Increased Expression of Insulin-Like Growth Factor I Is Associated with Ara-C Resistance in Leukemia.

Cytosine arabinoside (Ara-C) is one of the most common drugs used in the treatment of acute myeloid leukemia (AML). Therefore, the resistance to Ara-C is a major obstacle to improvement of the cure rate of AML. However, the molecular mechanism for Ara-C resistance has not been fully understood. In order to investigate the mechanism involved in Ara-C resistance, the gene expression profile of Ara-C-resistant human K562 leukemia cells (K562/AC cells) was compared with that of the Ara-C-sensitive K562 parental cells by using a cDNA microarray platform. Correspondence analysis demonstrated that the insulin-like growth factor I (IGF-I) gene was significantly upregulated in K562/AC cells. The increased expression of IGF-I in K562/AC cells was confirmed by quantitative RT-PCR (RQ-PCR). We then studied the biological significance of IGF-I upregulation in Ara-C resistance. First, K562/AC cells were treated by suramin, a nonspecific growth factor antagonist. The treatment with suramine reduced viability and induced apoptosis of K562/AC cells. Furthermore, the addition of IGF-I neutralizing antibody exhibited the same effect as suramine. Moreover, the IC50 of Ara-C sensitive K562 parental cells was increased by the addition of exogenous IGF-I ligand. These results strongly suggest that IGF-I-IGF-I-R pathway is directly involved in Ara-C resistance. Next, the activation status of phosphatidylinositol 3-kinase (PI3-kinase)/Akt pathway, which is the downstream of IGF-I-IGFI-R signal, in K562/AC cells was examined. Western blot analysis showed that the phosphorylation level of Akt is higher in K562/AC cells than K562 sensitive cells. In addition, LY294002, the specific inhibitor for PI3-kinase reduced the survival of K562/AC cells. Since activation of PI3-kinase/Akt pathway has been shown to exhibit an anti-apoptotic effect in tumor cells, it is possible that IGF-I induces Ara-C resistance by escaping from apoptosis through PI3-kinase/Akt pathway in an autocrine or paracrine fashion. Finally, the expression level of IGF-I was examined in 27 clinical samples by RQ-PCR. The results showed that IGF-I expression level was higher in leukemic cells in refractory or relapsed AML patients, who received Ara-C combined chemotherapy, than that at diagnosis. These results suggest that the inhibition of the IGF-I-IGF-I receptor pathway is a valuable therapeutic approach to overcome Ara-C resistance in AML.