責(zé)編丨迦溆
視網(wǎng)膜母細(xì)胞瘤蛋白（Retinoblastoma protein，RB）是一類腫瘤抑制蛋白，其主要參與了細(xì)胞周期調(diào)控、DNA損傷應(yīng)答、免疫檢查點(diǎn)激活與分化等【1, 2】。未磷酸化/低磷酸化的RB蛋白可與轉(zhuǎn)錄因子E2F相互作用從而抑制E2F靶向基因的表達(dá)及G1/S細(xì)胞周期轉(zhuǎn)換；而cyclin/CDK 誘導(dǎo)的RB蛋白多位點(diǎn)高磷酸化態(tài)可促使E2F轉(zhuǎn)錄因子從與RB結(jié)合的狀態(tài)解離，促進(jìn)細(xì)胞周期的發(fā)生【3】。小鼠遺傳研究表明RB可行使E2F非依賴的腫瘤抑制功能，但其參與的信號(hào)通路途徑目前尚未闡釋明晰【4】。
轉(zhuǎn)錄因子NF-κB在免疫中發(fā)揮了重要功能，同時(shí)在腫瘤調(diào)控中也具有重要作用。通過炎性因子等刺激，NF-κB可由MAP3K7-IKK信號(hào)通路激活【5】。除了調(diào)控經(jīng)典通路的基因，NF-κB可介導(dǎo)多種腫瘤類型的PD-L1 mRNA的表達(dá)【6, 7】。最近一項(xiàng)研究也表明NF-κB信號(hào)通路可作為腫瘤細(xì)胞逃避T細(xì)胞免疫監(jiān)測(cè)的主要機(jī)制之一【8, 9】。
PD-1（Programmed death-1）與其配體PD-L1（也稱為B7-H1）是兩個(gè)重要的免疫檢查點(diǎn)分子。腫瘤細(xì)胞表達(dá)的PD-1與活化T細(xì)胞表面的PD-L1結(jié)合，可誘導(dǎo)T細(xì)胞凋亡及功能缺失，從而引發(fā)腫瘤的免疫逃避【10-12】。此前的研究表明PD-L1蛋白豐度受到泛素化與蛋白酶途徑（例如去泛素修飾酶CSN5與E3連接酶銜接體SPOP）的調(diào)控，其異常表達(dá)會(huì)導(dǎo)致免疫檢查點(diǎn)療法的抵抗效應(yīng)【13, 14】。然而，PD-L1 表達(dá)的分子機(jī)制需要更多的研究來闡明用以提高臨床病人對(duì)免疫療法的應(yīng)答。
12月6日，梅奧診所醫(yī)學(xué)院黃浩杰、華中科技大學(xué)同濟(jì)醫(yī)學(xué)院吳河水、江蘇大學(xué)附屬醫(yī)院Runzhi Zhu等組成的合作團(tuán)隊(duì)在Molecular Cell 雜志在線發(fā)表題為Phosphorylated RB Promotes Cancer Immunity by Inhibiting NF-κB Activation and PD-L1 Expression 研究論文，發(fā)現(xiàn)了高磷酸態(tài)的RB蛋白可抑制NF-κB的激活與PD-L1的轉(zhuǎn)錄從而促進(jìn)腫瘤免疫。RB可與NF-κB的主要因子p65相互作用并促進(jìn)CDK4/6介導(dǎo)的RB磷酸化。RB敲低或CDK4/6抑制劑可誘導(dǎo)包括PD-L1在內(nèi)的NF-κB信號(hào)通路基因的表達(dá)增強(qiáng)。同時(shí)研究人員依據(jù)RB與p65的相互作用構(gòu)建了小分子肽，此肽可在體內(nèi)抑制放療誘導(dǎo)的PD-L1的表達(dá)升高并增強(qiáng)放射的體內(nèi)療效。因此，本研究為腫瘤免疫的逃避機(jī)制提供了新穎思路。
研究人員首先利用小分子抑制劑刺激PTEN/MAP3K7與PTEN/CHD1缺失的細(xì)胞（NF-κB信號(hào)通路被抑制），發(fā)現(xiàn)CDK4/6抑制劑palbociclib可恢復(fù)雙敲細(xì)胞的活性。由于RB是CDK4/6信號(hào)通路下游主要的效應(yīng)分子，研究人員進(jìn)一步發(fā)現(xiàn)RB的缺失可緩解MAP3K7與CHD1缺失帶來的不良效應(yīng)，提示RB是NF-κB的負(fù)調(diào)控分子。機(jī)制上，RB可與NF-κB的p65發(fā)生相互作用且RB的N端對(duì)于其相互作用不可或缺。RB的磷酸化可極大的提升RB-p65相互作用能力，這有賴于RB N端的第249位絲氨酸與252位蘇氨酸（S249與T252位點(diǎn)）以及p65的161FQVTV165基序。
此外，通過染色質(zhì)免疫沉淀結(jié)合定量PCR實(shí)驗(yàn)，研究人員發(fā)現(xiàn)p65蛋白易結(jié)合于RB影響的NF-κB靶基因的基因組位點(diǎn)，其中包括了PD-L1的位點(diǎn)。在PD-L1的啟動(dòng)子區(qū)，這個(gè)功能性的p65結(jié)合位點(diǎn)受到CDK4/6-RB信號(hào)通路的調(diào)控。RB通過依賴于NF-κB的途徑抑制PD-L1的表達(dá)。RB的S249與T252位點(diǎn)對(duì)于PD-L1表達(dá)的調(diào)控不可或缺，且在去勢(shì)抵抗性前列腺癌病人樣本中（mCRPC），pRB-S249/T252的表達(dá)與PD-L1的表達(dá)呈現(xiàn)負(fù)相關(guān)模式。研究人員依此合成了一個(gè)RB N端磷酸化模擬肽RL-S249/T252D peptide。將此小分子肽應(yīng)用于小鼠實(shí)驗(yàn)研究人員發(fā)現(xiàn)：γ射線誘導(dǎo)的PD-L1表達(dá)升高是通過RB-NF-κB途徑介導(dǎo)的，且小分子肽可顯著抑制PD-L1的表達(dá)。
因此，這項(xiàng)研究不僅發(fā)現(xiàn)了一個(gè)起源于RB S249/T252的磷酸化模擬肽，此小分子肽可通過抑制PD-L1的表達(dá)抵抗射線誘導(dǎo)的免疫耐受，也闡明了腫瘤抑制基因RB的磷酸化通過抑制NF-κB活性與PD-L1的表達(dá)來克服腫瘤免疫逃避的機(jī)制，為其臨床應(yīng)用提供了新型思路。
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