A potential mechanism for the resistance of interferon-alpha treatment in chronic myelogenous leukemia patients

Interferon-α has been used as a therapeutic agent in chronic myeloid leukemia (CML). About 70% of CML patients achieve partial or complete remission after interferon-α treatment of newly diagnosed CML patients. At least 10% of the responding patients develop late resistance to interferon-α (IFN-α) and 30% patients do not respond to interferon treatment at all, even at diagnosis. The reason for this variability in response to IFN-a among CML patients is not known. In an attempt to clarify the mechanism of sensitivity and resistance to IFN-α, several interferon-inducible genes have been examined, including 2′,5′A synthetase, methalothionein II, Class I histocompatibility antigens, 1-8, 6-16, ISG15 (IFI-15K), ISG54 (IFI-54K), IFI-56K, and IFI-78. The expression of these genes to IFN-a induction did not change in Interferon-sensitive or interferoo-resistant patients. We decided to examine the activation of Janus Associated Kinases (JAK) by IFN-α or interferon-y in several human leukemia cell lines based on the fact that (A) several proteins were pbosphorylated specifically at the tyrosine residues in CML and (B) mutation in JAKs could cause leukemia-like hematopoietic defects in Drosophlla. Among the JAK family of kinases discovered so far, JAK1 and Tyk2 were found by others to be involved in IFN-α/0 signal transduction pathways while JAK1 and JAK2 were associated with the interferon-γ signaling pathway. We prepared protein extracts from several leukemia cell lines, immunoprecipitated the JAK1, JAK2 or Tyk2 protein with antibodies against these proteins, and blotted against antiphosphotyrosine antibody for the detection of phosphorylated JAK kinases. We found that in the CML cell line K562, Tyk2 was phosphorylated constitutively. Exposure of cells to either IFN-α or interferon-γ did not further increase the phosphorylation of Tyk2 in K562 cells. The patterns for activation of JAK1 by exposure to either IFN-α or interferon-γ and of JAK2 by interferon-γ did not differ from normal cells or from other leukemia cell lines. These studies suggest that: (A) changes in Tyk2 may be playing a role in interferon-resistance in K562 cells; and (B) the interferon-γ pathway appeared to be intact but the IFN-α pathway response was partially altered at the level of phosphorylation of Tyk2. Further studies will be needed to establish a causal relationship between the relative interferon resistance of K562 blast crisis CML cells and altered phosphorylation of Tyk2.