Preliminary Functional Studies of hnRNPK in Progression of CML

The blast crisis(BC) is terminal phase of chronic myeloid leukemia (CML), which is accompanied by an increase in both BCR/ABL mRNA and protein level and imatinib(IM) resistance. The BCR/ABL oncoprotein is responsible for inducing and sustaining the leukemic phenotype through its deregulated tyrosine kinase activity which is essential for recruitment and induction of signaling pathways leading to cytokine-independent proliferation, resistance to apoptosis, and impaired differentiation of BCR/ABL-expressing myeloid and lymphoid progenitors. However, the molecular mechanisms responsible for blastic transformation remain poorly understood, although a reasonable assumption is that the unrestrained activity of BCR/ABL in hematopoietic stem/progenitor cells is the primary determinant of disease progression. We had analyzed the changes of protein expression between the bone marrow mononuclear cells in CML-CP and that in CML-BC by the technique of proteome. Compared to CML-CP, the protein expression of heterogeneous nuclear ribonucleoprotein K(hnRNPK) increases in CML-BC. In present study, the over-expression of hnRNPK in CML-BC was verified by western blotting. Furthermore, the expression of hnRNPK increases in a new imatinib-resistant BCR/ABL-positive cell line, K562-R, than in the wild-type K562. After K562 and K562-R were treated by different inhibitors such as PD98059, LY294002, AG490 and imatinib, the protein expression and mRNA level of hnRNPK were detected by western blotting and QRT-PCR. Inhibition of BCR-ABL by imatinib significantly decreased hnRNPK expression in K562 and inhibition of MEK by PD98059, downstream of the BCR-ABL signaling cascade, decreased hnRNPK expression in both K562 and K562-R cells. In addition, down-regulation of hnRNPK by small double-stranded RNA specifically complementary to hnRNPK (siRNA-hnRNPK) can inhibite cell growth and induce maximal G2/M block at 48 hours in both K562 and K562-R cells, whereas cell appototis was observed only at 72 hours by AnnexinV-PI assay. The combination of siRNA-hnRNPK and imatinib induce more cell death in K562-R cells than either treatment alone. Our data therefore suggest that hnRNPK is regulated by the BCR-ABL/MAPK cascade in Ph+ CML. The results that down-regulating hnRNPK expression induced cell-growth arrest and subsequent cell death suggest the potential therapeutic utility of this strategy in patients with CML.