CHOP Deficiency Enhances Hematopoietic Stem Cell Function via Reducing ATF3/ROS Induced Cell Apoptosis

Hematopoietic stem cells (HSCs) reside in a quiescent niche to reserve their capacity of self-renewal. Upon hematopoietic injuries, HSCs enter the cell cycle and encounter protein homeostasis problems caused by accumulation of mis-folded proteins. However, the mechanism by which protein homeostasis influences HSC function and maintenance remains poorly understood. Here, we show that C/EBP homologous protein (CHOP), which is demonstrated previously, induces cell death upon tunicamycin induced unfolded protein response (UPR), plays an important role in HSCs regeneration. CHOP-/- mice showed normal hematopoietic stem and progenitor cell frequencies in steady state. However, when treated with 5-FU, CHOP deficiency resulted in higher survival rates, associated with an increased number of HSCs and reduced level of apoptosis. In serial competitive transplantation experiments, CHOP-/- HSCs showed a dramatic enhancement of repopulation ability and a reduction of protein aggresomes. Mechanistically, CHOP deletion causes reduced ATF3 expression and further leads to decreased protein aggregation and ROS. In addition, CHOP-/- HSCs exhibited an increased resistance to IR-induced DNA damage and improved HSCs homeostasis and function in telomere-dysfunctional (G3Terc-/-) mice. In summary, these findings disclose a new role of CHOP in the regulation of the HSCs function and homeostasis through reducing ATF3 and ROS signaling.

造血干细胞（Hematopoietic stem cells, HSCs）定位于静息龛位以保留其自我更新能力。当发生造血损伤时，HSCs进入细胞周期，并遭遇因错误折叠蛋白积累所引发的蛋白质稳态紊乱问题。然而，蛋白质稳态如何影响HSC功能与维持的分子机制目前仍尚不明确。本研究证实，此前被报道可在衣霉素诱导的未折叠蛋白反应（unfolded protein response, UPR）中介导细胞死亡的C/EBP同源蛋白（CHOP），在HSC再生过程中发挥关键调控作用。CHOP敲除（CHOP-/-）小鼠在稳态条件下的造血干祖细胞频率并无异常。但经5-氟尿嘧啶（5-FU）处理后，CHOP缺陷小鼠的存活率显著升高，同时伴随HSC数量增加与凋亡水平降低。在连续竞争性移植实验中，CHOP-/- HSCs的重建造血能力显著增强，且细胞内蛋白聚集体水平明显减少。机制研究表明，CHOP缺失可下调激活转录因子3（ATF3）的表达，进而减少蛋白质聚集与活性氧（ROS）的生成。此外，CHOP-/- HSCs对电离辐射诱导的DNA损伤抗性增强，且在端粒功能障碍（G3Terc-/-）小鼠模型中展现出更优异的HSC稳态维持与功能恢复能力。综上，本研究揭示了CHOP通过调控ATF3与ROS信号通路，进而调控HSC功能与稳态的全新功能。