Calreticulin Is a Negative Regulator of Bronchial Smooth Muscle Cell Proliferation

Background . Calreticulin controls the C/EBP α p42/p30 at the translational level trough a cis-regulatory CNG rich loop in the CEBPA mRNA. We determined the effects of steroids and long-acting beta-agonists on the p42/p30 ratio and on calreticulin expression in primary human bronchial smooth muscle (BSM) cells. Methods . The effects of budesonide (10 −8  M) and formoterol (10 −8  M) were studied in BSM cells pre-treated with siRNA targeting calreticulin. The expression of C/EBP α and calreticulin was determined by immuno-blotting. Automated cell counts were performed to measure proliferation. Results . All tested BSM cell lines ( n = 5) expressed C/EBP α and calreticulin. In the presence of 5% FBS, the p42/p30 ratio significantly decreased ( n = 3, P < 0.05) and coincided with BSM cell proliferation. High levels of calreticulin were associated with a decreased p42/p30 isoform ratio. FBS induced the expression of calreticulin ( n = 3, P < 0.05), which was further increased by formoterol. siRNA targeting calreticulin increased the p42/p30 ratio in non-stimulated BSM cells and significantly inhibited the proliferation of PDGF-BB-stimulated BSM cells ( n = 5, P < 0.05). Neither budesonide nor formoterol restored the p42 isoform expression. Conclusions . Our data show calreticulin is a negative regulator of C/EBP α protein expression in BSM cells. Modulation of calreticulin levels may provide a novel target to reduce BSM remodeling.