Pulmonary toxicity and lung tumorigenic potential of surrogate metal oxides in gas metal arc welding–stainless steel fume: Iron as a primary mediator versus chromium and nickel

In 2017, the International Agency for Research on Cancer classified welding fumes as “car- cinogenic to humans” (Group 1). Both mild steel (MS) welding, where fumes lack carcino- genic chromium and nickel, and stainless steel (SS) increase lung cancer risk in welders; therefore, further research to better understand the toxicity of the individual metals is needed. The objectives were to (1) compare the pulmonary toxicity of chromium (as Cr(III) oxide [Cr2O3] and Cr (VI) calcium chromate [CaCrO4]), nickel [II] oxide (NiO), iron [III] oxide (Fe2O3), and gas metal arc welding-SS (GMAW-SS) fume; and (2) determine if these metal oxides can promote lung tumors. Lung tumor susceptible A/J mice (male, 4–5 weeks old) were exposed by oropharyngeal aspiration to vehicle, GMAW-SS fume (1.7 mg), or a low or high dose of surrogate metal oxides based on the respective weight percent of each metal in the fume: Cr2O3 + CaCrO4 (366 + 5 μg and 731 + 11 μg), NiO (141 and 281 μg), or Fe2O3 (1 and 2 mg). Bronchoalveolar lavage, histopathology, and lung/liver qPCR were done at 1, 7, 28, and 84 days post-aspiration. In a two-stage lung carcinogenesis model, mice were initi- ated with 3-methylcholanthrene (10 μg/g; intraperitoneal; 1x) or corn oil then exposed to metal oxides or vehicle (1 x/week for 5 weeks) by oropharyngeal aspiration. Lung tumors were counted at 30 weeks post-initiation. Results indicate the inflammatory potential of the metal oxides was Fe2O3 \u3e Cr2O3 + CaCrO4 \u3e NiO. Overall, the pneumotoxic effects were negligible for NiO, acute but not persistent for Cr2O3 + CaCrO4, and persistent for the Fe2O3 exposures. Fe2O3, but not Cr2O3 + CaCrO4 or NiO significantly promoted lung tumors. These results provide experimental evidence that Fe2O3 is an important mediator of welding fume toxicity and support epidemiological findings and the IARC classification

Experimental timeline and block design for experimental protocol 1: Bronchoalveolar lavage and histopathology/gene expression studies.

Two groups of 256 mice were used for parallel BAL and histopathology/gene expression studies. Each group of mice was separated into 4 blocks with 8 treatment groups corresponding to the low or high doses of metal oxides, sham or GMAW-SS fume (8 groups x 8 mice/group x 4 blocks = 256 mice). Mice were sacrificed at 1, 7, 28, and 84 days after a single oropharyngeal aspiration exposure.</p

A bronchiolo-alveolar adenoma in an Fe₂O₃-exposed mouse 30 weeks post-initiation with MCA at 10x magnification (panel A) and 20x magnification (panel B).

A bronchiolo-alveolar adenoma in an Fe2O3-exposed mouse 30 weeks post-initiation with MCA at 10x magnification (panel A) and 20x magnification (panel B).</p

BAL cytokine analysis at 28 days post-exposure to GMAW-SS fume or surrogate metal oxides.

Color scheme of yellow-orange-red represent fold change from sham (darker red represents greatest change).</p

BAL cytokine analysis at 1 day post-exposure to GMAW-SS fume or surrogate metal oxides.

Color scheme of yellow-orange-red represent fold change from sham (darker red represents greatest change).</p

Exudate and brown material in the bronchial lumen of a mouse exposed to GMAW-SS fume and sacrificed 1 day post-exposure (panel A; 10x magnification).

Mononuclear cell infiltrate of alveolar wall in a mouse exposed to GMAW-SS fume (panel B; 20x magnification) or low-dose Fe2O3 (panel C; 40x magnification) and sacrificed 7 days post-exposure.</p

Relative mRNA levels as mean fold change ± standard error compared to sham (mean fold change of 1) in the lungs and liver at 1 day post-exposure to GMAW-SS fume or Fe₂O₃.

Relative mRNA levels as mean fold change ± standard error compared to sham (mean fold change of 1) in the lungs and liver at 1 day post-exposure to GMAW-SS fume or Fe2O3.</p

Ability of alveolar macrophages to phagocytose <i>E</i>. <i>coli</i> GFP after exposure to GMAW-SS fume or metal oxides.

E. coli uptake by macrophages was quantified by flow cytometry. Data presented as percent change from sham (dashed line– 100%). *p<0.05 compared to sham.</p

A, B, C, D.

Scanning electron microscopy images of GMAW-SS (panel A), NiO (panel B), Fe2O3 (panel C), and Cr2O3 + CaCrO4 mixture (panel D).</p

Experimental protocol 2: Two-stage initiation-promotion lung tumor bioassay.

200 male A/J mice were organized into 5 groups: MCA/NiO, MCA/Cr2O3 + CaCrO4, MCA/ Fe2O3, MCA/sham, or CO/sham. Beginning 1 week post-initiation with MCA or CO, mice were exposed to the metal oxide or sham by oropharyngeal aspiration once per week for 5 weeks (panel A). Doses of metal oxides were the cumulative high doses from experimental protocol 1 (panel B).</p