Novel retinoid-related molecules as apoptosis inducers and effective inhibitors of human lung cancer cells in vivo

Design and synthesis of adamantyl-substituted flavonoid derivatives as anti-inflammatory Nur77 modulators: Compound B7 targets Nur77 and improves LPS-induced inflammation in vitro and in vivo

In continuing our study on discovering new Nur77-targeting anti-inflammatory agents with natural skeletons, we combined adamantyl group and hydroxamic acid moiety with flavonoid nucleus, synthesized three series of flavonoid derivatives with a similar structure like CD437, and evaluated their activities against LPS-induced inflammation. Compound B7 was found to be an excellent Nur77 binder (K_d = 3.55 × 10^-7 M) and a potent inhibitor of inflammation, which significantly decreased the production of cytokines in vitro, such as NO, IL-6, IL-1β, and TNF-α, at concentrations of 1.25, 2.5, and 5 μM. Mechanistically, B7 modulated the colocalization of Nur77 at mitochondria and inhibited the lipopolysaccharides (LPS)-induced inflammation via the blockade of NF-κB activation in a Nur77-dependent manner. Additionally, B7 showed in vivo anti-inflammatory activity in the LPS-induced mice model of acute lung injury (ALI). These data suggest that the Nur77-targeting flavonoid derivatives can be particularly useful for further pharmaceutical development for the treatment of inflammatory diseases such as ALI.

Minireview: Familiar Faces in Unfamiliar Places: The Emerging Role of Nuclear Receptors in Lung Cancer

Nuclear hormone receptors (NRs) are a superfamily of 48 transcription factors that are frequently modulated by ligands and control various cancer-relevant cellular pathways, such as differentiation, proliferation, migration, and metabolism. These properties make them excellent therapeutic targets in cancers dependent upon their activity, and as such, 3 NRs, estrogen receptor-α, androgen receptor, and retinoic acid receptor-α (more specifically, the promyelocytic leukemia-retinoic acid receptor-α translocation), have been targeted clinically in breast cancer, prostate cancer, and acute promyelocytic leukemia, respectively. Recently, a number of studies have highlighted a putative role for NRs in nonsmall cell lung cancer (NSCLC), a highly lethal type of lung cancer with relatively few targeted agents. Here, we review the potential roles of selected NRs in NSCLC and offer insights on how NRs may be leveraged in NSCLC to improve patient outcomes.

PI3K/AKT and ERK regulate retinoic acid-induced neuroblastoma cellular differentiation

Neuroblastoma, the most common extra-cranial solid tumor in infants and children, is characterized by a high rate of spontaneous remissions in infancy. Retinoic acid (RA) has been known to induce neuroblastoma differentiation; however, the molecular mechanisms and signaling pathways that are responsible for RA-mediated neuroblastoma cell differentiation remain unclear. Here, we sought to determine the cell signaling processes involved in RA-induced cellular differentiation. Upon RA administration, human neuroblastoma cell lines, SK-N-SH and BE(2)-C, demonstrated neurite extensions, which is an indicator of neuronal cell differentiation. Moreover, cell cycle arrest occurred in G1/G0 phase. The protein levels of cyclin-dependent kinase inhibitors, p21 and p27(Kip), which inhibit cell proliferation by blocking cell cycle progression at G1/S phase, increased after RA treatment. Interestingly, RA promoted cell survival during the differentiation process, hence suggesting a potential mechanism for neuroblastoma resistance to RA therapy. Importantly, we found that the PI3K/AKT pathway is required for RA-induced neuroblastoma cell differentiation. Our results elucidated the molecular mechanism of RA-induced neuroblastoma cellular differentiation, which may be important for developing novel therapeutic strategy against poorly differentiated neuroblastoma.

Adamantyl Retinoid-Related Molecules Induce Apoptosis in Pancreatic Cancer Cells by Inhibiting IGF-1R and Wnt/β-Catenin Pathways. J Oncol. 2012;2012:796729. [PMID: 22570653 PMCID: PMC3335256 DOI: 10.1155/2012/796729]

Pancreatic carcinoma has a dismal prognosis as it often presents as locally advanced or metastatic. We have found that exposure to adamantyl-substituted retinoid-related (ARR) compounds 3-Cl-AHPC and AHP3 resulted in growth inhibition and apoptosis induction in PANC-1, Capan-2, and MiaPaCa-2 pancreatic cancer cell lines. In addition, AHP3 and 3-Cl-AHPC inhibited growth and induced apoptosis in spheres derived from the CD44⁺/CD24⁺ (CD133⁺/EpCAM⁺) stem-like cell population isolated from the pancreatic cancer cell lines. 3-Cl-AHPC-induced apoptosis was preceded by decreasing expression of IGF-1R, cyclin D1, β-catenin, and activated Notch-1 in the pancreatic cancer cell lines. Decreased IGF-1R expression inhibited PANC-1 proliferation, enhanced 3-Cl-AHPC-mediated apoptosis, and significantly decreased sphere formation. 3-Cl-AHPC inhibited the Wnt/β-catenin pathway as indicated by decreased β-catenin nuclear localization and inhibited Wnt/β-catenin activation of transcription factor TCF/LEF. Knockdown of β-catenin using sh-RNA also induced apoptosis and inhibited growth in pancreatic cancer cells. Thus, 3-Cl-AHPC and AHP3 induce apoptosis in pancreatic cancer cells and cancer stem-like cells and may serve as an important potential therapeutic agent in the treatment of pancreatic cancer.

Role of retinoic acid in the modulation of benzo(a)pyrene-DNA adducts in human hepatoma cells: implications for cancer prevention

Carcinogen-DNA adducts could lead to mutations in critical genes, eventually resulting in cancer. Many studies have shown that retinoic acid (RA) plays an important role in inducing cell apoptosis. Here we have tested the hypothesis that levels of carcinogen-DNA adducts can be diminished by DNA repair and/or by eliminating damaged cells through apoptosis. Our results showed that the levels of total DNA adducts in HepG2 cells treated with benzo(a)pyrene (BP, 2 μM)+RA (1 μM) were significantly reduced compared to those treated with BP only (P=0.038). In order to understand the mechanism of attenuation of DNA adducts, further experiments were performed. Cells were treated with BP (4 μM) for 24h to initiate DNA adduct formation, following which the medium containing BP was removed, and fresh medium containing 1 μM RA was added. The cells were harvested 24h after RA treatment. Interestingly, the levels of total DNA adducts were lower in the BP/RA group (390 ± 34) than those in the BP/DMSO group (544 ± 33), P=0.032. Analysis of cell apoptosis showed an increase in BP+RA group, compared to BP or RA only groups. Our results also indicated that attenuation of BP-DNA adducts by RA was not primarily due to its effects on CYP1A1 expression. In conclusion, our results suggest a mechanistic link between cellular apoptosis and DNA adduct formation, phenomena that play important roles in BP-mediated carcinogenesis. Furthermore, these results help understand the mechanisms of carcinogenesis, especially in relation to the chemopreventive properties of nutritional apoptosis inducers.

Design and synthesis of novel derivatives of all-trans retinoic acid demonstrate the combined importance of acid moiety and conjugated double bonds in its binding to PML-RAR-alpha oncogene in acute promyelocytic leukemia

The binding of all-trans retinoic acid (ATRA) to retinoid receptor-alpha (RAR-alpha) relieves transcriptional repression induced by the promyelocytic leukemia-retinoic acid receptor (PML-RAR) oncoprotein. The ATRA molecule contains a cyclohexenyl ring, a polyene chain containing conjugated double alkene bonds, and a terminal carboxyl group. To determine the contributions of these structural components of ATRA to its clinical efficacy, we synthesized three novel retinoids. These consisted of either a modified conjugated alkene backbone with an intact acid moiety (13a) or a modified conjugated alkene backbone and conversion of the acid group to either an ester (13b) or an aromatic amide (13c). Reporter assays demonstrated that compound 13a successfully relieved transcriptional repression by RAR-alpha, while 13b and 13c could not, demonstrating the critical role of the acid moiety in this binding. However, only ATRA was able to significantly inhibit the proliferation of APL cells while 13a, 13b, or 13c was not. Furthermore, only 13a led to partial non-significant differentiation of NB4 cells, demonstrating the importance of C9-C10 double bonds in differentiation induced CD11 expression. Our results demonstrate that both the acid moiety and conjugated double bonds present in the ATRA molecule are important for its biological activity in APL and have important implications for the design of future novel retinoids.

SHP and Sin3A expression are essential for adamantyl-substituted retinoid-related molecule-mediated nuclear factor-kappaB activation, c-Fos/c-Jun expression, and cellular apoptosis

We previously found that the adamantyl-substituted retinoid-related molecules bind to the small heterodimer partner (SHP) as well as the Sin3A complex. In this report, we delineated the role of SHP and the Sin3A complex in 4-[3'-(1-adamantyl)-4'-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC)-mediated inhibition of cell growth and apoptosis. We examined the effect of loss of SHP and Sin3A expression in a number of cell types on 3-Cl-AHPC-mediated growth inhibition and apoptosis induction, 3-Cl-AHPC-mediated nuclear factor-kappaB (NF-kappaB) activation, and 3-Cl-AHPC-mediated increase in c-Fos and c-Jun expression. We found that loss of SHP or Sin3A expression, while blocking 3-Cl-AHPC-mediated apoptosis, had little effect on 3-Cl-AHPC inhibition of cellular proliferation. We have previously shown that 3-Cl-AHPC-mediated NF-kappaB activation is necessary for apoptosis induction. We have now shown that 3-Cl-AHPC-enhanced c-Fos and c-Jun expression is also essential for maximal 3-Cl-AHPC-mediated apoptosis. 3-Cl-AHPC induction of c-Fos and c-Jun expression as well as NF-kappaB activation was dependent on SHP protein levels. In turn, SHP levels are regulated by Sin3A because ablation of Sin3A resulted in a decrease in SHP expression. Thus, SHP and Sin3A play an important role in adamantyl-substituted retinoid-related induction of cellular apoptosis.

Adamantyl-substituted retinoid-related molecules bind small heterodimer partner and modulate the Sin3A repressor

6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (CD437/AHPN) and 4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC/MM002) are inducers of apoptosis of malignant cells both in vitro and in vivo. Numerous mechanisms have been proposed for how these compounds exert this effect. This report shows that AHPN/3-Cl-AHPC binds specifically to the orphan nuclear receptor small heterodimer partner (SHP; NR0B2), and this binding promotes interaction of the receptor with a corepressor complex that minimally contains Sin3A, N-CoR, histone deacetylase 4, and HSP90. Formation of the SHP-Sin3A complex is essential for the ability of AHPN and 3-Cl-AHPC to induce apoptosis, as both knockout SHP and knockdown of Sin3A compromise the proapoptotic activity of these compounds but not other apoptosis inducers. These results suggest that AHPN/3-Cl-AHPC and their analogues are SHP ligands and their induction of apoptosis is mediated by their binding to the SHP receptor.

Potential diagnostic value of methylation profile in pleural fluid and serum from cancer patients with pleural effusion

BACKGROUND

The objective of this study was to investigate the diagnostic value of methylation profiles for discrimination between malignant and benign pleural effusions. A secondary objective was to examine the concordance of methylation in samples of serum and pleural fluid.

METHODS

The authors used methylation-specific polymerase chain reaction (MSP) analysis to examine the promoter methylation status of 4 genes in patients with pleural effusion: death-associated protein kinase (DAPK), Ras association domain family 1A (RASSF1A), retinoic acid receptor beta (RARbeta), and p16/INK4a. Pleural effusions were collected from 87 patients who had their diagnoses confirmed on cytologic and/or histologic examinations and clinical evolution. Pleural effusions were classified as malignant (n = 53 patients) or benign (n = 34 patients).

RESULTS

Methylation was detected in serum from 45.3% of patients with malignant pleural effusions and from 0% of patients with benign pleural effusions, and it was detected in pleural fluid samples from 58.5% of patients with malignant pleural effusions and from 0% of patients with benign pleural effusions (P = .001). The sensitivity of MSP was greater than that of cytologic examination alone (39.1%; P = .001). When MSP was used together with cytologic examination, sensitivity increased to 69.8% (P = .001).

CONCLUSIONS

Cell-free methylated DNA in pleural fluid can be detected in patients with neoplastic malignancy in a single extraction by thoracocentesis. Adequate management of the extracted pleural fluid can provide a rapid and reliable diagnosis in patients with pleural effusions who have suspected malignancy. MSP, used together with cytologic examination, may obviate the need for other invasive diagnostic tests.

Influence of the level of γ-glutamyltranspeptidase activity on the response of poorly and moderately differentiated rhabdomyosarcoma cell lines to all-trans-retinoic acid

Differentiation therapy with retinoic acid has been considered a potential approach for treating rhabdomyosarcoma. Analysis of retinoids as differentiating agents for rhabdomyosarcoma is, however, rendered incomplete by the fact that some rhabdomyosarcoma cell lines are retinoic acid resistant. Therefore, the aim of the present work was to study the effect of all-trans-retinoic acid on two rat tumour cell lines, derived from the same rhabdomyosarcoma tumour model (i.e. the moderately differentiated low metastatic F21 cell line and the poorly differentiated high metastatic S4MH cell line), to discover how degree of differentiation and glutathione metabolism influence response to this retinoic acid derivative. We observed that whereas in the S4MH cell line all-trans-retinoic acid induced a significant inhibition of tumorigenic potential, in F21 cells all-trans-retinoic acid enhanced tumour growth and only at a higher dose was there a slight antiproliferative effect. These effects were in consonance with the activity level of gamma-glutamyltranspeptidase, which was significantly increased in F21 cells, but not in S4MH cells, in response to the all-trans-retinoic acid-induced increase in reactive oxygen species. The pro-tumour effect observed in F21 cells was reversed by adding buthionine sulphoximide, a specific cellular glutathione-depleting agent, to the all-trans-retinoic acid treatment. This combination produced a decrease in gamma-glutamyltranspeptidase activity, and an increase in oxidative stress and apoptosis. Our findings suggest that the response to all-trans-retinoic-acid of the tumour cell lines studied is influenced by the strong relationship between intracellular glutathione content, gamma-glutamyltranspeptidase activity and degree of differentiation of the rhabdomyosarcoma cell line, and that this relationship should be taken into account when identifying 'retinoid-sensitive' tumours.

Sp1 Is Up-regulated in Cellular and Transgenic Models of Huntington Disease, and Its Reduction Is Neuroprotective

Interactions between mutant huntingtin (Htt) and a variety of transcription factors including specificity proteins (Sp) have been suggested as a central mechanism in Huntington disease (HD). However, the transcriptional activity induced by Htt in neurons that triggers neuronal death has yet to be fully elucidated. In the current study, we characterized the relationship of Sp1 to Htt protein aggregation and neuronal cell death. We found increased levels of Sp1 in neuronal-like PC12 cells expressing mutant Htt, primary striatal neurons, and brain tissue of HD transgenic mice. Sp1 levels were also elevated when 3-nitropropionate (3-NP) was used to induce cell death in PC12 cells. To assess the effects of knocking down Sp1 in HD pathology, we used Sp1 siRNA, a heterozygous Sp1 knock-out mouse, and mithramycin A, a DNA-intercalating agent that inhibits Sp1 function. The three approaches consistently yielded reduced levels of Sp1 which ameliorated toxicity caused by either mutant Htt or 3-NP. In addition, when HD mice were crossed with Sp1 heterozygous knock-out mice, the resulting offspring did not experience the loss of dopamine D2 receptor mRNA characteristic of HD mice, and survived longer than their HD counterparts. Our data suggest that enhancement of transcription factor Sp1 contributes to the pathology of HD and demonstrates that its suppression is beneficial.

Hypervitaminosis A resulting in DNA aberration in fetal transgenic mice (Muta Mouse)

Treatment with excessive amounts of Vitamin A during maternity induces fetal malformations. However, it is unclear whether these malformations are due to gene mutations or not. Using transgenic mice (containing lacZ gene showing beta-galactosidase enzymatic activity), we planned to observe whether gene mutations occur in the fetal tissues after treatment during maternity with Vitamin A (retinol palmitate). On the 11th day of pregnancy, mothers were given 30 mg (group 2), 150 mg (group 3) and 300 mg (group 4) of Vitamin A/kg body weight orally. Fetuses obtained on the 18th day of gestation showed malformations, such as cleft palate, origodactyly, brachydactyly and ectromeria. Most notably, cleft palate occurred dose dependently. The incidental rates were 100% in group 4, 58% in group 3 and 6% in group 2. The number of dead and absorbed fetuses also increased dose dependently with the treatments. DNA (integrated vectors containing lacZ genes) extracted from each fetus showed Vitamin A-induced lacZ mutations, especially in the malformed fetuses. The mutation frequencies were 4.99x10(-5) in group 4, 5.28x10(-5) in group 3 and 4.26x10(-5) in group 2. The frequencies of group 3 were significantly higher (p<0.05) than that of the controls (group 1), 2.79x10(-5). Maternal treatment with Vitamin A (150 mg/kg of body weight) was carried out on the 11th day of pregnancy. Fetuses obtained on the 14th day of gestation showed a much higher incidence of mutation, approximately 8.91x10(-5) (group 6) that was significantly higher (p<0.0001) than those from the controls (group 5), 2.94x10(-5). The present study indicates a possibility that hypervitaminosis A-induced fetal malformation and death might be caused by gene mutations.

Induction of apoptosis by the synthetic retinoid MX3350-1 through extrinsic and intrinsic pathways in head and neck squamous carcinoma cells

Retinoids have shown promise in cancer prevention and therapy. As some retinoids exhibit undesirable side effects, new retinoid analogs have been synthesized. In this study, we examined the effects of the retinoid MX3350-1 on human head and neck squamous cell carcinoma (HNSCC) cell lines. MX3350-1 suppressed the growth of 7/8 HNSCC cell lines by >65%. This inhibition appeared to be due to induction of apoptosis as revealed by the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay. Studies with cell line UMSCC17B indicated that apoptosis was induced within 1-2 days and involved activation of caspase-8, -9, and -3. Inhibitors of these caspases suppressed MX3350-1-induced apoptosis. MX3350-1 decreased the levels of antiapoptotic Bcl-2 and Bcl-XL, increased proapoptotic Bax, induced mitochondrial membrane permeabilization (MMP), and cytochrome c release from mitochondria to cytosol. The antioxidant butylated hydroxyanisol and the MMP inhibitor cyclosporin A (Cs A) blocked apoptosis induced by MX3350-1. In contrast, retinoid receptor antagonists failed to inhibit apoptosis. MX3350-1 increased the levels of Fas-ligand, Fas, and Fas-associated death domain, and enhanced activation of procaspase-8 and cleavage of its substrate Bid. Soluble Fas rescued the cells from MX3350-1-induced apoptosis. These results demonstrate that MX3350-1 induces apoptosis by activating both extrinsic and intrinsic apoptosis pathways and suggest that further studies on the potential of this retinoid for prevention and therapy of HNSCCs are warranted.

Antimyeloma effects of a novel synthetic retinoid Am80 (Tamibarotene) through inhibition of angiogenesis

In multiple myeloma (MM), the interaction between myeloma cells and bone marrow microenvironment has an important role in the pathogenesis of MM. We first examined the inducing effect of myeloma cells on migration of human umbilical vein vascular endothelial cells (HUVECs). Five myeloma cell lines produced varying amounts of VEGF, and migration of HUVECs was induced by coculture with myeloma cells. We next examined the inhibitory effect of a novel synthetic retinoid Am80 (Tamibarotene) on both myeloma cells and HUVECs. Am80 is specific for the retinoic-acid receptor-alpha/beta, and has therapeutic effects in all-trans retinoic acid resistant acute promyelocytic leukemia. Am80 slightly inhibited the growth of both myeloma cells and HUVECs, and remarkably inhibited the growth of HUVECs stimulated by VEGF. Am80 showed little growth inhibition of bone marrow stromal cells (BMSCs), but it markedly inhibited migration of HUVECs by cocultured myeloma cells. Am80 inhibited VEGF-induced phosphorylation of VEGF receptor. In addition, VEGF-induced formation of tube-like structures in vitro and neovascularization in mouse corneas were significantly inhibited by Am80. These findings clearly demonstrate that Am80 is a potential inhibitor of angiogenesis caused by the interaction between vascular endothelial cells and myeloma cells, and might be a useful therapeutic agent against MM.

Retinoid related molecules: new promises against lung and breast cancer

Retinoids have long interested cancer researchers due to their well-documented antiproliferative and differentiation inducing activities. However, natural compounds such as all-trans and 9-cis retinoic acid, as well as related synthetic derivatives, show only moderate anticancer activities, while at the same time inducing a broad range of undesirable activities. By taking advantage of the newly gained understanding in retinoid action and screening large numbers of novel molecules, new potent anticancer agents have been discovered that often lack typical retinoid side effects. Two of these novel types of molecules, referred to here as retinoid related molecules (RRMs), are described, one of which effectively kills non small cell lung cancer cells by apoptosis and is effective and well tolerated in vivo, while the other one belongs to a class of molecules selective for the nuclear receptor, RXR, which promises to be more effective and more tolerable than presently used compounds in anti-breast cancer adjuvant therapy. These novel molecules demonstrate the potential of novel RRMs that vastly outreaches classical retinoids.

Retinoids arrest breast cancer cell proliferation: retinoic acid selectively reduces the duration of receptor tyrosine kinase signaling

Retinoic acid (RA) induces cell cycle arrest of hormone-dependent human breast cancer (HBC) cells. Previously, we demonstrated that RA-induced growth arrest of T-47D HBC cells required the activity of the RA-induced protein kinase, protein kinase Calpha (PKCalpha) [J. Cell Physiol. 172 (1997) 306]. Here, we demonstrate that RA treatment of T-47D cells interfered with growth factor signaling to downstream, cytoplasmic and nuclear targets. RA treatment did not inhibit epidermal growth factor (EGF) receptor activation but resulted in rapid inactivation. The lack of sustained EGFR activation was associated with transient rather than sustained association of the EGFR with the Shc adaptor proteins and activation of Erk 1/2 and with compromised induction of expression of immediate early response genes. Inhibiting the activity of PKCalpha, a retinoic acid-induced target gene, prevented the effects of RA on cell proliferation and EGF signaling. Constitutive expression of PKCalpha, in the absence of RA, decreased cell proliferation and decreased EGF signaling. RA treatment increased steady-state levels of the protein tyrosine phosphatase PTP-1C and all measured effects of RA on EGF receptor function were reversed by the tyrosine phosphate inhibitor orthovanadate. These results indicate that RA-induced target genes, particularly PKCalpha, prevent sustained growth factor signaling, uncoupling activated receptor tyrosine kinases and nuclear targets that are required for cell cycle progression.

Nonclassical Retinoids and Lung Carcinogenesis

The retinoids are natural and synthetic derivatives of vitamin A. These cancer therapeutic and chemopreventive agents exert antiproliferative, differentiation-inducing, proapoptotic, and other biologic effects. The retinoids act through nuclear retinoid receptors to activate target genes that signal biologic effects. Agents that specifically activate the nuclear retinoid X receptors (RXRs) are known as rexinoids. Rexinoid growth suppression of human bronchial epithelial cells was linked to triggering of G1 cell cycle arrest, concomitant growth suppression, and a decrease in expression of G1 cyclins through activation of a proteasome-dependent degradation pathway. Clinical studies have demonstrated prolonged survival of subsets of patients with non-small-cell lung cancer (NSCLC) treated with rexinoids as single agents or as part of combination regimens. The critical role of RXR in downstream signaling makes rexinoids especially attractive agents to consider in combination therapy. There is encouraging evidence for therapeutic benefit of combination regimens of rexinoids with other targeted agents, such as epidermal growth factor receptor inhibitors, and with chemotherapy. Results from randomized phase III clinical trials in NSCLC will ultimately determine the impact for rexinoid-based therapy or chemoprevention for lung cancer.

The GRIMs: a new interface between cell death regulation and interferon/retinoid induced growth suppression

Cytokines and vitamins play a central role in controlling neoplastic cell growth. The interferon (IFN) family of cytokines regulates antiviral, anti-tumor, antimicrobial, differentiation, and immune responses in mammals. Significant advances have been made with respect to IFN-induced signal transduction pathways and antiviral responses. However, the IFN-induced anti-tumor actions are poorly defined. Although IFNs themselves inhibit tumor growth, combination of IFNs with retinoids (a class of Vitamin A related compounds) strongly potentiates the IFN-regulated anti-tumor action in a number of cell types. To define the molecular mechanisms involved in IFN/retinoid (RA)-induced apoptosis we have employed a genetic approach and identified several critical genes. In this review, I provide the current picture of IFN- RA- and IFN/RA-regulated growth suppressive pathways. In particular, I focus on a novel set of genes, the genes-associated with retinoid-interferon induced mortality (GRIM). GRIMs may be novel types of tumor suppressors, useful as biological response markers and potentially novel targets for drug development.

Retinoid-related molecules require caspase 9 for the effective release of Smac and the rapid induction of apoptosis

Certain retinoid-related molecules (RRMs) with agonist or antagonist activities have been described to induce apoptosis in a variety of cancer cell lines and show promise for the treatment of cancer. Similar to other chemotherapeutic drugs, these retinoid analogs have been suggested to induce apoptosis through the intrinsic pathway, which requires the release of cytochrome c from the mitochondria for the effective activation of caspase 9. Expression of a catalytically inactive form of caspase 9, which functions as a dominant negative mutant, inhibits the induction of DEVDase activity and nuclear fragmentation by selective RRMs. Whereas the RRMs could induce the release of cytochrome c in the absence of caspase 9 activity, the later is necessary for the effective release of Smac/Diablo from the mitochondria. Furthermore, overexpression of Bcl-2 or Bcl-X(L) also inhibits RRM-induced apoptosis. We demonstrate that activation of caspase 2 by the agonist MX2870-1 requires caspase 9 activity and is inhibited by Bcl-2 overexpression. In contrast, the antagonist MX781 induces cleavage of procaspase 2 upstream of mitochondria and independently of caspase 9. Thus, two retinoid analogs with unique characteristics activate two distinct apical caspases (2 or 9) to initiate apoptosis. In addition to caspase-mediated cell death, sustained exposure to the RRMs can also lead to loss of cell viability in cells lacking caspase 9 activity or in cells stimulated in the presence of the caspase inhibitor Z-VAD-fmk. Moreover, MX2870-1 and MX781 produce cell cycle arrest independently of caspase activity and the retinoid receptors.

A Retinoid-Related Molecule that Does Not Bind to Classical Retinoid Receptors Potently Induces Apoptosis in Human Prostate Cancer Cells through Rapid Caspase Activation

Synthetic retinoid-related molecules, such as N-(4-hydroxyphenyl)retinamide (fenretinide) and 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) induce apoptosis in a variety of malignant cells. The mechanism(s) of action of these compounds does not appear to involve retinoic acid receptors (RARs) and retinoid X receptors (RXRs), although some investigators disagree with this view. To clarify whether some retinoid-related molecules can induce apoptosis without involving RARs and/or RXRs, we used 4-[3-(1-heptyl-4,4-dimethyl-2-oxo-1,2,3,4-tetrahydroquinolin-6-yl)-3-oxo-E-propenyl] benzoic acid (AGN193198) that neither binds effectively to RARs and RXRs nor transactivates in RAR- and RXR-mediated reporter assays. AGN193198 potently induced apoptosis in prostate, breast, and gastrointestinal carcinoma cells and in leukemia cells. AGN193198 also abolished growth (by 50% at 130-332 nM) and induced apoptosis in primary cultures established from prostatic carcinoma (13 patients) and gastrointestinal carcinoma (1 patient). Apoptosis was induced rapidly, as indicated by mitochondrial depolarization and DNA fragmentation. Molecular events provoked by AGN193198 included activation of caspase-3, -8, -9, and -10 (by 4-6 h) and the production of BID/p15 (by 6 h). These findings show that caspase-mediated induction of apoptosis by AGN193198 is RAR/RXR-independent and suggest that this compound may be useful in the treatment of prostate cancer.

Retinoid targets for apoptosis induction

Certain synthetic retinoid-related molecules induce apoptosis in cancer cells through a novel mechanism of retinoid action that is independent of the nuclear retinoid receptors. These compounds target protein kinases and protein phosphatases to trigger signal transduction pathways that lead to apoptosis. Whereas retinoid agonists such as CD437 activate stress kinases via inhibition of the phosphatase MKP-1, the retinoid antagonist MX781 inhibits the survival kinase IKK. These retinoid-mediated signaling pathways converge at the mitochondria, where they cause the release of cytochrome c and subsequent Apaf-1-dependent activation of caspases. Identification of the retinoid targets that mediate their apoptotic activity will enhance our understanding of the mechanism of this novel retinoid action, to allow appropriate optimization of currently available compounds to advance into the clinic as novel anticancer agents.

Reduced concentrations of serum enhance the antiproliferative activity of retinoid-related molecules and accelerate the onset of apoptosis

Retinoid-related molecules (RRMs) that are selective agonists for the retinoic acid receptor-gamma and one retinoid antagonist are potent inducers of apoptosis in various cancer cell lines. This cell-killing activity makes them promising candidates for their use as anticancer drugs. We have observed that reducing the amount of serum in the cell culture medium significantly increased the antiproliferative activity of these RRMs in a serum concentration dependent manner. The induction of caspase activity, DNA fragmentation, and externalization of phosphatidylserine by the RRMs was markedly reduced when cells were treated in medium containing 10% serum, as compared to cells treated in low serum. High concentrations of serum also inhibited the activation of stress kinases by RRMs and higher amounts of the retinoid derivatives were necessary to cause quantitatively similar effects as compared to treatments in medium containing low serum. We have demonstrated that high concentrations of serum in the culture medium prevented the intracellular accumulation of MX3350-1 (agonist). Moreover, pre-incubation of cells in low serum-containing medium accelerated the onset of apoptosis as evidenced by the rapid activation of caspases and formation of apoptotic bodies. The release of cytochrome c and Smac induced by RRMs occurred earlier in cells that had been pre-incubated in 0.5% serum, while the activation of JNK and p38 stress kinases was unaffected.

Aberrant DNA methylation in lung cancer: biological and clinical implications

Genetic abnormalities of proto-oncogenes and tumor suppressor genes are well-known changes that are frequently involved in lung cancer pathogenesis. However, another mechanism for inactivation of tumor suppressor genes is coming more and more into focus. Epigenetic inactivation of certain tumor suppressor genes by aberrant promoter methylation is frequently observed in lung carcinomas and seems to play an important role in the pathogenesis of this tumor type. While genetic abnormalities are associated with changes in DNA sequence, epigenetic events may lead to changes in gene expression that occur without changes in DNA sequence. Recent findings demonstrate that aberrant methylation can also be detected in the smoking-damaged bronchial epithelium from cancer-free heavy smokers, suggesting that aberrant methylation might be an ideal candidate biomarker for lung cancer risk assessment and monitoring of chemoprevention trials. Moreover, in vitro studies demonstrate that methylation can be reversed by demethylating agents resulting in gene re-expression. This concept is currently under investigation in clinical trials. In summary, recent studies demonstrate that aberrant methylation may be the most common mechanism of inactivating cancer-related genes in lung cancer, occurs already in smoking-damaged bronchial epithelium from cancer-free individuals, can be reversed in vitro by demethylating agents, and may be a useful biomarker for lung cancer risk assessment.

Differential regulation of nonsteroidal anti-inflammatory drug-activated gene in normal human tracheobronchial epithelial and lung carcinoma cells by retinoids

In this study, we analyze the effect of several retinoids on the expression of nonsteroidal anti-inflammatory drug-activated gene (NAG-1) in normal human tracheobronchial epithelial (HTBE) cells and several lung carcinoma cell lines. The retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (AHPN) greatly enhances the expression of NAG-1 mRNA and protein in a time- and dose-dependent manner in human lung adenocarcinoma H460 cells and several other carcinoma cell lines. This induction was specific for AHPN because retinoic acid, a retinoic acid receptor-, and a retinoid X receptor pan-agonist were unable to induce NAG-1, suggesting that this induction is not mediated through activation of retinoid receptors. Although NAG-1 is a p53-responsive gene, AHPN-induced NAG-1 expression does not require p53. The induction of NAG-1 expression by AHPN is caused at least in part by an 8-fold increase in the stability of NAG-1 mRNA. In contrast to carcinoma cells, NAG-1 expression is effectively induced by retinoic acid and the RAR-selective pan-agonist in normal HTBE cells and accompanies the inhibition of squamous differentiation and the initiation of normal differentiation. In vivo, NAG-1 expression was observed in the normal tracheobronchial epithelium, whereas no expression was found in either squamous metaplastic tracheal epithelium or in sections of human lung tumors. Our results suggest that the induction of NAG-1 expression by retinoids in normal HTBE and lung carcinoma cells is regulated by distinct mechanisms and is associated with different biological processes. The linkage between AHPN treatment and NAG-1 expression revealed in this study provides a new mechanism for the antitumorigenic activity of AHPN.

Inhibition of IkappaB kinase by a new class of retinoid-related anticancer agents that induce apoptosis

The transcription factor NF-kappaB is overexpressed or constitutively activated in many cancer cells, where it induces expression of antiapoptotic genes correlating with resistance to anticancer therapies. Small molecules that inhibit the NF-kappaB signaling pathway could therefore be used to induce apoptosis in NF-kappaB-overexpressing tumors and potentially serve as anticancer agents. We found that retinoid antagonist MX781 inhibited the activation of NF-kappaB-dependent transcriptional activity in different tumor cell lines. MX781 was able to completely inhibit tumor necrosis factor alpha-mediated activation of IkappaB kinase (IKK), the upstream regulator of NF-kappaB. Inhibition of IKK activity resulted from direct binding of MX781 to the kinase, as demonstrated by in vitro inhibition studies. Two other molecules, MX3350-1 and CD2325, which are retinoic acid receptor gamma-selective agonists, were capable of inhibiting IKK in vitro, although they exerted variable inhibition of IKK and NF-kappaB activities in intact cells in a cell type-specific manner. However, N-(4-hydroxyphenyl)-retinamide, another apoptosis-inducing retinoid, and retinoic acid as well as other nonapoptotic retinoids did not inhibit IKK. Inhibition of IKK by the retinoid-related compounds and other small molecules correlated with reduced cell proliferation and increased apoptosis. Reduced cell viability was also observed after overexpression of an IKKbeta kinase-dead mutant or the IkappaBalpha superrepressor. The induction of apoptosis by the retinoid-related molecules that inhibited IKK was dependent on caspase activity but independent of the retinoid receptors. Thus, the presence of an excess of retinoic acid or a retinoid antagonist did not prevent the inhibition of IKK activation by MX781 and CD2325, indicating a retinoid receptor-independent mechanism of action.

Retinoids in combination therapies for the treatment of cancer: mechanisms and perspectives

Retinoid derivatives have been of special interest in cancer research because of their antiproliferative and differentiation-inducing activities in premalignant and malignant cells. Some retinoids are clinically effective in cancer therapy and prevention, and all-trans-retinoic acid is being used for the treatment of acute promyelocytic leukemia. Unfortunately, classical retinoids are not effective against most advanced solid tumors and cause undesirable side effects, which have limited the full development of retinoids as chemopreventive and chemotherapeutic drugs. The recent identification of selective retinoid derivatives capable of inducing apoptosis and their combination with other anticancer therapies promises a more effective and less toxic manner to the successful use of retinoids in cancer therapy.

Molecular pathogenesis of lung cancer

Lung cancer is the most common cause of cancer death in the United States, killing more than 156,000 people every year. In the past two decades, significant progress has been made in understanding the molecular and cellular pathogenesis of lung cancer. Abnormalities of proto-oncogenes, genetic and epigenetic changes of tumor suppressor genes, the role of angiogenesis in the multistage development of lung cancer, as well as detection of molecular abnormalities in preinvasive respiratory lesions, have recently come into focus. Efforts are ongoing to translate these findings into new clinical strategies for risk assessment, chemoprevention, early diagnosis, treatment selection, and prognosis and to provide new targets and methods of treatment for lung cancer patients. All these strategies should aid in reducing the number of newly diagnosed lung cancer cases and in increasing the survival and quality of life of patients with lung cancer.

Classical and novel retinoids: their targets in cancer therapy

Retinoids are important mediators of cellular growth and differentiation. Retinoids modulate the growth of both normal and malignant cells through their binding to retinoid nuclear receptors and their subsequent activation. While retinoids have demonstrated therapeutic efficacy in the treatment of acute promyelocytic leukemia, their spectrum of activity remains limited. Other agents such as histone deacetylase inhibitors may significantly increase retinoid activity in a number of malignant cell types. The novel retinoids N-(4-hydroxyphenyl) retinamide (4-HPR) and 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437; AHPN) induce apoptosis in a wide variety of malignant cells. Their mechanism(s) of action remain unclear, although a number of potential targets have been identified. Whether the retinoid receptors are involved in 4-HPR and CD473/AHPN mediated apoptosis remains unclear. Both 4-HPR and CD437/AHPN display significant potential as therapeutic agents in the treatment of a number of premalignant and malignant conditions.

Apoptosis induction and cell cycle perturbation in established cell lines by peroxysomicine A1 (T-514)

Peroxysomicine A1, a novel potential anticancer compound induced cell death in established cell lines and in a primary culture of rat neonatal cardiomyocytes. Non-transformed cells are less sensitive to the compound than transformed cell lines. Fluorescent microscopy of dying cells stained with DNA-specific dyes revealed chromatin condensation and nuclear fragmentation as well as membrane blebbing characteristic of apoptosis. Flow cytometry of cells treated with peroxysomicine A1, demonstrated appearance of cells containing less than 2C DNA, that indicated degradation of nuclear DNA, another hallmark of apoptotic cell death. Z-VAD, a nonspecific caspase inhibitor, prevented DNA fragmentation but not cell death registered by permeabilization of cell outer membrane. Peroxysomicine A1 also inhibited proliferation of various cell lines. Flow cytometry analysis showed significant accumulation of dividing cells in G2/M phases of cell cycle indicating, most likely delay in G2. These results provide initial insight into the mechanisms of action of peroxysomicine A1 and suggest that peroxysomicine A1 is a potent inhibitor of cell proliferation and inducer of apoptosis and may be a useful antineoplastic chemotherapeutic agent.

Antagonists of retinoic acid receptors (RARs) are potent growth inhibitors of prostate carcinoma cells

Novel synthetic antagonists of retinoic acid receptors (RARs) have been developed. To avoid interference by serum retinoids when testing these compounds, we established serum-free grown sub-lines (>3 years) of the prostate carcinoma lines LNCaP, PC3 and DU145. A high affinity pan-RAR antagonist (AGN194310, K(d) for binding to RARs = 2-5 nM) inhibited colony formation (by 50%) by all three lines at 16-34 nM, and led to a transient accumulation of flask-cultured cells in G1 followed by apoptosis. AGN194310 is 12-22 fold more potent than all-trans retinoic acid (ATRA) against cell lines and also more potent in inhibiting the growth of primary prostate carcinoma cells. PC3 and DU145 cells do not express RARbeta, and an antagonist with predominant activity at RARbeta and RARgamma (AGN194431) inhibited colony formation at concentrations (approximately 100 nM) commensurate with a K(d)value of 70 nM at RARgamma. An RARalpha antagonist (AGN194301) was less potent (IC(50) approximately 200 nM), but was more active than specific agonists of RARalpha and of betagamma. A component(s) of serum and of LNCaP-conditioned medium diminishes the activity of antagonists: this factor is not the most likely candidates IGF-1 and EGF. In vitro studies of RAR antagonists together with data from RAR-null mice lead to the hypothesis that RARgamma-regulated gene transcription is necessary for the survival and maintenance of prostate epithelium. The increased potencies of RAR antagonists, as compared with agonists, suggest that antagonists may be useful in the treatment of prostate carcinoma.

Nuclear hormone receptors and gene expression

The nuclear hormone receptor superfamily includes receptors for thyroid and steroid hormones, retinoids and vitamin D, as well as different "orphan" receptors of unknown ligand. Ligands for some of these receptors have been recently identified, showing that products of lipid metabolism such as fatty acids, prostaglandins, or cholesterol derivatives can regulate gene expression by binding to nuclear receptors. Nuclear receptors act as ligand-inducible transcription factors by directly interacting as monomers, homodimers, or heterodimers with the retinoid X receptor with DNA response elements of target genes, as well as by "cross-talking" to other signaling pathways. The effects of nuclear receptors on transcription are mediated through recruitment of coregulators. A subset of receptors binds corepressor factors and actively represses target gene expression in the absence of ligand. Corepressors are found within multicomponent complexes that contain histone deacetylase activity. Deacetylation leads to chromatin compactation and transcriptional repression. Upon ligand binding, the receptors undergo a conformational change that allows the recruitment of multiple coactivator complexes. Some of these proteins are chromatin remodeling factors or possess histone acetylase activity, whereas others may interact directly with the basic transcriptional machinery. Recruitment of coactivator complexes to the target promoter causes chromatin decompactation and transcriptional activation. The characterization of corepressor and coactivator complexes, in concert with the identification of the specific interaction motifs in the receptors, has demonstrated the existence of a general molecular mechanism by which different receptors elicit their transcriptional responses in target genes.

Induction of Egr-1 expression by the retinoid AHPN in human lung carcinoma cells is dependent on activated ERK1/2

The novel retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (AHPN/CD437) inhibits cell proliferation and is a very effective inducer of apoptosis in a variety of carcinoma cell lines. In order to obtain greater insight into the mechanism of AHPN-induced growth arrest and apoptosis, we began to examine AHPN-induced changes in gene expression by cDNA array screening using human lung carcinoma H460 cells. This analysis identified several AHPN-inducible genes, including the immediate-early genes Egr-1 and Nur77. AHPN was able to increase Egr-1 and Nur77 mRNA expression and protein in a variety of carcinoma cell lines. This induction appeared to be regulated at the transcriptional level and was specific for AHPN since an RAR- and an RXR-selective retinoid were inactive. These results suggest that the induction of Egr-1 and Nur77 by AHPN is independent of nuclear retinoid receptors and involves a novel mechanism. Overexpression of Bcl-2, which inhibits AHPN-induced apoptosis but not growth arrest in human T cell lymphoma Molt-4 cells, did not block the induction of immediate-early gene expression. Treatment of H460 cells with AHPN induced activation of the p38 MAP-kinase but not the ERK1/2 signaling pathway. However, inhibition of the ERK1/2 signaling pathway by PD98059 blocked the induction of Egr-1 and Nur77 mRNA while the p38 MAPK inhibitor PD169316 had little effect. Expression of a dominant-negative ERK1 completely abolished the increase in Egr-1 mRNA. Treatment with MAPK inhibitors or expression of dnERK1 reduced but did not block AHPN-induced apoptosis. Our results suggest that the induction of Egr-1 in H460 by AHPN requires active ERK1/2 and is independent of p38 activation. Egr-1, in cooperation with several other growth-suppressor proteins, is likely involved in AHPN-induced inhibition of cell growth and cell death.

Effects of a novel synthetic retinoid on malignant glioma in vitro: inhibition of cell proliferation, induction of apoptosis and differentiation

Among six synthetic retinoids tested, the retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) was highly efficient in inducing growth inhibition of 8MG-BA and GL-15 human glioblastoma cell lines, with growth arrest at the S phase of the cell cycle. CD 437 also induced apoptosis in these cells, with 8MG-BA being the most sensitive. In these cells, induction of apoptosis by CD437 has been related to the downregulation of Bcl-2 expression and to CPP32 activation, but not to p53 expression. The remaining non-apoptotic cells presented a morphological pattern of astroglial differentiation with overexpression of glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS). The mechanism of action of CD437, originally developed as a RARgamma agonist, is not yet elucidated. However, our results suggest that it acts through an increase of the expression of retinoid-inducible genes, such as RARbeta2 and/or RARalpha2.

Chemical genetics: ligand-based discovery of gene function

Chemical genetics is the study of gene-product function in a cellular or organismal context using exogenous ligands. In this approach, small molecules that bind directly to proteins are used to alter protein function, enabling a kinetic analysis of the in vivo consequences of these changes. Recent advances have strongly enhanced the power of exogenous ligands such that they can resemble genetic mutations in terms of their general applicability and target specificity. The growing sophistication of this approach raises the possibility of its application to any biological process.

A combination treatment of c-myc antisense DNA with all-trans-retinoic acid inhibits cell proliferation by downregulating c-myc expression in small cell lung cancer

The dysregulation of both c-myc expression and retinoid signaling pathways commonly occurs in small cell lung cancers (SCLC), frequently accompanying tumor relapse, and contributing to the poor prognosis of patients with SCLC. In this study, we investigated whether c-myc antisense oligodeoxynucleoside phosphorothioate (OPT) covering the translational initiation site of c-myc mRNA used in combination with all-trans-retinoic acid (RA) would be more effective than either agent alone in inhibiting the growth of an SCLC cell line, NCI-H82, overexpressing c-myc with amplification of this gene, and whether this combination could be an experimental therapeutic tool against SCLC. c-myc antisense OPT decreased c-myc expression in Northern and Western blot analyses, thus inducing 40% and 20% cell growth inhibition compared with scrambled and sense OPT and with scrambled four guanosine-containing OPT (p < 0.01, and p < 0.01, respectively). All-trans-RA also inhibited cell proliferation at the rate of 40% by downregulating c-myc expression. Having obtained these results, we tested the hymothesis that c-myc antisense OPT in combination with all-trans-RA may further reduce c-myc expression and lead to improved cell growth control. This combination showed a greater inhibition of cell proliferation than either agent given alone (p < 0.01) (60% inhibition of cell growth compared with treatment of control scrambled or sense OPT alone, p < 0.01) through enhanced downregulation of c-myc expression. In conclusion, c-myc antisense DNA in combination with other modalities for c-myc downregulation may represent an attractive gene regulation-based therapy of SCLC in the future. Further efforts, however, using new oligodeoxynucleotide analogs, specific interventions for DNA delivery into cells, and more potent therapeutic agents are required to increase the potentiation of c-myc downregulation and cell growth inhibition.

Identification of a unique binding protein specific for a novel retinoid inducing cellular apoptosis

The retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN, CD437) induces apoptosis in a variety of cell types, many of which are cancer cells that resist the antiproliferative and/or differentiating effects of retinoids. While the retinoids exert their effects by binding to the retinoic acid nuclear receptors (RARs) or retinoid X receptors (RXRs), AHPN (CD437) binds to another protein with different ligand specificity. In nuclear extracts from HL-60R cells the binding of AHPN (CD437) was only minimally competed by either retinoic acid (tRA)or 9-cis-retinoic acid (9-cis-RA), the natural ligands for the RARs and RXRs, respectively. Moreover, AHPN (CD437) was unable to compete with either tRA or 9-cis-RA for binding to endogenous retinoid receptors in nuclear extracts from the MDA-MB-468 breast carcinoma cell line. Size exclusion chromatography revealed AHPN binding to a 95 kDa protein(s) which is neither an RAR or RXR. Our results suggest that apoptosis induction by AHPN (CD437) may occur through interaction with another protein and is independent of the RAR/RXR-signaling pathways.

Selective inhibition of human lung cancer cell growth by peptides derived from retinoblastoma protein

Peptides containing retinoblastoma protein (RB) fragment 649-654 (LFYKKV) were tested for their ability to block the proliferation of RB-negative and RB-positive human non-small cell lung cancer (NSCLC) cells. These peptides potently restrained the growth of both types of tumor cells, as measured by metabolic (MTT) and cellular viability (trypan blue exclusion) assays. As such, and remarkably, the peptides were able to overcome the resistance of RB-positive cells usually observed with RB gene or protein replacement therapy. Compared to the overall performance of conventional chemotherapy tested in parallel, the peptides were more cytotoxic against RB-negative neoplastic cells and equipotent toward RB-positive tumor cells, yet less toxic toward normal human cells. Thus, these new molecules hold great promise to evolve into an efficient therapy for human lung cancer, a common malignancy still defying treatment and holding a poor prognosis, as well as for other human neoplasias.

The biologic basis for the use of retinoids in cancer prevention and treatment

Retinoids (vitamin A and related molecules) are biologic agents that have demonstrated, in preclinical and clinical models, potent activity in the prevention and treatment of a variety of malignancies. Presented in this article is a review of recent clinical studies and correlative laboratory findings that advance our understanding of the biologic basis for the use of retinoids in cancer prevention and treatment.

Mitochondrial permeability transition and release of cytochrome c induced by retinoic acids

Retinoic acids, structurally related to vitamin A, inhibit the in vitro proliferation of different types of normal and neoplastic cells. The effects of all-trans, 9-cis, and 13-cis retinoic acids were tested on mitochondria isolated from rat liver. All the compounds were able to induce the membrane permeability transition observed as swelling and decrease in membrane potential, but 13-cis retinoic acid appeared to be the most effective. The latter was also shown to stimulate the release of cytochrome c from mitochondria, suggesting a potential target of retinoids in the induction of cell apoptosis. Interestingly, EGTA and cyclosporin A, which strongly inhibit the permeability transition induced by 13-cis retinoic acid, were without effect on the release of cytochrome c from the mitochondrial intermembrane space.

Expression of retinoid receptor genes and proteins in non-small-cell lung cancer

BACKGROUND

Retinoids can suppress carcinogenesis in high-risk non-neoplastic bronchial lesions and can reduce the risk of second primary non-small-cell lung cancer (NSCLC). The effects of retinoids are mediated by nuclear receptors, i.e., the retinoic acid receptors (RARalpha, RARbeta, and RARgamma) and the retinoid X receptors (RXRalpha, RXRbeta, and RXRgamma). We investigated whether abnormalities in the in vivo expression of retinoid receptors are observed in NSCLC.

METHODS

Expression of retinoid receptors in paired specimens of normal and cancerous tissues from the lungs of 76 patients with NSCLC was studied by use of antiretinoid receptor antibodies (except those against RXRgamma) and immunohistochemistry. RAR messenger RNAs were analyzed by use of in situ hybridization and by reverse transcription-polymerase chain reaction (RT-PCR). Samples were also studied for loss of heterozygosity (LOH) at chromosome 3p24. All P values are two-sided.

RESULTS

All studied receptors were expressed in normal lung cells and in high- risk non-neoplastic lesions. In tumor cells, overexpression of RXRalpha and RARalpha was frequently observed. In contrast, RXRbeta expression decreased in 18% of the tumor specimens. Furthermore, there was a marked decrease in the expression of RARbeta in 63% of the tumors (P<.0001). Decreased expression of RARgamma was observed by RT-PCR in 41% of the tumors (P<.0001). LOH at 3p24 was observed in 41% of the tumor specimens from informative patients and in 20% of the non-neoplastic lesions.

CONCLUSIONS

Expression of RARalpha and RXRalpha is either normal or elevated in NSCLC. In contrast, a large percentage of tumors show a marked decrease in the expression of RARbeta, RARgamma, and RXRbeta as well as a high frequency of LOH at 3p24, which was also observed in non-neoplastic lesions. These data suggest that altered retinoid receptor expression may play a role in lung carcinogenesis.

Optimization of treatment conditions for studying the anticancer effects of retinoids using pancreatic adenocarcinoma as a model

Retinoids are natural differentiation-inducing compounds that are promising as anticancer agents. Cancer cell lines are valuable in the investigation of the potential of retinoids for the treatment of specific cancers. However, using different treatment conditions but the same cell lines, investigators have produced markedly contradictory results for the effectiveness of retinoids. The present study examined different factors in the treatment conditions that may have masked or interfered with the effects of retinoids and, thereby, resulted in this conflict. Our studies revealed that the effects of retinoids on cancer cell proliferation were influenced by serum, the choice of vehicle (DMSO vs ethanol) and its concentration, phenol red, the degree of cellular confluence, and the method of assessing proliferation (cell number or [3H]thymidine uptake vs the MTT assay). Optimized conditions were the use of serum-free, ethanol-free, and phenol red-free media, investigating cells in the log phase of growth, using </=0.01% DMSO as the vehicle, and monitoring proliferation by cell number or [3H]thymidine incorporation into DNA measured after TCA precipitation. Using these conditions, retinoids were found to exhibit potent antiproliferative effects in pancreatic cancer cells with a variety of degrees of differentiation, even in cell lines previously documented as being retinoid resistant. Retinoids also induced morphological changes and cellular death that may indicate terminal differentiation and apoptosis.

Tumor promotion by hydrogen peroxide in rat liver epithelial cells

Reactive oxygen species, including H2O2, play an important role in the tumor promotion process. Using an in vitro model of tumor promotion involving the rat liver epithelial oval cell line T51B, the tumor promoting activity of H2O2 in N-methyl-N'-nitro-N-nitrosoguanidine-initiated cells was studied. In this assay system, the promoting effect of H2O2 is evidenced by the formation of colonies in soft agar, appearance of foci in monolayer culture, disruption of gap junction communication (GJC) in foci areas and growth at higher saturation densities. H2O2 preferentially induced the expression of c-fos, c-jun, c-myc and egr-1, while JunB and JunD levels remained almost unchanged. H2O2 also induced hyperphosphorylation of Cx43 and disruption of GJC. The effects of H2O2 on tumor promotion, induction of immediate early (IE) genes and disruption of GJC are blocked by antioxidants. These results suggest that H2O2 acts as a tumor promoter in rat liver non-neoplastic epithelial cells and that the induction of IE genes and disruption of GJC are two possible targets of H2O2 during the tumor promotion process.

A selective retinoid with high activity against an androgen-resistant prostate cancer cell type

Retinoic acid (RA) and its natural and synthetic analogs, the retinoids, regulate many biological processes, including development, differentiation, cell growth, morphogenesis, metabolism and homeostasis. Retinoid effects are mediated by specific nuclear receptors, the RARs and RXRs. Because of their ability to control cell growth and induce differentiation, retinoids are being examined for the prevention and treatment of several cancers. The majority of retinoids so far analyzed and available inhibit primarily cell proliferation and tumor progression but cannot eliminate cancer cells. In addition, the beneficial effects of the natural retinoids are undermined by undesirable side effects, possibly due to indiscriminate activation of all retinoid receptor subtypes and response pathways. Here, we show that a synthetic retinoid, CD-271, that activates selectively the RAR gamma subtype in a given context, shows increased anti-proliferative activity against certain carcinoma cells over all-trans-retinoic acid (tRA). CD-271 exhibits enhanced activity against DU-145 prostate adenocarcinoma cells through apoptosis-inducing activity, while tRA does not. The selective anti-cancer cell action appears to be receptor-mediated as an RAR antagonist reverses the inhibition. This profile was not seen with other selective retinoids, such as RAR alpha-selective agonists, anti-AP-1 compounds and a non-apoptosis inducing RAR gamma agonist. Our data point to a specific role for RAR gamma in controlling the growth of the prostate, consistent with previous RAR gamma gene knockout data. The identified retinoid represents a new class of compounds with potential for the treatment of prostate cancer.

The T-cell oncogenic protein HOX11 activates Aldh1 expression in NIH 3T3 cells but represses its expression in mouse spleen development

Hox11 is a homeobox gene essential for spleen formation in mice, since atrophy of the anlage of a developing spleen occurs in early embryonic development in Hox11 null mice. HOX11 is also expressed in a subset of T-cell acute leukemias after specific chromosomal translocations. Since the protein has a homeodomain and can activate transcription, it probably exerts at least some of its effects in vivo by regulation of target genes. Representational difference analysis has been used to isolate cDNA clones corresponding to mRNA species activated following stable expression of HOX11 in NIH 3T3 cells. The gene encoding the retinoic acid-synthesizing enzyme aldehyde dehydrogenase 1 (Aldh1), initially called Hdg-1, was found to be ectopically activated by HOX11 in this system. Study of Aldh1 gene expression during spleen development showed that the presence of Aldh1 mRNA inversely correlated with Hox11. Hox11 null mouse embryos have elevated Aldh1 mRNA in spleen primordia prior to atrophy, while Aldh1 seems to be repressed by Hox11 during organogenesis of the spleens of wild-type mice. This result suggests that expression of Aldh1 protein is negatively regulated by Hox11 and that abnormal expression of Aldh1 in Hox11 null mice may cause loss of splenic precursor cells by aberrant retinoic acid metabolism.