Given the ubiquitous expression and multiple regulatory roles of Atoh8 during embryonic development, it is not at all surprising to see its involvement in cancer and several other disorders. The high-grade gliomas are diagnosed on the basis of different molecular markers. The 1p19q codeletion is one characteristic marker for oligodendroglioma. In contrast, the epidermal growth factor receptor (EGFR) amplification is seen in glioblastoma. In 2008, a study performed to understand the two mutually exclusive groups of gliomas carrying ‘1p19q codeletion’ and ‘EGFR amplification’, identified Atoh8 as one of the multiple differentially regulated genes. Higher expression of Atoh8 was found to be correlated with the pro-neural group of gliomas particularly oligodendroglioma with ‘1p19q codeletion’ and lower expression of Atoh8 was found to be correlated with a proliferative and mesenchymal group of gliomas with ‘EGFR amplification’. Likewise, another study which was performed to ascertain the copy number variations in glioblastomas has also revealed aberrant expression of Atoh8 further suggesting it as a potential candidate involved in carcinogenesis [50,51]. Following this, another study which was investigating the effect of retinoic acid on glioblastoma stem cell-like cells also showed an increase in the expression of Atoh8 following treatment with retinoic acid which was found to be correlated with differentiation of the glioblastoma stem cell-like population [52]. In recent years, Atoh8 has been further studied in other types of cancers. In hepatitis B virus-associated hepatocellular carcinomas (HCC), reduced levels of Atoh8 were observed to be correlated with loss of tumor differentiation. Confirming the same, analysis of additional hepatic carcinoma cell lines such as HepG2, PLC8024 and CRL8064 also revealed a correlation between reduced Atoh8 expression and increased CD133 positive population. Additionally, the authors have also evaluated the effect of overexpression of Atoh8 in the hepatic carcinoma cell lines, which resulted in the reduction of the proliferation along with reduced invasive and migratory abilities of those cells. Lastly, overexpression of Atoh8 was also shown to reduce tumor formation and increase chemosensitivity in these cells. Most importantly, this study showed that Atoh8 could repress core regulators of pluripotency such as Oct4 and Nanog by binding to their E-Box sequences [22,53].

In nasopharyngeal carcinomas (NPC), similar to the findings in hepatic carcinomas, the inhibition of Atoh8 was shown to enhance the malignant phenotype, whereas its transgenic expression was shown to reverse the phenotype. In this study, the authors identified the silencing of Atoh8 in tumor cells as an epigenetic mechanism induced by the tumorigenic LMP1 (Latent Membrane Protein-1). In tumor cells, the occupancy of active H3K4me3 of the Atoh8 promoter sequence was found to be replaced by a repressive H3K27me3 mark. Lastly, this study further described Atoh8 as a regulator of epithelial-mesenchymal transition (EMT) because of its correlation with EMT markers [54].

In colorectal cancer (CRC), higher expression of Atoh8 was found to correlate with the poor prognosis of CRC patients. The knockdown of Atoh8 in CRC cells was shown to reduce proliferation with an accumulation of cells in the S phase of the cell cycle. Additionally, the depletion of Atoh8 in CRC cells was observed to increase apoptosis with no apparent changes to the migratory ability of the cells which is in clear contrast to the findings observed in HCC and NPC [55]. Another study, which used colorectal cancer cells as mimics of circulating tumor cells (m-CTCs), showed Atoh8 as a mechanosensor following their exposure to laminar shear stress. This study indicated that activation of Atoh8 in m-CTCs increased their plasticity and intravascular survival suggesting a promoting effect of Atoh8 [56].

In breast cancer, a study was performed to identify the enriched tissue-specific transcription factors which connected Atoh8 to the dysregulated transcriptional regulatory network which affected the proliferation, differentiation, cell adhesion and metastasis [57]. Interestingly, a most recent study has identified a novel isoform of Atoh8 called Atoh8-V1 in breast cancer, it was shown to be a negative prognostic marker with a high expression. Atoh8-V1 was further shown to bind and activate the RhoC promoter which in turn promotes metastasis in breast cancer. Similarly, downregulation of Atoh8-V1 was shown to correlate with reduced metastasis [13]. Given this new transcriptional variant, it would be interesting to see if the contradictory reports that are mentioned in the case of different cancers result from divergent functions of Atoh8 or because of the existence of more transcriptional variants.

In prostate cancer, Atoh8 was reported to be epigenetically active with high expression [58]. Similarly, in renal cell carcinoma, higher levels of Atoh8 was also shown as the worst prognostic marker [59].

So far, Atoh8 has been implicated to be involved in cellular stress response, hypoxia response, and iron metabolism. Atoh8 was identified to be activated by shear stress during endothelial cell differentiation. It was further reported that overexpression of Atoh8 mimicked shear-stress treatment attributing a major role to Atoh8 in stress management. In addition to shear stress, IFN-γ, TNF-α and oxidative stress were shown to act upstream and activate Atoh8 expression in endothelial cells. Furthermore, eNOS which plays a major role in endothelial cell protection, nitric oxide production was shown to be positively regulated, at the same time as a direct target of Atoh8 [21,47]. The phenotype observed in Atoh8 knockout mice which was similar to pulmonary arterial hypertension together with the reduced levels of the eNOS following knock-down or absence of Atoh8 correlates with the phenotype of defective placental development observed in Atoh8 knockout mice (Table 1) [21,43,48]. In addition to this, it would also be of great interest to check if Atoh8-associated stress response is restricted to the endothelial cells or if it could perform a similar function in other cell types as it is ubiquitously expressed.

Atoh8 was identified as a downstream factor for activating transcription factor 4 (ATF4) in pancreatic ß-cells. ATF4 is a transcriptional regulator for integrated stress response (ISR) and unfolded proteins. Severe diabetes is led by dysregulation of ISR. Thereby ATF4 senses endoplasmic reticulum stress in cellular pathologies and is responsible for the maintenance of homeostasis in the endoplasmic reticulum. The ISR enhancer Sephin1 increased the ATF4 expression in the pancreatic islets and increased the insulin secretion [61].

Lately, Atoh8 was also shown to regulate hypoxic response. A study by Morikawa’s group showed that Atoh8 binds hypoxia-inducible factor 2α (HIF-2α) and decreases its abundance leading to the attenuation of the hypoxic response. It was further shown that the Alk1/Smad/Atoh8 axis acts as a regulator of the hypoxic response. Interestingly, analysis performed on HPAECs showed the HLH domain of Atoh8 to be crucial for hypoxic response with no functional role of the basic domain [48]. Recently, another study performed on hypoxia adaptation of Tibetan pigs also showed Atoh8 as a potential regulator of hypoxia, which has the potential to modulate TGF-ß and PI3K-AKT signaling [62].

In addition to stress and hypoxia regulation, Atoh8 was also shown to regulate metabolism, especially iron metabolism. Atoh8 was observed to regulate hepcidin which prevents the overloading of iron in the body. In hepatocytes, BMP6 was shown to activate Atoh8 (Figure 3), which further binds to the promoter of hepcidin and promotes hepcidin production, ultimately regulating the uptake of iron by the duodenal epithelium [63,64]. Despite the existence of multiple Atoh8 knockout mouse models, no phenotype concerning iron metabolism or liver has been reported so far.