Microphthalmia transcription factor (MITF)is a basic-helix-loop-helix-leucine zippertype transcription factor. The mutant mi and Mi wh alleles encode MITFs with deletion and alteration of a single amino acid, respectively, whereas the tg is a null mutation. In coculture with NIH/3T3 fibroblasts, the numbers of cultured mast cells (CMCs) derived from C57BL/6 (B6)

TSLC1 (tumor suppressor in lung cancer-1) is an adhesion molecule of the Ig superfamily that binds homophilically and mediates cell-cell interactions. Originally, TSLC1 was cloned as a candidate tumor suppressor from the genomic region that frequently exhibits loss of heterogeneity in human non-small-cell lung cancer (NSCLC). However, there have been no studies on TSLC1 expression in normal lungs or NSCLC. Here we show that pulmonary epithelial cells express TSLC1 and its expression levels are often decreased or lost in primary pulmonary adenocarcinoma, a major histologic type of NSCLC. Immunohistochemistry revealed that TSLC1 was localized at cell-cell boundaries of all columnar epithelial cells in mouse embryonic lungs of 10.5 and 13 days postcoitus. Similar staining patterns were observed in bronchiolar and alveolar epithelial cells of adult human lungs, suggesting a physiologic role for TSLC1 in interactions of these cells. Next we performed Western blot analyses of TSLC1 in 47 primary pulmonary adenocarcinomas and judged each tumor as either decreased or nondecreased by comparing TSLC1 expression levels of the tumor with the levels of normal lungs. The expression profiles had a significant relation to histologic subtypes but not to other clinicopathologic parameters. Sixteen bronchioloalveolar carcinomas (BACs) were all judged nondecreased, while 19 of 31 (63%) adenocarcinomas other than BAC were judged decreased (p < 0.0001). Immunohistochemistry of tumors judged nondecreased revealed that not only BAC cells but also tumor cells in lepidic growth components of adenocarcinomas other than BAC expressed TSLC1 on their lateral plasma membranes. These tumor cells are considered less invasive because they proliferate in a lepidic growth pattern along alveolar walls. Thus, the present results not only support the hypothesis that TSLC1 is a tumor suppressor of NSCLC but also suggest that preserved integrity of TSLC1 may contribute to less invasive phenotypes of lepidic growth tumor cells.

SgIGSF (spermatogenic immunoglobulin superfamily) is a recently identified intercellular adhesion molecule of the immunoglobulin superfamily. In a mast-cell cDNA library, we found a clone that resulted from the retention of intron 7 within the mature SgIGSF message. This clone was predicted to encode a soluble isoform of SgIGSF (sSgIGSF) with 336 amino-acid residues because its open reading frame ended just before the transmembrane domain. We constructed a plasmid expressing sSgIGSF fused to the human IgG Fc fragment at its C-terminus (sSgIGSF-Fc), and transfected it into COS-7 cells. The fusion protein was readily detectable in the culture supernatant. Solid-phase binding assay showed that sSgIGSF interacted directly the extracellular domain of membrane-bound SgIGSF (mSgIGSF). We next examined whether this interaction inhibited homophilic binding of mSgIGSF by aggregation assays using L cells that did not express mSgIGSF. A stable L-cell clone that overexpressed mSgIGSF aggregated with each other but not with mock-transfected L cells, indicating that a homophilic interaction of mSgIGSF mediated the aggregation. Addition of sSgIGSF-Fc inhibited the aggregation of L cells overexpressing mSgIGSF in a dose-dependent manner. Moreover, FACScan analyses revealed the specific binding of sSgIGSF-Fc to mSgIGSF expressed in L cells. Binding of sSgIGSF-Fc to mSgIGSF appeared to inhibit homophilic interactions of mSgIGSF.