Overexpression of CD44 variant transcripts in rat transplantable thyroid carcinoma lines demonstrating lung metastasis.

Expression of CD44 isoforms has been reported to be involved in tumor invasion and metastasis in both rodents and man. We earlier documented establishment of rat transplantable thyroid carcinoma lines in vivo from primary lesions induced by a chemical carcinogen. Recently, two lines (L1a-M4 and L2a-M6) were found to spontaneously metastasize to the lung after subcutaneous transplantation. To determine whether CD44 splice variants contribute to their metastatic spread, carcinoma lines with and without lung metastasis were evaluated quantitatively and qualitatively using RT-PCR followed by hybridization and immunohistochemical analyses. The L1a-M4 and L2a-M6 metastatic lines showed significant overexpression of CD44 variant transcripts containing variant exons v4-v6 or v9-v10/v8-v10, respectively, with concomitant reduced levels of standard transcripts. Investigation of the precise composition of alternatively spliced mRNA in normal tissues and carcinoma lines using an exon-specific RT-PCR method, revealed major chain variant transcripts containing v2/v3, v4-v6, v7-v10 and v8-v10 in all specimens. Applying the same RT-PCR analysis to mRNAs derived from cultured cell lines, demonstrated essentially the same pattern. The results suggest that quantitative increase rather than qualitative change in CD44 variant isoforms is associated with the pathogenesis of lung metastasis of rat thyroid carcinomas.