Retinoids regulate stem cell differentiation

Retinoids are ubiquitous signaling molecules that influence nearly every cell type, exert profound effects on development, and complement cancer chemotherapeutic regimens. All-trans retinoic acid (RA) and other active retinoids are generated from vitamin A (retinol), but key aspects of the signaling pathways required to produce active retinoids remain unclear. Retinoids generated by one cell type can affect nearby cells, so retinoids also function in intercellular communication. RA induces differentiation primarily by binding to RARs, transcription factors that associate with RXRs and bind RAREs in the nucleus. Binding of RA: (1) initiates changes in interactions of RAR/RXRs with co-repressor and co-activator proteins, activating transcription of primary target genes; (2) alters interactions with proteins that induce epigenetic changes; (3) induces transcription of genes encoding transcription factors and signaling proteins that further modify gene expression (e.g., FOX03A, Hoxa1, Sox9, TRAIL, UBE2D3); and (4) results in alterations in estrogen receptor α signaling. Proteins that bind at or near RAREs include Sin3a, N-CoR1, PRAME, Trim24, NRIP1, Ajuba, Zfp423, and MN1/TEL. Interactions among retinoids, RARs/RXRs, and these proteins explain in part the powerful effects of retinoids on stem cell differentiation. Studies of this retinol signaling cascade enhance our ability to understand and regulate stem cell differentiation for therapeutic and scientific purposes. In cancer chemotherapeutic regimens retinoids can promote tumor cell differentiation and/or induce proteins that sensitize tumors to drug combinations. Mechanistic studies of retinoid signaling continue to suggest novel drug targets and will improve therapeutic strategies for cancer and other diseases, such as immune-mediated inflammatory diseases.

Induction of murine embryonic stem cell differentiation by medicinal plant extracts

Epidemiological evidence indicates that diets high in fruits and vegetables provide a measure of cancer chemoprevention due to phytochemical constituents. Natural products are a rich source of cancer chemotherapy drugs, and primarily target rapidly cycling tumor cells. Increasing evidence indicates that many cancers contain small populations of resistant, stem-like cells that have the capacity to regenerate tumors following chemotherapy and radiation, and have been linked to the initiation of metastases. Our goal is to discover natural product-based clinical or dietary interventions that selectively target cancer stem cells, inducing differentiation. We adapted an alkaline phosphatase (AP) stain to assay plant extracts for the capacity to induce differentiation in embryonic stem (ES) cells. AP is a characteristic marker of undifferentiated ES cells, and this represents a novel approach to screening medicinal plant extracts. Following a survey of approximately 100 fractions obtained from 12 species of ethnomedically utilized plants, we found fractions from 3 species that induced differentiation, decreasing AP and transcript levels of pluripotency markers (Nanog, Oct-4, Rex-1). These fractions affected proliferation of murine ES, and human embryonal, prostate, and breast carcinoma cells in a dose-dependent manner. Several phytochemical constituents were isolated; the antioxidant phytochemicals ellagic acid and gallic acid were shown to affect viability of cultured breast carcinoma cells.

Smoking-induced upregulation of AKR1B10 expression in the airway epithelium of healthy individuals

BACKGROUND

The aldo-keto reductase (AKR) gene superfamily codes for monomeric, soluble reduced nicotinamide adenine dinucleotide phosphate-dependent oxidoreductases that mediate elimination reactions. AKR1B10, an AKR that eliminates retinals, has been observed as upregulated in squamous metaplasia and non-small cell lung cancer and has been suggested as a diagnostic marker specific to tobacco-related carcinogenesis. We hypothesized that upregulation of AKR1B10 expression may be initiated in healthy smokers prior to the development of evidence of lung cancer.

METHODS

Expression of AKR1B10 was assessed at the mRNA level using microarrays with TaqMan confirmation in the large airway epithelium (21 healthy nonsmokers, 31 healthy smokers) and small airway epithelium (51 healthy nonsmokers, 58 healthy smokers) obtained by fiberoptic bronchoscopy and brushing.

RESULTS

Compared with healthy nonsmokers, AKR1B10 mRNA levels were significantly upregulated in both large and small airway epithelia of healthy smokers. Consistent with the mRNA data, AKR1B10 protein was significantly upregulated in the airway epithelium of healthy smokers as assessed by Western blot analysis and immunohistochemistry, with AKR1B10 expressed in both differentiated and basal cells. Finally, cigarette smoke extract mediated upregulation of AKR1B10 in airway epithelial cells in vitro, and transfection of AKR1B10 into airway epithelial cells enhanced the conversion of retinal to retinol.

CONCLUSIONS

Smoking per se mediates upregulation of AKR1B10 expression in the airway epithelia of healthy smokers with no evidence of lung cancer. In the context of these observations and the link of AKR1B10 to the metabolism of retinals and to lung cancer, the smoking-induced upregulation of AKR1B10 may be an early process in the multiple events leading to lung cancer.

The effects of DNA methyltransferase inhibitors and histone deacetylase inhibitors on digit regeneration in mice

METHOD

We injected two drugs that modify the epigenome, the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine (5-aza-dC) and the histone deacetylase inhibitor trichostatin A (TSA), alone or in combination, into C57Bl/6 mice subjected to amputation through the mid-second phalanx of the third digit. Wound-site tissue was collected.

RESULTS

We observed increased staining of the stem cell markers Rex1 (Zfp42) and stem cell antigen-1 at digit amputation sites from drug-treated mice. Samples from 5-aza-dC plus TSA and TSA treated mice also showed increased proliferating cell nuclear antigen staining, a measure of cell proliferation. Drug treatments increased Msx1, but not Cyp26a1 or ALDH1a2 (RALDH2) mRNA.

CONCLUSION

5-aza-dC and TSA treatments stimulated cell proliferation at the amputation site, possibly via increased expression of genes involved in digit development and regeneration.

Transcriptional regulation of Rex1 (zfp42) in normal prostate epithelial cells and prostate cancer cells

Rex1 (zfp42) was identified by our laboratory because of its reduced expression in F9 teratocarcinoma stem cells after retinoic acid (RA) treatment. The Rex1 (Zfp42) gene is currently widely used as a marker of embryonic stem cells. We compared the transcriptional regulation of the human Rex1 gene in NTera-2 (NT-2) human teratocarcinoma, normal human prostate epithelial cells (PrEC), and prostate cancer cells (PC-3) by promoter/luciferase analyses. Oct4, Sox2, Nanog, and Dax1 transcripts are expressed at higher levels in NT-2 and PrEC cells than in PC-3 cells. Co-transfection analyses showed that YY1 and Rex1 are positive regulators of hRex1 transcription in NT-2 and PrEC cells, whereas Nanog is not. Serial deletion constructs of the hRex1 promoter were created and analyzed, by which we identified a potential negative regulatory site that is located between -1 and -0.4 kb of the hRex1 promoter. We also delineated regions of the hRex1 promoter between -0.4 kb and the TSS that, when mutated, reduced transcriptional activation; these are putative Rex1 binding sites. Mutation of a putative Rex1 binding site in electrophoretic mobility shift assays (EMSA) resulted in reduced protein binding. Taken together, our results indicate that hRex1 binds to the hRex1 promoter region at -298 bp and positively regulates hRex1 transcription, but that this regulation is lost in PC-3 human prostate cancer cells. This lack of positive transcriptional regulation by the hRex1 protein may be responsible for the lack of Rex1 expression in PC-3 prostate cancer cells.

Phase I trial of ATRA-IV and Depakote in patients with advanced solid tumor malignancies

Retinoic acid derivatives have shown their greatest benefit in acute promyelocytic leukemia, but have also demonstrated pre-clinical anti-cancer effects in some solid tumors. Histone deacetylase inhibitors, by upregulating gene expression, are able to limit cancer cell proliferation and induce apoptosis. The combination of all-trans retinoic acid (ATRA) and the histone deacetylase inhibitor valproic acid has been previously studied in hematologic malignancies. We conducted a phase I two-step dose escalation trial of the liposomal ATRA analog ATRA-IV and divalproex sodium (Depakote) in nine patients with advanced solid tumors refractory to prior therapy. Side effects attributed to therapy had a severity <or=grade 2 and included skin toxicity and thrombocytopenia. The best disease response seen was disease stabilization in one patient. Expression of cellular retinoic acid binding protein-2 in peripheral blood mononuclear cells was detected as a marker of drug effect. The maximum tolerated dose (MTD) of both drugs in combination could not be established due to early closure of the trial resulting from a halt in the commercial availability of ATRA-IV.

Epigenetic regulatory mechanisms distinguish retinoic acid-mediated transcriptional responses in stem cells and fibroblasts

Retinoic acid (RA), a vitamin A metabolite, regulates transcription by binding to RA receptor (RAR) and retinoid X receptor (RXR) heterodimers. This transcriptional response is determined by receptor interactions with transcriptional regulators and chromatin modifying proteins. We compared transcriptional responses of three RA target genes (Hoxa1, Cyp26a1, RARbeta(2)) in primary embryo fibroblasts (mouse embryonic fibroblasts), immortalized fibroblasts (Balb/c3T3), and F9 teratocarcinoma stem cells. Hoxa1 and Cyp26a1 transcripts are not expressed, but RARbeta(2) transcripts are induced by RA in mouse embryonic fibroblasts and Balb/c3T3 cells. Retinoid receptors (RARgamma, RXRalpha), coactivators (pCIP (NCOA3, SRC3)), and p300 and RNA polymerase II are recruited only to the RARbeta(2) RA response element (RARE) in Balb/c3T3, whereas these proteins are recruited to RAREs of all three genes by RA in F9 cells. In F9, RA reduces polycomb (PcG) protein Suz12 and the associated H3K27me3 repressive epigenetic modification at the RAREs of all three genes. In contrast, in Balb/c3T3 cells cultured in the +/-RA, Suz12 is not associated with the Hoxa1, RARbeta(2), and Cyp26a1 RAREs, whereas slow levels of the H3K27me3 mark are seen at these RAREs. Thus, Suz12 is not required for gene repression in the absence of RA. Even though the Hoxa1 RARE and proximal promoter show high levels of H3K9,K14 acetylation in Balb/c3T3, the Hoxa1 gene is not transcriptionally activated by RA. In Balb/c3T3, CpG islands are methylated in the Cyp26a1 promoter region but not in the Hoxa1 promoter or in these promoters in F9 cells. We have delineated the complex mechanisms that control RA-mediated transcription in fibroblasts versus stem cells.

Regulation of stem cell pluripotency and differentiation involves a mutual regulatory circuit of the NANOG, OCT4, and SOX2 pluripotency transcription factors with polycomb repressive complexes and stem cell microRNAs

Coordinated transcription factor networks have emerged as the master regulatory mechanisms of stem cell pluripotency and differentiation. Many stem cell-specific transcription factors, including the pluripotency transcription factors, OCT4, NANOG, and SOX2 function in combinatorial complexes to regulate the expression of loci, which are involved in embryonic stem (ES) cell pluripotency and cellular differentiation. This review will address how these pathways form a reciprocal regulatory circuit whereby the equilibrium between stem cell self-renewal, proliferation, and differentiation is in perpetual balance. We will discuss how distinct epigenetic repressive pathways involving polycomb complexes, DNA methylation, and microRNAs cooperate to reduce transcriptional noise and to prevent stochastic and aberrant induction of differentiation. We will provide a brief overview of how these networks cooperate to modulate differentiation along hematopoietic and neuronal lineages. Finally, we will describe how aberrant functioning of components of the stem cell regulatory network may contribute to malignant transformation of adult stem cells and the establishment of a "cancer stem cell" phenotype and thereby underlie multiple types of human malignancies.

A DNA methyltransferase inhibitor and all-trans retinoic acid reduce oral cavity carcinogenesis induced by the carcinogen 4-nitroquinoline 1-oxide

The transcriptional silencing of some cell cycle inhibitors and tumor suppressors, such as p16 and retinoic acid receptor beta(2), by DNA hypermethylation at CpG islands is commonly found in human oral squamous carcinoma cells. We examined the effects of the DNA methyltransferase inhibitor 5-Aza-2'-deoxycytidine (5-Aza; 0.25 mg/kg body weight), all-trans retinoic acid (RA; given at 100 microg/kg body weight and 1 mg/kg body weight), and the combination of 5-Aza and the low-dose RA on murine oral cavity carcinogenesis induced by the carcinogen 4-nitroquinoline 1-oxide (4-NQO) in a mouse model. All the drug treatments were done for 15 weeks after a 10-week 4-NQO treatment. Mice in all drug treatment groups showed decreases in the average numbers of neoplastic tongue lesions. The combination of 5-Aza and RA effectively attenuated tongue lesion severity. Although all drug treatments limited the increase in the percentage of proliferating cell nuclear antigen-positive cells and the decrease in the percentage of p16-positive cells caused by the 4-NQO treatment in mouse tongue epithelial regions without visible lesions and in the neoplastic tongue lesions, the combination of 5-Aza and RA was the most effective. Collectively, our results show that the combination of a DNA demethylating drug and RA has potential as a strategy to reduce oral cavity cancer in this 4-NQO model.

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.

Analysis of Rex1 (zfp42) function in embryonic stem cell differentiation

Rex1 (zfp42) is a zinc finger protein expressed primarily in undifferentiated stem cells, both in the embryo and the adult. Upon all-trans retinoic acid induced differentiation of murine embryonic stem (ES) cells, Rex1 mRNA levels decrease several fold. To characterize the function(s) of Rex1 more extensively, we generated Rex1 double knockout ES cell lines. The disruption of the Rex1 gene enhanced the expression of ectoderm, mesoderm, and endoderm markers as compared to wild-type (Wt) cells. We propose that Rex1 acts to reduce retinoic acid induced differentiation in ES cells. We performed microarray analyses on Wt and Rex1-/- cells cultured in the presence or absence of LIF to identify potential Rex1 targets. We also evaluated gene expression in a Wt line that overexpresses Rex1 and in a Rex1-/- line in which Rex1 expression was restored. These data, taken together, suggest that Rex1 influences differentiation, cell cycle regulation, and cancer progression.

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.

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.

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.

Chemoattractant activity of degradation products of fetal and adult skin extracellular matrix for keratinocyte progenitor cells

Biological scaffolds composed of naturally occurring extracellular matrix (ECM) have been utilized as templates for the constructive remodelling of numerous tissues in preclinical studies and human clinical applications. The mechanisms by which ECM induces constructive remodelling are not well understood, but it appears that the degradation products of ECM scaffolds may play key roles in cell recruitment. The objective of the present study was to investigate the effects of age and species of the tissue from which ECM is harvested on the chemoattractant activity of degradation products of ECM for human keratinocyte stem and progenitor cells. Adult human skin ECM, fetal human skin ECM and adult porcine skin ECM were prepared, enzymatically digested, characterized by SDS-PAGE and evaluated for in vitro chemoattractant activity for human keratinocyte progenitor and stem cells (HEKn). Degradation products of human fetal skin ECM showed greater chemoattractant activity than human adult skin ECM degradation products for the HEKn. Degradation products of porcine adult skin ECM showed greater chemoattractant activity than human adult skin ECM. The human fetal skin ECM degradation products showed the strongest chemoattractant activity for the HEKn. The findings of this study support the concept that the mechanism of ECM scaffold remodelling involves the recruitment of lineage-directed progenitor cells by scaffold degradation products, and that both the age and species of the tissue from which the ECM is harvested have an effect upon this chemoattractant potential.

The truncated Hoxa1 protein interacts with Hoxa1 and Pbx1 in stem cells

Hox genes contain a homeobox encoding a 60-amino acid DNA binding sequence. The Hoxa1 gene (Hox1.6, ERA1) encodes two alternatively spliced mRNAs that encode distinct proteins, one with the homeodomain (Hoxa1-993), and another protein lacking this domain (Hoxa1-399). The functions of Hoxa1-399 are unknown. We detected Hoxa1-993 and Hoxa1-399 by immunoprecipitation using Hoxa1 antibodies. To assess whether Hoxa1-399 functions in cellular differentiation, we analyzed Hoxb1, a Hoxa1 target gene. Hoxa1-993 and its cofactor, Pbx1, bind to the Hoxb1 SOct-R3 promoter to transcriptionally activate a luciferase reporter. Results from F9 stem cells that stably express ectopic Hoxa1-399 (the F9-399 line) show that Hoxa1-399 reduces this transcriptional activation. Gel shift assays demonstrate that Hoxa1-399 reduces Hoxa1-993/Pbx1 binding to the Hoxb1 SOct-R3 region. GST pull-down experiments suggest that Hoxa1-399, Hoxa1-993, and Pbx1 form a trimer. However, the F9-399 line exhibits no differences in RA-induced proliferation arrest or endogenous Hoxb1, Pbx1, Hoxa5, Cyp26a1, GATA4, or Meis mRNA levels when compared to F9 wild-type.

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.

Hoxa1 is required for the retinoic acid-induced differentiation of embryonic stem cells into neurons

The ability of embryonic stem (ES) cells to differentiate into different cell fates has been extensively evaluated, and several protocols exist for the generation of various types of cells from mouse and human ES cells. We used a differentiation protocol that involves embryoid body formation and all-trans-retinoic acid (RA, 5 microM) treatment (EB/5 microM RA) to test the ability of Hoxa1 null ES cells to adopt a neuronal fate. Hoxa1(-/-) ES cells, when treated in this EB/5 microM RA protocol, failed to differentiate along a neural lineage; Hoxa1(-/-) ES cells express severalfold lower levels of many neuronal differentiation markers, including nestin, beta-tubulin III, and MAP2, and conversely, higher levels of endodermal differentiation markers (i.e., Sox17, Col4a1) than wild type (Wt) cells. Reintroduction of exogenous Hoxa1, under the control of the metallothionein I promoter, into Hoxa1(-/-) ES cells restored their capacity to generate neurons. Moreover, overexpression of Sox17, a gene that regulates endodermal differentiation, in Wt ES cells resulted in endodermal differentiation and in a complete abolition of beta-tubulin III expression. Thus, Hoxa1 activity is essential for the neuronal differentiation of ES cells in the presence of all-trans-RA, and Hoxa1 may promote neural differentiation by inhibiting Sox17 expression. Pharmacological manipulation of Hoxa1 levels may provide a method for promoting neuronal differentiation for therapeutic uses. Furthermore, because mutations in the Hoxa1 gene can cause autism spectrum disorder in humans, these data also provide important mechanistic insights into the early developmental processes that may result in this disorder.

Gene microarray analysis of human renal cell carcinoma: the effects of HDAC inhibition and retinoid treatment

Histone deacetylase (HDAC) inhibitor treatments can augment the anti-tumor effects of retinoids in renal cancer cells. We studied the effects of the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) and 13-cis retinoic acid (cRA) on two human renal cell carcinoma (RCC) lines. Cells were cultured in the presence of each drug for six days to determine the responses to monotherapy and to combination therapy. The proliferation of SKRC06 was inhibited with cRA treatment; the proliferation of SKRC39 was not. However, both RCC lines were sensitive to growth inhibition by dibutyryl cyclic AMP, with or without 13-cis RA. SAHA alone also reduced cell proliferation in both cell lines. To identify the alterations in gene expression that correlate with the responsiveness to treatment, gene microarray analyses were performed. Several retinoid-regulated genes exhibited much higher mRNA levels in SKRC06 than in SKRC39, even in the absence of drugs; these included crabp2, rargamma and cyp26A1. Combination treatment of cells with both SAHA and cRA induced several transcripts with known anti-cancer/immunomodulatory effects, including dhrs9, gata3, il1beta, phlda1, txk and vhl. Immunostaining confirmed the decreased expression of gata3 in human RCC specimens compared to normal kidney. Together, our results show that treatment of RCC with cRA and/or SAHA increases the expression of several genes and gene families that result in reduced cell proliferation.

Retinoic acid receptor gamma activates receptor tyrosine kinase Tie1 gene transcription through transcription factor GATA4 in F9 stem cells

OBJECTIVE

The retinoic acid receptors (RARs) alpha, beta2, and gamma regulate specific subsets of target genes during all-trans retinoic acid (RA) induced differentiation of F9 teratocarcinoma stem cells. The Tie1 gene exhibited reduced expression in RA-treated F9 RARgamma-/- cells as compared to wild-type (WT) by microarray analysis. Our goal was to analyze the Tie1 gene, which encodes a surface receptor tyrosine kinase expressed in the hematovascular system.

MATERIALS AND METHODS

We assessed Tie1, Tie2, Flk1, Runx1, Peg/Mest2, and angiopoietin-1 and 2 mRNA levels and Tie1 promoter activity.

RESULTS

We showed that RARgamma, but not RARalpha or RARbeta2, is required for Tie1 promoter activation by RA. Treatment with a RARgamma selective agonist plus a retinoid X receptor agonist (LGD1069) increased Tie1 mRNA levels by 11- +/- 2.5-fold 48 hours after RA addition in F9 WT, but not in F9 RARgamma-/- cells, by quantitative reverse transcription polymerase chain reaction. Multiple putative GATA elements were identified in the Tie1 proximal promoter. RA increased GATA4 transcripts by 12- +/- 1-fold in F9 WT at 48 hours, but not in F9 RARgamma-/- cells. In addition, transfection of a GATA4 expression vector increased Tie1 promoter/luciferase activity in both RA-treated F9 WT and RARgamma-/- cells. Tie1 promoter deletion analyses indicated that a region of the promoter that possessed multiple GATA sites mediated the RA-associated Tie1 transcriptional increase.

CONCLUSIONS

Our results indicate that GATA4 plays a role in the RA/RARgamma-associated transcriptional activation of the Tie1 promoter. An understanding of RAR specificity in RA signaling should result in insights into hematopoietic stem cell signaling and potentially in improved therapies for several human diseases.

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.

Gene expression profiling elucidates a specific role for RARgamma in the retinoic acid-induced differentiation of F9 teratocarcinoma stem cells

The biological effects of all-trans-retinoic acid (RA), a major active metabolite of retinol, are mainly mediated through its interactions with retinoic acid receptor (RARs alpha, beta, gamma) and retinoid X receptor (RXRs alpha, beta, gamma) heterodimers. RAR/RXR heterodimers activate transcription by binding to RA-response elements (RAREs or RXREs) in the promoters of primary target genes. Murine F9 teratocarcinoma stem cells have been widely used as a model for cellular differentiation and RA signaling during embryonic development. We identified and characterized genes that are differentially expressed in F9 wild type (Wt) and F9 RARgamma-/- cells, with and without RA treatment, through the use of oligonucleotide-based microarrays. Our data indicate that RARgamma, in the absence of exogenous RA, modulates gene expression. Genes such as Sfrp2, Tie1, Fbp2, Emp1, and Emp3 exhibited higher transcript levels in RA-treated Wt, RARalpha-/- and RARbeta2-/- lines than in RA-treated RARgamma-/- cells, and represent specific RARgamma targets. Other genes, such as Runx1, were expressed at lower levels in both F9 RARbeta2-/- and RARgamma-/- cell lines than in F9 Wt and RARalpha-/-. Genes specifically induced by RA at 6h with the protein synthesis inhibitor cycloheximide in F9 Wt, but not in RARgamma-/- cells, included Hoxa3, Hoxa5, Gas1, Cyp26a1, Sfrp2, Fbp2, and Emp1. These genes represent specific primary RARgamma targets in F9 cells. Several genes in the Wnt signaling pathway were regulated by RARgamma. Delineation of the receptor-specific actions of RA with respect to cell proliferation and differentiation should result in more effective therapies with this drug.

Transcriptional activation of the suppressor of cytokine signaling-3 (SOCS-3) gene via STAT3 is increased in F9 REX1 (ZFP-42) knockout teratocarcinoma stem cells relative to wild-type cells

Rex1 (Zfp42), first identified as a gene that is transcriptionally repressed by retinoic acid (RA), encodes a zinc finger transcription factor expressed at high levels in F9 teratocarcinoma stem cells, embryonic stem cells, and other stem cells. Loss of both alleles of Rex1 by homologous recombination alters the RA-induced differentiation of F9 cells, a model of pluripotent embryonic stem cells. We identified Suppressor of Cytokine Signaling-3 (SOCS-3) as a gene that exhibits greatly increased transcriptional activation in RA, cAMP, and theophylline (RACT)-treated F9 Rex1(-/-) cells (approximately 25-fold) as compared to wild-type (WT) cells ( approximately 2.5-fold). By promoter deletion, mutation, and transient transfection analyses, we have shown that this transcriptional increase is mediated by the STAT3 DNA-binding elements located between -99 to -60 in the SOCS-3 promoter. Overexpression of STAT3 dominant-negative mutants greatly diminishes this SOCS-3 transcriptional increase in F9 Rex1(-/-) cells. This increase in SOCS-3 transcription is associated with a four- to fivefold higher level of tyrosine-phosphorylated STAT3 in the RACT-treated F9 Rex1(-/-) cells as compared to WT. Dominant-negative Src tyrosine kinase, Jak2, and protein kinase A partially reduce the transcriptional activation of the SOCS 3 gene in RACT-treated F9 Rex1 null cells. In contrast, parathyroid hormone peptide enhances the effect of RA in F9 Rex1(-/-) cells, but not in F9 WT. Thus, Rex1, which is highly expressed in stem cells, inhibits signaling via the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway, thereby modulating the differentiation of F9 cells.