Tumor necrosis factor-alpha (TNF-alpha) is a multifunctional, proinflammatory cytokine that is capable of activating a diverse number of target genes within multiple cell types. Little information is known regarding the role of TNF-alpha in the regulation of human airway mucin hypersecretion and MUC-2 gene expression. To assess the effect of TNF-alpha exposure on mucin secretion, human airway organ cultures and primary cultures of human airway epithelial cells were stimulated with 20 ng/ml of recombinant human TNF-alpha and mucin secretion quantitated by an enzyme-linked immunosorbent assay using a specific monoclonal antibody directed against human airway mucin. Significant increases in mucin secretion from human airway organ cultures were initially detected at 1 h, peaked at 8 h, and persisted for 24 h. The TNF-alpha-mediated mucin hypersecretion at 8 h was concentration dependent. Significant increases in mucin secretion from primary cultures of human airway epithelial cells were initially detected at 4 h, peaked at 48 h, and persisted for 72 h after stimulation with 20 ng/ml of recombinant human TNF-alpha. The TNF-alpha-mediated mucin hypersecretion at 48 h from primary cultures of human airway epithelial cells was inhibited by coincubation with soluble 55 kD, type I TNF receptors. Using reverse transcription-polymerase chain reaction and a human pulmonary mucoepidermoid carcinoma cell line (NCI-H292), increases in MUC-2 steady-state mRNA levels were first detectable after 30 min of TNF-alpha stimulation and persisted for 24 h. Cycloheximide did not inhibit TNF-alpha-mediated MUC-2 mRNA expression at 1 h, suggesting that new protein translation was not required.(ABSTRACT TRUNCATED AT 250 WORDS)
Human serum albumin (HSA) is a cystine-rich serum protein taken up by many cells through receptor-mediated and fluid-phase endocytosis. We hypothesized that HSA may play a role in modulating cellular antioxidant redox signaling. Lung epithelial cells (A549), fibroblasts (HFL1), and blood lymphocytes had increased glutathione (GSH) levels after 8 h incubation with HSA. Similar GSH increases were observed with either plasma-derived or recombinant HSA. Serum depleted of HSA had no effect on cellular GSH. The GSH increase was also observed in normal murine lungs upon in vivo airway instillation of HSA. GSH enhancement was not related to the redox state of the free cysteine residue (Cys-34) on HSA, however, reduction of disulfide bonds in HSA inhibited the increase in cellular GSH. In addition, the albumin-mediated increase in GSH was inhibited by the vacuolar (H(+))-ATPase inhibitors, bafilomycin A(1) and concanamycin, as well as by the membrane pH-disrupting ionophore monensin, but not by 20 mM NH(4)Cl. The degree to which albumin increased GSH levels was sufficient to protect cells against H(2)O(2)-mediated cytotoxicity and to decrease TNF-alpha-mediated NF-kappaB activation. We conclude that albumin specifically modulates cellular GSH levels, an effect sufficient to protect cells against oxidant injury and regulate NF-kappaB activation.
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