastomas induced by mouse papillomavirus 1

Skin cancer burden is significant as treatment costs have skyrocketed to $8.1 million annually and some forms metastasize, such as cutaneous squamous cell carcinoma (cSCC) and melanoma. cSCC is caused by altered growth factor signaling induced by chemical carcinogens, ultraviolet light (UV) exposure, and infections with papillomaviruses (PVs). One of the few options for preventing cSCC in high-risk patients is oral retinoids. While much is understood about retinoid treatments and metabolism in mouse models of chemically and UV exposure induced cSCC, little is known about the role of retinoids in PV-induced cSCC. To better understand how retinoid metabolism is altered in cSCC, we examined the expression of this pathway in the newly discovered mouse papillomavirus (MmuPV1), which produces trichoblastomas in dorsal skin but not cSCC. We found significant increases in a rate-limiting enzyme involved in retinoic acid synthesis and retinoic acid binding proteins, suggestive of increased RA synthesis, in MmuPV1-induced tumors in B6.Cg-Foxn1(nu)/J mice. Similar increases in these proteins were seen after acute UVB exposure in Crl:SKH1-Hr(hr) mice and in regressing pre-cancerous lesions in a chemically-induced mouse model, suggesting a common mechanism in limiting the progression of papillomas to full blown cSCC.

Endogenous Retinoic Acid Required to Maintain the Epidermis Following Ultraviolet Light Exposure in SKH-1 Hairless Mice

Ultraviolet light B (UVB) exposure induces cutaneous squamous cell carcinoma (cSCC), one of the most prevalent human cancers. Reoccurrence of cSCC in high-risk patients is prevented by oral retinoids. But oral retinoid treatment causes significant side effects; and patients develop retinoid resistance. Exactly how retinoids prevent UVB-induced cSCC is currently not well understood. Retinoid resistance blocks mechanistic studies in the leading mouse model of cSCC, the UVB-exposed SKH-1 hairless mouse. To begin to understand the role of retinoids in UVB-induced cSCC we first examined the localization pattern of key retinoid metabolism proteins by immunohistochemistry 48 h after UVB treatment of female SKH-1 mice. We next inhibited retinoic acid (RA) synthesis immediately after UVB exposure. Acute UVB increased RA synthesis, signaling and degradation proteins in the stratum granulosum. Some of these proteins changed their localization; while other proteins just increased in intensity. In contrast, acute UVB reduced the retinoid storage protein lectin:retinol acyltransferase (LRAT) in the epidermis. Inhibiting RA synthesis disrupted the epidermis and impaired differentiation. These data suggest that repair of the epidermis after acute UVB exposure requires endogenous RA synthesis.

Knockdown of lecithin retinol acyltransferase increases all-trans retinoic acid levels and restores retinoid sensitivity in malignant melanoma cells

Retinoids such as all-trans retinoic acid (ATRA) influence cell growth, differentiation and apoptosis and may play decisive roles in tumor development and progression. An essential retinoid-metabolizing enzyme known as lecithin retinol acyltransferase (LRAT) is expressed in melanoma cells but not in melanocytes catalysing the esterification of all-trans retinol (ATRol). In this study, we show that a stable LRAT knockdown (KD) in the human melanoma cell line SkMel23 leads to significantly increased levels of the substrate ATRol and biologically active ATRA. LRAT KD restored cellular sensitivity to retinoids analysed in cell culture assays and melanoma 3D skin models. Furthermore, ATRA-induced gene regulatory mechanisms drive depletion of added ATRol in LRAT KD cells. PCR analysis revealed a significant upregulation of retinoid-regulated genes such as CYP26A1 and STRA6 in LRAT KD cells, suggesting their possible involvement in mediating retinoid resistance in melanoma cells. In conclusion, LRAT seems to be important for melanoma progression. We propose that reduction in ATRol levels in melanoma cells by LRAT leads to a disturbance in cellular retinoid level. Balanced LRAT expression and activity may provide protection against melanoma development and progression. Pharmacological inhibition of LRAT activity could be a promising strategy for overcoming retinoid insensitivity in human melanoma cells.

High incidence of LRAT promoter hypermethylation in colorectal cancer correlates with tumor stage

Lecithin:retinol acyltransferase (LRAT) is a major enzyme involved in vitamin A/retinol metabolism, which regulates various physiological processes like cell proliferation and differentiation. LRAT expression is reduced in numerous cancers, yet the underlying mechanisms have remained undefined. We hypothesized that methylation silencing may contribute to decreased LRAT gene expression in colorectal cancer (CRC). LRAT hypermethylation status was analyzed in five CRC cell lines, 167 colorectal tumors, and 69 adjacent normal colonic mucosae, using a quantitative bisulfite/PCR/LDR/Universal Array assay. LRAT transcription levels were determined by real-time RT-PCR in a subset of tumors and matched normal tissues and in CRC cell lines that were treated with a demethylating agent, 5-aza-2'-deoxycytidine. The incidence of LRAT hypermethylation was significantly higher in colorectal tumors than in adjacent normal mucosae (p = 0.0025). Aberrant methylation occurred in 51 % of microsatellite-stable CRCs, in 84 % of microsatellite-unstable CRCs, and in 12 out of 13 colonic polyps. The number of hypermethylated LRAT events was inversely correlated with CRC stage (p < 0.0001). Importantly, LRAT hypermethylation was associated with decreased mRNA level in CRC clinical specimens, and demethylation treatment resulted in LRAT transcriptional reactivation. Our data support the idea that LRAT promoter hypermethylation associates with LRAT gene expression in CRC. The higher frequency of LRAT hypermethylation in colonic polyps and early-stage CRCs indicates that it may occur early in malignant progression.

Retinoic acid signaling in cancer: The parable of acute promyelocytic leukemia

Inevitably fatal some 40 years, acute promyelocytic leukemia (APL) can now be cured in more than 95% of cases. This clinical success story is tightly linked to tremendous progress in our understanding of retinoic acid (RA) signaling. The discovery of retinoic acid receptor alpha (RARA) was followed by the cloning of the chromosomal translocations driving APL, all of which involve RARA. Since then, new findings on the biology of nuclear receptors have progressively enlightened the basis for the clinical efficacy of RA in APL. Reciprocally, the disease offered a range of angles to approach the cellular and molecular mechanisms of RA action. This virtuous circle contributed to make APL one of the best-understood cancers from both clinical and biological standpoints. Yet, some important questions remain unanswered including how lessons learnt from RA-triggered APL cure can help design new therapies for other malignancies.

A method to identify and isolate pluripotent human stem cells and mouse epiblast stem cells using lipid body-associated retinyl ester fluorescence

We describe the use of a characteristic blue fluorescence to identify and isolate pluripotent human embryonic stem cells and human-induced pluripotent stem cells. The blue fluorescence emission (450–500 nm) is readily observed by fluorescence microscopy and correlates with the expression of pluripotency markers (OCT4, SOX2, and NANOG). It allows easy identification and isolation of undifferentiated human pluripotent stem cells, high-throughput fluorescence sorting and subsequent propagation. The fluorescence appears early during somatic reprogramming. We show that the blue fluorescence arises from the sequestration of retinyl esters in cytoplasmic lipid bodies. The retinoid-sequestering lipid bodies are specific to human and mouse pluripotent stem cells of the primed or epiblast-like state and absent in naive mouse embryonic stem cells. Retinol, present in widely used stem cell culture media, is sequestered as retinyl ester specifically by primed pluripotent cells and also can induce the formation of these lipid bodies.

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Human pluripotent stem cells exhibit a characteristic blue fluorescence

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It arises from the sequestration of retinyl esters in cytoplasmic lipid bodies

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It is associated with pluripotency and allows for easy high throughput propagation

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It marks cells of primed or epiblast-like state and is absent in naive cells

Vitamin A/retinol and maintenance of pluripotency of stem cells

Retinol, the alcohol form of vitamin A is a key dietary component that plays a critical role in vertebrate development, cell differentiation, reproduction, vision and immune system. Natural and synthetic analogs of retinol, called retinoids, have generally been associated with the cell differentiation via retinoic acid which is the most potent metabolite of retinol. However, a direct function of retinol has not been fully investigated. New evidence has now emerged that retinol supports the self-renewal of stem cells including embryonic stem cells (ESCs), germ line stem cells (GSCs) and cancer stem cells (CSCs) by activating the endogenous machinery for self-renewal by a retinoic acid independent mechanism. The studies have also revealed that stem cells do not contain enzymes that are responsible for metabolizing retinol into retinoic acid. This new function of retinol may have important implications for stem cell biology which can be exploited for quantitative production of pure population of pluripotent stem cells for regenerative medicine as well as clinical applications for cancer therapeutics.

Vitamin A metabolism in benign and malignant melanocytic skin cells: importance of lecithin/retinol acyltransferase and RPE65

Disturbance in vitamin A metabolism seems to be an important attribute of cancer cells. Retinoids, particularly retinoic acid, have critical regulatory functions and appear to modulate tumor development and progression. The key step of vitamin A metabolism is the esterification of all-trans retinol, catalyzed by lecithin/retinol acyltransferase (LRAT). In this work, we show that malignant melanoma cells are able to esterify all-trans retinol and subsequently isomerize all-trans retinyl esters (RE) into 11-cis retinol, whereas their benign counterparts-melanocytes are not able to catalyze these reactions. Besides, melanoma cell lines express lecithin/retinol acyltranseferase both at the mRNA and protein levels. In contrast, melanocytes do not express this enzyme at the protein level, but mRNA of lecithin/retinol acyltransefrase could still be present at mRNA level. RPE65 is expressed in both melanocytic counterparts, and could be involved in the subsequent isomerization of RE produced by lecithin/retinol acyltransefrase to 11-cis retinol. Cellular retinol-binding protein 2 does not appear to be involved in the regulation of all-trans retinol esterification in these cells. Expression of LRAT and RPE65 can be modulated by retinoids. We propose that the post-transcriptional regulation of lecithin/retinol acyltransefrase could be involved in the differential expression of this enzyme. Besides, activities of LRAT and RPE65 may be important for removal of all-trans retinal which is the substrate for retinoic acid production in skin cells. Consequently, the decreasing cellular amount of retinoic acid and its precursor molecules could result in a change of gene regulation.

Retinyl ester storage particles (retinosomes) from the retinal pigmented epithelium resemble lipid droplets in other tissues

Levels of many hydrophobic cellular substances are tightly regulated because of their potential cytotoxicity. These compounds tend to self-aggregate in cytoplasmic storage depots termed lipid droplets/bodies that have well defined structures that contain additional components, including cholesterol and various proteins. Hydrophobic substances in these structures become mobilized in a specific and regulated manner as dictated by cellular requirements. Retinal pigmented epithelial cells in the eye produce retinyl ester-containing lipid droplets named retinosomes. These esters are mobilized to replenish the visual chromophore, 11-cis-retinal, and their storage ensures proper visual function despite fluctuations in dietary vitamin A intake. But it remains unclear whether retinosomes are structures specific to the eye or similar to lipid droplets in other organs/tissues that contain substances other than retinyl esters. Thus, we initially investigated the production of these lipid droplets in experimental cell lines expressing lecithin:retinol acyltransferase, a key enzyme involved in formation of retinyl ester-containing retinosomes from all-trans-retinol. We found that retinosomes and oleate-derived lipid droplets form and co-localize concomitantly, indicating their intrinsic structural similarities. Next, we isolated native retinosomes from bovine retinal pigmented epithelium and found that their protein and hydrophobic small molecular constituents were similar to those of lipid droplets reported for other experimental cell lines and tissues. These unexpected findings suggest a common mechanism for lipid droplet formation that exhibits broad chemical specificity for the hydrophobic substances being stored.

Oral carcinogenesis induced by 4-nitroquinoline 1-oxide in lecithin:retinol acyltransferase gene knockout mice

Lecithin:retinol acyltransferase (LRAT) regulates retinol (vitamin A) metabolism by esterifying retinol. LRAT expression is decreased in cultured human squamous cell carcinoma cells of the head and neck relative to normal epithelial cells. We investigated whether the carcinogen 4-nitroquinoline 1-oxide (4-NQO) induced a higher incidence of oral cancer in LRAT knockout (LRAT(-/-)) than in wild-type (Wt) mice. We also investigated retinol deprivation during 4-NQO treatment in LRAT(-/-) mice as a model for rapid retinol deficiency. We observed higher levels of secreted frizzled-related protein (Sfrp) 2, an inhibitor of WNT signaling, in tongue tumors in LRAT(-/-) versus Wt. LRAT(-/-) embryonic stem cells also expressed higher Sfrp2 transcripts, indicating an interaction between retinol and WNT signaling. Cox-2, Cyclin D1, p21, Trop2 and RARβ2 were not differentially expressed in Wt versus LRAT(-/-) tongue tumors. Wt and LRAT(-/-) mice fed a retinol-sufficient diet showed the same oral tumor incidence after 4-NQO treatment. In contrast, tongue tumors developed in 60% of Wt mice and in 100% of LRAT(-/-) mice fed a retinol-deficient diet during 4-NQO treatment (P=.22); moreover, the bromodeoxyuridine labeling index was 21.0 ± 2.4% in LRAT(-/-) normal tongue epithelium as compared to 9.9 ± 0.8% in Wt normal tongue epithelium (P<.001). Thus, partial retinol deficiency during carcinogen treatment (achieved in LRAT(-/-)) resulted in more proliferating cells in tongue epithelia from LRAT(-/-) mice and, ultimately, a greater probability of carcinogenesis.

Retinoid metabolism and ALDH1A2 (RALDH2) expression are altered in the transgenic adenocarcinoma mouse prostate model

Retinoids, which include vitamin A (retinol) and metabolites such as retinoic acid, can inhibit tumor growth and reverse carcinogenesis in animal models of prostate cancer. We analyzed retinoid signaling and metabolism in the TRAMP (transgenic adenocarcinoma mouse prostate) model. We detected increased retinol and retinyl esters in prostates pooled from 24 to 36 week TRAMP transgenic positive mice compared to nontransgenic littermates by HPLC. We used quantitative RT-PCR to measure transcripts for genes involved in retinoid signaling and metabolism, including ALDH1A1, ALDH1A2, ALDH1A3, CYP26A1, LRAT, and RARbeta(2), in prostate tissue from TRAMP positive (+) and age-matched littermate control mice ranging from 18 to 36 weeks. Transcript levels of ALDH1A1, a putative stem cell marker, were decreased in ventral and lateral lobes of prostates from TRAMP mice compared to age-matched, nontransgenic mice. ALDH1A2 (RALDH2) mRNA levels in dorsal and anterior lobes of TRAMP+ mice were lower than in age-matched (24 week) nontransgenic mice. We detected lower RARbeta(2) mRNA levels in dorsal prostate lobes of 36 week TRAMP mice relative to nontransgenic mice. We detected high levels of ALDH1A2 protein in the cytoplasm and nucleus in nontransgenic murine prostate paraffin sections, and lower ALDH1A2 protein levels in all prostate lobes of TRAMP mice compared to nontransgenic mice by immunohistochemistry. We also detected much lower cytoplasmic ALDH1A2 protein levels in all human prostate cancer paraffin sections stained (19 total) relative to normal human prostate tissue on the same sections. Our data indicate that this reduction in ALDH1A2 protein is an early event in human prostate cancer.

An essential set of basic DNA response elements is required for receptor-dependent transcription of the lecithin:retinol acyltransferase (Lrat) gene

Lecithin:retinol acyltransferase (LRAT) is essential for vitamin A storage. Nuclear run-on assays demonstrated transcriptional regulation of the Lrat gene in vivo by all-trans-retinoic acid (RA) and other retinoids. Analysis of a 2.5 kb segment of rat genomic DNA revealed that the region approximately 300 bp upstream from the transcription start site (TSS) is necessary for high luciferase (Luc) reporter activity in HEK293T and HepG2 cells. Although this region lacks retinoid receptor binding elements, it responded to the nuclear receptors RARalpha, RARbeta or RARgamma, with RXRalpha, with and without ligand. Removal of -111 bp from the TSS, which is well conserved in human, rat and mouse genomes, completely eliminated activity. This region contains several basic elements (TATA box, SP3 site, AP-1 site, CAAT box), all of which were essential. Nuclear extracts from RA-treated cells exhibited enhanced binding. Therefore, this proximal region together with basal transcription factors may be sufficient to drive Lrat expression.

Overexpression of lecithin:retinol acyltransferase in the epithelial basal layer makes mice more sensitive to oral cavity carcinogenesis induced by a carcinogen

Lecithin:retinol acyltransferase (LRAT) is an enzyme that converts retinol (vitamin A) to retinyl esters. Its expression is often reduced in human cancers, including oral cavity cancers. We investigated the effects of ectopic expression of human lecithin:retinol acyltransferase (LRAT) on murine oral cavity carcinogenesis induced by the carcinogen 4-nitroquinoline 1-oxide (4-NQO). We targeted human LRAT expression specifically to the basal layer of mouse skin and oral cavity epithelia by using a portion of the human cytokeratin 14 (K14) promoter. High levels of human LRAT transgene transcripts were detected in the tongues and skin of adult transgenic positive (TG+) mice, but not in transgenic negative (TG-) mice. The retinyl ester levels in skin of LRAT TG+ mice were 32% +/- 5.4% greater than those in TG- mice, and topical treatment of the back skin with retinol resulted in greater increases in retinyl esters (from 6.9- to 14.3-fold in different TG+ mice) in TG+ mouse skin than in TG- mouse skin (1.3 fold). While carcinogen (4-NQO) treatment induced multifocal precancerous and cancer lesions in the tongues of both TG positive (n=16) and negative mice (n=22), higher percentages of transgenic positive mice (62.5%) developed more severe tongue lesions (grades 3 and 4) than transgenic negative mice (24.8%) after 4-NQO treatment (p < 0.05). Carcinogen treatment also resulted in greater percentages of transgenic positive mouse tongues with hyperplasia (71.4%), dysplasia (85.7%, p < 0.05), and carcinoma (28.6%) than transgenic negative mouse tongues (53.3%, 46.7%, and 20%, respectively). Moreover, we observed higher cyclooxygenase-2 (Cox-2) and lower RARbeta(2) mRNA levels in TG+ mouse tongues as compared to TG- mouse tongues after 4-NQO treatment (p < 0.05). Taken together, these data show that overexpression of human LRAT specifically in oral basal epithelial cells makes these cells more sensitive to carcinogen induced tumorigenesis.

Retinoic acid receptors and GATA transcription factors activate the transcription of the human lecithin:retinol acyltransferase gene

Lecithin:retinol acyltransferase (LRAT) catalyzes the esterification of retinol (vitamin A). Retinyl esters and LRAT protein levels are reduced in many types of cancer cells. We present data that both the LRAT and retinoic acid receptor beta(2) (RARbeta(2)) mRNA levels in the human prostate cancer cell line PC-3 are lower than those in cultured normal human prostate epithelial cells (PrEC). The activity of the human LRAT promoter (2.0 kb) driving a luciferase reporter gene in PC-3 cells is less than 40% of that in PrEC cells. Retinoic acid (RA) treatment increased this LRAT promoter-luciferase activity in PrEC cells, but not in PC-3 cells. Deletion of various regions of the human LRAT promoter demonstrated that a 172-bp proximal promoter region is essential for LRAT transcription and confers RA responsiveness in PrEC cells. This 172-bp region, contained within the 186 bp pLRAT/luciferase construct, has five putative GATA binding sites. Cotransfection of RARbeta(2) or RARgamma and the transcription factor GATA-4 increased LRAT (pLRAT186) promoter activity in both PrEC and PC-3 cells. In addition, we found that both retinoic acid and retinol induced transcripts for the STRA6 gene, which encodes a membrane receptor involved in retinol (vitamin A) uptake, in PrEC cells but not in PC-3 cells. In summary, our data show that the transcriptional regulation of the human LRAT gene is aberrant in human prostate cancer cells and that GATA transcription factors are involved in the transcriptional activation of LRAT in PrEC cells.

Biological and pharmacological activities of squalene and related compounds: potential uses in cosmetic dermatology

Squalene is a triterpene that is an intermediate in the cholesterol biosynthesis pathway. It was so named because of its occurrence in shark liver oil, which contains large quantities and is considered its richest source. However, it is widely distributed in nature, with reasonable amounts found in olive oil, palm oil, wheat-germ oil, amaranth oil, and rice bran oil. Squalene, the main component of skin surface polyunsaturated lipids, shows some advantages for the skin as an emollient and antioxidant, and for hydration and its antitumor activities. It is also used as a material in topically applied vehicles such as lipid emulsions and nanostructured lipid carriers (NLCs). Substances related to squalene, including β-carotene, coenzyme Q10 (ubiquinone) and vitamins A, E, and K, are also included in this review article to introduce their benefits to skin physiology. We summarize investigations performed in previous reports from both in vitro and in vivo models.

Homeostasis of retinol in lecithin: retinol acyltransferase gene knockout mice fed a high retinol diet

We analyzed the retinoid levels and gene expression in various tissues after wild-type (Wt) and lecithin:retinol acyltransferase (LRAT-/-) knockout mice were fed a high retinol diet (250 IU/g). As compared to Wt, LRAT-/- mice exhibited a greater and faster increase in serum retinol concentration (mean+/-S.D., Wt, 1.3 +/- 0.2 microM to 1.5 +/- 0.3 microM in 48 h, p > 0.05; LRAT-/-, 1.3 +/- 0.2 microM to 2.2+/-0.3 microM in 48 h, p < 0.01) and a higher level of retinol in adipose tissue (17.2 +/- 2.4 pmol/mg in Wt vs. 34.6 +/- 8.0 pmol/mg in LRAT-/-). In the small intestines of Wt mice higher levels of retinol (96.4 +/- 13.0 pmol/mg in Wt vs. 13.7 +/- 7.6 pmol/mg in LRAT-/- and retinyl esters (2493.4 +/- 544.8 pmol/mg in Wt vs. 8.2 +/- 2.6 pmol/mg in LRAT-/- were detected. More retinol was detected in the feces of LRAT-/- mice (69.3 +/- 32.6 pmol/mg in LRAT-/- vs. 24.1 +/- 8.6 pmol/mg in Wt). LRAT mRNA levels increased in the lungs, small intestines, and livers of Wt mice on the high retinol diet, while CYP26A1 mRNA levels increased greatly only in the LRAT-/- mice. After 4 weeks, no significant differences between Wt mice and LRAT-/- mice were observed in either the serum retinol level or in the prevalence of Goblet cells in jejunal crypts. Our data indicate that the LRAT-/- mice maintain the homeostasis of retinol as the dietary retinol increases by increasing the excretion of retinol from the gastrointestinal tract, increasing the distribution of retinol to adipose tissue, and enhancing the catabolism by CYP26A1. We show that LRAT plays a role in maintaining a stable serum retinol concentration when dietary retinol concentration fluctuates.

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.

Delivery of retinoid-based therapies to target tissues

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

Mechanisms of the HRSL3 tumor suppressor function in ovarian carcinoma cells

HRSL3 (also known as H-REV107-1) belongs to a class II tumor suppressor gene family and is downregulated in several human tumors including ovarian carcinomas. To unravel the mechanism of HRSL3 tumor suppressor action, we performed a yeast two-hybrid screen and identified the alpha-isoform of the regulatory subunit A of protein phosphatase 2A (PR65alpha) as a new interaction partner of HRSL3. Interaction between HRSL3 and PR65alpha was confirmed in vitro and by co-immunoprecipitation in mammalian cells. We demonstrate that HRSL3 binds to the endogenous PR65alpha, thereby partially sequestering the catalytic subunit PR36 from the PR65 protein complex, and inhibiting PP2A catalytic activity. Furthermore, binding of HRSL3 to PR65 induces apoptosis in ovarian carcinoma cells in a caspase-dependent manner. Using several mutant HRSL3 constructs, we identified the N-terminal proline-rich region within the HRSL3 protein as the domain that is relevant for both binding of PR65alpha and induction of programmed cell death. This suggests that the negative impact of HRSL3 onto PP2A activity is important for the HRSL3 pro-apoptotic function and indicates a role of PP2A in survival of human ovarian carcinomas. The analysis of distinct PP2A target molecules revealed PKCzeta as being involved in HRSL3 action. These data implicate HRSL3 as a signaling regulatory molecule, which is functionally involved in the oncogenic network mediating growth and survival of ovarian cancer cells.

Cell proliferation inhibition and alterations in retinol esterification induced by phytanic acid and docosahexaenoic acid

We investigated the effects of two natural dietary retinoid X receptor (RXR) ligands, phytanic acid (PA) and docosahexaenoic acid (DHA), on proliferation and on the metabolism of retinol (vitamin A) in both cultured normal human prostate epithelial cells (PrECs) and PC-3 prostate carcinoma cells. PA and DHA inhibited the proliferation of the parental PC-3 cells and PC-3 cells engineered to overexpress human lecithin:retinol acyltransferase (LRAT) in both the absence and presence of retinol. A synthetic RXR-specific ligand also inhibited PC-3 cell proliferation, whereas all-trans retinoic acid (ATRA) did not. PA and DHA treatment increased the levels of retinyl esters (REs) in both PrECs and PC-3 cells and generated novel REs that eluted on reverse-phase HPLC at 54.0 and 50.5 min, respectively. Mass spectrometric analyses demonstrated that these novel REs were retinyl phytanate (54.0 min) and retinyl docosahexaenoate (50.5 min). Neither PA nor DHA increased LRAT mRNA levels in these cells. In addition, we demonstrate that retinyl phytanate was generated by LRAT in the presence of PA and retinol; however, retinyl docosahexaenoate was produced by another enzyme in the presence of DHA and retinol.

Tumor-suppressive activity of retinoic acid receptor-beta in cancer

Retinoids, a group of structural and functional analogs of vitamin A, are known to regulate a large number of essential biological processes and to suppress carcinogenesis. The effects of retinoids are mainly mediated by nuclear retinoid receptors, which include retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Each receptor has three subtypes (alpha, beta, and gamma) and each subtype has different isoforms. Retinoic acid receptor-beta (RAR-beta) has four isoforms that have different affinities to retinoids and different biological functions. Loss of expression of RAR-beta(2) during cancer development is associated with tumorigenesis and retinoid resistance; induction of its expression, on the other hand, can suppress carcinogenesis. Expression of another isoform, RAR-beta(4), is increased in various types of cancer. RAR-beta(4) transgenic mice develop hyperplasia and neoplasia in various tissues, and induction of RAR-beta(4) expression increases the growth of tumor cells that do not express RAR-beta(2). Future studies will focus on molecular pathways involving RAR-beta(2) and the role of RAR-beta(4) in cancer development.

A Repressive Epigenetic Domino Effect Confers Susceptibility to Breast Epithelial Cell Transformation: Implications for Predicting Breast Cancer Risk

Retinoic acid (RA) is a master epigenetic regulator that plays a pivotal role in both breast morphogenesis and development. Here, we show for the first time that RA, via the RA receptor alpha (RARalpha), epigenetically regulates in a concerted fashion the transcription of two RA-responsive genes, the RA receptor beta2 (RARbeta2) and the cellular retinol-binding protein 1 (CRBP1). Specifically, an impaired RA signal through RARalpha in human breast epithelial cells triggers a repressive epigenetic domino effect, involving first RARbeta2 and second CRBP1. The phenotype acquired by breast epithelial cells clearly implies that the resistance to RA-mediated growth inhibition precedes the acquisition of morphological epithelial transformation, thus supporting the occurrence of sequential transcriptional silencing of first RARbeta2 and second CRBP1. The identification of this epigenetic network mechanistically linking RARbeta2 and CRBP1 transcription provides the basis for devising more accurate epigenetic tests for the prediction of breast cancer risk.

Impaired retinoic acid (RA) signal leads to RARbeta2 epigenetic silencing and RA resistance

Resistance to the growth-inhibitory action of retinoic acid (RA), the bioactive derivative of vitamin A, is common in human tumors. One form of RA resistance has been associated with silencing and hypermethylation of the retinoic acid receptor beta2 gene (RARbeta2), an RA-regulated tumor suppressor gene. The presence of an epigenetically silent RARbeta2 correlates with lack of the RA receptor alpha (RARalpha). Normally, RARalpha regulates RARbeta2 transcription by mediating dynamic changes of RARbeta2 chromatin in the presence and absence of RA. Here we show that interfering with RA signal through RARalpha (which was achieved by use of a dominant-negative RARalpha, by downregulation of RARalpha by RNA interference, and by use of RARalpha antagonists) induces an exacerbation of the repressed chromatin status of RARbeta2 and leads to RARbeta2 transcriptional silencing. Further, we demonstrate that RARbeta2 silencing causes resistance to the growth-inhibitory effect of RA. Apparently, RARbeta2 silencing can also occur in the absence of DNA methylation. Conversely, we demonstrate that restoration of RA signal at a silent RARbeta2 through RARalpha leads to RARbeta2 reactivation. This report provides proof of principle that RARbeta2 silencing and RA resistance are consequent to an impaired integration of RA signal at RARbeta2 chromatin.

Evidence that sequence homologous region in LRAT-like proteins possesses anti-proliferative activity and DNA binding properties: translational implications and mechanism of action

LRAT (lecithin:retinol acyltransferase), an enzyme whose levels are modulated during malignant conversion, has been reported as the founder member of a new LRAT-like family that includes tumor suppressors TIG-3(1-164) and Ha-Rev107(1-162). The mechanisms that link these three proteins to carcinogenesis as well as the significance of a reported shared sequence homologous region remain unclear. This begs the question if the tumor suppressors possess enzyme properties and/or if the LRAT enzyme possesses tumor suppressor properties. We use the reported homologous region as a first approach to address the question from the perspective that all three proteins can possess tumor suppressor properties. We postulated that the homologous sequence harbors an anti-proliferation domain within the full-length proteins and that dodecapeptides of this sequence possess anti-proliferative activity. We report that H-TIG-3(111-123), H-Ha-Rev107-1(111-123) and H-LRAT160-171:C168L exhibited in vitro growth inhibitory activity in a human cutaneous melanoma (HCM) model and affected tumor growth in a nude mouse model. Further, in peptide-sensitive HCM cells, these peptides crossed the plasma membrane and localized to the nucleus, where they could bind and activate promoters of transcription factors involved in G1-->S transition. Moreover, peptide-induced abrogation of cyclin dependent kinase-2 expression was concomitant with sub-cellular re-distribution of cyclins E and A. Indeed, the sequence homologous region within each full-length wild-type protein as well as the growth inhibitory peptides can form alpha helices, a likely configuration for binding to DNA. This is the first report that this sequence homologous region (AA111-123) within these LRAT-like proteins harbors an anti-proliferative domain with DNA binding properties. Sequences from this sequence homologous region can be used as templates for anti-tumor drug design and as probes to investigate disease-related mechanisms and structure-activity relationships of the full-length proteins, TIG-3(1-164), Ha-Rev107(1-162) and LRAT160-171.

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.

Clinical significance of altered expression of retinoid receptors in oral precancerous and cancerous lesions: Relationship with cell cycle regulators

Alterations in expression of retinoid receptors are implicated in human cancers. We hypothesized that altered expression of retinoic acid receptors (RARalpha,beta,gamma) and retinoid X receptor RXRalpha and their relationship with cell cycle regulators (p53, p16, p21) is associated with development, progression and prognosis of oral cancer. Immunohistochemical analysis of RAR alpha, beta, gamma and RXRalpha proteins was carried out on serial sections from 244 oral squamous cell carcinomas (OSCCs), 102 potentially malignant lesions (65 hyperplasias, 37 dysplasias), 83 matched histologically normal oral tissues and 29 normal mucosa from non-exposed individuals without oral lesions and correlated with expression of cell cycle regulators p53, p16 and p21 as well as with clinicopathological parameters. Expression of retinoid receptors RARbeta, RARgamma, RXRalpha and cell cycle regulators p16 and p21 was decreased in majority of oral SCCs as well as in potentially malignant lesions. Multivariate stepwise logistic regression analysis carried out for comparison of non-exposed normal oral mucosa with histologically normal oral tissues from patients with oral lesions showed significant loss of RARbeta or p53 accumulation (RARbeta(-)/p53(+) Odd's ratio, OR = 266.6, p = 0.000); non-exposed normal mucosa from individuals without oral lesions with potentially malignant lesion was RARbeta(-)/p21(-)/p53(+) (OR = 215.7, p = 0.000); matched normal to potentially malignant stage was RARalpha(+)/p21(-) (OR = 4.414, p = 0.005); hyperplasia to dysplasia was RARalpha(+)/p53(+) (OR = 4.72, p = 0.005) and potentially malignant to malignant phenotype was RARalpha(+) (OR = 2.061, p = 0.004). The prognostic relevance of these factors was assessed in 115 of these SCC patients who were followed-up for a maximum period of 94 months (median 21 months). Multivariate analysis using Cox's proportional Hazard's model showed that RARalpha(+)/p21(-) phenotype was associated with shorter disease-free survival (Hazard's ratio, HR = 1.863, p = 0.0471). To our knowledge, this is the first large study showing alterations in expression of retinoid receptors at the protein level at different stages in development and progression of oral SCC. It also underscored the prognostic significance of retinoid receptors and their interactions with cell cycle regulators in multistep oral tumorigenesis.

Retinoid absorption and storage is impaired in mice lacking lecithin:retinol acyltransferase (LRAT)

Lecithin:retinol acyltransferase (LRAT) is believed to be the predominant if not the sole enzyme in the body responsible for the physiologic esterification of retinol. We have studied Lrat-deficient (Lrat-/-) mice to gain a better understanding of how these mice take up and store dietary retinoids and to determine whether other enzymes may be responsible for retinol esterification in the body. Although the Lrat-/- mice possess only trace amounts of retinyl esters in liver, lung, and kidney, they possess elevated (by 2-3-fold) concentrations of retinyl esters in adipose tissue compared with wild type mice. These adipose retinyl ester depots are mobilized in times of dietary retinoid insufficiency. We further observed an up-regulation (3-4-fold) in the level of cytosolic retinol-binding protein type III (CRBPIII) in adipose tissue of Lrat-/- mice. Examination by electron microscopy reveals a striking total absence of large lipid-containing droplets that normally store hepatic retinoid within the hepatic stellate cells of Lrat-/- mice. Despite the absence of significant retinyl ester stores and stellate cell lipid droplets, the livers of Lrat-/- mice upon histologic analysis appear normal and show no histological signs of liver fibrosis. Lrat-/- mice absorb dietary retinol primarily as free retinol in chylomicrons; however, retinyl esters are also present within the chylomicron fraction obtained from Lrat-/- mice. The fatty acyl composition of these (chylomicron) retinyl esters suggests that they are synthesized via an acyl-CoA-dependent process suggesting the existence of a physiologically significant acyl-CoA:retinol acyltransferase.

Expression of estrogen receptor alpha, retinoic acid receptor alpha and cellular retinoic acid binding protein II genes is coordinately regulated in human breast cancer cells

Human breast cancer cell lines expressing the estrogen receptor alpha (ERalpha), all-trans-retinoic acid (ATRA) receptor alpha (RARalpha) and cellular retinoic acid binding protein II (CRABPII) genes are sensitive to ATRA-mediated growth inhibition. To study the relationship among ERalpha, RARalpha and CRABPII expression, the protein levels of each member were compared in five breast cancer cell lines (T47D, MCF-7, ZR-75-1, Hs587 T and MDA-MB-231 cells) and two immortalized nontumorigenic breast epithelial cell lines (MTSV1.7 and MCF-10A). ERalpha, RARalpha and CRABPII proteins were detected in T47D, MCF-7 and ZR-75-1 cells but not in other tested cell lines. RARalpha and CRABPII proteins were either reduced or undetectable in T47D/C4:2W and MCF-7/ADR cells with lost expression of ERalpha. Estradiol increased and anti-estrogens (tamoxifen and ICI 164,384) downregulated the expression of both RARalpha and CRABPII proteins in T47D and MCF-7 cells. RARalpha antagonist Ro-41-5253 inhibited CRABPII expression, but not RARalpha expression in estradiol-treated T47D and MCF-7 cells. Suppression of ERalpha by small interfering RNA (siRNA) reduced RARalpha and CRABPII gene expression and siRNA suppression of RARalpha reduced CRABPII expression while having no effect on ERalpha in T47D cells. Transient transfection of either RARalpha or ERalpha expression vectors increased CRABPII expression in MDA-MB-231 cells but only RARalpha, not ERalpha, activated hCRABPII promoter reporter. These results indicate that there is a gene activation pathway in which ERalpha drives RARalpha transcription and RARalpha drives CRABPII transcription in ERalpha-positive human breast cancer cells.

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.

Retinoic acid and the histone deacetylase inhibitor trichostatin a inhibit the proliferation of human renal cell carcinoma in a xenograft tumor model

PURPOSE

Therapy for advanced renal cell carcinoma (RCC) is ineffective in the majority of patients. We have previously reported that retinoid-induced up-regulation of retinoic acid receptor beta (RARbeta) correlated with antitumor effects in RCCs. Recent studies show that there is a reduction in the level of RARbeta2 expression in cancer cells due in part to histone hypoacetylation. Therefore, we tested whether combining histone deacetylase inhibitors with retinoic acid (RA) would restore RARbeta2 receptor expression, leading to increased growth inhibition in RCC cells.

EXPERIMENTAL DESIGN

Cell proliferation, Western blot, and reverse transcription-PCR analyses of two RA-resistant RCC cell lines, SK-RC-39 and SK-RC-45, were assessed in the presence of all-trans retinoic acid (ATRA), trichostatin A (TSA), or the combination of ATRA and TSA. Analysis of apoptosis was also done on SK-RC-39 cells treated with these combinations. Additionally, a xenograft tumor model (SK-RC-39) was used in this study to investigate the efficacy of a liposome-encapsulated, i.v. form of ATRA (ATRA-IV) plus TSA combination therapy.

RESULTS

Enhanced inhibition of the proliferation of RCC cell lines and of tumor growth in a xenograft model was observed with the combination of ATRA plus TSA. Reactivation of RARbeta2 mRNA expression was observed in SK-RC-39 and SK-RC-45 cells treated with TSA alone or TSA in combination with ATRA. A partial G0-G1 arrest and increased apoptosis were observed with SK-RC-39 cells on treatment with ATRA and TSA.

CONCLUSIONS

The combination of ATRA and the histone deacetylase inhibitor TSA elicits an additive inhibition of cell proliferation in RCC cell lines. These results indicate that ATRA and histone deacetylase inhibitor therapies should be explored for the treatment of advanced RCC.

Cellular retinol-binding protein I, a regulator of breast epithelial retinoic acid receptor activity, cell differentiation, and tumorigenicity

BACKGROUND

Retinoic acid receptor (RAR) activation induces cell differentiation and may antagonize cancer progression. Cellular retinol-binding protein I (CRBP-I) functions in retinol storage and its expression is lower in human cancers than in normal cells. We hypothesized that retinol storage might be linked to RAR activation and thus that lowered CRBP-I function might impair RAR activity and cell differentiation.

METHODS

Sarcoma virus 40-immortalized human mammary epithelial cells (MTSV1-7) devoid of CRBP-I were transfected with wild-type CRBP-I or CRBP-I point mutants with low RA binding affinity. The subcellular localization of CRBP-I was investigated in these cells and in wild-type or CRBP-I null mouse mammary epithelial cells (MECs), using indirect immunofluorescence and sucrose gradient fractionation. RAR activity was assessed using reporter gene assays. Acinar differentiation and in vivo tumor growth were assessed in reconstituted basement membrane and athymic mice, respectively.

RESULTS

In cells expressing wild-type CRBP-I but not the CRBP-I mutants, CRBP-I was found mainly in lipid droplets, the retinol storage organelle, and this localization was associated with promotion of retinol storage by wild-type CRBP-I only. RAR activity was higher and acinar differentiation was observed in cells expressing wild-type but not mutant CRBP-I. RAR antagonist treatment blocked and chronic RA treatment mimicked, the CRBP-I induction of cell differentiation. Finally, CRBP-I suppressed tumorigenicity in athymic mice.

CONCLUSIONS

Physiologic RAR activation is dependent on CRBP-I-mediated retinol storage, and CRBP-I downregulation chronically compromises RAR activity, leading to loss of cell differentiation and tumor progression.

Cloning, gene organization and identification of an alternative splicing process in lecithin:retinol acyltransferase cDNA from human liver

Lecithin:retinol acyltransferase (LRAT) catalyzes the synthesis of retinyl esters in many tissues and is crucial for the transport and intracellular storage of vitamin A. LRAT expression is highly regulated in the liver. In this study, we have cloned and sequenced the full-length LRAT mRNA from human liver and identified its 5'- and 3'-ends. Full-length LRAT mRNA comprises 5023 nt with a predicted ORF of 230 amino acids, a short 5'UTR, and a relatively long 3'UTR of 4 kb containing several polyadenylation signals and AU-rich regions. Based on alignment of this mRNA with human genomic DNA in the GenBank database, the human LRAT gene spans about 9.1 kbp and consists of two exons and a relatively long 4-kbp intron. Further analysis of normal liver revealed a minor alternative splicing variant which lacks a 103 nt polynucleotide contained in the 5'UTR of the full-length LRAT transcript. This variant predicts that the LRAT gene is organized into three exons and two introns, as reported for LRAT cloned from retinal pigment epithelium (RPE) cells. These two LRAT mRNA variants are also present in testis, which is known to express LRAT and contain retinyl esters. Major and minor transcription start sites for human liver LRAT mRNA were identified and the sequence of the upstream proximal promoter region was retrieved from the GenBank database and physically analyzed for the presence of putative cis-acting elements essential for basal transcription. This region contains a TATA box, CCAAT box and Sp1 site, which are apparently conserved in mouse and rat LRAT genes. Our results provide evidence that multiple LRAT mRNA transcripts, which are expressed in a tissue-specific manner, may result from several mechanisms including differential splicing of the 5'UTR region and the use of multiple polyadenylation signals in the 3'UTR.

Reduced Lecithin

PURPOSE

Retinoids, which include vitamin A (retinol; ROL) and its derivatives, have been investigated in the treatment of bladder cancer. We have shown that expression of the enzyme lecithin:ROL acyltransferase (LRAT), which converts ROL to retinyl esters, is reduced in several human cancers. Here we evaluated expression of LRAT protein and mRNA in normal and malignant bladder tissue specimens from human patients. We also examined the effect of retinoids on LRAT expression in bladder cancer cell lines.

EXPERIMENTAL DESIGN

We evaluated 49 bladder cancer specimens for LRAT protein expression using immunohistochemistry with affinity-purified antibodies to human LRAT. LRAT mRNA expression was assessed using reverse transcription-PCR in bladder specimens from an additional 16 patients. We examined the effect of retinoic acid and ROL on LRAT mRNA expression in five human bladder cancer cell lines.

RESULTS

LRAT protein was detected throughout the nonneoplastic bladder epithelium in all of the specimens. In bladder tumors, LRAT protein expression was reduced compared with the nonneoplastic epithelium or was completely absent in 7 of 32 (21.9%) superficial tumors versus 16 of 17 (94.1%) invasive tumors (P < 0.001). All of the non-neoplastic bladder specimens tested (11 of 11) showed LRAT mRNA expression, compared with 5 of 8 (62%) superficial tumors and 0 of 5 (0%) invasive tumors (P = 0.001). Three of five human bladder cancer cell lines expressed LRAT mRNA independent of retinoid exposure, whereas in two cell lines LRAT mRNA expression was induced by retinoid treatment.

CONCLUSIONS

We report a significant reduction in LRAT expression in bladder cancer. Moreover, we demonstrate an inverse correlation of LRAT mRNA and protein expression with increasing tumor stage. These data suggest that loss of LRAT expression is associated with invasive bladder cancer.

Epigenetic CRBP downregulation appears to be an evolutionarily conserved (human and mouse) and oncogene-specific phenomenon in breast cancer

Background

The cellular retinol binding protein I gene (CRBP) is downregulated in a subset of human breast cancers and in MMTV-Myc induced mouse mammary tumors. Functional studies suggest that CRBP downregulation contributes to breast tumor progression. What is the mechanism underlying CRBP downregulation in cancer? Here we investigated the hypothesis that CRBP is epigenetically silenced through DNA hypermethylation in human and mouse breast cancer.

Results

Bisulfite sequencing of CRBP in a panel of 6 human breast cancer cell lines demonstrated that, as a rule, CRBP hypermethylation is closely and inversely related to CRBP expression and identified one exception to this rule. Treatment with 5-azacytidine, a DNA methyltransferase inhibitor, led to CRBP reexpression, supporting the hypothesis that CRBP hypermethylation is a proximal cause of CRBP silencing. In some cells CRBP reexpression was potentiated by co-treatment with retinoic acid, an inducer of CRBP, and trichostatin A, a histone deacetylase inhibitor. Southern blot analysis of a small panel of human breast cancer specimens identified one case characterized by extensive CRBP hypermethylation, in association with undetectable CRBP mRNA and protein. Bisulfite sequencing of CRBP in MMTV-Myc and MMTV-Neu/NT mammary tumor cell lines extended the rule of CRBP hypermethylation and silencing (both seen in MMTV-Myc but not MMTV-Neu/NT cells) from human to mouse breast cancer and suggested that CRBP hypermethylation is an oncogene-specific event.

Conclusion

CRBP hypermethylation appears to be an evolutionarily conserved and principal mechanism of CRBP silencing in breast cancer. Based on the analysis of transgenic mouse mammary tumor cells, we hypothesize that CRBP silencing in human breast cancer may be associated with a specific oncogenic signature.

Vitamin A metabolism in the retinal pigment epithelium: genes, mutations, and diseases

Mutations in the genes necessary for the metabolism of vitamin A (all-trans retinol) and cycling of retinoids between the photoreceptors and retinal pigment epithelium (RPE) (the visual cycle) have recently emerged as an important class of genetic defects responsible for retinal dystrophies and dysfunctions. Research into the causes and treatment of diseases resulting from defects in retinal vitamin A metabolism is currently the subject of intense interest, since disorders affecting the RPE are, in principle, more accessible to therapeutic intervention than those affecting the proteins of photoreceptor cells. This chapter presents an overview of the visual cycle, as well as the function of the RPE genes involved in the conversion of vitamin A to 11-cis retinal, the chromophore of the visual pigments. The identification of disease-causing mutations in this group of genes is described as well as the associated phenotypes that range from stationary night blindness to childhood-onset severe visual handicap. Consideration is also given to alternative genetic paradigms potentially relevant to defects in vitamin A metabolism, including a discussion of the relationship of this pathway to age-related macular degeneration, a non-Mendelian disease of late onset. Finally, progress and prospects for targeted therapeutic intervention in vitamin A metabolism are presented, including retinoid and gene replacement therapy. On the basis of early successes in animal models, and plans underway for Phase I/II clinical trials, it is hoped that the near future will bring effective therapies for many retinal dystrophy patients with defects in vitamin A metabolism.

Inhibition of adenovirus infection and adenain by green tea catechins

Green tea catechins have been reported to inhibit proteases involved in cancer metastasis and infection by influenza virus and HIV. To date there are no effective anti-adenoviral therapies. Consequently, we studied the effect of green tea catechins, and particularly the predominant component, epigallocatechin-3-gallate (EGCG), on adenovirus infection and the viral protease adenain, in cell culture. Adding EGCG (100 microM) to the medium of infected cells reduced virus yield by two orders of magnitude, giving and IC(50) of 25 microM and a therapeutic index of 22 in Hep2 cells. The agent was the most effective when added to the cells during the transition from the early to the late phase of viral infection suggesting that EGCG inhibits one or more late steps in virus infection. One of these steps appears to be virus assembly because the titer of infectious virus and the production of physical particles was much more affected than the synthesis of virus proteins. Another step might be the maturation cleavages carried out by adenain. Of the four catechins tested on adenain, EGCG was the most inhibitory with an IC(50) of 109 microM, compared with an IC(50) of 714 microM for PCMB, a standard cysteine protease inhibitor. EGCG and different green teas inactivated purified adenovirions with IC(50) of 250 and 245-3095, respectively. We conclude that the anti-adenoviral activity of EGCG manifests itself through several mechanisms, both outside and inside the cell, but at effective drug concentrations well above that reported in the serum of green tea drinkers.

Cloning and molecular expression analysis of large and small lecithin:retinol acyltransferase mRNAs in the liver and other tissues of adult rats

Retinyl ester, the most abundant form of vitamin A (retinol), is synthesized by the enzyme lecithin:retinol acyltransferase (LRAT). Previously, we cloned a 2.5 kb LRAT cDNA from rodent liver which codes for functional LRAT activity. However, Northern blots of tissues probed with the 2.5 kb cDNA revealed the presence of a larger transcript of approximately 5 kb as well as several smaller transcripts. To elucidate the nature of the large LRAT transcript, a high-molecular-mass adrenal gland cDNA library was screened. Two similar clones of 3962 and 3187 nt were identified which appeared to be part of the 3'-untranslated region (UTR) of a 5358 nt LRAT mRNA. The 5.3 kb cDNA was then amplified from liver by reverse transcriptase PCR (RT-PCR) and demonstrated to encode functional LRAT activity. The 3'-UTR of the 5.3 kb cDNA contains several AAUAAA polyadenylation signals. Analysis of the 3' ends of LRAT mRNA transcripts from liver, intestine and testis showed the usage of both canonical and non-canonical polyadenylation signals. To further analyse the LRAT mRNAs expressed in vivo, Northern blot analysis was performed using four probes spanning sections from the 5' end to the distal 3' end of the 5.3 kb LRAT cDNA. The results show that the major 5.3 kb transcript uses the canonical signal AAUAAA located at nt 5308, and the major short transcript of approximately 1.5 kb uses the non-canonical signal AUUAAA located at nt 1330. The 5.3 kb LRAT transcript was predominant in the liver of retinoic acid-repleted vitamin A-deficient rats, coincident with increased quantitative expression of LRAT mRNA and enzyme activity. The differential usage of these polyadenylation signals can explain the presence of multiple LRAT mRNA transcripts which are expressed in different tissue-specific patterns.

Retinoic acid induces expression of the interleukin-1beta gene in cultured normal human mammary epithelial cells and in human breast carcinoma lines

Retinoic acid (RA) and its derivatives inhibit the proliferation of normal human mammary epithelial cells (HMEC) and some breast carcinoma lines by mechanisms which are not fully understood. To identify genes that mediate RA-induced cell growth arrest, an HMEC cDNA library was synthesized and subtractive screening was performed. We identified the interleukin-1beta (IL-1beta) gene as an RA induced gene in HMEC. Northern blot analyses showed that the IL-1beta gene was up-regulated as early as 2 h after RA treatment. Results from the treatment of HMEC with cycloheximide and actinomycin D indicated that the regulation of the IL-1beta gene by RA occurred at the transcriptional level and that the IL-1beta gene is a direct, downstream target gene of RA. To evaluate the effects of IL-1beta on cell proliferation, the proliferation of HMEC was measured in the presence of RA or IL-1beta, or both. Either RA or IL-1beta could significantly inhibit the proliferation of HMEC. However, the addition of soluble IL-1 receptor antagonist (sIL-1ra) to the cell culture medium did not block RA-induced HMEC growth inhibition, whereas sIL-1ra did block the growth inhibition of HMEC by IL-1beta. IL-1beta expression was not observed in the three carcinoma cell lines, MCF-7, MDA-MB-231, and MDA-MB-468, as compared to the HMEC. Growth curves of the breast carcinoma cell lines showed strong inhibitory effects of RA and IL-1beta on the growth of the estrogen receptor (ER) positive MCF-7 cell line, but only a small effect on the ER negative MDA-MB-231 cells. The expression of the IL-1beta gene was also transcriptionally activated by RA in normal epithelial cells of prostate and oral cavity. Our results suggest that: (a) the IL-1beta gene is a primary target of RA receptors in HMEC; (b) the enhanced expression of the IL-1beta gene does not mediate the RA-induced growth arrest of HMEC; and (c) the expression of the IL-1beta gene is low or absent in all three human breast carcinoma cell lines examined, but the defect in the IL-1beta signaling pathway may be different in ER positive versus ER negative carcinoma cells.

Phase I trial of interferon alpha2b and liposome-encapsulated all-trans retinoic acid in the treatment of patients with advanced renal cell carcinoma

BACKGROUND

Studies suggest that retinoic acid (RA) can augment the antitumor effects of interferon-based therapy in patients with advanced renal cell carcinoma (RC); however, this benefit has not been achieved convincingly using oral formulations of 13-cis RA and all-trans RA. Liposome-encapsulated all-trans RA (ATRA-IV) has improved pharmacokinetics with increased and prolonged ATRA serum levels compared with oral retinoids.

METHODS

Cohorts of 3-6 patients with progressive metastatic RC received a dose of 3 MU interferon alpha2b per day subcutaneously, which was escalated weekly to 5 MU and then to 10 MU, plus ATRA-IV beginning at a dose of 90 mg/m(2) intravenously three times per week (Monday, Wednesday, and Friday), with a planned escalation to a maximum of 140 mg/m(2).

RESULTS

Two of the initial five patients experienced Grade 3 leukopenia while receiving 3 MU interferon and 90 mg/m(2) ATRA-IV. Therefore, the trial was amended to begin ATRA-IV at a dose of 15 mg/m(2) three times per week with a planned escalation by 15 mg/m(2) per cohort plus interferon-alpha at a dose of 3 MU subcutaneously 5 days per week (Monday through Friday), which was escalated weekly to 5 MU and then to 10 MU. Twelve patients were treated on the revised schedule. Toxicity was mild and included Grade 2 anemia (n = 7 patients), leukopenia (n = 2 patients), nausea (n = 2 patients), fatigue (n = 2 patients), fever (n = 2 patients), hepatic toxicity (n = 1 patient), edema (n = 1 patient), neurocortical toxicity (n = 1 patient), headache (n = 1 patient), and infection (n = 1 patient). One patient developed hyperthyroidism, and one patient required admission for bacteremia from a line infection. Dose limiting toxicity was Grade 3 hepatic toxicity, which was observed at a dose of 30 mg/m(2) ATRA-IV in 2 of 6 patients. Only 2 of 12 patients agreed to a dose escalation up to 10 MU interferon-alpha. Of 12 patients who were evaluable for response, 2 patients (17%) had a partial response in bone and lung, including 1 partial response of > 91 weeks' duration, at a dose of 15 mg/m(2) ATRA-IV three times per week and 5 MU interferon-alpha. Five additional patients experienced stable disease, two of whom had disease progression in bone only.

CONCLUSIONS

The acceptable toxicity profile and preliminary efficacy results suggest that this regimen warrants further evaluation. ATRA-IV (15 mg/m(2) TIW) and interferon-alpha (3 MU Monday through Friday escalated weekly to 5 MU and to 7 MU) are recommended for further study in patients with advanced RC.

HEK293S cells have functional retinoid processing machinery

Rhodopsin activation is measured by the early receptor current (ERC), a conformation-associated charge motion, in human embryonic kidney cells (HEK293S) expressing opsins. After rhodopsin bleaching in cells loaded with 11-cis-retinal, ERC signals recover in minutes and recurrently over a period of hours by simple dark adaptation, with no added chromophore. The purpose of this study is to investigate the source of ERC signal recovery in these cells. Giant HEK293S cells expressing normal wild-type (WT)-human rod opsin (HEK293S) were regenerated by solubilized 11-cis-retinal, all-trans-retinal, or Vitamin A in darkness. ERCs were elicited by flash photolysis and measured by whole-cell recording. Visible flashes initially elicit bimodal (R(1), R(2)) ERC signals in WT-HEK293S cells loaded with 11-cis-retinal for 40 min or overnight. In contrast, cells regenerated for 40 min with all-trans-retinal or Vitamin A had negative ERCs (R(1)-like) or none at all. After these were placed in the dark overnight, ERCs with outward R(2) signals were recorded the following day. This indicates conversion of loaded Vitamin A or all-trans-retinal into cis-retinaldehyde that regenerated ground-state pigment. 4-butylaniline, an inhibitor of the mammalian retinoid cycle, reversibly suppressed recovery of the outward R(2) component from Vitamin A and 11-cis-retinal-loaded cells. These physiological findings are evidence for the presence of intrinsic retinoid processing machinery in WT-HEK293S cells similar to what occurs in the mammalian eye.

Expression of a retinol dehydrogenase (hRoDH-4), a member of the retinol/steroid dehydrogenase family implicated in retinoic acid biosynthesis, in normal and neoplastic endometria

OBJECTIVE

Retinoic acid plays an essential role in epithelial differentiation, and retinoid homeostasis is disrupted in cancers of epithelial origin. The goal of this study was to determine whether hRoDH-4, an enzyme that can catalyze the first and rate-limiting step in retinoic acid biosynthesis, is expressed in normal endometrium and, if so, whether its expression is altered in endometrial cancer.

STUDY DESIGN

Proliferative, secretory, hyperplastic, and neoplastic endometria were examined by immunocytochemistry for hRoDH-4 protein and by reverse transcriptase-polymerase chain reaction for the hRoDH-4 transcript.

RESULTS

In proliferative and secretory glandular epithelia, immunoreactive hRoDH-4 was uniformly present. In endometrial cancers, hRoDH-4 immunoreactivity was markedly reduced in many neoplastic epithelial cells. Expression of hRoDH-4 in normal and neoplastic endometrium was confirmed by findings on reverse transcriptase-polymerase chain reaction.

CONCLUSION

These findings are consistent with the hypothesis that altered expression of enzymes essential for in situ retinoic acid biosynthesis is an important phenotypic change associated with the development of endometrial cancer.