Calreticulin Is a Negative Regulator of Bronchial Smooth Muscle Cell Proliferation

Background. Calreticulin controls the C/EBPαp42/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/EBPα 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/EBPα 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/EBPα protein expression in BSM cells. Modulation of calreticulin levels may provide a novel target to reduce BSM remodeling.