Molecular Mechanism Underlying TNFα-Induced Proliferation of Human Airway Smooth Muscle Cells

Airway inflammation occurs as a defense mechanism in response to pathogens, allergens, toxins, or pollutants. The physiological responses of airway inflammation are mediated by proinflammatory cytokines such as tumor necrosis factor alpha (TNFα), that causes airway remodeling, characterized by airway smooth muscle (ASM) hyperreactivity and proliferation. The molecular mechanism underlying TNFα-induced ASM cell proliferation is not fully understood. Previous studies from our lab demonstrated that TNFα exposure induces accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggering an ER stress response that involves autophosphorylation of inositol requiring enzyme 1α at Serine 724 (pIRE1α S724 ). pIRE1α S724 promotes alternative splicing of X-box binding protein 1 (XBP1s) mRNA. XBP1s acts as a ubiquitous transcription factor in ASM cells, targets a number of genes associated with different cellular responses. In the present study, we hypothesized that TNFα promotes XBP1s mediated transcriptional activation of the Cyclin B1, a protein essential for the control of the cell cycle, thus promoting ASM cell proliferation. Human ASM (hASM) cells were dissociated from bronchiolar tissue samples collected during lung surgery from female and male patients (49 to 61 years of age) with no history of chronic respiratory diseases or current smoking. Isolated hASM cells from the same patient were serum-deprived for 48 h and separated into two groups: 1) untreated controls, and 2) treated with TNFα (20 ng/ ml) for 6 h. To confirm the effect of TNFα on hASM cell proliferation, loss of function experiments were employed by: 1) transfecting hASM cells with a non-spliceable XBP1 mutant, and 2) treating the cells with 4μ8C, a pharmacological inhibitor for IRE1α mediated XBP1 splicing. hASM cell proliferation was measured using CyQuant cell proliferation assay. The binding site sequence of XBP1s to the Cyclin B1 promoter was identified by bioinformatic analysis and confirmed by chromatin immunoprecipitation (ChIP) assay. mRNA and protein expressions of Cyclin B1 were determined by qRT-PCR and Western blot respectively. Nuclear localization of Cyclin B1 was measured by nuclear fractionation, followed by western blot of nuclear and cytosolic fraction. Results were analyzed by paired t-tests. TNFα treatment for 6 h induced pIRE1α S724 phosphorylation and XBP1s splicing. Bioinformatic analysis revealed the presence of an XBP1s binding site on the Cyclin B1 promoter. The ChIP assay confirmed that XBP1s binds to the promoter region of the Cyclin B1 genes, and the binding affinity was increased in TNFα-treated hASM cells. Consistent with these results, we found that TNFα treatment increased both mRNA and protein expressions of Cyclin B1 in hASM cells. Nuclear localization of Cyclin B1 increased significantly in TNFα treated cells consistent with an increase in hASM cell proliferation. Inhibition of XBP1 splicing reduced the expression of Cyclin B1 in hASM cells and hASM cell proliferation. In summary, TNFα induces an ER stress response involving pIRE1α S724 phosphorylation and XBP1s splicing. In turn, XBP1s transcriptionally targets expression of Cyclin B1 that mediates hASM cell proliferation. Supported by NIH grant HL157984 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.