[Molecular biology of L-histidine decarboxylase]

L-Histidine decarboxylase (HDC) catalyzes the formation of histamine from L-histidine. This biogenic amine is known to exert various effects in physiological and pathological reactions. In contrast to the well-known mechanism of histamine action through its interaction with specific receptors, the mechanisms regulating HDC gene expression are not elucidated. We have purified HDC from mouse mastocytoma cells, and isolated mouse HDC cDNA, and found that the primary translated product is posttranslationally processed to yield a mature active enzyme. In mastocytoma cells, we demonstrated that the induction of HDC activity and HDC mRNA synergistically occurred on treatment with dexamethasone+TPA, and also cAMP+Ca2+. To clarify the mechanism of up-regulation by these stimuli of the transcription of the HDC gene, we have isolated a genomic DNA clone encoding 5'-flanking region sequence and the first two exons. The transcription start site and the nucleotide sequences of the promoter regions including TATA- and GC-boxes were determined. With mastocytoma cells transiently transfected with 5' deletion constructs of HDC-CAT fusion gene, it was found that the sequences from -132 to -53 and -267 to -53 are essential for the regulatory elements involved in the increased transcription of the HDC gene with dexamethasone+TPA and cAMP+Ca2+, respectively. Furthermore, we have isolated a genomic DNA from human basophilic cells, and analysed its structure to elucidate the mechanisms regulating the tissue specificity of HDC gene expression.