Autophagy and Ubiquitin-Mediated Proteolytic Degradation of PML/RarÎ± Fusion Protein in Matrine-Induced Differentiation Sensitivity Recovery of ATRA-Resistant APL (NB4-LR1) Cells: In Vitro and In Vivo studies
Authors: Wu Dijiong , Shao Keding , Zhou Qihao , Sun Jie , Wang Ziqi , Yan Fei , Liu Tingting , Wu Xiangping , Ye Baodong , Huang He , Zhou Yuhong 
Published: October 2019

Abstract
The major feature that distinguishes acute promyelocytic leukemia (APL) cells from other malignant hematopoietic cells is the expression of promyelocytic leukemia-retinoicacid receptor α (PML-RARα) fusion protein, which contributes to the inhibition of RARα-regulated hematopoietic cellular differentiation. The complete remission rate of APL exceeded 90% with the application of all-trans retinoic acid (ATRA), arsenic trioxide (ATO), and anthracycline-based chemotherapy. However, the 7-year cumulative incidence of relapses was reported as 28.6% in APL maintenance with ATRA and daunorubicin and even reached 33% in the ATRA maintenance treatment group. The relapse/refractory patients showed resistance to ATRA and/or ATO, which has been identified as a clinically significant problem. Currently, no effective drugs are available to reverse the ATRA resistance.
Matrine (MAT) is the main active component of Sophora flavescens. It has been used to treat chronic hepatitis for several years in China. Recently, the molecule has been proven to exhibit an anti-leukemic effect. Previous studies have revealed that MAT can reverse the ATRA resistance of NB4-LR1 cells when coupled with ATRA. The treatment with 0.1mmol/L MAT and 1μmol/L ATRA can restore the ability of NB4-LR1 cells to differentiate, which might be related to the increased level of cyclic adenosine monophosphate and protein kinase A activity in NB4-LR1 cells, reduced telomerase activity and downregulated expression of topoisomerase II beta (TopoIIβ) (Wu, et al. Planta Med 2014). Subsequently, this study aimed to investigate the mechanism underlying the degradation of the PML/RARα fusion protein in the presence of MAT and ATRA in NB4 and NB4-LR1 cell lines.
ATRA-sensitive (NB4) and ATRA-resistant (NB4-LR1) cell lines were used. Nitroblue tetrazolium reduction assay and flow cytometry were used to detect the differentiation ability. The activity of ubiquitin-proteasome and autophagy-mediated pathways in both cells treated with ATRA with or without MAT were compared in protein and mRNA level (Western blot analysis, qRT-PCR), the Fluorescent substrate Suc-LLVY-AMC detection was used to detect the activity of proteasome, and electron microscope for observing autophagosome. MG 132(proteasome inhibitor), rapamycin (autophagy activator, RAPA), hydroxychloroquine (lysosomal inhibitor, HCQ) and STI571 [retinoic acid receptor alpha (RARα) ubiquitin stabilizer] were used as positive controls. The effect of MAT was observed in vivo using xenografts. Results showed that MAT improved the sensitivity of NB4-LR1cells to ATRA treatment, which was consistent with the expression of PML-RARα fusion protein (Fig.A-C).The ubiquitin proteasome pathway plays a crucial role in protein degradation. MAT promoted the ubiquitylation level in NB4-LR1 by stabilized the 20S protein expression and enhanced the activity of the proteasome (Fig.D). ATRA inhibited the expression of RARα in NB4-LR1 cells, which was contradictory to that in NB4 cells. MAT can stabilize the expression of RARα in NB4-LR1 cells, whereas MG132 downregulated the expression of RARα in both cell lines, which hampered the differentiation of NB4 cells (Fig.E-G). In addition to UPP, the autophagy pathway also had a significant role in arsenious acid- or ATRA-mediated PML-RARα fusion protein degradation. MAT could promoted the autophagy in NB4-LR1 cells, with an increase in microtubule-associated protein 1 light chain3 (LC3)-II and LC3-II/LC3-I ratio and exhaustion of P62 (Fig.H-K). A similar phenomenon was observed in mouse xenografts (Fig.L-N).
In summary, the present study revealed the difference in the chain reaction of sensitive and resistant APL cell lines (NB4 and NB4-LR1, respectively) to the treatment of ATRA, explaining the mechanism underlying the resistance to ATRA. ATRA decrease the level of 20S core subunit and the RARα in NB4-LR1 cells, but could not activate the autophagy process. These effects were reversed by the combination of MAT. The proteasome inhibitor might hamper the RARα stabilization and hence was not advantageous for the differentiation of cells. It also induced autophagy in NB4-LR1 cells. MAT induced the activation of UPP and mediated the autophagic degradation process, which synergistically induced the degradation of PML-RARα fusion protein and promote the differentiation of NB4-LR1 cells.
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Disclosures
No relevant conflicts of interest to declare.