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Growth factor receptors are frequently amplified and over-expressed in various human cancers. Recently, a Drosophila cell surface protein, Kekkon-1, was found to participate in an epidermal growth factor (EGF) driven negative feedback loop. Kekkon-1 is induced by EGF, binds to the EGF-receptor, and inhibits receptor-mediated signaling. Here, we have searched for human genes with homologies to Kekkon-1 and identified human LIG1. The gene is the human homologue of mouse Lig-1 and is located on chromosome band 3p14, a region frequently deleted in various human cancers. It is predicted to encode a transmembrane cell-surface protein with extracellular leucine-rich repeats and immunoglobulin-like domains. LIG1 mRNA was detected in all tissues analyzed. The highest and lowest relative expression levels were found in brain and spleen, respectively, and differed by more than 200-fold. Taken together, our data are compatible with a role for LIG1 as a growth and tumor suppressor in human tissues.

We have recently identified and cloned the human LRIG1 gene (formerly LIG1). LRIG1 is a predicted integral membrane protein with a domain organization reminiscent of the Drosophila epidermal growth factor (EGF)-receptor antagonist Kekkon-1. We have searched for additional members of the human LRIG family and identified LRIG2 (KIAA0806). The LRIG2 gene was localized to chromosome 1p13 and had an open reading frame of 1065 amino acids. The LRIG2 protein was predicted to have the same domain organization as LRIG1 with a signal peptide, an extracellular part containing15 leucine-rich repeats and three immunoglobulin-like domains, a transmembrane domain, and a cytoplasmic tail. The LRIG2 amino acid sequence was 47% identical to human LRIG1 and mouse Lrig1 (also known as Lig-1). Northern blotting and RT-PCR revealed LRIG2 transcripts in all tissues analyzed. Quantitative real-time RT-PCR showed the most prominent RNA expression in skin, uterus, ovary, kidney, brain, small intestine, adrenal gland, and stomach. Immunoblotting of COS-7 cell lysates demonstrated that heterologously expressed human LRIG2 had an apparent molecular weight of 132 kDa under reducing gel-running conditions. N-glycosidase F treatment resulted in a reduction of the apparent molecular weight to 107 kDa, showing that LRIG2 was a glycoprotein carrying N-linked oligosaccharides. Cell surface biotinylation experiments and confocal fluorescence laser microscopy demonstrated expression of LRIG2 both at the cell surface and in the cytoplasm. LRIG2 was detected in tissue lysates from stomach, prostate, lung, and fetal brain by immunoblotting. In conclusion, LRIG2 was found to be a glycoprotein which was encoded by a gene on human chromosome 1p13 and its mRNA was present in all tissues analyzed.

We have previously characterized three human leucine-rich repeats and immunoglobulin-like domains (LRIG) genes and proteins, named LRIG1-3 and proposed that they may act as suppressors of tumor growth. The LRIG1 transmembrane protein antagonizes the activity of epidermal growth factor receptor family receptor tyrosine kinases. In this study, we evaluated the mRNA expression level of LRIG1-3 in human glioma cell lines and control-matched glioma tissues, characterized the sub-cellular localization of an LRIG3–GFP fusion protein, and analyzed the relationship between sub-cellular localization of LRIG1-3 and clinical parameters in 404 astrocytic tumors by immunohistochemistry. LRIG1-3 mRNA was detected in all human glioma cell lines and matched glioma samples, with large differences in the expression levels. Ectopically expressed LRIG3–GFP localized to perinuclear and cytoplasmic compartments, and to the cell surface of transfected glioma cells. Perinuclear staining of LRIG1-3 was associated with low WHO grade and better survival of the patients. Perinuclear staining of LRIG3 was associated with a lower proliferation index and was in addition to tumor grade, an independent prognostic factor. Furthermore, within the groups of grade III and grade IV tumors, perinuclear staining of LRIG3 significantly correlated with better survival. These results indicate that expression and sub-cellular localization of LRIG1-3 might be of importance in the pathogenesis and prognosis of astrocytic tumors.