Expression of Tropomyosin 1 Gene Isoforms in Human Breast Cancer Cell Lines

Nine malignant breast epithelial cell lines and 3 normal breast cell lines were examined for stress fiber formation and expression of TPM1 isoform-specific RNAs and proteins. Stress fiber formation was strong (++++) in the normal cell lines and varied among the malignant cell lines (negative to +++). Although TPM1.. and TPM1.. were the dominant transcripts of TPM1, there was no clear evidence for TPM1.. protein expression. Four novel human TPM1 gene RNA isoforms were discovered (lambda, mu, v, and xi), which were not identified in adult and fetal human cardiac tissues. TPM1.. was the most frequent isoform expressed in themalignant breast cell lines, and it was absent in normal breast epithelial cell lines. By western blotting, we were unable to distinguish between TPM1..,.., and] protein expression, which were the only TPM1 gene protein isoforms potentially expressed. Some malignant cell lines demonstrated increased or decreased expression of these isoforms relative to the normal breast cell lines. Stress fiber formation did not correlate with TPM1.. RNA expression but significantly and inversely correlated with TPM1.. and TPM1.. expression, respectively. The exact differences in expression of these novel isoforms and their functional properties in breast epithelial cells will require further study.