cAMP-Mediated Autophagy Promotes Cell Survival via ROS-Induced Activation of PARP1: Implications for Treatment of Acute Lymphoblastic Leukemia.

DNA-damaging therapy is the basis for treatment of most cancers, including B-cell precursor acute lymphoblastic leukemia (BCP-ALL, hereafter ALL). We have previously shown that cAMP-activating factors present in the bone marrow render ALL cells less sensitive to DNA damage-induced apoptosis, by enhancing autophagy and suppressing p53. To sensitize ALL cells to DNA-damaging therapy, we have searched for novel targets that may counteract the effects induced by cAMP signaling. In the current study, we have identified PARP1 as a potential target. We show that the PARP1 inhibitors olaparib or PJ34 inhibit cAMP-mediated autophagy and thereby potentiate the DNA-damaging treatment. Furthermore, we reveal that cAMP-mediated PARP1 activation is preceded by induction of reactive oxygen species (ROS) and results in depletion of nicotinamide adenine dinucleotide (NAD), both of which are autophagy-promoting events. Accordingly, we demonstrate that scavenging ROS by N-acetylcysteine and repleting NAD independently reduce DNA damage-induced autophagy. In addition, olaparib augmented the effect of DNA-damaging treatment in a human xenograft model of ALL in NOD-scidIL2Rgammanull mice. On the basis of the current findings, we suggest that PARP1 inhibitors may enhance the efficiency of conventional genotoxic therapies and thereby provide a novel treatment strategy for pediatric patients with ALL. IMPLICATIONS: PARP1 inhibitors augment the DNA damage-induced killing of ALL cells by limiting the opposing effects of cAMP-mediated autophagy, which involves ROS-induced PARP1 activation and depletion of cellular NAD levels.

Targeting cyclooxygenase by indomethacin decelerates progression of acute lymphoblastic leukemia in a xenograft model.

Acute lymphoblastic leukemia (ALL) develops in the bone marrow in the vicinity of stromal cells known to promote tumor development and treatment resistance. We previously showed that the cyclooxygenase (COX) inhibitor indomethacin prevents the ability of stromal cells to diminish p53-mediated killing of cocultured ALL cells in vitro, possibly by blocking the production of prostaglandin E2 (PGE2). Here, we propose that PGE2 released by bone marrow stromal cells might be a target for improved treatment of pediatric ALL. We used a xenograft model of human primary ALL cells in nonobese diabetic-scid IL2rγnull mice to show that indomethacin delivered in the drinking water delayed the progression of ALL in vivo. The progression was monitored by noninvasive in vivo imaging of the engrafted leukemic cells, as well as by analyses of CD19+CD10+ leukemic blasts present in spleen or bone marrow at the termination of the experiments. The indomethacin treatment increased the level of p53 in the leukemic cells, implying that COX inhibition might reduce progression of ALL by attenuating protective paracrine PGE2 signaling from bone marrow stroma to leukemic cells.

cAMP-mediated autophagy inhibits DNA damage-induced death of leukemia cells independent of p53.

Autophagy is important in regulating the balance between cell death and survival, with the tumor suppressor p53 as one of the key components in this interplay. We have previously utilized an in vitro model of the most common form of childhood cancer, B cell precursor acute lymphoblastic leukemia (BCP-ALL), to show that activation of the cAMP signaling pathway inhibits p53-mediated apoptosis in response to DNA damage in both cell lines and primary leukemic cells. The present study reveals that cAMP-mediated survival of BCP-ALL cells exposed to DNA damaging agents, involves a critical and p53-independent enhancement of autophagy. Although autophagy generally is regarded as a survival mechanism, DNA damage-induced apoptosis has been linked both to enhanced and reduced levels of autophagy. Here we show that exposure of BCP-ALL cells to irradiation or cytotoxic drugs triggers autophagy and cell death in a p53-dependent manner. Stimulation of the cAMP signaling pathway further augments autophagy and inhibits the DNA damage-induced cell death concomitant with reduced nuclear levels of p53. Knocking-down the levels of p53 reduced the irradiation-induced autophagy and cell death, but had no effect on the cAMP-mediated autophagy. Moreover, prevention of autophagy by bafilomycin A1 or by the ULK-inhibitor MRT68921, diminished the protecting effect of cAMP signaling on DNA damage-induced cell death. Having previously proposed a role of the cAMP signaling pathway in development and treatment of BCP-ALLs, we here suggest that inhibitors of autophagy may improve current DNA damage-based therapy of BCP-ALL - independent of p53.

The thyroid hormone nuclear receptors and the Wnt/ß-catenin pathway: An intriguing liaison.

The thyroid hormones, T3 and T4, control several developmental and homeostatic processes. From a molecular point of view, most of their actions depend on the activity of the thyroid hormone nuclear receptors (TRs), which are T3-modulated transcription factors. Recent studies have not only highlighted that the physiological response induced by T3 within a cell depends on the expression of specific TRs, but also that the functions of TRs are coordinated by and integrated in other signalling pathways. This is particularly the case for the multilevel interactions between TRs and the Wnt signalling pathway. Interestingly both signals are involved in development and homeostasis, and their alterations are responsible for the development of pathologies, such as cancer. Here, we present findings on the complex crosstalk between TRs and Wnt in several organisms and in different tissue contexts, and speculate on the biological relevance of modulating TR-Wnt functionality in therapeutic approaches aimed to target cancer cells or applications for regenerative medicine.

Bone marrow stroma-derived PGE2 protects BCP-ALL cells from DNA damage-induced p53 accumulation and cell death.

BACKGROUND: B cell precursor acute lymphoblastic leukaemia (BCP-ALL) is the most common paediatric cancer. BCP-ALL blasts typically retain wild type p53, and are therefore assumed to rely on indirect measures to suppress transformation-induced p53 activity. We have recently demonstrated that the second messenger cyclic adenosine monophosphate (cAMP) through activation of protein kinase A (PKA) has the ability to inhibit DNA damage-induced p53 accumulation and thereby promote survival of the leukaemic blasts. Development of BCP-ALL in the bone marrow (BM) is supported by resident BM-derived mesenchymal stromal cells (MSCs). MSCs are known to produce prostaglandin E(2) (PGE(2)) which upon binding to its receptors is able to elicit a cAMP response in target cells. We hypothesized that PGE(2) produced by stromal cells in the BM microenvironment could stimulate cAMP production and PKA activation in BCP-ALL cells, thereby suppressing p53 accumulation and promoting survival of the malignant cells. METHODS: Primary BCP-ALL cells isolated from BM aspirates at diagnosis were cocultivated with BM-derived MSCs, and effects on DNA damage-induced p53 accumulation and cell death were monitored by SDS-PAGE/immunoblotting and flow cytometry-based methods, respectively. Effects of intervention of signalling along the PGE(2)-cAMP-PKA axis were assessed by inhibition of PGE(2) production or PKA activity. Statistical significance was tested by Wilcoxon signed-rank test or paired samples t test. RESULTS: We demonstrate that BM-derived MSCs produce PGE(2) and protect primary BCP-ALL cells from p53 accumulation and apoptotic cell death. The MSC-mediated protection of DNA damage-mediated cell death is reversible upon inhibition of PGE(2) synthesis or PKA activity. Furthermore our results indicate differences in the sensitivity to variations in p53 levels between common cytogenetic subgroups of BCP-ALL. CONCLUSIONS: Our findings support our hypothesis that BM-derived PGE(2), through activation of cAMP-PKA signalling in BCP-ALL blasts, can inhibit the tumour suppressive activity of wild type p53, thereby promoting leukaemogenesis and protecting against therapy-induced leukaemic cell death. These novel findings identify the PGE(2)-cAMP-PKA signalling pathway as a possible target for pharmacological intervention with potential relevance for treatment of BCP-ALL.

The secreted Frizzled-Related Protein 2 modulates cell fate and the Wnt pathway in the murine intestinal epithelium.

The secreted Frizzled-Related Proteins (sFRPs) are generally considered antagonistic to Wnt signaling. However, several studies have described their synergy and/or activation of this pathway. Our own data indicated that in the intestinal epithelium, thyroid hormone induced-expression of sFRP2 stabilizes ß-catenin, leading to induction of Wnt. The aim of this work was to investigate the role of sFRP2 in the intestinal epithelium homeostasis and its specific effect on canonical Wnt pathway. In wild type animals we observed a restricted pattern of sFRP2 protein expression at the level of the intestinal crypts. Interestingly, sFRP2(-/-) mice displayed increased apoptosis within the crypts together with a defect in cell migration. Because of altered proportion of lineage-specific committed progenitors, the sFRP2(-/-) animals also showed a decrease of absorptive differentiation counterbalanced by an increase of secretory differentiation. Regarding the action of sFRP2 on canonical Wnt pathway, the lack of sFRP2 expression in sFRP2(-/-)/TopGal animals in vivo reduced the Wnt activity. This positive action of sFRP2 on Wnt was further confirmed by in vitro studies. In conclusion, in accordance with its restricted expression profile, sFRP2 contributes to the physiology of the intestinal epithelial crypt progenitors by controlling apoptosis, cell fate decisions and the Wnt pathway.

WNT/ß-catenin signalling is activated in aldosterone-producing adenomas and controls aldosterone production.

Primary aldosteronism (PA) is the main cause of secondary hypertension, resulting from adrenal aldosterone-producing adenomas (APA) or bilateral hyperplasia. Here, we show that constitutive activation of WNT/ß-catenin signalling is the most frequent molecular alteration found in 70% of APA. We provide evidence that decreased expression of the WNT inhibitor SFRP2 may be contributing to deregulated WNT signalling and APA development in patients. This is supported by the demonstration that mice with genetic ablation of Sfrp2 have increased aldosterone production and ectopic differentiation of zona glomerulosa cells. We further show that ß-catenin plays an essential role in the control of basal and Angiotensin II-induced aldosterone secretion, by activating AT1R, CYP21 and CYP11B2 transcription. This relies on both LEF/TCF-dependent activation of AT1R and CYP21 regulatory regions and indirect activation of CYP21 and CYP11B2 promoters, through increased expression of the nuclear receptors NURR1 and NUR77. Altogether, these data show that aberrant WNT/ß-catenin activation is associated with APA development and suggest that WNT pathway may be a good therapeutic target in PA.

Thyroid hormone's action on progenitor/stem cell biology: new challenge for a classic hormone?

BACKGROUND: Thyroid hormones are involved in developmental and homeostatic processes in several tissues. Their action results in different outcomes depending on the developmental stage, tissue and/or cellular context. Interestingly, their pleiotropic roles are conserved across vertebrates. It is largely documented that thyroid hormones act via nuclear receptors, the TRs, which are transcription factors and whose activity can be modulated by the local availability of the hormone T3. In the "classical view", the T3-induced physiological response depends on the expression of specific TR isoforms and the iodothyronine deiodinase selenoenzymes that control the local level of T3, thus TR activity. SCOPE OF THE REVIEW: Recent data have clearly established that the functionality of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. Here, we summarize these data and propose a new and intriguing role for thyroid hormones in two selected examples. MAJOR CONCLUSIONS: In the intestinal epithelium and the retina, TRα1 and TRß2 are expressed at the level of the precursors where they induce cell proliferation and differentiation, respectively. Moreover, these different functions result from the integration of the hormone signal with other intrinsic pathways, which play a fundamental role in progenitor/stem cell physiology. GENERAL SIGNIFICANCE: Taken together, the interaction of TRs with other signaling pathways, specifically in stem/progenitor cells, is a new concept that may have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Multi-level interactions between the nuclear receptor TRα1 and the WNT effectors ß-catenin/Tcf4 in the intestinal epithelium.

Intestinal homeostasis results from complex cross-regulation of signaling pathways; their alteration induces intestinal tumorigenesis. Previously, we found that the thyroid hormone nuclear receptor TRα1 activates and synergizes with the WNT pathway, inducing crypt cell proliferation and promoting tumorigenesis. Here, we investigated the mechanisms and implications of the cross-regulation between these two pathways in gut tumorigenesis in vivo and in vitro. We analyzed TRα1 and WNT target gene expression in healthy mucosae and tumors from mice overexpressing TRα1 in the intestinal epithelium in a WNT-activated genetic background (vil-TRα1/Apc mice). Interestingly, increased levels of ß-catenin/Tcf4 complex in tumors from vil-TRα1/Apc mice blocked TRα1 transcriptional activity. This observation was confirmed in Caco2 cells, in which TRα1 functionality on a luciferase reporter-assay was reduced by the overexpression of ß-catenin/Tcf4. Moreover, TRα1 physically interacted with ß-catenin/Tcf4 in the nuclei of these cells. Using molecular approaches, we demonstrated that the binding of TRα1 to its DNA target sequences within the tumors was impaired, while it was newly recruited to WNT target genes. In conclusion, our observations strongly suggest that increased ß-catenin/Tcf4 levels i) correlated with reduced TRα1 transcriptional activity on its target genes and, ii) were likely responsible for the shift of TRα1 binding on WNT targets. Together, these data suggest a novel mechanism for the tumor-promoting activity of the TRα1 nuclear receptor.

The overexpression of the putative gut stem cell marker Musashi-1 induces tumorigenesis through Wnt and Notch activation.

The RNA-binding protein Musashi-1 (Msi1) has been proposed as a marker of intestinal epithelial stem cells. These cells are responsible for the continuous renewal of the intestinal epithelium. Although the function of Msi1 has been studied in several organs from different species and in mammalian cell lines, its function and molecular regulation in mouse intestinal epithelium progenitor cells are still undefined. We describe here that, in these cells, the expression of Msi1 is regulated by the canonical Wnt pathway, through a mechanism involving a functional Tcf/Lef binding site on its promoter. An in vitro study in intestinal epithelium primary cultures showed that Msi1 overexpression promotes progenitor proliferation and activates Wnt and Notch pathways. Moreover, Msi1-overexpressing cells exhibit tumorigenic properties in xenograft experiments. These data point to a positive feedback loop between Msi1 and Wnt in intestinal epithelial progenitors. They also suggest that Msi1 has oncogenic properties in these cells, probably through induction of both the Wnt and Notch pathways.

Cooperation between the thyroid hormone receptor TRalpha1 and the WNT pathway in the induction of intestinal tumorigenesis.

BACKGROUND & AIMS: Colorectal tumorigenesis is a multistep process involving the alteration of oncogenes and tumor suppressor genes, leading to the deregulation of molecular pathways that govern intestinal homeostasis. We have previously shown that the thyroid hormone receptor alpha1 (TRalpha1) controls intestinal development and homeostasis through the WNT pathway. More precisely, TRalpha1 directly enhances the transcription of several components of this pathway, allowing increased expression of beta-catenin/Tcf4 target genes and stimulation of cell proliferation. Because the WNT pathway is a major player in controlling intestinal homeostasis, we addressed whether the TRalpha1 receptor has tumor-inducing potential. METHODS: We generated mice overexpressing TRalpha1 specifically in the intestinal epithelium in a wild-type (vil-TRalpha1) or a WNT-activated (vil-TRalpha1/Apc(+/1638N)) genetic background. RESULTS: The intestine of vil-TRalpha1 mice presents aberrant intestinal mucosal architecture and increased cell proliferation and develops adenoma at a low rate. However, TRalpha1 overexpression is unable to induce cancer development. On the contrary, we observed accelerated tumorigenesis in vil-TRalpha1/Apc(+/1638N) mice compared with the Apc(+/1638N) mutants. CONCLUSION: Our results suggest that this phenotype is due to cooperation between the activated TRalpha1 and WNT pathways. This is the first report describing the tumor-inducing function of TRalpha1 in the intestine.