Autophagy inhibition by TSSC4 (tumor suppressing subtransferable candidate 4) contributes to sustainable cancer cell growth

Cancer cell growth is dependent upon the sustainability of proliferative signaling and resisting cell death. Macroautophagy/autophagy promotes cancer cell growth by providing nutrients to cells and preventing cell death. This is in contrast to autophagy promoting cell death under some conditions. The mechanism regulating autophagy-mediated cancer cell growth remains unclear. Herein, we demonstrate that TSSC4 (tumor suppressing subtransferable candidate 4) is a novel tumor suppressor that suppresses cancer cell growth and tumor growth and prevents cell death induction during excessive growth by inhibiting autophagy. The oncogenic proteins ERBB2 (erb-b2 receptor tyrosine kinase 2) and the activation EGFR mutant (EGFRvIII, epidermal growth factor receptor variant III) promote cell growth and TSSC4 expression in breast cancer and glioblastoma multiforme (GBM) cells, respectively. In EGFRvIII-expressing GBM cells, <i>TSSC4</i> knockout shifted the function of autophagy from a pro-cell survival role to a pro-cell death role during prolonged cell growth. Furthermore, the interaction of TSSC4 with MAP1LC3/LC3 (microtubule associated protein 1 light chain 3) via its conserved LC3-interacting region (LIR) contributes to its inhibition of autophagy. Finally, TSSC4 suppresses tumorsphere formation and tumor growth by inhibiting autophagy and maintaining cell survival in tumorspheres. Taken together, sustainable cancer cell growth can be achieved by autophagy inhibition via TSSC4 expression. <b>Abbreviations:</b> 3-MA: 3-methyladenine; ACTB: actin beta; CQ: chloroquine; EGFRvIII: epidermal growth factor receptor variant III; ERBB2: erb-b2 receptor tyrosine kinase 2; GBM: glioblastoma multiforme; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule Associated protein 1 light chain 3; TSSC4: tumor suppressing subtransferable candidate 4

癌细胞的生长依赖于增殖信号的持续维持以及对细胞死亡的抵抗。巨自噬（macroautophagy）/自噬（autophagy）通过为细胞提供营养并抑制细胞死亡，从而促进癌细胞生长，这与部分条件下自噬促进细胞死亡的作用相悖。调控自噬介导的癌细胞生长的具体机制目前仍不明确。

本研究证实，TSSC4（tumor suppressing subtransferable candidate 4）是一种新型肿瘤抑制因子，可通过抑制自噬通路抑制癌细胞生长与肿瘤增殖，并阻断过度增殖状态下的细胞死亡诱导。致癌蛋白ERBB2（erb-b2 receptor tyrosine kinase 2）与激活型EGFR突变体EGFRvIII（epidermal growth factor receptor variant III）分别可在乳腺癌及多形性胶质母细胞瘤（GBM，glioblastoma multiforme）细胞中促进细胞生长与TSSC4的表达。

在表达EGFRvIII的GBM细胞中，敲除TSSC4会使自噬在细胞长期增殖过程中的功能从促细胞存活转变为促细胞死亡。此外，TSSC4通过其保守的LC3相互作用区域（LIR，LC3-interacting region）与MAP1LC3/LC3（microtubule associated protein 1 light chain 3）结合，这一相互作用是其抑制自噬的关键分子基础。最后，TSSC4可通过抑制自噬并维持肿瘤球的细胞存活，从而抑制肿瘤球形成与肿瘤生长。

综上，通过TSSC4介导的自噬抑制，可实现癌细胞生长的持续维持。

**缩写说明：**
3-MA：3-甲基腺嘌呤（3-methyladenine）；ACTB：肌动蛋白β（actin beta）；CQ：氯喹（chloroquine）；EGFRvIII：表皮生长因子受体变体III（epidermal growth factor receptor variant III）；ERBB2：erb-b2受体酪氨酸激酶2（erb-b2 receptor tyrosine kinase 2）；GBM：多形性胶质母细胞瘤（glioblastoma multiforme）；LIR：LC3相互作用区域（LC3-interacting region）；MAP1LC3/LC3：微管相关蛋白1轻链3（microtubule associated protein 1 light chain 3）；TSSC4：肿瘤抑制可转移候选因子4（tumor suppressing subtransferable candidate 4）