Melanoma dormancy in a mouse model is linked to GILZ/FOXO3A-dependent quiescence of disseminated stem-like cells.

Metastatic cancer relapses following the reactivation of dormant, disseminated tumour cells; however, the cells and factors involved in this reactivation are just beginning to be identified. Using an immunotherapy-based syngeneic model of melanoma dormancy and GFP-labelled dormant cell-derived cell lines, we determined that vaccination against melanoma prevented tumour growth but did not prevent tumour cell dissemination or eliminate all tumour cells. The persistent disseminated melanoma tumour cells were quiescent and asymptomatic for one year. The quiescence/activation of these cells in vitro and the dormancy of melanoma in vivo appeared to be regulated by glucocorticoid-induced leucine zipper (GILZ)-mediated immunosuppression. GILZ expression was low in dormant cell-derived cultures, and re-expression of GILZ inactivated FOXO3A and its downstream target, p21CIP1. The ability of dormancy-competent cells to re-enter the cell cycle increased after a second round of cellular dormancy in vivo in association with shortened tumour dormancy period and faster and more aggressive melanoma relapse. Our data indicate that future cancer treatments should be adjusted according to the stage of disease progression.

Transient TNF regulates the self-renewing capacity of stem-like label-retaining cells in sphere and skin equivalent models of melanoma.

BACKGROUND: It is well established that inflammation promotes cancer, including melanoma, although the exact mechanisms involved are less known. In this study, we tested the hypothesis that inflammatory factors affect the cancer stem cell (CSC) compartment responsible for tumor development and relapse. RESULTS: Using an inducible histone 2B-GFP fusion protein as a tracer of cell divisional history, we determined that tumor necrosis factor (TNF), which is a classical pro-inflammatory cytokine, enlarged the CSC pool of GFP-positive label-retaining cells (LRCs) in tumor-like melanospheres. Although these cells acquired melanoma stem cell markers, including ABCB5 and CD271, and self-renewal ability, they lost their capacity to differentiate, as evidenced by the diminished MelanA expression in melanosphere cells and the loss of pigmentation in a skin equivalent model of human melanoma. The undifferentiated cell phenotype could be reversed by LY294002, which is an inhibitor of the PI3K/AKT signaling pathway, and this reversal was accompanied by a significant reduction in CSC phenotypic markers and functional properties. Importantly, the changes induced by a transient exposure to TNF were long-lasting and observed for many generations after TNF withdrawal. CONCLUSIONS: We conclude that pro-inflammatory TNF targets the quiescent/slow-cycling melanoma SC compartment and promotes PI3K/AKT-driven expansion of melanoma SCs most likely by preventing their asymmetrical self-renewal. This TNF effect is maintained and transferred to descendants of LRC CSCs and is manifested in the absence of TNF, suggesting that a transient exposure to inflammatory factors imprints long-lasting molecular and/or cellular changes with functional consequences long after inflammatory signal suppression. Clinically, these results may translate into an inflammation-triggered accumulation of quiescent/slow-cycling CSCs and a post-inflammatory onset of an aggressive tumor.

Insulin-like growth factor 1 receptor and p38 mitogen-activated protein kinase signals inversely regulate signal transducer and activator of transcription 3 activity to control human dental pulp stem cell quiescence, propagation, and differentiation.

Dental pulp stem cells (DPSCs) remain quiescent until activated in response to severe dental pulp damage. Once activated, they exit quiescence and enter regenerative odontogenesis, producing reparative dentin. The factors and signaling molecules that control the quiescence/activation and commitment to differentiation of human DPSCs are not known. In this study, we determined that the inhibition of insulin-like growth factor 1 receptor (IGF-1R) and p38 mitogen-activated protein kinase (p38 MAPK) signaling commonly activates DPSCs and promotes their exit from the G0 phase of the cell cycle as well as from the pyronin Y(low) stem cell compartment. The inhibition of these two pathways, however, inversely determines DPSC fate. In contrast to p38 MAPK inhibitors, IGF-1R inhibitors enhance dental pulp cell sphere-forming capacity and reduce the cells' colony-forming capacity without inducing cell death. The inverse cellular changes initiated by IGF-1R and p38 MAPK inhibitors were accompanied by inverse changes in the levels of active signal transducer and activator of transcription 3 (STAT3) factor, inactive glycogen synthase kinase 3, and matrix extracellular phosphoglycoprotein, a marker of early odontoblast differentiation. Our data suggest that there is cross talk between the IGF-1R and p38 MAPK signaling pathways in DPSCs and that the signals provided by these pathways converge at STAT3 and inversely regulate its activity to maintain quiescence or to promote self-renewal and differentiation of the cells. We propose a working model that explains the possible interactions between IGF-1R and p38 MAPK at the molecular level and describes the cellular consequences of these interactions. This model may inspire further fundamental study and stimulate research on the clinical applications of DPSC in cellular therapy and tissue regeneration.

The PI3K/AKT signaling pathway controls the quiescence of the low-Rhodamine123-retention cell compartment enriched for melanoma stem cell activity.

Melanoma is one of the most aggressive and extremely resistant to conventional therapies neoplasms. Recently, cellular resistance was linked to the cancer stem cell phenotype, still controversial and not well-defined. In this study, we used a Rhodamine 123 (Rh123) exclusion assay to functionally identify stem-like cells in metastatic human melanomas and melanoma cell lines. We demonstrate that a small subset of Rh123-low-retention (Rh123(low)) cells is enriched for stem cell-like activities, including the ability to self-renew and produce nonstem Rh123(high) progeny and to form melanospheres, recapitulating the phenotypic profile of the parental tumor. Rh123(low) cells are relatively quiescent and chemoresistant. At the molecular level, we show that melanoma Rh123(low) cells overexpress HIF1α, pluripotency factor OCT4, and the ABCB5 marker of melanoma stem cells and downregulate the expression of Cyclin D1 and CDK4. Interestingly, a short treatment with LY294002, an inhibitor of the PI3K/AKT pathway, specifically reverts a subset of Rh123(high) cells to the Rh123(low) phenotype, whereas treatment with inhibitors of mammalian target of rapamycin, phosphatase and tensin homolog or mitogen-activated protein kinase signaling does not. This phenotypic switching was associated with reduced levels of the HIF1α transcript and an increase in the level of phosphorylated nuclear FOXO3a preferentially in Rh123(low) cells. Moreover, the Rh123(low) cells became less quiescent and displayed a significant increase in their melanosphere-forming ability. All the above indicates that the Rh123(low) melanoma stem cell pool is composed of cycling and quiescent cells and that the PI3K/AKT signaling while maintaining the quiescence of Rh123(low) G0 cells promotes the exit of cycling cells from the stem cell compartment.

The glucocorticoid receptor is a co-regulator of the orphan nuclear receptor Nurr1.

Nurr1 (NR4A2) is an atypical nuclear receptor (NR) because of its inability to bind a ligand and to activate transcription following canonical NR rules. An affinity chromatography-based screen identified the glucocorticoid receptor (GR) as an interactant of Nurr1. The co-localization of these two NRs in the hippocampus and the substantia nigra, as well as their involvement in similar neurological processes led us to investigate the functional consequences of such a physical interaction. GR interfered with Nurr1 transcriptional activity, and Nurr1 association to GR confers glucocorticoid regulation to this orphan receptor. The N-terminal domain of Nurr1 interacts directly with GR, whereas several domains of GR can associate to Nurr1. The GR-mediated increase in Nurr1 transcriptional activity requires the N-terminal domain of GR, but not a functional DNA binding domain. Finally, SMRT and SRC2, two co-regulators of GR, modulated the transcriptional activity of the Nurr1-GR complex, but not that of Nurr1 alone. Our results therefore establish GR as a transcriptional regulator of Nurr1, and open new opportunities in the pharmacological regulation of Nurr1 by glucocorticoids in the CNS.

Multiple signaling pathways regulate the transcriptional activity of the orphan nuclear receptor NURR1.

The orphan nuclear receptor nurr1 (NR4A2) is an essential transcription factor for the acquisition and maintenance of the phenotype of dopamine (DA)-synthesizing neurons in the mesencephalon. Although structurally related to ligand-regulated nuclear receptors, nurr1 is functionally atypical due to its inability to bind a cognate ligand and to activate transcription following canonical nuclear receptor (NR) rules. Importantly, the physiological stimuli that activate this NR and the signaling proteins that regulate its transcriptional activity in mesencephalic neurons are unknown. We used an affinity chromatography approach and CSM14.1 cells of mesencephalic origin to isolate and identify several proteins that interact directly with nurr1 and regulate its transcriptional activity. Notably, we demonstrate that the mitogen-activated protein kinases, ERK2 and ERK5, elevate, whereas LIM Kinase 1 inhibits nurr1 transcriptional activity. Furthermore, nurr1 recruits ERK5 to a NBRE-containing promoter and is a potential substrate for this kinase. We have identified amino acids in the A/B domain of nurr1 important for mediating the ERK5 activating effects on nurr1 transcriptional activity. Our results suggest that nurr1 acts as a point of convergence for multiple signaling pathways that likely play a critical role in differentiation and phenotypic expression of dopaminergic (DAergic) neurons.

Selective alteration of gene expression in response to natural and synthetic retinoids.

BACKGROUND: Retinoids are very potent inducers of cellular differentiation and apoptosis, and are efficient anti-tumoral agents. Synthetic retinoids are designed to restrict their toxicity and side effects, mostly by increasing their selectivity toward each isotype of retinoic acids receptors (RARalpha,beta, gamma and RXRalpha, beta, gamma). We however previously showed that retinoids displayed very different abilities to activate retinoid-inducible reporter genes, and that these differential properties were correlated to the ability of a given ligand to promote SRC-1 recruitment by DNA-bound RXR:RAR heterodimers. This suggested that gene-selective modulation could be achieved by structurally distinct retinoids. RESULTS: Using the differential display mRNA technique, we identified several genes on the basis of their differential induction by natural or synthetic retinoids in human cervix adenocarcinoma cells. Furthermore, this differential ability to regulate promoter activities was also observed in murine P19 cells for the RARbeta2 and CRABPII gene, showing conclusively that retinoid structure has a dramatic impact on the regulation of endogenous genes. CONCLUSIONS: Our findings therefore show that some degree of selective induction or repression of gene expression may be achieved when using appropriately designed ligands for retinoic acid receptors, extending the concept of selective modulators from estrogen and peroxisome proliferator activated receptors to the class of retinoid receptors.