Hypertonicity-enhanced TNF-α release from activated human monocytic THP-1 cells requires ERK activation

Background: Hypertonic stress enhances tumor necrosis factor (TNF)-alpha expression in activated monocytes. However, the underlying mechanism is unknown. The produced TNF-alpha is primarily cleaved and released by TNF-alpha-converting enzyme (TACE), and the surface expression of TACE is down-regulated by endocytosis. As hypertonicity inhibits endocytosis, we evaluated the mechanism of hypertonicity-induced TNF-alpha release from activated human monocytic THP-1 cells. Methods: THP-1 cells were stimulated with lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA) in the presence or absence of hypertonic agents (150 mM sucrose or 150-300 mM NaCl). The amount of TNF-alpha mRNA and protein, surface expression of TACE and activation of signaling pathways (mitogen-activated protein kinase, Akt and NF-kappa B) were assayed. Results: Hypertonic sucrose and NaCl significantly enhanced TNF-alpha release from THP-1 cells upon LPS or PMA stimulation. Hypertonic sucrose and other endocytosis inhibitors increased surface expression of TACE, but their effects on TNF-alpha release were inconsistent. This enhancement effect by hypertonicity was not attenuated by inhibition of TACE or I kappa B kinase, but it was blocked by cycloheximide and a MAP/ERK kinase inhibitor. The LPS- or PMA-induced TNF-alpha mRNA expression was not increased; rather, it was inhibited by hypertonicity. ERK1/2 was re-activated after sucrose treatment in LPS-stimulated THP-1 cells. Conclusions: Hypertonicity-enhanced TNF-alpha protein synthesis from LPS- or PMA-activated THP-1 cells requires ERK activation and may proceed without TACE. General significance: A vast amount of TNF-alpha production was regulated by a crucial post-transcriptional manner in activated human monocytic leukemia cells, and it may possibly be contributed to the cachexia condition. (C) 2011 Elsevier B.V. All rights reserved.