Expression of a splice variant of KAI1, a tumor metastasis suppressor gene, influences tumor invasion and progression.

KAI1 (CD82) belongs to the transmembrane 4 superfamily in which members have inhibitory effects on tumor cell motility and metastasis. During reverse transcription-PCR analysis, we found a splice variant of KAI1 (spliced-KAI1) in which exon 7 was deleted. This exon encodes the 28 amino acids that span from the distal part of the second extracellular loop to the proximal part of the fourth transmembrane region. Expression of spliced-KAI1 was observed in metastatic tissues of gastric cancer patients with poor prognosis after operation. Genomic DNA analysis revealed that this variant was derived from the alternative splicing of exon 7. Immunoprecipitation showed that the interaction of spliced-KAI1 with integrin alpha(3)beta(1) was weaker than that of wild-type KAI1. Wild-type KAI1, but not spliced-KAI1, colocalized with E-cadherin, an adherens junction protein. Also, mouse colon adenocarcinoma cells stably expressing spliced-KAI1 (CT-26/spliced-KAI1) showed increased in vivo tumorigenicity, as well as increased in vitro invasive potential and cell-extracellular matrix adhesion compared with wild-type KAI1-expressing cells. In metastatic lung and liver tissues from mice inoculated with CT-26/spliced-KAI1 cells, the expression of wild-type KAI1 was nearly absent and spliced-KAI1 was dominant, and weak interaction of KAI1 with integrin alpha(3)beta(1) was observed. These results indicate that there is a functional difference between wild-type KAI1 and spliced-KAI1 in respect to cell motility, adhesion, tumor growth and metastasis, and expression of spliced-KAI1 may be a marker for poor prognostic factors in gastric and other cancers.

KAI1 COOH-terminal interacting tetraspanin (KITENIN), a member of the tetraspanin family, interacts with KAI1, a tumor metastasis suppressor, and enhances metastasis of cancer.

We cloned recently an alternatively spliced variant of KAI1 mRNA that lacked exon 7 at the COOH-terminal region and showed differences in metastasis suppression when compared with the wild-type KAI1. These findings indicated that the COOH-terminal region of KAI1 is critical for its metastasis suppressor function. In this study, we isolated a cDNA clone of VANGL1, a member of the tetraspanin protein family, which interacted specifically with the COOH-terminal cytoplasmic domain of KAI1 in the yeast two-hybrid system. We renamed it KAI1 COOH-terminal interacting tetraspanin (KITENIN). We found that KITENIN-overexpressing CT-26 mouse colon cancer cells showed increased tumorigenicity and early hepatic metastasis in vivo, as well as increased invasiveness and adhesion to fibronectin in vitro compared with parental cells. Moreover, increased levels of KITENIN were observed in a human gastric tumor and its metastatic tissues, compared with the normal adjacent mucosa. Our results indicate that KITENIN promotes adhesion and invasion of cancer cells in vitro and in vivo, and suggest that KITENIN participates in the regulation of the tumor formation and metastasis by interacting with KAI1, a metastasis suppressor and antisense KITENIN strategy that can be used to inhibit metastasis in various cancers.