Cadmium stimulates myofibroblast differentiation and mouse lung fibrosis

Increasing evidence suggests that Cd at levels found in the human diet can cause oxidative stress and activate redox-sensitive transcription factors in inflammatory signaling. Following inflammation, tissue repair often involves activation of redox-sensitive transcription factors in fibroblasts. In lungs, epithelial barrier remodeling is required to restore gas exchange and barrier function, and aberrant myofibroblast differentiation leads to pulmonary fibrosis. Contributions of exogenous exposures, such as dietary Cd, to pulmonary fibrosis remain incompletely defined. In the current study, we tested whether Cd activates fibrotic signaling in human fetal lung fibroblasts (HFLF) at micromolar and submicromolar Cd concentrations that do not cause cell death. Exposure of HFLF to low-dose Cd (<= 1.0 mu M) caused an increase in stress fibers and increased protein levels of myofibroblast differentiation markers, including a-smooth muscle actin (alpha-SMA) and extra-domain-A-containing fibronectin (ED-A-FN). Assay of transcription factor (TF) activity using a 45-TF array showed that Cd increased activity of 12 TF, including SMAD2/3/4 (mothers against decapentaplegic homolog) signaling differentiation and fibrosis. Results were confirmed by real-time PCR and supported by increased expression of target genes of SMAD2/3/4. Immunocytochemistry of lungs of mice exposed to low-dose Cd (0.3 and 1.0 mg/L in drinking water) showed increased a-SMA protein level with lung Cd accumulation similar to lung Cd in non-smoking humans. Together, the results show that relatively low Cd exposures stimulate pulmonary fibrotic signaling and myofibroblast differentiation by activating SMAD2/3/4-dependent signaling. The results indicate that dietary Cd intake could be an important variable contributing to pulmonary fibrosis in humans.