Abstract
CD26 is a 110-kDa multifunctional molecule having dipeptidyl peptidase IV (DPPIV) enzyme activity and is present on the surface of human T cells. Soluble CD26 (sCD26) exists in human blood and enhances the proliferation of peripheral T lymphocytes induced by tetanus toxoid (TT). The mechanisms by which CD26 enhances the activation of T cells and monocytes remain to be fully elucidated. In this study, we compared the stimulation of THP-1 cells and isolated human monocytes with a combination of recombinant sCD26 and lipopolysaccharide (LPS) and the stimulation of these cells with LPS alone. We found that addition of sCD26 increased TNF-α and IL-6 mRNA and protein expression and enhanced ERK1/2 levels in the cytosol as well as c-Fos, NF-κB p50, NF-κB p65, and CUX1 levels in the nuclei of these cells. On the other hand, the selective DPPIV inhibitor sitagliptin inhibited the increase in TNF-α mRNA and protein expression as well as the increase in ERK, c-Fos, NF-κB p50, NF-κB p65, and CUX1 levels. However, it did not inhibit the increase in IL-6 mRNA and protein expression. We then demonstrated that sCD26 enhanced binding of transcription factors to the TNF- and IL-6 promoters and used reporter assays to demonstrate that transcription factor binding enhanced promoter activity. Once again, we observed differential activities at the TNF- and IL-6 promoters. Finally, we demonstrated that CUX-1 overexpression enhanced TNF- production on sCD26/LPS stimulation, while CUX-1 depletion had no effect. Neither CUX-1 overexpression nor CUX-1 depletion had an effect on IL-6 stimulation. These results are discussed in the context of a model that describes the mechanisms by which stimulation of monocytic cells by sCD26 and LPS leads to elevation of TNF- and IL-6 expression. CUX-1 is identified as a new transcription factor that differently regulates the activities of the TNF- and IL-6 promoters.