The E3 Ubiquitin Ligases Siah2 Contributes To Imatinib Resistance In Chronic Myeloid Leukemia In Hypoxia Condition

Severe hypoxia has been shown to favor the self-renewal of human hematopoietic stem cells. Recent studies demonstrate that hypoxia via hypoxia-inducible factor (Hif-1a) can modify the proliferation and differentiation of CML stem cells. The ubiquitin E3 ligase SIAH2 is an important regulator of the hypoxic response as it leads to the ubiquitin/proteasomal degradation of prolyl hydroxylases such as PHD3, which in turn increases the stability of Hif-1a. The Hif-1a has been linked to chemosensitivity while the underlying molecular mechanism remains elusive. Therefore, we comprehensively analysed SIAH2 and Hif-1a role in determining chemosensitivity via signal molecule vascular epithelial growth factor (VEGF) pathway.

We tested the level of Siah2, Hif-1a and VEGF in Imatinb-sensitive CML Patients (n=15) and insensitive CML patients(n=10) by real-time reverse transcription PCR and western blot analysis. K562-wild type (K562-W) and K562-imatinib-resistance type (K562-R) cell were cultured for 24h and 48h under the condition of normal and hypoxia concentration of oxygen (1%). Knockdown of SAIH2 by RNA interference ,Cell viability and IC50 under 1% concentration oxygen were tested by cck-8;detected cell cycle and apoptosis by flow cytometry (FCM) ; analyzed the expression levels of Siah2, Hif-1a and VEGF respectively by real-time PCR and western blot.

The level of mRNA and protein of SIAH2 ,Hif-1a and VEGF were significantly higher in IM- resistant CML patients compared to IM sensitive CML patients, respectively (P<0.05). The similar results were observed in K562-R and K562-W cells(P<0.01). Under 21%,1% oxygen concentration cultured for 24h ,the IC50 of K562-W and K562-R cells was no significant difference (P<0.05),but there was significant difference after cultured for 48h Cell cycle analysis showed that more G0/G1 cells in K562-R than K562-W after cultured for 48h under hypoxia condition(P<0.05). After being cultured in 1% oxygen concentration for 48 hours, we confirmed SIAH2, Hif-1a and VEGF were up-regulated in both cell lines, moreover, it was more obvious in K562-R cells. In SIAH2-sh K562-R cells, the apoptosis was higher than K562-R cells obviously under 1% oxygen concentration for 48 hours(P<0.05);the level of Hif-1a Hardly monitored, and the level of VEGF was also lower.

There were higher level of SIAH2, Hif-1a and VEGF in IM-resistant CML patients. Under hypoxia condition, K562 cells were likely to improve their resistance to Imatinib .After we Knockdown SAIH2, K562-R Cell apoptosis rate increased significantly, along with low level Hif-1a and VEGF. It indicated that in hypoxia micro-environment, Siah-2 might be one of the critical molecules that induce Imatinib-resistance in CML in the way of maintaining leukemic cell survival and stimulating them into quiescence phase.

No relevant conflicts of interest to declare.