Molecular mechanism for growth suppression of human hepatocellular carcinoma cells by acyclic retinoid

Serum MYCN as a predictive biomarker of prognosis and therapeutic response in the prevention of hepatocellular carcinoma recurrence

The proto-oncogene MYCN expression marked a cancer stem-like cell population in hepatocellular carcinoma (HCC) and served as a therapeutic target of acyclic retinoid (ACR), an orally administered vitamin A derivative that has demonstrated promising efficacy and safety in reducing HCC recurrence. This study investigated the role of MYCN as a predictive biomarker for therapeutic response to ACR and prognosis of HCC. MYCN gene expression in HCC was analyzed in the Cancer Genome Atlas and a Taiwanese cohort (N = 118). Serum MYCN protein levels were assessed in healthy controls (N = 15), patients with HCC (N = 116), pre- and post-surgical patients with HCC (N = 20), and a subset of patients from a phase 3 clinical trial of ACR (N = 68, NCT01640808). The results showed increased MYCN gene expression in HCC tumors, which positively correlated with HCC recurrence in non-cirrhotic or single-tumor patients. Serum MYCN protein levels were higher in patients with HCC, decreased after surgical resection of HCC, and were associated with liver functional reserve and fibrosis markers, as well as long-term HCC prognosis (>4 years). Subgroup analysis of a phase 3 clinical trial of ACR identified serum MYCN as the risk factor most strongly associated with HCC recurrence. Patients with HCC with higher serum MYCN levels after a 4-week treatment of ACR exhibited a significantly higher risk of recurrence (hazard ratio 3.27; p = .022). In conclusion, serum MYCN holds promise for biomarker-based precision medicine for the prevention of HCC, long-term prognosis of early-stage HCC, and identification of high-response subgroups for ACR-based treatment.

The phosphorylated retinoid X receptor-α promotes diethylnitrosamine-induced hepatocarcinogenesis in mice through the activation of β-catenin signaling pathway

Previous studies have shown that phosphorylation of the retinoid X receptor-α (RXRα) is associated with the development of hepatocellular carcinoma (HCC). However, these findings were revealed using HCC cell lines that express phosphorylated-RXRα (p-RXRα) proteins; therefore, it remains unclear whether p-RXRα affects hepatocarcinogenesis in vivo. Therefore, to investigate the biological function of p-RXRα in vivo, we developed a doxycycline-inducible ES cell line and transgenic mouse, both of which overexpress the phosphomimetic mutant form of RXRα, T82D/S260D, in a doxycycline-dependent manner. We found that the development of liver tumors, especially high-grade adenoma and HCC, was enhanced in diethylnitrosamine (DEN)-induced T82D/S260D-inducible mice. Moreover, the increased incidence of liver tumors in the transgenic mice was attributable to the promotion of cell cycle progression. Interestingly, the expression of β-catenin protein and its target gene cyclin D1 was elevated in the liver tumors of DEN-treated T82D/S260D-inducible mice, concurrent with increased cytoplasmic and nuclear β-catenin protein expression, indicating its stabilization and transcriptional activation. These results indicate that p-RXRα promotes DEN-induced hepatocarcinogenesis in mice through the activation of the β-catenin signaling pathway, suggesting that p-RXRα may serve as a possible therapeutic target for HCC.

Sulfarotene, a synthetic retinoid, overcomes stemness and sorafenib resistance of hepatocellular carcinoma via suppressing SOS2-RAS pathway

BACKGROUND

Recurrent hepatocellular carcinoma (HCC) shows strong resistance to sorafenib, and the tumor-repopulating cells (TRCs) with cancer stem cell-like properties are considered a driver for its high recurrent rate and drug resistance.

METHODS

Suppression of TRCs may thus be an effective therapeutic strategy for treating this fatal disease. We evaluated the pharmacology and mechanism of sulfarotene, a new type of synthetic retinoid, on the cancer stem cell-like properties of HCC TRCs, and assessed its preclinical efficacy in models of HCC patient-derived xenografts (PDXs).

RESULTS

Sulfarotene selectively inhibited the growth of HCC TRCs in vitro and significantly deterred TRC-mediated tumor formation and lung metastasis in vivo without apparent toxicity, with an IC₅₀ superior to that of acyclic retinoid and sorafenib, to which the recurrent HCC exhibits significant resistance at advanced stage. Sulfarotene promoted the expression and activation of RARα, which down-regulated SOS2, a key signal mediator associated with RAS activation and signal transduction involved in multiple downstream pathways. Moreover, sulfarotene selectively inhibited tumorigenesis of HCC PDXs with high expression for SOS2.

CONCLUSIONS

Our study identified sulfarotene as a selective inhibitor for the TRCs of HCC, which targets a novel RARα-SOS2-RAS signal nexus, shedding light on a new, promising strategy of target therapy for advanced liver cancer.

Natural and synthetic retinoid X receptor ligands and their role in selected nuclear receptor action

Important key players in the regulatory machinery within the cells are nuclear retinoid X receptors (RXRs), which compose heterodimers in company with several diverse nuclear receptors, playing a role as ligand inducible transcription factors. In general, nuclear receptors are ligand-activated, transcription-modulating proteins affecting transcriptional responses in target genes. RXR molecules forming permissive heterodimers with disparate nuclear receptors comprise peroxisome proliferator-activated receptors (PPARs), liver X receptors (LXRs), farnesoid X receptor (FXR), pregnane X receptor (PXR) and constitutive androstan receptor (CAR). Retinoid receptors (RARs) and thyroid hormone receptors (TRs) may form conditional heterodimers, and dihydroxyvitamin D₃ receptor (VDR) is believed to form nonpermissive heterodimer. Thus, RXRs are the important molecules that are involved in control of many cellular functions in biological processes and diseases, including cancer or diabetes. This article summarizes both naturally occurring and synthetic ligands for nuclear retinoid X receptors and describes, predominantly in mammals, their role in molecular mechanisms within the cells. A focus is also on triorganotin compounds, which are high affinity RXR ligands, and finally, we present an outlook on human microbiota as a potential source of RXR activators. Nevertheless, new synthetic rexinoids with better retinoid X receptor activity and lesser side effects are highly required.

Beyond the Warburg Effect: N-Myc Contributes to Metabolic Reprogramming in Cancer Cells

Cancer cells generate large amounts of lactate derived from glucose regardless of the available oxygen level. Cancer cells finely control ATP synthesis by modulating the uptake of substrates and the activity of enzymes involved in aerobic glycolysis (Warburg effect), which enables them to adapt to the tumor microenvironment. However, increasing evidence suggests that mitochondrial metabolism, including the tricarboxylic acid (TCA) cycle, oxidative phosphorylation (OXPHOS), and glutaminolysis, is paradoxically activated in MYCN-amplified malignancies. Unlike non-amplified cells, MYCN-amplified cancer cells significantly promote OXPHOS-dependent ATP synthesis. Furthermore, tumor cells are differentially dependent on fatty acid β-oxidation (FAO) according to N-Myc status. Therefore, upregulation of FAO-associated enzymes is positively correlated with both N-Myc expression level and poor clinical outcome. This review explores therapeutic strategies targeting cancer stem-like cells for the treatment of tumors associated with MYCN amplification.

Prediagnostic dietary intakes of vitamin A and β-carotene are associated with hepatocellular-carcinoma survival

Vitamin A and its precursor (β-carotene) have been linked with cancer incidence and mortality.

Tissue and serum metabolomic phenotyping for diagnosis and prognosis of hepatocellular carcinoma

More than two-thirds of patients with hepatocellular carcinoma (HCC) cannot receive curative therapy and have poor survival due to late diagnosis and few prognostic directions. In our study, nontargeted and targeted metabolomics analyses were conducted by liquid chromatography-mass spectrometry to characterize metabolic features of HCC and identify diagnostic and prognostic biomarker candidate incorporating liver tissue and serum metabolites. A total of 552 subjects, including 432 with liver tissue and 120 with serum specimens, were recruited in China. In the discovery cohort, a series of 138 metabolites were identified to discriminate HCC tissues from matched nontumor tissues. Retinol presented with the highest area under the curve (AUC) of 0.991 and associated with Edmondson grade. In the validation cohort, all metabolites in retinol metabolism pathway were examined and the levels of retinol and retinal in tumor tissue and serum decreased in the order of normal to cirrhosis to HCC of Edmondson Grades I to IV. Retinol and retinal levels could also differentiate between HCC and cirrhosis, with AUCs of 0.996 and 0.994, respectively, in tissue and 0.812 and 0.744, respectively, in serum. The AUC of the combined retinol and retinal panel in serum was 0.852. Univariate and multivariate Cox regression identified this panel as an independent predictor for HCC and showed that low expression of retinol and retinal correlated with decreased survival time. In conclusion, the retinol metabolic signature had considerable diagnostic and prognostic value for identifying HCC patients who would benefit from prompt therapy and optimal prognostic direction.

Prevention of hepatocellular carcinoma by targeting MYCN-positive liver cancer stem cells with acyclic retinoid

Hepatocellular carcinoma (HCC) is a highly lethal cancer, partly because of its high rate of recurrence, which is caused by the presence of liver cancer stem cells (CSCs). Here, using a selective chemopreventive agent, acyclic retinoid (ACR), as a bioprobe, we identified MYCN, which is mostly recognized as an oncogene in neuroblastoma, as a therapeutic target of ACR for HCC through a selective deletion of MYCN⁺ liver CSCs. We also demonstrated that the expression of MYCN in HCC served as a prognostic biomarker and positively correlated with recurrence of de novo HCC after curative treatment. Our study highlighted MYCN as a biomarker and therapeutic target in drug discovery for screening chemopreventive agents against the recurrence of HCC.

Hepatocellular carcinoma (HCC) is a highly lethal cancer that has a high rate of recurrence, in part because of cancer stem cell (CSC)-dependent field cancerization. Acyclic retinoid (ACR) is a synthetic vitamin A-like compound capable of preventing the recurrence of HCC. Here, we performed a genome-wide transcriptome screen and showed that ACR selectively suppressed the expression of MYCN, a member of the MYC family of basic helix–loop–helix–zipper transcription factors, in HCC cell cultures, animal models, and liver biopsies obtained from HCC patients. MYCN expression in human HCC was correlated positively with both CSC and Wnt/β-catenin signaling markers but negatively with mature hepatocyte markers. Functional analysis showed repressed cell-cycle progression, proliferation, and colony formation, activated caspase-8, and induced cell death in HCC cells following silencing of MYCN expression. High-content single-cell imaging analysis and flow cytometric analysis identified a MYCN⁺ CSC subpopulation in the heterogeneous HCC cell cultures and showed that these cells were selectively killed by ACR. Particularly, EpCAM⁺ cells isolated using a cell-sorting system showed increased MYCN expression and sensitivity to ACR compared with EpCAM⁻ cells. In a long-term (>10 y) follow-up study of 102 patients with HCC, MYCN was expressed at higher levels in the HCC tumor region than in nontumor regions, and there was a positive correlation between MYCN expression and recurrence of de novo HCC but not metastatic HCC after curative treatment. In summary, these results suggest that MYCN serves as a prognostic biomarker and therapeutic target of ACR for liver CSCs in de novo HCC.

Peretinoin, an acyclic retinoid, inhibits hepatocarcinogenesis by suppressing sphingosine kinase 1 expression in vitro and in vivo

Sphingosine-1-phospate is a potent bioactive lipid metabolite that regulates cancer progression. Because sphingosine kinase 1 and sphingosine kinase 2 (SPHK 1/2) are both essential for sphingosine-1-phospate production, they could be a therapeutic target in various cancers. Peretinoin, an acyclic retinoid, inhibits post-therapeutic recurrence of hepatocellular carcinoma via unclear mechanisms. In this study, we assessed effects of peretinoin on SPHK expression and liver cancer development in vitro and in vivo. We examined effects of peretinoin on expression, enzymatic and promoter activity of SPHK1 in a human hepatoma cell line, Huh-7. We also investigated effects of SPHK1 on hepatocarcinogenesis induced by diethylnitrosamine using SPHK1 knockout mice. Peretinoin treatment of Huh-7 cells reduced mRNA levels, protein expression and enzymatic activity of SPHK1. Peretinoin reduced SPHK1 promoter activity; this effect of peretinoin was blocked by overexpression of Sp1, a transcription factor. Deletion of all Sp1 binding sites within the SPHK1 promoter region abolished SPHK1 promoter activity, suggesting that peretinoin reduced mRNA levels of SPHK1 via Sp1. Additionally, diethylnitrosamine-induced hepatoma was fewer and less frequent in SPHK1 knockout compared to wild-type mice. Our data showed crucial roles of SPHK1 in hepatocarcinogenesis and suggests that peretinoin prevents hepatocarcinogenesis by suppressing mRNA levels of SPHK1.

Phosphorylation of Human Retinoid X Receptor α at Serine 260 Impairs Its Subcellular Localization, Receptor Interaction, Nuclear Mobility, and 1α,25-Dihydroxyvitamin D3-dependent DNA Binding in Ras-transformed Keratinocytes

Human retinoid X receptor α (hRXRα) plays a critical role in DNA binding and transcriptional activity through heterodimeric association with several members of the nuclear receptor superfamily, including the human vitamin D receptor (hVDR). We previously showed that hRXRα phosphorylation at serine 260 through the Ras-Raf-MAPK ERK1/2 activation is responsible for resistance to the growth inhibitory effects of 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3), the biologically active metabolite of vitamin D3 To further investigate the mechanism of this resistance, we studied intranuclear dynamics of hVDR and hRXRα-tagged constructs in living cells together with endogenous and tagged protein in fixed cells. We find that hVDR-, hRXRα-, and hVDR-hRXRα complex accumulate in the nucleus in 1α,25(OH)2D3-treated HPK1A cells but to a lesser extent in HPK1ARas-treated cells. Also, by using fluorescence resonance energy transfer (FRET), we demonstrate increased interaction of the hVDR-hRXRα complex in 1α,25(OH)2D3-treated HPK1A but not HPK1ARas cells. In HPK1ARas cells, 1α,25(OH)2D3-induced nuclear localization and interaction of hRXRα are restored when cells are treated with the MEK1/2 inhibitor UO126 or following transfection of the non-phosphorylatable hRXRα Ala-260 mutant. Finally, we demonstrate using fluorescence loss in photobleaching and quantitative co-localization with chromatin that RXR immobilization and co-localization with chromatin are significantly increased in 1α,25(OH)2D3-treated HPK1ARas cells transfected with the non-phosphorylatable hRXRα Ala-260 mutant. This suggests that hRXRα phosphorylation significantly disrupts its nuclear localization, interaction with VDR, intra-nuclear trafficking, and binding to chromatin of the hVDR-hRXR complex.

Metabolome Analyses Uncovered a Novel Inhibitory Effect of Acyclic Retinoid on Aberrant Lipogenesis in a Mouse Diethylnitrosamine-Induced Hepatic Tumorigenesis Model

Acyclic retinoid (ACR) is a promising drug under clinical trials for preventing recurrence of hepatocellular carcinoma. The objective of this study was to gain insights into molecular basis of the antitumorigenic action of ACR from a metabolic point of view. To achieve this, comprehensive cationic and lipophilic liver metabolic profiling was performed in mouse diethylnitrosamine (DEN)-induced hepatic tumorigenesis model using both capillary electrophoresis time-of-flight mass spectrometry and liquid chromatography time-of-flight mass spectrometry. ACR significantly counteracted against acceleration of lipogenesis but not glucose metabolism in DEN-treated mice liver, suggesting an important role of lipid metabolic reprogramming in the initiation step of hepatic tumorigenesis. Knowledge-based pathway analysis suggested that inhibition of linoleic acid metabolites such as arachidonic acid, a proinflammatory precursor, played a crucial role in the prevention by ACR of DEN-induced chronic inflammation-mediated tumorigenesis of the liver. As a molecular mechanism of the ACR's effect to prevent the aberrant lipogenesis, microarray analysis identified that a key transcription regulator of both embryogenesis and tumorigenesis, COUP transcription factor 2, also known as NR2F2, was associated with the metabolic effect of ACR in human hepatocellular carcinoma cells. Our study provided potential therapeutic targets for the chemoprevention of hepatocellular carcinoma as well as new insights into the mechanisms underlying prevention of hepatic tumorigenesis.

Chemoprevention of obesity-related liver carcinogenesis by using pharmaceutical and nutraceutical agents

Obesity and its related metabolic disorders are serious health problems worldwide, and lead to various health-related complications, including cancer. Among human cancers, hepatocellular carcinoma (HCC) is one of the most common malignancies affected by obesity. Therefore, obesity and its related disorders might be a key target for the prevention of HCC. Recently, new research indicates that the molecular abnormalities associated with obesity, including insulin resistance/hyperinsulinemia, chronic inflammation, adipokine imbalance, and oxidative stress, are possible molecular mechanisms underlying the pathogenesis of obesity-related hepatocarcinogenesis. Green tea catechins and branched-chain amino acids, both of which are classified as nutraceutical agents, have been reported to prevent obesity-related HCC development by improving metabolic abnormalities. The administration of acyclic retinoid, a pharmaceutical agent, reduced the incidence of HCC in obese and diabetic mice, and was also associated with improvements in insulin resistance and chronic inflammation. In this article, we review the detailed molecular mechanisms that link obesity to the development of HCC in obese individuals. We also summarize recent evidence from experimental and clinical studies using either nutraceutical or pharmaceutical agents, and suggest that nutraceutical and pharmaceutical approaches targeting metabolic abnormalities might be a promising strategy to prevent the development of obesity-related HCC.

Molecular mechanism by which acyclic retinoid induces nuclear localization of transglutaminase 2 in human hepatocellular carcinoma cells

Nuclear accumulation of transglutaminase 2 (TG2) is an important step in TG2-dependent cell death. However, the underlying molecular mechanisms for nuclear translocation of TG2 are still poorly understood. In this study, we demonstrated that acyclic retinoid (ACR) induced nuclear accumulation of TG2 in JHH-7 cells, a hepatocellular carcinoma (HCC) leading to their apoptosis. We further demonstrated molecular mechanism in nuclear-cytoplasmic trafficking of TG2 and an effect of ACR on it. We identified a novel 14-amino acid nuclear localization signal (NLS) (466)AEKEETGMAMRIRV(479) in the 'C' domain and a leucine-rich nuclear export signal (NES) (657)LHMGLHKL(664) in the 'D' domain that allowed TG2 to shuttle between the nuclear and cytosolic milieu. Increased nuclear import of GAPDH myc-HIS fused with the identified NLS was observed, confirming its nuclear import ability. Leptomycin B, an inhibitor of exportin-1 as well as point mutation of all leucine residues to glutamine residues in the NES of TG2 demolished its nuclear export. TG2 formed a trimeric complex with importin-α and importin-β independently from transamidase activity which strongly suggested the involvement of a NLS-based translocation of TG2 to the nucleus. ACR accelerated the formation of the trimeric complex and that may be at least in part responsible for enhanced nuclear localization of TG2 in HCC cells treated with ACR.

Retinoid roles in blocking hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a major health issue in many countries. The prognosis of HCC is still poor due to its high recurrence rate and its resistance to chemotherapy. Retinoids have long been known to have a connection with liver diseases, including HCC. Many experimental and clinical investigations have demonstrated associations between retinoids and hepatic disease, including the loss of retinoid activity in HCC cell lines and decreased hepatic retinoid stores and altered retinoid signaling in patients with cirrhosis and HCC. Based on these findings, preclinical and clinical investigations of retinoid effects on HCC have been undertaken. Recently, clinical trial results for the use of a synthetic retinoid, acyclic retinoid (ACR), to prevent HCC recurrence were published. In addition, extensive experimental studies on the action of retinoids in liver disease, including chronic viral hepatitis and non-alcoholic fatty liver disease (NAFLD), which lead to HCC have been performed. In the first section of this review, we will summarize the effectiveness and roles of retinoid for treating liver disease and blocking HCC. Subsequently, we will focus on ACR actions in blocking HCC.

Differentiation of bel-7402 human hepatocarcinoma cells induced by aqueous extracts of fresh gecko (AG) and its anti-tumor activity in vivo

ETHNOPHARMACOLOGICAL RELEVANCE

Gecko, a kind of reptile, has been widely used as a traditional Chinese medicine to treat various diseases including cancer in China for thousands of years. The aim of this study was to investigate the anti-tumor effect of AG (aqueous extracts of fresh gecko) on human hepatocellular carcinoma cell Bel-7402 in vitro and mouse H22 hepatocellular in vivo. Further to underlie the molecular mechanism of AG inducing the differentiation of Bel-7402 cells.

MATERIALS AND METHODS

AG was obtained by water extracting method and qualitatively analyzed through High Performance Liquid Chromatography. The total protein concentration of AG was measured by BCA (bicinchoninic acid disodium) assay. The anti-tumor activities in vivo were analyzed through H22 (mouse hepatocellular carcinoma cell line H22) tumor xenografts mice. The cytotoxic activity of AG on Bel-7402 cells was evaluated by MTT assays. AFP (alpha fetoprotein) was detected by radioimmunoassay. ALB (albumin), ALP (alkaline phosphatase) and γ-GT (γ-glutamyl transpeptidase) were detected by biochemical methods with commercial kits. While morphological changes were observed through an inverted microscope. Moreover, the expression level of the proteins involved in MAPK (mitogen-activated protein kinase) signal pathway which was closely related to cellular differentiation was assessed by Western blot.

RESULTS

AG showed obviously anti-tumor activity in vivo and anti-proliferative activity on Bel-7402 cells in vitro both dose-dependently. The number of clones of Bel-7402 cells treated with AG reduced and the cells were displaying differentiation state such as relatively bigger size and dispersed growth. The biochemical function markers of the cells were significantly changed after being treated with AG. The data showed that AFP secretion of the cells decreased 42.5%, ALB secretion increased 58.9%, the activity of ALP and γ-GT markedly decreased 67.0% and 48.5% separately when the concentration of AG was 10μl/ml, and those effects were all in a dose-dependent manner. The major original and phosphorylated signal proteins (ERK1/2 (extracellular sigal-regualted kinase 1/2), P38 (p38 MAPK) and JNK1/2 (c-Jun N-terminal kinase 1/2)) involved in MAPK signal pathway were measured and the results showed that AG activated the ERK1/2 of Bel-7402 cells.

CONCLUSIONS

AG has anti-tumor activity in vivo and inhibits Bel-7402 cell proliferation in vitro through inducing cell differentiation, and the mechanism involves the activation of ERK1/2.

Activation of RAS/ERK alone is insufficient to inhibit RXRα function and deplete retinoic acid in hepatocytes

Activation of RAS/ERK signaling pathway, depletion of retinoid, and phosphorylation of retinoid X receptor alpha (RXRα) are frequent events found in liver tumors and thought to play important roles in hepatic tumorigenesis. However, the relationships among them still remained to be elucidated. By exploring the transgenic mouse model of hepatic tumorigenesis induced by liver-specific expression of H-ras12V oncogene, the activation of RAS/ERK, the mRNA expression levels of retinoid metabolism-related genes, the contents of retinoid metabolites, and phosphorylation of RXRα were determined. RAS/ERK signaling pathway was gradually and significantly activated in hepatic tumor adjacent normal liver tissues (P) and hepatic tumor tissues (T) of H-ras12V transgenic mice compared with normal liver tissues (Wt) of wild type mice. On the contrary, the mRNA expression levels of retinoid metabolism-related genes were significantly reduced in P and T compared with Wt. Interestingly, the retinoid metabolites 9-cis-retinoic acid (9cRA) and all-trans-retinoic acid (atRA), the well known ligands for nuclear transcription factor RXR and retinoic acid receptor (RAR), were significantly decreased only in T compared with Wt and P, although the oxidized polar metabolite of atRA, 4-keto-all-trans-retinoic-acid (4-keto-RA) was significantly decreased in both P and T compared with Wt. To our surprise, the functions of RXRα were significantly blocked only in T compared with Wt and P. Namely, the total protein levels of RXRα were significantly reduced and the phosphorylation levels of RXRα were significantly increased only in T compared with Wt and P. Treatment of H-ras12V transgenic mice at 5-week-old or 5-month-old with atRA had no effect on the prevention of tumorigenesis or cure of developed nodules in liver. These events imply that the depletion of 9cRA and atRA and the inhibition of RXRα function in hepatic tumors involve more complex mechanisms besides the activation of RAS/ERK pathway.

Retinoid X receptor ligands: a patent review (2007 - 2013)

INTRODUCTION

Retinoid X receptors (RXRs) are nuclear receptors that act as ligand-dependent transcription factors. RXRs function as homodimers or as heterodimers with other nuclear receptors, such as retinoic acid receptors, PPARs, liver X receptors, farnesoid X receptor, vitamin D receptor or thyroid hormone receptors. RXR ligands (agonists or antagonists) show various physiological effects, depending on their partner receptors. RXR agonist bexarotene (Targretin®) is used for the treatment of cutaneous T-cell lymphoma in clinical practice. RXR agonists were also reported to be useful for treatment of type 2 diabetes, autoimmune disease and Alzheimer's disease. RXR antagonists were also reported to be effective in type 2 diabetes treatment.

AREAS COVERED

Here patent applications (2007 - 2013) concerning RXR ligands are summarized, and the usefulness of RXR ligands as pharmaceutical agents is discussed.

EXPERT OPINION

RXR agonists show a wide variety of biological effects. However, they cause serious side effects, such as blood triglyceride elevation, hypothyroidism and others. Thus, for clinical application of RXR agonists, abrogation of these side effects is required. RXR heterodimer-selective agonists and RXR partial agonists exhibiting desired effects without side effects are expected to find clinical application.

The effect of acyclic retinoid on the metabolomic profiles of hepatocytes and hepatocellular carcinoma cells

Background/Purpose

Acyclic retinoid (ACR) is a promising chemopreventive agent for hepatocellular carcinoma (HCC) that selectively inhibits the growth of HCC cells (JHH7) but not normal hepatic cells (Hc). To better understand the molecular basis of the selective anti-cancer effect of ACR, we performed nuclear magnetic resonance (NMR)-based and capillary electrophoresis time-of-flight mass spectrometry (CE-TOFMS)-based metabolome analyses in JHH7 and Hc cells after treatment with ACR.

Methodology/Principal Findings

NMR-based metabolomics revealed a distinct metabolomic profile of JHH7 cells at 18 h after ACR treatment but not at 4 h after ACR treatment. CE-TOFMS analysis identified 88 principal metabolites in JHH7 and Hc cells after 24 h of treatment with ethanol (EtOH) or ACR. The abundance of 71 of these metabolites was significantly different between EtOH-treated control JHH7 and Hc cells, and 49 of these metabolites were significantly down-regulated in the ACR-treated JHH7 cells compared to the EtOH-treated JHH7 cells. Of particular interest, the increase in adenosine-5′-triphosphate (ATP), the main cellular energy source, that was observed in the EtOH-treated control JHH7 cells was almost completely suppressed in the ACR-treated JHH7 cells; treatment with ACR restored ATP to the basal levels observed in both EtOH-control and ACR-treated Hc cells (0.72-fold compared to the EtOH control-treated JHH7 cells). Moreover, real-time PCR analyses revealed that ACR significantly increased the expression of pyruvate dehydrogenase kinases 4 (PDK4), a key regulator of ATP production, in JHH7 cells but not in Hc cells (3.06-fold and 1.20-fold compared to the EtOH control, respectively).

Conclusions/Significance

The results of the present study suggest that ACR may suppress the enhanced energy metabolism of JHH7 cells but not Hc cells; this occurs at least in part via the cancer-selective enhancement of PDK4 expression. The cancer-selective metabolic pathways identified in this study will be important targets of the anti-cancer activity of ACR.

Pharmaceutical and nutraceutical approaches for preventing liver carcinogenesis: chemoprevention of hepatocellular carcinoma using acyclic retinoid and branched-chain amino acids

The poor prognosis for patients with hepatocellular carcinoma (HCC) is associated with its high rate of recurrence in the cirrhotic liver. Therefore, more effective strategies need to be urgently developed for the chemoprevention of this malignancy. The malfunction of retinoid X receptor α, a retinoid receptor, due to phosphorylation by Ras/mitogen-activated protein kinase is closely associated with liver carcinogenesis and may be a promising target for HCC chemoprevention. Acyclic retinoid (ACR), a synthetic retinoid, can prevent HCC development by inhibiting retinoid X receptor α phosphorylation and improve the prognosis for this malignancy. Supplementation with branched-chain amino acids (BCAA), which are used to improve protein malnutrition in patients with liver cirrhosis, can also reduce the risk of HCC in obese cirrhotic patients. In experimental studies, both ACR and BCAA exert suppressive effects on HCC development and the growth of HCC cells. In particular, combined treatment with ACR and BCAA cooperatively inhibits the growth of HCC cells. Furthermore, ACR and BCAA inhibit liver tumorigenesis associated with obesity and diabetes, both of which are critical risk factors for HCC development. These findings suggest that pharmaceutical and nutraceutical approaches using ACR and BCAA may be promising strategies for preventing HCC and improving the prognosis of this malignancy.

Synergistic growth inhibition by acyclic retinoid and phosphatidylinositol 3-kinase inhibitor in human hepatoma cells

BACKGROUND

A malfunction of RXRα due to phosphorylation is associated with liver carcinogenesis, and acyclic retinoid (ACR), which targets RXRα, can prevent the development of hepatocellular carcinoma (HCC). Activation of PI3K/Akt signaling plays a critical role in the proliferation and survival of HCC cells. The present study examined the possible combined effects of ACR and LY294002, a PI3K inhibitor, on the growth of human HCC cells.

METHODS

This study examined the effects of the combination of ACR plus LY294002 on the growth of HLF human HCC cells.

RESULTS

ACR and LY294002 preferentially inhibited the growth of HLF cells in comparison with Hc normal hepatocytes. The combination of 1 μM ACR and 5 μM LY294002, in which the concentrations used are less than the IC₅₀ values of these agents, synergistically inhibited the growth of HLF, Hep3B, and Huh7 human HCC cells. These agents when administered in combination acted cooperatively to induce apoptosis in HLF cells. The phosphorylation of RXRα, Akt, and ERK proteins in HLF cells were markedly inhibited by treatment with ACR plus LY294002. Moreover, this combination also increased RXRE promoter activity and the cellular levels of RARβ and p21(CIP1), while decreasing the levels of cyclin D1.

CONCLUSION

ACR and LY294002 cooperatively increase the expression of RARβ, while inhibiting the phosphorylation of RXRα, and that these effects are associated with the induction of apoptosis and the inhibition of cell growth in human HCC cells. This combination might therefore be effective for the chemoprevention and chemotherapy of HCC.

Retinoids: novel immunomodulators and tumour-suppressive agents?

Retinoids play important roles in the transcriptional activity of normal, degenerative and tumour cells. Retinoid analogues may be promising therapeutic agents for the treatment of immune disorders as different as type I diabetes and systemic lupus erythematosus. In addition, the use of retinoids in cancer treatment has progressed significantly in the last two decades; thus, numerous retinoid compounds have been synthesized and tested. In this paper, the actual or potential use of retinoids as immunomodulators or tumour-suppressive agents is discussed.

Combination of acyclic retinoid with branched-chain amino acids inhibits xenograft growth of human hepatoma cells in nude mice

AIM

  Combination chemoprevention is a promising strategy to improve the prognosis of hepatocellular carcinoma (HCC). A malfunction of retinoid X receptor-α (RXR-α) due to phosphorylation by Ras/mitogen-activated protein kinase is closely associated with liver carcinogenesis and acyclic retinoid (ACR) can prevent HCC development by inhibiting RXR-α phosphorylation. The present study examined the possible combined effects of ACR plus branched-chain amino acids (BCAA), which can also prevent the development of HCC in obese patients with liver cirrhosis, in human HCC xenografts in nude mice.

METHODS

  This study examined the effects of the combination of ACR plus BCAA on the growth of Huh7 human HCC xenografts in nude mice. The effects of the combination on the phosphorylation of RXR-α, extracellular signal-regulated kinase (ERK), Akt and insulin-like growth factor-1 receptor (IGF-1R) proteins, and on the expression levels of retinoic acid receptor-β (RAR-β) and p21(CIP1) mRNA, were also examined by western blot and real-time reverse transcription polymerase chain reaction analyses, respectively.

RESULTS

  The combined treatment with ACR plus BCAA significantly inhibited the growth of Huh7 xenografts. The combination of these agents caused a marked inhibition of the phosphorylation of RXR-α, ERK, Akt and IGF-1R proteins in the xenografts. In addition, the expression levels of RAR-β and p21(CIP1) mRNA significantly increased by these agents.

CONCLUSION

  The combination of ACR and BCAA restores the function of RXR-α by inhibiting its phosphorylation and increasing the level of RAR-β, a heterodimeric partner for RXR-α, and thus suppresses the growth of HCC xenografts. Therefore, this combination might be an effective regimen for the treatment and, probably, chemoprevention of HCC.

Obesity and hepatocellular carcinoma: targeting obesity-related inflammation for chemoprevention of liver carcinogenesis

Obesity and related metabolic abnormalities, including a state of chronic inflammation, increase the risk of hepatocellular carcinoma (HCC). Adipose tissue constitutively expresses the proinflammatory cytokine tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), which are important tumor promoters in inflammation-related carcinogenesis. Dysregulation of TNF-α and IL-6 is associated with the development of steatosis and inflammation within the liver. These cytokines also lie at the core of the association between obesity and insulin resistance, which is a key factor in the development of obesity-related HCC. Here we present a detailed review of the relationship between metabolic abnormalities and the development of HCC, focusing on the role played by inflammation. Drawing from our basic and clinical research, the present report also reviews evidence that targeting metabolic abnormalities, such as attenuation of chronic inflammation and improvement of insulin resistance by either pharmaceutical or nutritional intervention, may be an effective strategy in preventing the development of HCC in obese individuals.

Acyclic retinoid in chemoprevention of hepatocellular carcinoma: Targeting phosphorylated retinoid X receptor-α for prevention of liver carcinogenesis

One of the key features of hepatocellular carcinoma (HCC) is the high rate of intrahepatic recurrence that correlates with poor prognosis. Therefore, in order to improve the clinical outcome for patients with HCC, development of a chemopreventive agent that can decrease or delay the incidence of recurrence is a critical issue for urgent investigation. Acyclic retinoid (ACR), a synthetic retinoid, successfully improves HCC patient survival by preventing recurrence and the formation of secondary tumors. A malfunction of the retinoid X receptor-α (RXRα) due to phosphorylation by the Ras-MAPK signaling pathway plays a critical role in liver carcinogenesis, and ACR exerts chemopreventive effects on HCC development by inhibiting RXRα phosphorylation. Here, we review the relationship between retinoid signaling abnormalities and liver disease, the mechanisms of how RXRα phosphorylation contributes to liver carcinogenesis, and the detailed effects of ACR on preventing HCC development, especially based on the results of our basic and clinical research. We also outline the concept of “clonal deletion and inhibition” therapy, which is defined as the removal and inhibition of latent malignant clones from the liver before they expand into clinically detectable HCC, because ACR prevents the development of HCC by implementing this concept. Looking toward the future, we discuss “combination chemoprevention” using ACR as a key drug since it can generate a synergistic effect, and may thus be an effective new strategy for the prevention of HCC.

Synergistic growth inhibition of human hepatocellular carcinoma cells by acyclic retinoid and GW4064, a farnesoid X receptor ligand

Abnormalities in the expression and function of retinoid X receptor (RXR), a master regulator of the nuclear receptor superfamily, are associated with the development of hepatocellular carcinoma (HCC). Dysfunction of farnesoid X receptor (FXR), one of the nuclear receptors that forms a heterodimer with RXR, also plays a role in liver carcinogenesis. In the present study, we examined the effects of acyclic retinoid (ACR), a synthetic retinoid targeting RXRα, plus GW4064, a ligand for FXR, on the growth of human HCC cells. We found that ACR and GW4064 preferentially inhibited the growth of HLE, HLF, and Huh7 human HCC cells in comparison with Hc normal hepatocytes. The combination of 1μM ACR plus 1μM GW4064 synergistically inhibited the growth of HLE cells by inducing apoptosis. The combined treatment with these agents acted cooperatively to induce cell cycle arrest in the G(0)/G(1) phase and inhibit the phosphorylation of RXRα, which is regarded as a critical factor for liver carcinogenesis, through inhibition of ERK and Stat3 phosphorylation. This combination also increased the expression levels of p21(CIP1) and SHP mRNA, while decreasing the levels of c-myc and cyclin D1 mRNA in HLE cells. In addition, a reporter assay indicated that the FXRE promoter activity was significantly increased by treatment with ACR plus GW4064. Our results suggest that ACR and GW4064 cooperatively inhibit RXRα phosphorylation, modulate the expression of FXR-regulated genes, thus resulting in the induction of apoptosis and the inhibition of growth in HCC cells. This combination might therefore be effective for the chemoprevention and chemotherapy of HCC.

A therapeutic method for the direct reprogramming of human liver cancer cells with only chemicals

Various methods for the direct reprogramming of human somatic cells have been developed. However, a therapeutic method to reprogram and eliminate human solid tumor cells has not been developed. Here we show a novel therapeutic method to reprogram and eliminate human solid tumor cells with chemicals. This therapeutic method may be applicable to various human solid tumor cells that express aldo-keto reductase family 1 member B10 (AKR1B10) and retinoid X receptors (RXRs).

Hepatic metabolism of retinoids and disease associations

The liver is the most important tissue site in the body for uptake of postprandial retinoid, as well as for retinoid storage. Within the liver, both hepatocytes and hepatic stellate cells (HSCs) are importantly involved in retinoid metabolism. Hepatocytes play an indispensable role in uptake and processing of dietary retinoid into the liver, and in synthesis and secretion of retinol-binding protein (RBP), which is required for mobilizing hepatic retinoid stores. HSCs are the central cellular site for retinoid storage in the healthy animal, accounting for as much as 50-60% of the total retinoid present in the entire body. The liver is also an important target organ for retinoid actions. Retinoic acid is synthesized in the liver and can interact with retinoid receptors which control expression of a large number of genes involved in hepatic processes. Altered retinoid metabolism and the accompanying dysregulation of retinoid signaling in the liver contribute to hepatic disease. This is related to HSCs, which contribute significantly to the development of hepatic disease when they undergo a process of cellular activation. HSC activation results in the loss of HSC retinoid stores and changes in extracellular matrix deposition leading to the onset of liver fibrosis. An association between hepatic disease progression and decreased hepatic retinoid storage has been demonstrated. In this review article, we summarize the essential role of the liver in retinoid metabolism and consider briefly associations between hepatic retinoid metabolism and disease. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Dual induction of caspase 3- and transglutaminase-dependent apoptosis by acyclic retinoid in hepatocellular carcinoma cells

Background

Hepatocellular carcinoma has a high mortality rate due to its rate of recurrence. Acyclic retinoid prevents recurrence of hepatocellular carcinoma in patients after surgical removal of their primary tumors by inducing apoptosis in hepatocellular carcinoma cells, although the molecular mechanisms of action are not understood.

Methods

Human hepatocellular carcinoma cells in culture, as well as nude mice transplanted with hepatocellular carcinoma cells and rats given with N-diethylnitrosamine were treated with acyclic retinoid. Changes in activated caspase 3 and transglutaminase 2 (TG2) levels, Sp1 cross-linking and its activities, expression of epidermal growth factor receptor, and apoptotic levels were measured.

Results

Acyclic retinoid simultaneously stimulated the activation of caspase 3, and the expression, nuclear localization and crosslinking activity of TG2, resulting in crosslinking and inactivation of the transcription factor, Sp1, thereby reducing expression of epidermal growth factor receptor and cell death in three hepatocellular carcinoma cell lines. These effects were partially restored by a caspase inhibitor, transfection of antisense TG2, restoration of functional Sp1, or an excess of epidermal growth factor. Nuclear expression of TG2 and crosslinked Sp1, as also activated caspase 3 were found in both hepatocellular carcinoma cells transplanted into nude mice and cancerous regions within the liver in N-diethylnitrosamine-induced hepatocarcinogenesis model in rats, following treatment of animals with acyclic retinoid.

Conclusions

Treatment with acyclic retinoid produces a dual activation of caspase 3 and TG2 induced apoptosis of hepatocellular carcinoma cells via modification and inactivation of Sp1, resulting in reduced expression of epidermal growth factor receptor.

Acyclic retinoid inhibits diethylnitrosamine-induced liver tumorigenesis in obese and diabetic C57BLKS/J- +(db)/+Lepr(db) mice

Obesity and the related metabolic abnormalities are associated with increased risk of hepatocellular carcinoma (HCC). Malfunctioning of retinoid X receptor (RXR) α due to phosphorylation by Ras/MAPK also plays a critical role in liver carcinogenesis. In the present study, we examined the effects of acyclic retinoid (ACR), which targets RXRα, on the development of diethylnitrosamine (DEN)-induced liver tumorigenesis in C57BLKS/J- +Lepr(db)/+Lepr(db) (db/db) obese mice. Male db/db mice were given tap water containing 40 ppm DEN for 2 weeks, after which they were fed a diet containing 0.03% or 0.06% of ACR throughout the experiment. In mice treated with either dose of ACR for 34 weeks, the development of liver cell adenomas was significantly inhibited as compared with basal diet-fed mice. ACR markedly inhibited the activation of Ras and phosphorylation of the ERK (extracellular signal-regulated kinase) and RXRα proteins in the livers of experimental mice. It also increased the expression of RAR β and p21(CIP1) mRNA while decreasing the expression of cyclin D1, c-Fos, and c-Jun mRNA in the liver, thereby restoring RXRα function. Administration of ACR improved liver steatosis and activated the AMPK protein. The serum levels of insulin decreased by ACR treatment, whereas the quantitative insulin sensitivity check index (QUICKI) values increased, indicating improved insulin sensitivity. The serum levels of TNF-α and the expression levels of TNF- α, IL-6, and IL-1 β mRNA in the livers of DEN-treated db/db mice were decreased by ACR treatment, suggesting attenuation of the chronic inflammation induced by excessive fatty deposits. ACR may be, therefore, useful in the chemoprevention of obesity-related HCC.

Synergistic growth inhibition in HL-60 cells by the combination of acyclic retinoid and vitamin K2

PURPOSE

The aim of this study was to assess the effects of acyclic retinoid (ACR) and vitamin K(2) (VK(2)) in HL-60 cells.

METHODS

We used HL-60 cells, and the Trypan Blue dye exclusion method was used for cell proliferation assays. For detection of apoptosis, the Annexin V-binding capacity of treated cells was examined by flow cytometry. To evaluate the cell cycle, we used a FITC BrdU Flow KIT and flow cytometry. Total extracted and equivalent amounts of protein were examined by Western blotting using specific antibodies.

RESULTS

ACR and VK(2) dose dependently inhibited the proliferation of HL-60 cells. These two agents in combination synergistically inhibited cell growth and induced apoptosis. VK(2) inhibited activation of the Ras/MAPK signaling pathway, and ACR plus VK(2) cooperatively inhibited phosphorylation of RXRα and the growth of HL-60 cells. Moreover, ACR and VK(2) induced increases in G0/G1 phase HL-60 cells, alone and synergistically in combination.

CONCLUSION

The synergistic effects of ACR and VK(2) on HL-60 cells may provide a novel strategy for treating leukemia.

Chemopreventive anti-cancer agent acyclic retinoid suppresses allogeneic immune responses in rats

Acyclic retinoid NIK-333 (ACR) is a chemopreventive agent that acts by suppressing the recurrence of hepatocellular carcinoma (HCC) following initial treatment and is now being employed in clinical trials. The chemopreventive effects of ACR have been analyzed from various aspects, and it is well known that some retinoic acid (RA) derivatives affect host immunity. The objective of this study is to investigate the effects of ACR on host immunity. The results demonstrated that ACR prolonged heart and liver graft and recipient survival in rat allogeneic organ transplantation. The immunosuppressive effect of ACR administered at 100mg/kg/day was almost equivalent to that of CsA administered at 1mg/kg/day in vivo. In the mixed lymphocyte reaction (MLR), ACR suppressed lymphocyte proliferation non-specifically. Gene expression analysis of splenic lymphocytes from ACR-treated recipient rats revealed no distinct change in Interleukin (IL)-2 and increases in Interferon (IFN)-gamma. In conclusion, ACR possesses immunosuppressive potential in vivo and is a promising chemopreventive drug for long term use against HCC.

Retinoic acid and its binding protein modulate apoptotic signals in hypoxic hepatocellular carcinoma cells

Hypoxia induces survival signals in hepatocellular carcinoma (HCC). We attempted to find a hypoxia-induced signal that could be used for modulating HCC cell death. Cellular retinoic acid binding protein-II (CRABP-II) expression was significantly increased in hypoxic HCC cells. Treatment with retinoic acid (RA), a ligand for CRABP-II, induced HCC cell apoptosis more effectively in hypoxia than in normoxia, whereas hypoxia-induced CRABP-II expression attenuated RA-induced apoptosis. Inhibition of CRABP-II enhanced RA-induced apoptosis and sensitized RA-resistant HCC cells to RA cytotoxicity by attenuating p42/44 MAPK and Akt activation. Therefore, RA/CRABP-II signal modulation is therapeutically implicated in infiltrative HCCs exposed to hypoxia.

Acyclic retinoid inhibits angiogenesis by suppressing the MAPK pathway

Acyclic retinoid (ACR) is currently under clinical trial as an agent to suppress the recurrence of hepatocellular carcinoma (HCC) through its ability to induce apoptosis in premature HCC cells. ACR has an anticancer effect in vivo as well, although it shows weak apoptosis-inducing activity against mature HCC cells, suggesting the existence of an additional action mechanism. In this study, we investigated the antiangiogenic activity of ACR. ACR inhibited angiogenesis within chicken chorioallantoic membrane (CAM) in as similar a manner as all-trans retinoic acid (atRA). Although suppression of angiogenesis by atRA was partially rescued by the simultaneous addition of angiopoietin-1, suppression of angiogenesis by ACR was not rescued under the same condition at all. Conversely, although suppression of angiogenesis by ACR was partially inverted by the simultaneous addition of vascular endothelial growth factor (VEGF), suppression of angiogenesis by atRA was not affected under the same condition. These results suggested that mechanisms underlying the suppression of angiogenesis by ACR and atRA were different. ACR selectively inhibited the phosphorylation of VEGF receptor 2 (VEGFR2) and of extracellular signal-regulated kinase (ERK) without changing their protein expression levels, and inhibited endothelial cell growth, migration, and tube formation. The inhibition of the phosphorylation of ERK, endothelial growth, migration, tube formation, and angiogenesis by ACR was rescued by the overexpression of constitutively active mitogen-activated protein kinase (MAPK). Finally, ACR, but not atRA, inhibited HCC-induced angiogenesis in a xenografted CAM model. These results delineate the novel activity of ACR as an antiangiogenic through a strong inhibition of the VEGFR2 MAPK pathway.

Acyclic retinoid synergises with valproic acid to inhibit growth in human hepatocellular carcinoma cells

A malfunction of retinoid X receptor-alpha (RXRalpha) due to phosphorylation is associated with the development of hepatocellular carcinoma (HCC) and acyclic retinoid (ACR), which targets RXRalpha, can prevent the development of second primary HCC. Valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, induces apoptosis and cell cycle arrest in cancer cells. VPA can also enhance the sensitivity of cancer cells to retinoids. The present study examined the possible combined effects of ACR plus VPA in HepG2 human HCC cell line. The combination of 5muM ACR and 1mM VPA, about the IC(25) value for both compounds, synergistically inhibited the growth of HepG2 cells without affecting the growth of Hc normal human hepatocytes. The combined treatment with ACR plus VPA also acted synergistically to induce apoptosis and G(0)-G(1) cell cycle arrest in HepG2 cells. This combination further exerted a synergistic inhibition of the phosphorylation of RXRalpha, ERK, Akt and GSK-3beta proteins and caused an accumulation of acetylated histones H3 and H4 proteins. VPA enhanced the ability of ACR to raise the cellular levels of RARbeta and p21(CIP1). The combination of these agents markedly increased both the RARE and RXRE promoter activities in HepG2 cells. These results suggest that ACR and VPA cooperatively increase the expression of RARbeta and p21(CIP1), while inhibiting the phosphorylation of RXRalpha, and these effects were associated with induction of apoptosis and the inhibition of cell growth in HepG2 cells. This combination might therefore be an effective regimen for the chemoprevention and chemotherapy of HCC.

Strategy and mechanism for the prevention of hepatocellular carcinoma: phosphorylated retinoid X receptor alpha is a critical target for hepatocellular carcinoma chemoprevention

Hepatocellular carcinoma (HCC) is a major health care problem worldwide. The prognosis of patients with HCC is poor because even in the early stages when surgical treatment might be expected to be curative, the incidence of recurrence in patients with underlying cirrhosis is very high due to multicentric carcinogenesis. Therefore, strategies to prevent recurrence and second primary HCC are required to improve the prognosis. One of the most practical approaches to prevent the multicentric development of HCC is 'clonal deletion' therapy, which is defined as the removal of latent (i.e. invisible) (pre)malignant clones from the liver in a hypercarcinogenic state. Retinoids, a group of structural and functional analogs of vitamin A, exert their biological function primarily through two distinct nuclear receptors, retinoic acid receptors and retinoid X receptors (RXR), and abnormalities in the expression and function of these receptors are highly associated with the development of various cancers, including HCC. In particular, a malfunction of RXRalpha due to phosphorylation by the Ras-mitogen-activated protein kinase signaling pathway is profoundly associated with the development of HCC and thus may be a critical target for HCC chemoprevention. Acyclic retinoid, which has been clinically shown to reduce the incidence of a post-therapeutic recurrence of HCC, can inhibit Ras activity and phosphorylation of the extracellular signal-regulated kinase and RXRalpha proteins. In conclusion, the inhibition of RXRalpha phosphorylation and the restoration of its physiological function as a master regulator for nuclear receptors may be a potentially effective strategy for HCC chemoprevention and clonal deletion. Acyclic retinoid, which targets phosphorylated RXRalpha, may thus play a critical role in preventing the development of multicentric HCC.

A phase I/II trial of TAC-101, an oral synthetic retinoid, in patients with advanced hepatocellular carcinoma

PURPOSE

Preclinical models showed TAC-101 (4-[3,5-bis(trimethylsilyl) benzamide] benzoic acid), an oral synthetic retinoid, has anti-tumor activity in hepatocellular carcinoma (HCC). A phase I/II study was performed in advanced HCC patients (pts).

PATIENTS AND METHODS

Thirty-three patients were enrolled. During Phase I, pts received 40 mg daily for 14 days q3 weeks; 2 of 5 patients developed DLT so dose was reduced to 20 mg/day. Twenty-eight patients received 20 mg/day.

RESULTS

No pt had a CR or PR, but 12 of 21 (57%) had SD. Two pts (9.5%) had late PR after discontinuing TAC-101. Median survival (MS) for all 28 pts treated with 20 mg/day was 12.7 months (95% CI 8.8-22.7); MS for 21 evaluable pts was 19.2 months (95% CI 10.4-27.6).

CONCLUSIONS

20 mg of TAC- was well tolerated. Significant disease stabilization (12/21 pts, 57%), 2 late PRs, and prolonged MS (19.2 months) suggest that TAC-101 provides meaningful patient benefit.

Inhibition of Balb/c 3T3 cell transformation by synthetic acyclic retinoid NIK-333; possible involvement of enhanced gap junctional intercellular communication

BACKGROUND

In an attempt to develop effective and non-cytotoxic cancer chemopreventive derivatives of retinoids, a novel acyclic retinoid has previously been synthesized. This acyclic retinoid, NIK-333, had been reported to suppress recurrence of primary hepatocellular carcinomas. We explored the molecular mechanisms by which NIK-333 exerts anti-proliferative effects.

METHODS

We employed Balb/c 3T3 cells, since they can be used as a quantitative transformation assay and since we can study a possible involvement of gap-junctional intercellular communication (GJIC) in their growth control. We included all-trans-retinoic acid (ATRA) for comparison.

RESULTS

We first confirmed that these cells express the retinoid receptors, RARalpha, gamma and RXRalpha, suggesting that they respond to NIK-333 and ATRA. When NIK-333 or ATRA was added to Balb/c 3T3 cells during the induction of cell transformation by a standard (3-methylcholanthrene (MCA) alone) or two-stage (low dose of MCA plus 12-O-tetradecanoylphorbol-13-acetate (TPA)) protocol, there was a significant decrease in the number of transformed foci. The extent of inhibition of transformation by NIK-333 was similar to that exerted by ATRA. Employing the microinjection dye-transfer assay, we found that both retinoids increase GJIC level when measured 24h after treatment. The extent of GJIC increase by NIK-333 was similar to that of ATRA. These retinoids also increased the amount of connexin 43 (Cx43) on the plasma membrane as revealed by immunostaining.

CONCLUSION

These data indicate that NIK-333 suppresses chemical carcinogenesis in vitro and support the hypothesis that enhancement of GJIC is involved in this process.

Synergistic Effects of PPARgamma Ligands and Retinoids in Cancer Treatment

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily. The activation of PPARs by their specific ligands is regarded as one of the promising strategies to inhibit cancer cell growth. However, recent clinical trials targeting several common cancers showed no beneficial effect when PPAR ligands are used as a monotherapy. Retinoid X receptors (RXRs), which play a critical role in normal cell proliferation as a master regulator for nuclear receptors, preferentially form heterodimers with PPARs. A malfunction of RXRalpha due to phosphorylation by the Ras/MAPK signaling pathway is associated with the development of certain types of human malignancies. The activation of PPARgamma/RXR heterodimer by their respective ligands synergistically inhibits cell growth, while inducing apoptosis in human colon cancer cells when the phosphorylation of RXRalpha was inhibited. We herein review the synergistic antitumor effects produced by the combination of the PPAR, especially PPARgamma, ligands plus other agents, especially retinoids, in a variety of human cancers. We also focus on the phosphorylation of RXRalpha because the inhibition of RXRalpha phosphorylation and the restoration of its physiological function may activate PPAR/RXR heterodimer and, therefore, be a potentially effective and critical strategy for the inhibition of cancer cell growth.

Retinoid X receptor alpha is highly phosphorylated in retinoic acid-resistant HL-60R cells and the combination of 9-cis retinoic acid plus MEK inhibitor induces apoptosis in the cells

We examined the effects of 9-cis retinoic acid (RA) on the expression levels of retinoid X receptor alpha (RXR alpha) and its phosphorylated form (p-RXR alpha) in HL-60 and HL-60R cells. 9-cis RA reduced both RXR alpha and p-RXR alpha in HL-60 cells, but did neither in HL-60R cells. However, when the HL-60R cells were treated with the combination of 9-cis RA plus PD98059, MEK inhibitor, the p-RXR alpha and RXR alpha proteins all markedly decreased. Moreover, the combination of those agents induced apoptosis in HL-60R cells. Phosphorylation of RXR alpha might be associated with RA-resistance in HL-60R cells.

Phosphorylated retinoid X receptor alpha loses its heterodimeric activity with retinoic acid receptor beta

A malfunction in retinoid X receptor (RXR) alpha due to phosphorylation is associated with the development of hepatocellular carcinoma. However, the precise mechanisms by which phosphorylated RXRalpha loses its physiological function remain unclear. In the present study we examined whether phosphorylation of RXRalpha affects its dimeric activity. Fluorescence resonance energy transfer studies and immunoprecipitation assays showed that the physical interaction between RXRalpha and retinoic acid receptor beta was impaired when 293T cells were transfected with phosphomimic mutant RXRalpha (T82D/S260D), whereas this interaction was activated at a level similar to wild-type RXRalpha-transfected cells when the cells were transfected with an unphosphorylated mutant RXRalpha (T82A/S260A). Treating the T82A/S260A-transfected cells with retinoid resulted in a significant increase in the transcriptional activities of the retinoic acid receptor responsive element and RXR responsive element promoters, whereas these transcriptional activities did not increase in the T82D/S260D-transfected cells. Transfection with T82A/S260A enhanced both the inhibition of cell growth and the induction of apoptosis caused by retinoid, although the T82D/S260D-transfected cells lost their responsiveness to retinoid. Moreover, transfection with T82A/S260A caused an inhibition of cell growth and a reduction of colony-forming ability in soft agar in HuH7 human hepatocellular carcinoma cells. These findings suggest that phosphorylation of RXRalpha abolishes its ability to form homodimers and heterodimers with RXR and retinoic acid receptor beta, thus resulting in the loss of cell growth control and the acceleration of cancer development. In conclusion, the inhibition of RXRalpha phosphorylation and the restoration of its original function as a master regulator of nuclear receptors might therefore be an effective strategy for controlling cancer cell growth.

Chemoprevention of liver carcinogenesis with retinoids: Basic and clinical aspects

The strategy to prevent liver carcinogenesis consists of: (i) antiviral modalities such as vaccination, lamivudin, and interferon; (ii) anti-inflammatory modality; and (iii) chemoprevention using such compounds as retinoid analog and vitamin K. Cancer chemoprevention is defined as an approach where natural or synthetic chemical compound works to arrest or reverse premalignant cells by using physiological pathways. As a consequence, such a clone of premalignant cells is eradicated (clonal deletion) by differentiation induction or apoptosis, and thus the process toward the development of clinically detectable cancer is disrupted. A particularly effective candidate target of chemoprevention in liver diseases is an advanced stage of chronic hepatitis, that is supposed to contain transformed hepatocyte clone(s); that is, primary prevention from liver cirrhosis and prevention of recurrent and second primary hepatocellular carcinoma following the treatment of the initial cancer. Retinoid is a collective term of vitamin A analog that binds to nuclear retinoid receptors;retinoic acid receptors (RAR) and retinoid X receptors (RXR). After ligand coupling, these receptors form homo- or heterodimers, bind to the response element (RARE or RXRE) upstream of the target gene, and regulate the gene expression as a transcriptional factor. Biological phenotypes of such transcriptional regulation by retinoid include cellular differentiation, tissue morphogenesis, and programmed cell death (apoptosis). Due to these functions, retinoid analogs are clinically tried to prevent/treat carcinoma in a wide variety of organs including head and neck cancer, uterine cervical cancer, certain leukemia and liver cancer. In this article, clinical trials of retinoid analog to inhibit second primary hepatoma, supposed molecular mechanism of the action of the compound, and aberrant metabolism of RXR and its role in liver carcinogenesis are briefly reviewed.

Synergistic growth inhibition by acyclic retinoid and vitamin K2 in human hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is one of the most prevalent cancers worldwide. However, effective chemopreventive and chemotherapeutic agents for this cancer have not yet been developed. In clinical trials acyclic retinoid (ACR) and vitamin K(2) (VK(2)) decreased the recurrence rate of HCC. In the present study we examined the possible combined effects of ACR or another retinoid 9-cis retinoic acid (9cRA) plus VK(2) in the HuH7 human HCC cell line. We found that the combination of 1.0 microM ACR or 1.0 microM 9cRA plus 10 microM VK(2) synergistically inhibited the growth of HuH7 cells without affecting the growth of Hc normal human hepatocytes. The combined treatment with ACR plus VK(2) also acted synergistically to induce apoptosis in HuH7 cells. Treatment with VK(2) alone inhibited phosphorylation of the retinoid X receptor (RXR)alpha protein, which is regarded as a critical factor for liver carcinogenesis, through inhibition of Ras activation and extracellular signal-regulated kinase phosphorylation. Moreover, the inhibition of RXRalpha phosphorylation by VK(2) was enhanced when the cells were cotreated with ACR. The combination of retinoids plus VK(2) markedly increased both the retinoic acid receptor responsive element and retinoid X receptor responsive element promoter activities in HuH7 cells. Our results suggest that retinoids (especially ACR) and VK(2) cooperatively inhibit activation of the Ras/MAPK signaling pathway, subsequently inhibiting the phosphorylation of RXRalpha and the growth of HCC cells. This combination might therefore be effective for the chemoprevention and chemotherapy of HCC.

Chemoprevention of spontaneous development of hepatocellular carcinomas in fatty liver Shionogi mice by a cyclooxygenase-2 inhibitor

Cyclooxygenase 2 (COX-2) and retinoid X receptor alpha (RXRalpha) are suggested to have roles in carcinogenesis. COX-2 inhibitors have been reported to suppress growth of hepatocellular carcinoma (HCC) cell lines in vitro. However, little is known about the preventive effect of these drugs on spontaneous hepatocarcinogenesis in vivo. Etodolac exists in a racemic mixture containing S- and R-etodolac. S-etodolac is responsible for COX-2 inhibitory activity and R-etodolac is related to the downregulation of RXRalpha. Here, the effect of etodolac on spontaneous development of HCC in fatty liver Shionogi mice is evaluated. Etodolac was administered at a low (2 mg/kg) or high (10 mg/kg) dose three times a week for 16 months starting at the age of 3 months. The development of HCC was suppressed slightly in the high-dose group, and suppressed markedly in the low-dose group, although the development of fatty liver was not inhibited in either group. Plasma prostaglandin E2 levels were also decreased significantly in the low-dose group, consistent with the suppression of HCC. The expression of RXRalpha and proliferating cell nuclear antigen in non-tumorous liver tissues was decreased significantly in both the low-dose and high-dose groups. These findings show that etodolac treatment at an optimum dose suppresses hepatocarcinogenesis in vivo, and may be useful for preventing the development of HCC in humans.

Met-regulated expression signature defines a subset of human hepatocellular carcinomas with poor prognosis and aggressive phenotype

Identification of specific gene expression signatures characteristic of oncogenic pathways is an important step toward molecular classification of human malignancies. Aberrant activation of the Met signaling pathway is frequently associated with tumor progression and metastasis. In this study, we defined the Met-dependent gene expression signature using global gene expression profiling of WT and Met-deficient primary mouse hepatocytes. Newly identified transcriptional targets of the Met pathway included genes involved in the regulation of oxidative stress responses as well as cell motility, cytoskeletal organization, and angiogenesis. To assess the importance of a Met-regulated gene expression signature, a comparative functional genomic approach was applied to 242 human hepatocellular carcinomas (HCCs) and 7 metastatic liver lesions. Cluster analysis revealed that a subset of human HCCs and all liver metastases shared the Met-induced expression signature. Furthermore, the presence of the Met signature showed significant correlation with increased vascular invasion rate and microvessel density as well as with decreased mean survival time of HCC patients. We conclude that the genetically defined gene expression signatures in combination with comparative functional genomics constitute an attractive paradigm for defining both the function of oncogenic pathways and the clinically relevant subgroups of human cancers.

Prevention of rat hepatocarcinogenesis by acyclic retinoid is accompanied by reduction in emergence of both TGF-alpha-expressing oval-like cells and activated hepatic stellate cells

We investigated the preventive effects of a synthetic acyclic retinoid, NIK-333, on the early and late events of hepatocarcinogenesis in male F344 rats treated with 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB). NIK-333 was administered once a day on consecutive days at a dose of 10, 40, or 80 mg/kg body weight along with the supplementation with 3'-MeDAB-containing diet for 16 wk. Animals from each group were sacrificed at 4 and 16 wk after the commencement of the experiment to determine the effect of NIK-333 on the early and late stages of carcinogenesis, respectively. NIK-333 suppressed the emergence of both oval-like cells expressing transforming growth factor (TGF)-alpha, putative progenitors of hepatocellular carcinoma (HCC), and activated hepatic stellate cells, major matrix-producing cells of the liver, in the early stage and inhibited the incidence of HCC in the late phase. These results suggest that NIK-333 is a promising drug for the chemoprevention of HCC by uniquely suppressing the early events of hepatocarcinogenesis, that is, development of both oval-like cells and fibrogenesis.

Prevention of second primary tumors by an acyclic retinoid in patients with hepatocellular carcinoma. Updated analysis of the long-term follow-up data

Oral administration with acyclic retinoid, a synthetic vitamin A analog, for a limited period of 12 months (48 weeks) prevented the development of second primary hepatocellular carcinoma (HCC) and also improved the survival of patients who underwent curative treatments of the initial tumor. Following that randomized controlled study reported in 1996 and 1999, we have continued to follow up the patients by medical imaging and blood chemical analyses, and found that the preventive effect of acyclic retinoid lasted up to 199 weeks after randomization (or 151 weeks after completion of retinoid administration). The retinoid's effect was not mediated by reduction in hepatic necro-inflammation since no significant decrease in serum aminotransferase activity was seen in the retinoid group. Such observation seems quite distinct from the cancer-preventive mechanism of interferon, a potent immunopreventive agent for HCC. We have also shown here the reduction by the retinoid in serum levels of lectin-reactive alpha-fetoprotein (AFP-L3) and protein induced by vitamin K absence or antagonist-II (PIVKA-II), both of which indicate the presence of latent HCC cells. These results suggest that acyclic retinoid may delete such malignant clones before they expand to clinically detectable tumors and thereby inhibited second primary HCC. Once such latent clones are eradicated, it may well take at least several years for the next cancer clone to arise clinically. This may possibly explain a reason for the long-term effect of the retinoid even after the limited period of administration.