Free LIF receptor α-chain distal cytoplasmic motifs enhance Jak2-independent STAT3 phosphorylation and induce differentiation in HL-60 cells.

The leukemia inhibitory factor (LIF) affects multiple types of leukemia cells in vitro through the functional LIF receptor (LIFR), which comprises a complex of the LIFR α-chain (LIFR α) and gp130. As Jak2/STAT3 has been proven to be a significant mediator in the LIF-induced differentiation of promyeloid leukemia cells, we constructed a recombinant vector, pcDNA3.0-CT3 (containing the structurally conserved triple YXXQ motifs of LIFR α, termed LIFR α-CT3), and its specific tyrosine-mutated counterpart, pcDNA3.0-MUT, to determine the sites and examine the corresponding mechanisms involved in STAT3 phosphorylation. We found that the triple YXXQ motifs of LIFR α-CT3 are capable of up-regulating phosphorylated levels of STAT3 in a Jak2-independent manner prior to the induction of myeloid differentiation by LIFR α-CT3 in the human promyeloid cell line HL-60. By specifically blocking Jak2 using the AG-490 inhibitor, we observed that the LIFR α-CT3 group of HL-60 cells still demonstrated up-regulation of phosphorylated STAT3 and this up-regulation could result in the myeloid differentiation of HL-60 cells. These results may shed light on acute promyeloid leukemia therapy in future clinical practice.