Chemotherapeutic drugs change actin skeleton organization and the expression of beta-thymosins in human breast cancer cells.

PURPOSE: Elevated expression of the beta-thymosin isotypes T beta(4), T beta(10), and T(15) appears to be involved in the manifestation of a malignant phenotype of human tumor cells, including those of mammary carcinomas. This has evoked an interest in these peptides as diagnostic/prognostic tumor markers. If increased levels of beta-thymosins correspond to tumor malignancy, the question arises whether tumor growth inhibition induced by chemotherapeutic drugs would reduce their expression.
METHODS: Two human breast cancer cell lines, the estrogen receptor(ER)-positive MCF-7 and the ER-negative MDA-MB231, were thus analyzed for the amount of beta-thymosin mRNAs by RNase protection assay and for the respective peptide levels by HPLC following different hormonal and drug treatments.
RESULTS: Both cell lines, growing in medium with 10% FCS, contain T beta(4) (400-500 fg/cell) and Tbeta(10) (about 100 fg/cell), but no T beta(15). Incubating MCF-7 cells with tamoxifen (1 microM) for 5 days resulted in about 80% growth inhibition and in reduction of intracellular T beta(4) and T beta(10) concentrations by about 40%. Levels of T beta(4) and T beta(10)-mRNA were reduced by about 60%. In contrast, cisplatin (2 microM) changed neither the peptide concentrations nor the mRNA levels of beta-thymosins, in spite of marked growth inhibition. In addition, no changes in beta-thymosin expression were observed in MDA-MB231 cells treated with either drug. MCF-7 cells maintained in estrogen-poor medium (10% horse serum) or stimulated to grow with estradiol (1 nM) had Tbeta(4) and T beta(10) concentrations reduced by about 30%, but changes in T beta(4)- and T beta(10)-mRNA levels did not correspond to those of the peptide.
CONCLUSION: Expression of T beta(4) and T beta(10) mRNAs and their peptides is differentially regulated and does not correlate with growth. Instead, reduced beta-thymosin expression may be linked to more intensive TRITC-phalloidin staining of F-actin lining the membrane at sites of intimate cell-cell contacts, while increased beta-thymosin levels appear in cells with more extensive substrate adhesion. This suggests that beta-thymosins play a role in cell surface dynamics.