Scientists discovered that breast cancer cells (MCF-7) make the peptide thymosin‑alpha‑1 themselves, and it may help those cells grow. This suggests that taking thymosin‑alpha‑1 could unintentionally support estrogen‑sensitive cancer growth, so caution is advised for anyone at risk.

Growth of normal and malignant cells is controlled by the interplay of several hormones and growth factors present in the body. The growth of the human breast cancer cell line MCF-7 is stimulated in vitro by estradiol which has been shown to induce the release of numerous polypeptide growth factors into the culture media. In a search to identify polypeptide growth factors for MCF-7 breast cancer cells we have detected the thymic hormone, thymosin alpha one (TA1) in culture supernatants and cytosol preparations of MCF-7 cells grown in TA1 free media. TA1 was identified by reverse phase-high performance liquid chromatography (RP-HPLC) followed by a specific radioimmunoassay for TA1 (TA1-RIA). Indirect immune fluorescence localized TA1 on, or within, the cytoplasm of MCF-7 cells grown in TA1 free media. The results of this data along with our previously published findings fulfills three of the four criteria needed to establish thymosin alpha-1 as an autocrine growth factor for MCF-7 cells in culture.