Tunicamycin (Tm) caused a marked increase in HSPA5/BiP (Fig. 2b), DDIT3/CHOP, PPP1R15A/GADD34 (Additional file 2: Figure S2A and S2B) and spliced XBP1 (data not shown), indicating that all pathways of the UPR are activated in both COPD and non-COPD epithelial cells. Here, XBP1 is linked to chronic obstructive pulmonary disease.