ATO Increases ROS Production and Apoptosis of Cells by Enhancing Calpain-Mediated Degradation of the Cancer Survival Protein TG2

Transglutaminase 2 (TG2) is a critical cancer cell survival factor that activates several signalling pathways to foster drug resistance, cancer stem cell survival, metastasis, inflammation, epithelial-mesenchymal transition, and angiogenesis. All-trans retinoic acid (ATRA) and chemotherapy have been the standard treatments for acute promyelocytic leukaemia (APL), but clinical studies have shown that arsenic trioxide (ATO), alone or in combination with ATRA, can improve outcomes. ATO exerts cytotoxic effects in a variety of ways by inducing oxidative stress, genotoxicity, altered signal transduction, and/or epigenetic modification. In the present study, we showed that ATO increased ROS production and apoptosis ratios in ATRA-differentiated NB4 leukaemia cells, and that these responses were enhanced when TG2 was deleted. The combined ATRA + ATO treatment also increased the amount of nuclear factor erythroid 2-related factor 2 (NRF2) transcription factor, an adaptive regulator of the cellular oxidative stress response, and calpain proteolytic activity, resulting in TG2 degradation and the reduced survival of WT leukaemia cells. We further showed that the induced TG2 protein expression was degraded in the MCF-7 epithelial cell line and primary peripheral blood mononuclear cells upon ATO treatment, thereby sensitising these cell types to apoptotic signals.

Stretch-Induced Down-Regulation of HCN2 Suppresses Contractile Activity

Although hyperpolarization-activated and cyclic nucleotide-gated 2 channels (HCN2) are expressed in multiple cell types in the gut, the role of HCN2 in intestinal motility is poorly understood. HCN2 is down-regulated in intestinal smooth muscle in a rodent model of ileus. Thus, the purpose of this study was to determine the effects of HCN inhibition on intestinal motility. HCN inhibition with ZD7288 or zatebradine significantly suppressed both spontaneous and agonist-induced contractile activity in the small intestine in a dose-dependent and tetrodotoxin-independent manner. HCN inhibition significantly suppressed intestinal tone but not contractile amplitude. The calcium sensitivity of contractile activity was significantly suppressed by HCN inhibition. Inflammatory mediators did not affect the suppression of intestinal contractile activity by HCN inhibition but increased stretch of the intestinal tissue partially attenuated the effects of HCN inhibition on agonist-induced intestinal contractile activity. HCN2 protein and mRNA levels in intestinal smooth muscle tissue were significantly down-regulated by increased mechanical stretch compared to unstretched tissue. Increased cyclical stretch down-regulated HCN2 protein and mRNA levels in primary human intestinal smooth muscle cells and macrophages. Overall, our results suggest that decreased HCN2 expression induced by mechanical signals, such as intestinal wall distension or edema development, may contribute to the development of ileus.

Transglutaminase 2 associated with PI3K and PTEN in a membrane-bound signalosome platform blunts cell death

Atypically expressed transglutaminase 2 (TG2) has been identified as a poor prognostic factor in a variety of cancers. In this study, we evaluated the contribution of TG2 to the prolonged cell survival of differentiated acute promyelocytic leukaemia (APL) cells in response to the standard treatment with combined retinoic acid (ATRA) and arsenic trioxide (ATO). We report that one advantage of ATRA + ATO treatment compared to ATRA alone diminishes the amount of activated and non-activated CD11b/CD18 and CD11c/CD18 cell surface integrin receptors. These changes suppress ATRA-induced TG2 docking on the cytosolic part of CD18 β2-integrin subunits and reduce cell survival. In addition, TG2 overexpresses and hyperactivates the phosphatidylinositol-3-kinase (PI3K), phospho-AKT S473, and phospho-mTOR S2481 signalling axis. mTORC2 acts as a functional switch between cell survival and death by promoting the full activation of AKT. We show that TG2 presumably triggers the formation of a signalosome platform, hyperactivates downstream mTORC2-AKT signalling, which in turn phosphorylates and inhibits the activity of FOXO3, a key pro-apoptotic transcription factor. In contrast, the absence of TG2 restores basic phospho-mTOR S2481, phospho-AKT S473, PI3K, and PTEN expression and activity, thereby sensitising APL cells to ATO-induced cell death. We conclude, that atypically expressed TG2 may serve as a hub, facilitating signal transduction via signalosome formation by the CD18 subunit with both PI3K hyperactivation and PTEN inactivation through the PI3K-PTEN cycle in ATRA-treated APL cells.

Abstract P006: Targeting the zymogen granule protein 16B (ZG16B) to suppress breast cell growth and invasiveness

The induction of tumor suppressing effects by retinoid X receptor-selective agonists in premalignant breast cells may become ineffective in the course of malignant transformation. Therefore, this study aimed to identify pro-tumorigenic factors suppressed in a rexinoid-dependent manner, hypothesizing that such a mechanism may remain active and relevant for cancer interception throughout the various stages of breast cell transformation. To minimize toxicity due to long-term rexinoid treatment in the prevention setting, an additional goal was to identify synergistic growth suppressive combinations of bexarotene (Bex) in normal mammary epithelial cells. The selected combination of Bex and a non-selective beta-adrenergic inhibitor carvedilol (Carv) from an unbiased high throughput screen demonstrated marked cancer preventive activity in a mouse model of Her2-induced ER-negative breast cancer at subtherapeutic doses. To elucidate the molecular underpinnings of this chemopreventive activity, genes selectively suppressed by the drug combination over the individual agents were sought. We determined that the zymogen granule protein 16B (ZG16B), a secreted prognostic marker in breast cancer, was suppressed in normal breast cells by the combination of Bex and Carv. Chromatin immunoprecipitation following Bex+Carv treatment revealed enrichment of the chromatin remodeler protein ARID1A at sites of decreased H3K27 acetylation within the enhancer of the ZG16B gene, resulting in a marked down-regulation of ZG16B. Cell-free supernatant containing overexpressed recombinant ZG16B triggered Erk phosphorylation, increased the proliferation and migration of immortalized HME-hTert cells and induced the expression of extracellular matrix proteins. Furthermore, ZG16B promoted mesenchymal characteristics similar to TGF-b, which was inhibited by an antibody against ZG16B. When administered in tandem, ZG16B neutralized the effects of Bex+Carv. In breast cancer cells, ZG16B stimulated the migration of all tested cell lines, but enhanced proliferation only in ER-positive MCF-7 and T47D cells. Anchorage independent 3D growth of both ER-positive T47D and ER-negative MDA-MB231 cells was stimulated by ZG16B, while Bex+Carv reduced colony formation and suppressed ZG16B levels. Taken together, ZG16B is a potent factor in the growth, mesenchymal transition and invasiveness of breast epithelial cells at all stages of transformation. Our data further suggest that ZG16B may represent a relevant target of the tumor suppressing activity of rexinoids and the combination of Bex+Carv.

Citation Format: Máté Lengyel, Iván P. Uray. Targeting the zymogen granule protein 16B (ZG16B) to suppress breast cell growth and invasiveness. [abstract]. In: Proceedings of the AACR Special Conference: Precision Prevention, Early Detection, and Interception of Cancer; 2022 Nov 17-19; Austin, TX. Philadelphia (PA): AACR; Can Prev Res 2023;16(1 Suppl): Abstract nr P006.

Redistribution of the SWI/SNF Complex Dictates Coordinated Transcriptional Control over Epithelial–Mesenchymal Transition of Normal Breast Cells through TGF-β Signaling

Therapeutic targets in cancer cells defective for the tumor suppressor ARID1A are fundamentals of synthetic lethal strategies. However, whether modulating ARID1A function in premalignant breast epithelial cells could be exploited to reduce carcinogenic potential remains to be elucidated. In search of chromatin-modulating mechanisms activated by anti-proliferative agents in normal breast epithelial (HME-hTert) cells, we identified a distinct pattern of genome-wide H3K27 histone acetylation marks characteristic for the combined treatment by the cancer preventive rexinoid bexarotene (Bex) and carvedilol (Carv). Among these marks, several enhancers functionally linked to TGF-β signaling were enriched for ARID1A and Brg1, subunits within the SWI/SNF chromatin-remodeling complex. The recruitment of ARID1A and Brg1 was associated with the suppression of TGFBR2, KLF4, and FoxQ1, and the induction of BMP6, while the inverse pattern ensued upon the knock-down of ARID1A. Bex+Carv treatment resulted in fewer cells expressing N-cadherin and dictated a more epithelial phenotype. However, the silencing of ARID1A expression reversed the ability of Bex and Carv to limit epithelial–mesenchymal transition. The nuclear levels of SMAD4, a canonical mediator of TGF-β action, were more effectively suppressed by the combination than by TGF-β. In contrast, TGF-β treatment exceeded the ability of Bex+Carv to lower nuclear FoxQ1 levels and induced markedly higher E-cadherin positivity, indicating a target-selective antagonism of Bex+Carv to TGF-β action. In summary, the chromatin-wide redistribution of ARID1A by Bex and Carv treatment is instrumental in the suppression of genes mediating TGF-β signaling, and, thus, the morphologic reprogramming of normal breast epithelial cells. The concerted engagement of functionally linked targets using low toxicity clinical agents represents an attractive new approach for cancer interception.

The Role of ARID1A in the Nonestrogenic Modulation of IGF-1 Signaling

Gaining pharmacologic access to the potential of ARID1A, a tumor suppressor protein, to mediate transcriptional control over cancer gene expression is an unresolved challenge. Retinoid X receptor ligands are pleiotropic, incompletely understood tools that regulate breast epithelial cell proliferation and differentiation. We found that low-dose bexarotene (Bex) combined with the nonselective beta-blocker carvedilol (Carv) reduces proliferation of MCF10DCIS.com cells and markedly suppresses ARID1A levels. Similarly, Carv synergized with Bex in MCF-7 cells to suppress cell growth. Chromatin immunoprecipitation sequencing analysis revealed that under nonestrogenic conditions Bex + Carv alters the concerted genomic distribution of the chromatin remodeler ARID1A and acetylated histone H3K27, at sites related to insulin-like growth factor (IGF) signaling. Several distinct sites of ARID1A enrichment were identified in the IGF-1 receptor and IRS1 genes, associated with a suppression of both proteins. The knock-down of ARID1A increased IGF-1R levels, prevented IGF-1R and IRS1 suppression upon Bex + Carv, and stimulated proliferation. In vitro IGF-1 receptor neutralizing antibody suppressed cell growth, while elevated IGF-1R or IRS1 expression was associated with poor survival of patients with ER-negative breast cancer. Our study demonstrates direct impact of ARID1A redistribution on the expression and growth regulation of IGF-1-related genes, induced by repurposed clinical drugs under nonestrogenic conditions.

IMPLICATIONS

This study underscores the possibility of the pharmacologic modulation of the ARID1A factor to downregulate protumorigenic IGF-1 activity in patients with postmenopausal breast cancer undergoing aromatase inhibitor treatment.

Mechanotransduction at the Plasma Membrane-Cytoskeleton Interface

Mechanical cues are crucial for survival, adaptation, and normal homeostasis in virtually every cell type. The transduction of mechanical messages into intracellular biochemical messages is termed mechanotransduction. While significant advances in biochemical signaling have been made in the last few decades, the role of mechanotransduction in physiological and pathological processes has been largely overlooked until recently. In this review, the role of interactions between the cytoskeleton and cell-cell/cell-matrix adhesions in transducing mechanical signals is discussed. In addition, mechanosensors that reside in the cell membrane and the transduction of mechanical signals to the nucleus are discussed. Finally, we describe two examples in which mechanotransduction plays a significant role in normal physiology and disease development. The first example is the role of mechanotransduction in the proliferation and metastasis of cancerous cells. In this system, the role of mechanotransduction in cellular processes, including proliferation, differentiation, and motility, is described. In the second example, the role of mechanotransduction in a mechanically active organ, the gastrointestinal tract, is described. In the gut, mechanotransduction contributes to normal physiology and the development of motility disorders.

Benefits of Combined All-Trans Retinoic Acid and Arsenic Trioxide Treatment of Acute Promyelocytic Leukemia Cells and Further Enhancement by Inhibition of Atypically Expressed Transglutaminase 2

Randomized trials in acute promyelocytic leukemia patients have shown that treatment with a combination of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) is superior in efficacy to monotherapy, with significantly decreased mortality. So far, there are little data available to explain the success of the ATRA and ATO combination treatment in molecular terms. We showed that ATRA- and ATO-treated cells had the same capacity for superoxide production, which was reduced by two-thirds in the combined treatment. Secreted inflammatory biomarkers (monocyte chemoattractant protein-1 [MCP-1], interleukin-1 beta [IL-1β] and tumor necrosis factor-α [TNF-α]) were significantly decreased and were further reduced in a transglutaminase 2 (TG2) expression-dependent manner. The amount of secreted TNF-α in the supernatant of NB4 TG2 knockout cells was close to 50 times lower than in ATRA-treated differentiated wild-type NB4 cells. The irreversible inhibitor of TG2 NC9 not only decreased reactive oxygen species production 28-fold, but decreased the concentration of MCP-1, IL-1β and TNF-α 8-, 15- and 61-fold, respectively in the combined ATRA + ATO-treated wild-type NB4 cell culture. We propose that atypical expression of TG2 leads to the generation of inflammation, which thereby serves as a potential target for the prevention of differentiation syndrome.

Abstract 5068: Genomic regulation of the cell cycle as a function of ARID1A occupancy in breast epithelial cells

The idea of chemoprevention introduced a new concept both in clinical practice and experimental approaches. While single agent ER-independent chemopreventive therapy based on RXR-selective retinoids and other agents has produced promising results, even minor toxicity remains an obstacle to their clinical use. We performed unbiased high throughput screening to identify synergistic growth suppressive combinations of agents in normal mammary epithelial cells. A RXR-selective agonist and a beta-adrenergic inhibitor, as a candidate combination, at low concentrations demonstrated marked cancer preventive activity in a mouse model of Her2-induced ER-negative breast cancer. To elucidate the molecular mechanisms of this chemopreventive activity proteomic analyses of the candidate drug combination ensued. These revealed increased levels and phosphorylation of the DNA damage sensor ATM, along with elevated levels of the ARID1A subunit of the SWI/SNF chromatin remodeler in normal breast epithelial cells, but not in in situ breast carcinoma cells. Thus, we hypothesized that the maintenance of genomic integrity through ARID1A is a critical factor in the effectiveness of cancer preventive agents and helps reduce the odds of transformation. Genomic regions of increased accessibility are mapped by high throughput sequencing enabling multidimensional assays of the regulatory chromatin regions. Enhancers identified through chromatin immunoprecipitation followed by deep sequencing (ChIP-Seq) for H3K27 acetyl complemented with histones H3K4 will be discussed. We propose that ARID1A may mediate antiproliferative effects through genome-wide pausing of transcription of cell cycle regulatory genes. These results will help establish a role for the crosstalk of rexinoid and adrenergic signaling in the maintenance of a conserved chromatin remodeling mechanism and thus, premalignancy in breast epithelial cells.

Citation Format: Sham Jdeed, Balint L. Balint, Iván P. Uray. Genomic regulation of the cell cycle as a function of ARID1A occupancy in breast epithelial cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5068.

Transglutaminase 2 programs differentiating acute promyelocytic leukemia cells in all-trans retinoic acid treatment to inflammatory stage through NF-κB activation

Differentiation syndrome (DS) is a life-threatening complication arising during retinoid treatment of acute promyelocytic leukemia (APL). Administration of all-trans retinoic acid leads to significant changes in gene expression, among the most induced of which is transglutaminase 2, which is not normally expressed in neutrophil granulocytes. To evaluate the pathophysiological function of transglutaminase 2 in the context of immunological function and disease outcomes, such as excessive superoxide anion, cytokine, and chemokine production in differentiated NB4 cells, we used an NB4 transglutaminase knock-out cell line and a transglutaminase inhibitor, NC9, which inhibits both transamidase- and guanosine triphosphate-binding activities, to clarify the contribution of transglutaminase to the development of potentially lethal DS during all-trans retinoic acid treatment of APL. We found that such treatment not only enhanced cell-surface expression of CD11b and CD11c but also induced high-affinity states; atypical transglutaminase 2 expression in NB4 cells activated the nuclear factor kappa (κ)-light-chain-enhancer of the activated B-cell pathway, driving pathogenic processes with an inflammatory cascade through the expression of numerous cytokines, including tumor necrosis factor alpha (TNF-α), interleukin 1 beta (IL-1β), and monocyte chemoattractant protein 1. NC9 decreased the amount of transglutaminase 2, p65/RelA, and p50 in differentiated NB4 cells and their nuclei, leading to attenuated inflammatory cytokine synthesis. NC9 significantly inhibits transglutaminase 2 nuclear translocation but accelerates its proteasomal breakdown. This study demonstrates that transglutaminase 2 expression induced by all-trans retinoic acid treatment reprograms inflammatory signaling networks governed by nuclear factor κ-light-chain-enhancer of activated B-cell activation, resulting in overexpression of TNF-α and IL-1β in differentiating APL cells, suggesting that atypically expressed transglutaminase 2 is a promising target for leukemia treatment.

Retinoids and rexinoids in cancer prevention: from laboratory to clinic

Early in the age of modern medicine the consequences of vitamin A deficiency drew attention to the fundamental link between retinoid-dependent homeostatic regulation and malignant hyperproliferative diseases. The term "retinoid" includes a handful of endogenous and a large group of synthetic derivatives of vitamin A. These multifunctional lipid-soluble compounds directly regulate target genes of specific biological functions and critical signaling pathways to orchestrate complex functions from vision to development, metabolism, and inflammation. Many of the retinoid activities on the cellular level have been well characterized and translated to the regulation of processes like differentiation and cell death, which play critical roles in the outcome of malignant transformation of tissues. In fact, retinoid-based differentiation therapy of acute promyelocytic leukemia was one of the first successful examples of molecularly targeted treatment strategies. The selectivity, high receptor binding affinity and the ability of retinoids to directly modulate gene expression programs present a distinct pharmacological opportunity for cancer treatment and prevention. However, to fully exploit their potential, the adverse effects of retinoids must be averted. In this review we provide an overview of the biology of retinoid (activated by nuclear retinoic acid receptors [RARs]) and rexinoid (engaged by nuclear retinoid X receptors [RXRs]) action concluded from a long line of preclinical studies, in relation to normal and transformed states of cells. We will also discuss the past and current uses of retinoids in the treatment of malignancies, the potential of rexinoids in the cancer prevention setting, both as single agents and in combinations.

Abstract 2804: The thiazolidinedione pioglitazone enhances the cancer preventive activity of the rexinoid LG100268 in Her2/neu induced breast cancer

Two major unresolved issues for breast cancer prevention are the lack of effective preventive agents for estrogen receptor (ER)-negative disease and the potential toxicity of long-term pharmacologic treatment. The ability of retinoid X receptor (RXR)-selective retinoids (rexinoids) to inhibit breast cancer formation independently of ER-status was previously demonstrated in various preclinical models. The goal of our study was to identify ideal druggable targets for the synergistic amplification of the cancer preventive effect of synthetic rexinoids and test the efficacy of a low-dose combination treatment in vivo. We identified the nuclear hormone receptors essential for rexinoid-dependent growth suppression by a high throughput siRNA screen of mammary epithelial cells. Knock-down of ELP, COUP-TF II, GR, TR2, VDR and PPARgamma partially reversed the anti-proliferative effect of the rexinoid bexarotene. Conversely, expression profiling showed that the largest fraction of genes up-regulated by the synthetic rexinoid LG100268 represented regulators of lipid metabolism and putative target genes of the RXR partner receptor PPARgamma. Combination dose-response experiments showed that the PPARgamma agonist pioglitazone was synergistic with low doses (100-300 nM) of LG100268. Long-term daily treatment of MMTV-ErbB2 mice with LG100268 (25 mg/kg) or pioglitazone (20 mg/kg), or their combination resulted in a significant delay of tumor formation, while serum triglyceride levels were not significantly elevated by the combination. Suppression of cell proliferation measured by reduced Ki67 staining was enhanced by the addition of pioglitazone, in both lesions and intact mammary epithelium, while pioglitazone also increased the frequency of cells containing cleaved caspase 3. Furthermore, pioglitazone treatment was associated with a marked induction of microvesicle formation in the adipose issue. These data suggest that the combination of LG100268 and pioglitazone, while potentially targeting the same heterodimer, can invoke distinct biological processes resulting in more effective cancer prevention than achieved by either agent alone, and may be an effective strategy for the prevention of ER-negative breast cancer.

Citation Format: Iván P. Uray, Jennifer M. Rodenberg, Yun Zhang, Jamal L. Hill, Powel H. Brown. The thiazolidinedione pioglitazone enhances the cancer preventive activity of the rexinoid LG100268 in Her2/neu induced breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2804. doi:10.1158/1538-7445.AM2015-2804

Atypical antipsychotics induce both proinflammatory and adipogenic gene expression in human adipocytes in vitro

Schizophrenia requires lifelong treatment, potentially causing systemic changes in metabolic homeostasis. In the clinical setting, antipsychotic treatment may differentially lead to weight gain among individual patients, although the molecular determinants of such adverse effects are currently unknown. In this study, we investigated changes in the expression levels of critical regulatory genes of adipogenesis, lipid metabolism and proinflammatory genes during the differentiation of primary human adipose-derived stem cells (ADSCs). These cells were isolated from patients with body mass indices <25 and treated with the second-generation antipsychotics olanzapine, ziprasidone, clozapine, quetiapine, aripiprazole and risperidone and the first-generation antipsychotic haloperidol. We found that antipsychotics exhibited a marked effect on key genes involved in the regulation of cell cycle, signal transduction, transcription factors, nuclear receptors, differentiation markers and metabolic enzymes. In particular, we observed an induction of the transcription factor NF-KB1 and NF-KB1 target genes in adipocytes in response to these drugs, including the proinflammatory cytokines TNF-α, IL-1β, IL-8 and MCP-1. In addition, enhanced secretion of both IL8 and MCP-1 was observed in the supernatant of these cell cultures. In addition to their remarkable stimulatory effects on proinflammatory gene transcription, three of the most frequently prescribed antipsychotic drugs, clozapine, quetiapine and aripiprazole, also induced the expression of essential adipocyte differentiation genes and the adipocyte hormones leptin and adiponectin, suggesting that both glucose and fat metabolism may be affected by these drugs. These data further suggest that antipsychotic treatments in patients alter the gene expression patterns in adipocytes in a coordinated fashion and priming them for a low-level inflammatory state.

Cancer-preventive rexinoid modulates neutral lipid contents of mammary epithelial cells through a peroxisome proliferator-activated receptor γ-dependent mechanism

Synthetic rexinoids effectively suppress both estrogen receptor-positive and estrogen receptor-negative mammary tumors in animal models, which makes them prime candidates for a novel class of cancer-preventive agents. When used in combination with chemotherapy for non-small-cell lung cancer, the rexinoid bexarotene was most effective for patients who developed hypertriglyceridemia as a side effect. Although serum triglycerides originate from the liver, the effect of bexarotene on lipogenesis in breast epithelial cells is not known. Gene expression studies with normal mammary epithelial cells indicated that rexinoids modulate lipid metabolism, particularly enzymes involved in triglyceride synthesis. High-content analysis revealed dose-dependent accumulation of neutral lipids within adipocyte differentiation-related protein-associated cytoplasmic lipid droplets after long-term bexarotene treatment. Bexarotene also induced mRNA and protein levels for peroxisome proliferator-activated receptor (PPAR) γ, whereas selective knockdown of PPARγ attenuated the induction of both lipid droplets and adipocyte differentiation-related protein. Pharmacological activation of PPARγ, but not PPARα or retinoic acid receptors, effectively induced lipid accumulation. Furthermore, the combination of the PPARγ agonist rosiglitazone with bexarotene synergistically suppressed the growth of human mammary epithelial cells and revealed a strong, nonlinear, inverse correlation of cell growth with lipid droplet accumulation in the cell population. These findings indicate that rexinoids activate a lipogenic program in mammary epithelial cells through a retinoid X receptor/PPARγ-mediated mechanism. It is noteworthy that combining low doses of bexarotene with the PPARγ agonist rosiglitazone provides effective growth suppression of mammary epithelial cells, potentially dissociating systemic adverse effects associated with standard bexarotene treatment from the antiproliferative effects on mammary epithelium.

Chemoprevention of hormone receptor-negative breast cancer: new approaches needed

Results from clinical trials have demonstrated that it is possible to prevent estrogen-responsive breast cancers by targeting the estrogen receptor with selective estrogen receptor modulators (SERMs) (tamoxifen, raloxifene, or lasofoxifene) or with aromatase inhibitors (AIs) (anastrozole, letrozole, or exemestene). Results from breast cancer treatment trials suggest that aromatase inhibitors may be even more effective in preventing breast cancer than SERMs. However, while SERMs and aromatase inhibitors do prevent the development of many ER-positive breast cancers, these drugs do not prevent ER-negative breast cancer. These results show that new approaches are needed for the prevention of this aggressive form of breast cancer. Our laboratory and clinical efforts have been focused on identifying critical molecular pathways in breast cells that can be targeted for the prevention of ER-negative breast cancer. Our preclinical studies have demonstrated that other nuclear receptors, such as RXR receptors, vitamin D receptors, as well as others are critical for the growth of ER-negative breast cells and for the transformation of these cells into ER-negative cancers. Other studies show that growth factor pathways including those activated by EGFR, Her2, and IGFR, which are activated in many ER-negative breast cancers, can be targeted for the prevention of ER-negative breast cancer in mice. Clinical studies have also shown that PARP inhibitors are effective for the treatment of breast cancers arising in BRCA-1 or -2 mutation carriers, suggesting that targeting PARP may also be useful for the prevention of breast cancers arising in these high-risk individuals. Most recently, we have demonstrated that ER-negative breast cancers can be subdivided into four distinct groups based on the kinases that they express. These groups include ER-negative/Her-2-positive groups (the MAPK and immunomodulatory groups) and ER-negative/Her2-negative groups (the S6K and the cell cycle checkpoint groups). These groups of ER-negative breast cancers can be targeted with kinase inhibitors specific for each subgroup. These preclinical studies have supported the development of several clinical trials testing targeted agents for the prevention of breast cancer. The results of a completed Phase II cancer prevention trial using the RXR ligand bexarotene in women at high risk of breast cancer will be reviewed, and the current status of an ongoing Phase II trial using the EGFR and Her2 kinase inhibitor lapatinib for the treatment of women with DCIS breast cancer will be presented. It is anticipated that in the future these molecularly targeted drugs will be combined with hormonal agents such as SERMs or aromatase inhibitors to prevent all forms of breast cancer.

Rexinoid-induced expression of IGFBP-6 requires RARbeta-dependent permissive cooperation of retinoid receptors and AP-1

The synthetic rexinoid bexarotene (Targretin, LGD1069) inhibits the formation of both estrogen receptor-negative and estrogen receptor-positive breast cancer in preclinical models and controls the expression of growth-regulatory biomarkers, such as IGFBP-6 (insulin-like growth factor-binding protein 6), RARbeta, or cyclin D1. In this study, we identified a classical retinoic acid-responsive element in the first intron in the IGFBP-6 gene adjacent to a consensus AP-1 binding site, both elements essential for rexinoid-induced expression of IGFBP-6. In chromatin binding experiments, bexarotene increased the occupancy of the identified enhancer element by RXRalpha, RARbeta, cJun, cFos, and p300. In normal mammary epithelial cells and T47D breast cancer cells, small interfering RNA-mediated knockdown of all RXR isoforms or RARbeta, but not RARalpha or RARgamma alone, blocked the induction of IGFBP-6 by bexarotene. Simultaneous knockdown of RARalpha and RARgamma abrogated both the induction of RARbeta and the up-regulation and secretion of IGFBP-6. The suppression of either RARbeta or cJun by small interfering RNA blocked the recruitment of RXRalpha and cJun to the enhancer. These results demonstrate a novel cooperative interaction between retinoid receptors and AP-1 orchestrated by RARbeta and highlight a novel mechanism by which RARbeta can mediate the cancer-preventive effects of rexinoids.

The AP-1 transcription factor regulates breast cancer cell growth via cyclins and E2F factors

The activating protein-1 (AP-1) transcription factor transduces growth signals through signal transduction pathways to the nucleus, leading to the expression of genes involved in growth and malignant transformation in many cell types. We have previously shown that overexpression of a dominant negative form of the cJun proto-oncogene, a cJun dominant negative mutant (Tam67), blocks AP-1 transcriptional activity, induces a G(1) cell cycle block and inhibits breast cancer cell growth in vitro and in vivo. We found that AP-1 blockade by Tam67 in MCF-7 breast cancer cells downregulates cyclin D1 transcriptional activity by at least two mechanisms: by suppressing transcription at the known AP-1 binding site (-934/-928) and by suppressing growth factor-induced expression through suppressing E2F activation at the E2F-responsive site (-726/-719). AP-1 blockade also led to reduced expression of E2F1 and E2F2, but not E2F4, at the mRNA and protein levels. Chromatin immunoprecipitation and supershift assays demonstrated that AP-1 blockade caused decreased binding of E2F1 protein to the E2F site in the cyclin D1 promoter. We also found that Tam67 suppressed the expression of the E2F1 dimerizing partner, DP1 and E2F-upregulated cell cycle genes (cyclins E, A, B and D3) and enhanced the expression of E2F-downregulated cell cycle genes (cyclins G(2) and I). Reduced expression of other E2F-regulated genes was also seen with AP-1 blockade and E2F suppression. Thus, the AP-1 factor regulates the expression of cyclin D and E2F (the latter in turn regulates E2F-downstream genes), leading to cell cycle progression and breast cancer cell proliferation.

Retinoid X receptors: X-ploring their (patho)physiological functions

Retinoid X receptor (RXR) belongs to a family of ligand-activated transcription factors that regulate many aspects of metazoan life. A class of nuclear receptors requires RXR as heterodimerization partner for their function. This places RXR in the crossroad of multiple distinct biological pathways. This and the fact that the debate on the endogenous ligand requirement for RXR is not yet settled make RXR still an enigmatic transcription factor. Here, we review some of the biology of RXR. We place RXR into the evolution of nuclear receptors, review structural details and ligands of the receptor. Then processes regulated by RXR are discussed focusing on the developmental roles deduced from studies on knockout animals and metabolic roles in diseases such as diabetes and atherosclerosis deduced from pharmacological studies. Finally, aspects of RXR's involvement in myeloid differentiation and apoptosis are summarized along with issues on RXR's suitability as a therapeutic target.

Mechanical unloading increases caveolin expression in the failing human heart

OBJECTIVE

Implantation of a left ventricular assist device (LVAD) in the failing human heart initiates structural and functional changes termed reverse remodeling. Mechanical unloading improves cardiac adrenergic responsiveness and lipid metabolism, processes regulated by caveolar function. We tested the hypothesis that mechanical unloading alters the expression of caveolins and these changes are linked to altered expression of markers of reverse remodeling.

METHODS

Paired human myocardial samples were obtained from patients who received an LVAD as a bridge to cardiac transplantation. Transcript levels were measured using real-time Q-RT-PCR in RNA prepared from 34 pairs of formalin-fixed myocardial tissue blocks. Caveolin-1 and -3 protein levels were determined from frozen tissue (n=5) by Western blots. Caveolin-3 localization was demonstrated by immunohistochemistry.

RESULTS

Caveolin-1 protein levels were upregulated in all LVAD-patients after mechanical unloading (P=0.002). Caveolin-1 mRNA was increased in 76% of the patients (n=34, P<0.001). Larger induction of caveolin-1 was associated with greater suppression of ANF. Caveolin-3 transcript levels increased in 82% of the cohort, along with a 2.5-fold induction of caveolin-2. Sarcolemmal caveolin-3 staining was increased after LVAD-support, although no change in total caveolin-3 protein was detected. The mRNA levels of the caveolin-associated CD36 also increased with unloading. Patients with ischemic cardiomyopathy showed greater induction of CD36 (P<0.05) than non-ischemic cases, as well as highly correlated changes in the expression of caveolin isoforms.

CONCLUSION

Mechanical unloading induces the expression of caveolins and CD36. The induction of caveolin-1 and the reciprocal suppression of ANF suggest that the changes in the expression of both genes are linked to decreased hemodynamic load. Enhanced caveolin expression during mechanical unloading of failing human hearts may be a part of the reverse remodeling of lipid metabolism, nitric oxide production and adrenergic signaling.

Left ventricular unloading alters receptor tyrosine kinase expression in the failing human heart

BACKGROUND

Experimental and clinical data suggest that the loss of membrane receptor tyrosine kinase (RTK) activity in cardiac myocytes results in increased frequency of apoptotic cell death and progression of heart failure. The goal of our study was to examine the expression characteristics of RTKs in ventricular myocardium obtained from patients before and after mechanical unloading.

METHODS

We extracted RNA from paired formalin-fixed, paraffin-embedded left ventricular tissue blocks obtained at the time of left ventricular assist device (LVAD) implantation and explantation from a cohort of 36 patients (median age 51 years). The duration of LVAD support ranged from 1 to 314 days (median 95 days), 17 patients had ischemic and 19 non-ischemic cardiomyopathy at the time of LVAD implantation. Using real-time reverse transcription-polymerase chain reaction (RT-PCR) we quantitated transcripts for atrial natriuretic factor (ANF) and tumor necrosis factor-alpha (TNF-alpha), markers of heart failure, and the RTKs Her2/neu, Her4 and gp130, regulators of cardiac cell survival.

RESULTS

In patients undergoing mechanical unloading, ANF and TNF-alpha mRNA levels were independently suppressed. Her2/neu, along with Her4 was upregulated, mostly in cases of ischemic cardiomyopathy, whereas gp130 levels decreased. Post-LVAD transcript levels of Her2 correlated tightly with gp130 in patients with non-pathologic entry values of gp130. Duration of treatment and age were also determining factors in the change of expression of these genes.

CONCLUSION

Real-time quantitative (Q)-RT-PCR can be used to quantitate gene expression in archival myocardial tissue blocks. Mechanical unloading leads to a re-adjustment of RTK transcript levels, but not their reverting to control values in heart failure patients.

Pharmacological separation of the expression of tissue transglutaminase and apoptosis after chemotherapeutic treatment of HepG2 cells

Chemotherapeutic drugs are known to eliminate cancer cells by inducing apoptosis. Tissue transglutaminase (tTG), a frequent player in apoptotic processes, is markedly induced in drug-resistant cancer cells. To better understand the action of apoptosis-inducing drugs, our study elucidates changes in the expression of tTG in the early phase of cell death, before the downstream events of apoptosis. We demonstrate that HepG2 cells uniformly induce both tTG mRNA and enzyme activity upon treatment with cisplatin, doxorubicin, and bleomycin, chemotherapeutic agents with different modes of action. The expression of fas ligand, caspase3 and baxalpha changes differentially or remain unaffected. tTG expression did not change significantly after administration of either the peroxisome proliferator activated receptor-alpha agonist WY14643 or the retinoid X receptor-specific analog LG 100268. However, both compounds blocked drug-induced tTG induction without affecting the extent of cell death. The pleiotropic cytokine interleukin-6 effectively rescued hepatoma cells from apoptosis while tTG induction still took place, along with the induction of antiapoptotic transcripts bcl-x(L), gp130, and her2/neu. These results suggest that the induction of tTG, although present in drug-induced apoptosis, is pharmacologically dissociable from the early, initiating events of apoptosis. Blocking the induction of tTG during drug-induced cell death may alleviate limiting side effects of anticancer agents, including fibrosis and neuropathies.