sh-Aldh1a1 cells had a significant reduction in mRNA levels (Fig.?2a) and Aldh1a1 activity (Fig.?2b) with respect to wild type B16F10 cells. AhR-expressing B16F10 cells did not significantly impact tumor growth in vivo. Aldh1a1 knockdown reduced the high levels of CD133+/CD29+/CD44+ cells, melanosphere size and the expression of the pluripotency marker Sox2 in sh-AhR cells. Interestingly, Sox2 increased Aldh1a1 expression in sh-AhR but not in sh-AhR?+?sh-Aldh1a1 cells, suggesting that Aldh1a1 and Sox2 may be co-regulated in melanoma cells. In vivo imaging revealed that mice inoculated with AhR?+?Aldh1a1 knockdown cells had reduced tumor burden and enhanced survival than those receiving Aldh1a1-expressing sh-AhR cells. Conclusions Aldh1a1 overactivation in an AhR-deficient background enhances melanoma progression. Since AhR may antagonize the protumoral effects of Aldh1a1, the AhRlow-Aldh1a1high phenotype could be indicative of bad end result in melanoma. Electronic supplementary material The online version of this article (doi:10.1186/s12943-015-0419-9) contains supplementary material, which is available to authorized users. [3] and [4, 5] genes have been suggested as potentially relevant for the medical center. Aldehyde dehydrogenases (Aldh) are ITM2A enzymes responsible for intracellular aldehyde metabolism [6] that have gained recent interest as potential diagnostic markers in melanoma. The Aldh1a1 isoform, which metabolizes retinal to retinoic acid, appears particularly important because of its ability to regulate melanogenesis [7]. Aldh1a1 has been associated to the malignancy stem/tumor initiating cell phenotype in human sarcomas [8], nasopharylgeal carcinomas [9], breast carcinomas [10] and melanoma [11C13], and its level of expression and/or activity could represent a potential tool to identify stem-like cells in melanoma tumors [11, 14]. In vivo xenografts of Aldh1a1high human melanoma cells in immunodeficient nude [15, 16], NGS [11] or NOD/SCID [12] mice produced larger a more aggressive tumors, suggesting that Aldh1a1 activity favoured tumorigenesis. Nevertheless, the molecular mechanisms by which Aldh1a1 influences melanoma progression are mostly unknown. The dioxin receptor (AhR) integrates signaling pathways controlling not only xenobiotic metabolism but also tissue and organ homeostasis [17]. AhR expression has opposite functions in tumor progression increasing the growth of liver [18] and belly tumors [19] while inhibiting intestinal carcinogenesis [20] in mice. In addition, AhR blocked the epithelial-to-mesenchymal transition (EMT) associated to tumor invasion [21] and its levels were reduced by promoter hypermethylation in acute lymphoblastic leukemia cells [22]. AhR has a role in melanoma main tumorigenesis and lung metastasis. Indeed, we have recently reported that stable AhR knockdown in B16F10 melanoma cells enhanced their tumorigenicity and their metastatic potential to the lungs whereas constitutive AhR activation strongly blocked melanoma progression. AhR knockdown increased melanoma cell migration and invasion and the expression of mesenchymal markers -easy muscle mass actin and Snail. Interestingly, the pro-tumoral phenotype caused by AhR depletion Nandrolone propionate in the tumor cell required AhR expression in the microenvironment as mice could not support tumor growth and metastatization of melanoma cells interfered for AhR [23]. The cell-autonomous effects of AhR depletion appeared to involve an EMT process and an increased content of malignancy stem-like cells. Consistently, human melanoma cells and biopsies from melanoma patients experienced reduced AhR expression as compared to bening nevi [23]. Nevertheless, the molecular intermediates regulating the protumoral effects of AhR deficiency could not be determined. In this study, we have found that Aldh1a1 upregulation is likely an intermediate factor promoting melanoma growth and metastasis in AhR depleted cells. Consistent with that hypothesis, AhR knockdown failed to exert a pro-tumoral effect when Aldh1a1 was simultaneously inactivated. Interestingly, depletion of basal Aldh1a1 levels in AhR-expressing melanoma cells did not significantly impact tumor growth, suggesting that this overactivation of Aldh1a1 is likely Nandrolone propionate a causal factor increasing the tumorigenicity of AhR deficient melanoma cells. Therefore, Nandrolone propionate the tumor suppresor role of AhR in melanoma [23] could take place by antagonizing the Aldh1a1 activity. We suggest that the coordinated expression of AhR and Aldh1a1 could be a useful molecular marker in melanoma. Results AhR levels inversely correlated with Aldh1a1 expression in melanoma cells: AhR knockdown increased Aldh1a1 activity We have shown that stable AhR knockdown (sh-AhR) increases main tumorigenesis and lung metastasis of mouse melanoma cells and that AhR expression was reduced in advanced human melanomas [23]. The increased tumorigenic potential of sh-AhR melanoma cells correlated with higher levels of malignancy stem-like markers, suggesting a more undifferentiated status [23]. On the other hand, aldehyde dehydrogenase 1a1 (Aldh1a1) has been recently identified as a potential melanoma promoter and a regulator of the malignancy stem cell phenotype [11C13, 24]. Here, we have investigated the contribution of Aldh1a1 to the pro-tumorigenic effects associated to AhR deficiency. AhR knockdown in mouse melanoma B16F10 cells significantly increased mRNA and protein expression as compared to wild type B16F10 cells Nandrolone propionate (Fig.?1a). In contrast,.