EXPRESSION OF A MAST-CELL TRYPTASE IN THE HUMAN MONOCYTIC CELL-LINES U-937 AND MONO MAC-6

Expression of a mast cell tryptase mRNA was detected in two human mono cytic cell lines, the U-937 and the Mono Mac 6, and in normal human pe ripheral blood(PB) monocytes. In the U-937 cell line but not in normal PB monocytes, the tryptase expression was upregulated 3-50 fold follo wing phorbol ester (PMA)-induced differentiation, but no such inductio n was seen after retinoic acid, interferon-gamma or vitamin D3 exposur e. The tryptases expressed in PMA-induced and non-induced U-937 and in Mono Mac 6 were characterized by PCR amplification and nucleotide seq uence analysis. The U-937 cell line was found to express a tryptase id entical to one of the previously cloned mast-cell beta tryptases (Tryp tase I), and the tryptase expressed in Mono Mac 6 was found to be near ly identical to the previously cloned alpha tryptase. By northern blot analysis with oligonucleotide probes specific for the alpha and beta tryptases both cell lines were found to express only one type of trypt ase. Densitometric quantifications of tryptase mRNA levels, in the two cell lines, showed approximately 80 times higher mRNA levels in Mono Mac 6 compared to non-induced U-937. Immunohistochemical staining for tryptase showed a marked heterogeneity in the Mono Mac 6 cell line. On ly one out of 10 cells were positive for the protein but the levels in these cells were very high, equivalent, or even higher than the level s seen in the human mast cell line HMC-1. This shows that the expressi on of a single tryptase, in this case the alpha tryptase, is sufficien t for the production of a stable protein and probably also a stable pr oteolytically active tetramer. The family of human mast-cell tryptases has been considered to represent a class of proteases specifically ex pressed in mast cells and basophilic leucocytes. The expression of try ptases in two monocytic cell lines and in normal PB monocytes indicate that in humans, the lineage specificity of these serine proteases is less restricted than earlier expected. The cloning of a full length cD NA for the murine counterpart to the human mast cell tryptases, the MM CP-6, is presented. No expression of the MMCP-6 was detected in a pane l of mouse monocyte or macrophage cell lines indicating a species diff erence in the lineage specificity of the 'mast cell tryptases'.