4-1BB is up-regulated in human mast cells, when exposed to tumor conditioned medium
Mast cells have for a long time been known to accumulate around tumors (Maltby et al., 2009). Studies show that they may be important and sometimes essential in tumor angiogenesis, tumor growth and metastasis of tumors (Soucek, et al., 2007; Xiang et al., 2010). In an expression array study (Wensman et al, submitted manuscript) performed on mouse mast cells exposed to tumor conditioned medium, the gene 4-1BB was among the most up-regulated genes compared to control medium. 4-1BB codes for a membrane receptor protein of the tumor necrosis factor superfamily and has been shown to be important in T cell regulation in tumor diseases. This study was performed to investigate if human mast cells up-regulate 4-1BB when they get exposed to tumor conditioned medium. Two human mast cell lines were used: HMC-1 and Lad-2. When comparing them, Lad-2 had a more clear response to tumor conditioned medium and showed higher expression of 4-1BB. Two different tumor cell lines were used for studying the mast cell response to tumors: The thyroid carcinoma cell line Kat-4 and the melanoma cell line SK-MEL-30. Using Kat-4 to stimulate the human mast cells led to an up-regulated gene expression of 4-1BB while the use of SK-MEL-30 as a stimulant did not. The gene expression of 4-1BB in Lad-2 peaked after four hours of stimulation with serum free medium conditioned with Kat-4 cells. After twenty-four hours the gene expression of 4-1BB decreased, which is a pattern that followed what Nishimoto Et al. showed in 2005 where expression of 4-1BB was investigated in mouse mast cells. Protein expression of 4-1BB was measured after zero, four and twenty-four hours. No up-regulated protein expression of 4-1BB was observed in neither HMC-1 cells nor Lad-2 cells after stimulation with tumor cell line conditioned medium, however there was an increase of 4-1BB protein in response to calcium ionophore. In this study human mast cells responded to stimulation of tumor factors by significantly up-regulate the gene 4-1BB. Further, a correlation between expression of 4-1BB mRNA and 4-1BB protein was not observed.