Chromium (VI) promotes lung cancer initiation by activating EGF/ALDH1A1 signalling

COMMENTARY

Lung cancer is the leading cause of cancer-related mortality, accounting for only 12% of cancer diagnoses but 20% of cancer-related deaths.¹ This is in part due to the relative lack of adequate screening, as well as late onset of symptoms. Accordingly, most patients will present with advanced disease and demonstrate incomplete responses to chemo-, radio- and immunotherapy.² Given the lack of an effective treatment for patients with disseminated disease, there is an ever-increasing interest in chemoprevention, specifically efforts to limit exposure to the known predisposing factors for lung cancer development. Tobacco use is the leading preventable cause of lung cancer and is implicated in 55% of lung cancer deaths in women and over 70% in men.³ Although cigarette smoke is the best studied and most important carcinogen in lung cancer development, there are several occupational carcinogens that have also been shown to increase lung cancer risk.⁴

To this end, hexavalent chromium [Cr(VI)] is emerging as an underappreciated industrial carcinogen. Cr(VI) is a metal ion often added to materials to enhance durability and corrosive resistance and is used extensively in manufacturing. Welders have a particularly high risk of occupational exposure, as high temperatures oxidize the chromium in stainless steel to the Cr(VI) form, which can be expelled in fumes and inhaled. Cr(VI) can subsequently accumulate in the bronchial epithelium over time. Although proper protective equipment and/or fume extractors can reduce Cr(VI) inhalation, they do not eliminate it entirely, and Cr(VI) exposure remains an occupational hazard for select sectors of the manufacturing industry.

Although the link between Cr(VI) exposure and lung cancer risk is generally well accepted,⁵ the mechanisms through which Cr(VI) enhances lung tumourigenesis are unclear. Recent evidence suggests that intracellular processes reduce Cr(VI) to Cr(III), which becomes trapped within cells and promotes an increase in reactive oxygen species (ROS). It has been proposed that this increase in ROS leads to genomic instability, thereby predisposing to lung cancer development.⁶ Although this is a plausible mechanism for the means through which Cr(VI) promotes or accelerates tumour development, the downstream effects of Cr(VI) on tumour-permissive cell signalling events are largely unexplored.

In the current study by Ge et al. published in Clinical and Translational Medicine, the authors describe a novel means through which Cr(VI) alters cell signalling to drive lung cancer initiation. In brief, the authors first established an in vitro system to model prolonged exposure to Cr(VI). They utilized the non-tumourigenic BEAS-2B human lung epithelial cell line, incubating cultures with gradually increasing concentrations of Cr(VI) over an 18-month period. After confirming the increased malignant potential of these cells, the authors referred to this new cell line as Cr(VI)-transformed (CrT) cells. They then characterized changes in gene expression and found that the CrT cell line displayed a significant increase in the Aldehyde dehydrogenase 1 family member A1 (ALDH1A1), an enzyme critical for detoxification of aldehyde substrates through NAD(P)+-dependent oxidation, with central roles in genesis, maintenance and self-renewal of cancer stem cells.⁷ Accordingly, these CrT cells demonstrated increased sphere-forming capacity and other features of stemness. Those with the highest stem cell character/ALDH1A1 expression were named CrT/tumour initiating cells, or CrT/TICs.

After demonstrating that ALDH1A1 is required for the CrT/TIC phenotype, the authors also showed that these cells harbor increased expression of the pro-stemness transcription factor Krüppel-like factor 4 (KLF4). KLF4 was required for the observed increase in ALDH1A1 expression, and the authors therefore proposed that Cr(VI) induces ALDH1A1 expression via KLF4-mediated transcriptional up-regulation. Interestingly, they found that CrT and particularly CrT/TIC cells exhibited an increase in secreted epidermal growth factor (EGF), a mitogen with known roles in the genesis and maintenance of cancer stem cells.⁸ Conditioned media from CrT and CrT/TIC cells was sufficient to stimulate Mitogen-activated protein kinase (MAPK) signalling in HCC95 and H226 tumour cells, as well as enhance their proliferation in vitro. The observed increase in EGF expression was mitigated with either siRNA against ALDH1A1, pharmacologic inhibition of ALDH1A1, or siRNA against KLF4. In each case, the reduction in EGF was associated with a parallel reduction in stem cell characteristics and a reduced capacity to promote the growth of tumour cells in conditioned media experiments.

Similar results were observed in orthotopic xenografts, in which CrT/TIC cells accelerated tumour formation and enhanced downstream EGF signalling in an ALDH1A1-dependent manner. Importantly, ALDH1A1 inhibition sensitized xenografted tumours to the chemotherapy agent Gemcitabine, reducing tumour burden and extending overall survival. Finally, in primary lung squamous cell carcinoma specimens, ALDH1A1 expression positively correlated with EGF receptor activation. ALDH1A1 expression was also increased in patients with more advanced disease, and accordingly, patients with high ALDH1A1 expression had worse overall survival compared to those with low ALDH1A1 expression.

In addition to describing a novel mechanism through which Cr(VI) promotes lung cancer initiation, this study also offers insight into a potential means through which these events can be overcome. Accordingly, ALDH1A1 inhibition may be a viable strategy to treat lung cancer patients with a history of extensive Cr(VI) exposure, particularly when combined with Gemcitabine chemotherapy for patients with advanced disease. As Gemcitabine has been shown to both enhance antigen presentation and lead to extensive immune remodeling in other cancers,^{9, 10} the combination of ALDH1A1 inhibition, Gemcitabine and immune checkpoint inhibitors also warrants consideration.

Finally, this study underscores the importance of either limiting Cr(VI) use or taking steps to reduce exposure in the industrial setting. Although those in the manufacturing industry are at the highest risk of hazardous Cr(VI) exposure, over 250 million Americans rely on tap water containing unsafe levels of Cr(VI) (https://www.ewg.org). Hence, there needs to be a concerted effort to address the widespread Cr(VI) contamination in the United States, as well as to identify safer alternatives for the manufacturing industry. Should the latter not prove feasible, at minimum, we recommend comprehensive safety training and improved personal protective equipment for those most at risk, particularly given the results of this study.

CONFLICT OF INTEREST

The authors declare no conflict of interest.