TGFB1 and chronic obstructive pulmonary disease: Nevertheless, other research has demonstrated that TGFβ1 is not the sole factor inducing fibrosis in small airways in COPD, as endothelin-1 (ET-1), which is a peptide secreted by the endothelial cells of airways, may also stimulate epithelial cells to activate fibroblasts, and it is markedly detected in the sputum of COPD patients.