Upregulation of interferon-alpha receptor expression in hydroxyurea-treated leukemia cell lines

Interferon-alpha (IFN-alpha) shows its antitumor effect through binding to specific cell surface receptors. A DNA synthesis inhibitor, hydroxyurea (HU), has been successfully combined with IFN-alpha to improve the efficiency of IFN therapy for chronic myelogenous leukemia (CML). To understand the mechanism of this combination effect, expression of IFN-alpha receptors on the CML cell line, K562, was studied before and after treatment with HU.

Cells were treated with HU at a dose of 0, 0.1, 0.2, or 0.4 mmol/L for 48 hours. Binding assays were performed using 125I-labeled IFN-alpha at 4 degrees C. Cell cycle analysis was carried out using flow cytometer following staining cellular DNA with propidium iodide. Northern blot analysis was performed to evaluate the inducibility of interferon regulatory factor-1 (IRF-1) gene expression by IFN-alpha.

Hydroxyurea-treated cells showed a dose- and time-dependent increase in binding of 125I-labeled IFN-alpha (maximal 2.5-fold). The increase of binding was caused by an increase in the number of binding sites with a constant receptor affinity. Similar results were obtained in the Burkitt's lymphoma cell line, Daudi. Cell cycle analyses suggested that upregulation of the IFN receptor may have occurred as a result of the alteration in the cell cycle distribution. Furthermore, IFN-alpha induction of the IFN-inducible gene IRF-1 mRNA in HU-treated K562 cells was 2-fold higher than that in untreated cells.

Thus, HU may have an ability to enhance the response to IFN-alpha probably because of its ability to upregulate the IFN-alpha receptors, suggesting that this may be involved in the mechanism of effective combination therapy of IFN-alpha with HU.