['Tobacco smoking is considered to be one of the major risk factors for periodontitis. For example, about half the risk of periodontitis can be attributable to smoking in the USA. It is evident that smokers have greater bone loss, greater attachment loss and deeper periodontal pockets than nonsmoking patients. It has recently been reported that endoplasmic reticulum (ER) stress markers are upregulated in periodontitis patients; however, the direct effects of nicotine on ER stress in regard to extracellular matrix (ECM) degradation are unclear. The purpose of this study was to examine the effects of nicotine on cytotoxicity and expression of ER stress markers, selected ECM molecules and MMPs, and to identify the underlying mechanisms in human periodontal ligament cells. We also examined whether ER stress was responsible for the nicotine-induced cytotoxicity and ECM degradation.', 'Cytotoxicity and cell death were measured by 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide assay and flow cytometric annexin V and propidium iodide staining. The mRNA and protein expressions of MMPs and ER markers were examined by RT-PCR and western blot analysis.', 'Treatment with nicotine reduced cell viability and increased the proportion of annexin V-negative, propidium iodide-positive cells, an indication of cell death. Nicotine induced ER stress, as evidenced by survival molecules, such as phosphorylated protein kinase-like ER-resident kinase, phosphorylated eukaryotic initiation factor-2α and glucose-regulated protein-78, and apoptotic molecules, such as CAAT/enhancer binding protein homologous protein (CHOP). Nicotine treatment led to the downregulation of ECM molecules, including collagen type I, elastin and fibronectin, and upregulation of MMPs (MMP-1, MMP-2, MMP-8 and MMP-9). Inhibition of ER stress by salubrinal and transfection of CHOP small interfering RNA attenuated the nicotine-induced cell death, ECM degradation and production of MMPs. Salubrinal and CHOP small interfering RNA inhibited the effects of nicotine on the activation of Akt, JNK and nuclear factor-κB.'] Does endoplasmic reticulum stress modulate nicotine-induced extracellular matrix degradation in human periodontal ligament cells?