Bassani, G.A., Fleming, J.V., Sousa-Gallagher, M.J., Pico, G.A.

Importance of the proteases Caspase-4 and Caspase-12 in L-histidine decarboxylase

2008

September

Published

1

()

J. A. TEIXEIRA

31

33

Histamine is a mediator of various physiological processes. The formation of histamine is catalyzed by the enzyme L-histidine decarboxylase (HDC; EC 4.1.1.22). Catalytically active mammalian HDC is a homodimer containing two subunits. ER resident proteases (caspases 4 and 12) would be responsible for HDC processing. Alternatively, the 502-504 site identified as a putative cleavage site in the rat HDC protein shows significant sequence similarity to the consensus cleavage site for site 1 protease (S1P). We attempted to develop an experimental cell model expressing high levels of unprocessed and processed forms of the enzyme so we could look at cleavage. Stably transfected HEK293 and PT67 packaging cell lines were generated. We explored the use of endogenous HDC expressing cell lines like RAW and RBL-2H3 cells, and looked at expression and processing in transiently transfected HEK293T cells. Cells were incubated in the presence or absence of a variety of drugs that we expected to affect the ER expression or processing of HDC. While we could detect expression of unprocessed ~74kDa HDC in many of these models we were not satisfied that sufficient levels of cleavage could be detected to support experimental studies specifically on processing. Cos7 cells that were transiently transfected to express rat HDC isoforms containing 502-504, 518/519 or 553/554 mutations. Only the 502-504 mutations significantly affected processing pointing towards a role for S1P in the cleavage of rat HDC. Our data suggest that aspartate cleaving caspases are not important for rat HDC processing in Cos7 cells.