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

Abstract.

Purpose: Elevated expression of the β-thymosin isotypes Tβ₄, Tβ₁₀, and T₁₅ 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 β-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 β-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 Tβ₄ (400–500 fg/cell) and Tβ₁₀ (about 100 fg/cell), but no Tβ₁₅. Incubating MCF-7 cells with tamoxifen (1 µM) for 5 days resulted in about 80% growth inhibition and in reduction of intracellular Tβ₄ and Tβ₁₀ concentrations by about 40%. Levels of Tβ₄ and Tβ₁₀-mRNA were reduced by about 60%. In contrast, cisplatin (2 µM) changed neither the peptide concentrations nor the mRNA levels of β-thymosins, in spite of marked growth inhibition. In addition, no changes in β-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 Tβ₄ and Tβ₁₀ concentrations reduced by about 30%, but changes in Tβ₄- and Tβ₁₀-mRNA levels did not correspond to those of the peptide. Conclusion: Expression of Tβ₄ and Tβ₁₀ mRNAs and their peptides is differentially regulated and does not correlate with growth. Instead, reduced β-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 β-thymosin levels appear in cells with more extensive substrate adhesion. This suggests that β-thymosins play a role in cell surface dynamics.