Expression of Inositol, 1,4,5-trisphosphate Receptor in the Nucleus of Human Benign and Malignant Squamous Cells and Its Significance

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Inositol 1,4,5-trisphosphate (IP_3) is one of the important messengers of the intracellular signal transduction, and controls the cytoplasmic flow of calcium ion in conjunction with its receptor (IP_3-R). The Ip_3-R protein and its coding gene had been revealed by Mikoshiba et al., and monoclonal antibodies reactive with mouse IP_3-R protein have been produced as well. Using cross-reactivity of these monoclonal antibodies (10A6,4C11,18A10) with the human counterpart we have shown the distribution of IP_3-R in human tissues and cells, especially in normal skin including benign squamous epithel, squamous cell carcinoma-tissues and cultured cell lines of squamous cell carcinoma. In the present researchproject we examined peculiar reactivity of 10A6 monoclonal antibody with an antigen in the nucleus. Presence of the antigen was most clearly shown in the nuclei of cultured squamous carcinoma cells. Observation by confocal laser scanning microscopy revealed its fine granular distribution in the nucleus but not in the nucleolus. The amount of the antigen as assessed by fluorescence intensity of immunostaining differed among cell lines, but there were no correlation with the stage of differentiation in squamous cell lineage or to the tissues where carcinoma arose. Attempt to reveal ultrastructural localization did not succeed because of the weaknees of the antigenicity againt various fixatives. Since funcitonal analysis indicates the presence of IP_3-R only in the nuclear membrane but not in the nucleus, 10A6 monoclonal antibody might react with crossreacting antigen other than IP_3-R protein itself. Further investigation to reveal nature of the antigen in the nucleus of human cells is now in progress.