Paracaspase MALT1於Arg-781的自我切割減弱 NF-κB訊號並調控活化型瀰漫性大B細胞淋巴瘤細胞生長

Abstract

Paracaspase mucosa-associated lymphoid tissue lymphoma translocation 1 (MALT1)能透過其鷹架蛋白功能來調控數種由不同接受器所引起的NF-κB訊息傳遞路徑。MALT1的蛋白水解活性被認為可以對數種NF-κB負向調控因子進行切割使其去活性，進而強化NF-κB活性。本研究發現，MALT1與具有聚合作用能力的BCL10共同表現時會引發本身的自我切割。MALT1自我切割的位點是在C端的Arg-781。在試管中切割試驗可以證實MALT1的自我切割能力；使用TPA/ionomycin或anti-CD3抗體刺激淋巴細胞，可偵測到微量的MALT1自我切割。然而在活化性瀰漫性大B細胞淋巴瘤細胞OCI-Ly3及HBL-1上可以觀察到明顯的MALT1於Arg-781的自我切割。自我切割後的MALT1與TRAF6結合的能力會降低，進而會使其引起NF-κB活化的能力下降。在內生性MALT1被knockdown的Jurkat細胞(Jurkat-S-MD)中表現野生型的MALT1、去蛋白水解活性的MALT1_C464A、自我切割能力缺乏的MALT1_R781L或C端被截短的MALT1_1-781，並以TPA/ionomycin激刺後，缺乏自我切割能力的MALT1_R781L保有蛋白水解活性並能夠誘發與MALT1相同的初始IκBα磷酸化活性。C端被截短的MALT1誘發較弱的初始IκBα磷酸化，並且使IL-2與IFN-γ的表現量下降。MALT1的蛋白水解活性對活化型瀰漫性大B細胞淋巴瘤細胞的生長及存活是必需的，在HBL-1細胞中誘發表現去蛋白水解活性的MAL1_C464A或缺乏自我切割能力的MALT1_R781L，會使細胞生長受到抑制。此一結果顯示MALT1於Arg-781的自我切割參與並調控活化型瀰漫性大B細胞淋巴瘤細胞的生長。

Paracaspase mucosa-associated lymphoid tissue lymphoma translocation 1 (MALT1) can regulate several NF-κB signaling pathways caused by different receptors through its scaffold protein function. MALT1 protease activity is shown to inactivate several negative regulators of NF-κB signaling and augment NF-κB activation ability. In this study, MALT1 was demonstrated to autoprocess itself in the presence of oligomerization-competent BCL10. Cleavage occurred after Arg-781 located in the C-terminus of MALT1. The autocleavage of MALT1 could be observed in in-vitro cleavage assay. Cleavage at Arg-781 was detectable but marginal after activation with TPA/ionomycin or anti-CD3 antibody in lymphocytes. However, cleavage at Arg-781 was evident in ABC-DLBCL cells such as OCI-Ly3, HBL-1. Shortened MALT1 cleavage products showed attenuated binding ability with TRAF6. Its NF-κB activation ability was also weakened. Various MALT1 constructs including wild type, catalytically-inactive (MALT1_C464A), cleavage-defective (MALT1_R781L), or truncated (MALT1_1-781) form of MALT1 was introduced into MALT1-knocked-down-Jurkat T cells. Cleavage-defective MALT1_R781L retained its proteolytic and initial IκBα phosphorylation activity as MALT1. Truncated MALT1_1-781 mutant showed weakness in IBα phosphorylation and the expression of NF-κB targets IL-2 and IFN-γ. The proteolytic activity of MALT1 is essential for the survival of ABC-DLBCL cells. HBL-1 cells with induced expression of catalytically-inactive MALT1_C464A or cleavage-defective MALT1_R781L exhibited characteristic of retarded-growth. These findings suggested that cleavage at Arg-781 of MALT1 played a role in the survival of ABC-DLBCL cells.