Department of Cellular & Molecular Physiology, Penn State College of Medicine, Hershey, PA 17033, United States.
Department of Dermatology, Penn State College of Medicine, Hershey, PA 17033, United States.
Cell Signal. 2018 Dec;52:35-47. doi: 10.1016/j.cellsig.2018.08.018. Epub 2018 Aug 30.
The primary cause of non-melanoma skin cancer (NMSC) is ultraviolet B (UVB) radiation. We have shown previously that mTORC2 inhibition sensitizes keratinocytes to UVB-induced apoptosis mediated by the transcription factor FOXO3a. FOXO3a is a key regulator of apoptosis and a tumor suppressor in several cancer types. Activation of FOXO3a promotes apoptosis through the coordinated expression of a variety of target genes, including TRAIL and NOXA. We hypothesized that in the setting of mTORC2 inhibition, the UVB-induced expression of these factors would lead to apoptosis in a FOXO3a-dependent manner. Using spontaneously immortalized human keratinocytes (HaCaT cells), we observed that both TRAIL and NOXA expression increased in cells exposed to UVB and the TOR kinase inhibitor Torin 2. Similar to knockdown of FOXO3a, NOXA knockdown reversed the sensitization to UVB-induced apoptosis caused by mTORC2 inhibition. In contrast, loss of TRAIL by either knockdown or knockout actually enhanced expression of nuclear FOXO3a, which maintained apoptosis. These surprising results are not due to faulty death receptor signaling in HaCaT cells, as we found that the cells undergo extrinsic apoptosis in response to treatment with recombinant TRAIL. Even more striking, TRAIL knockout cells were sensitized to recombinant TRAIL-induced apoptosis compared to wild-type HaCaT cells, with the largest increase occurring in the presence of mTORC2 inhibition. Taken together, these studies provide strong evidence that mTORC2 controls UVB-induced apoptosis by regulating NOXA expression downstream of FOXO3a. Moreover, FOXO3a transcriptional activation by mTORC2 inhibitors may be a valuable target for prevention or therapy of NMSC, especially in cases with low endogenous TRAIL.
非黑素瘤皮肤癌（NMSC）的主要病因是紫外线 B（UVB）辐射。我们之前已经表明，mTORC2 抑制使角质形成细胞对转录因子 FOXO3a 介导的 UVB 诱导的细胞凋亡敏感。FOXO3a 是多种癌症类型中凋亡的关键调节因子和肿瘤抑制因子。FOXO3a 的激活通过协调表达多种靶基因，包括 TRAIL 和 NOXA，促进细胞凋亡。我们假设，在 mTORC2 抑制的情况下，这些因子的 UVB 诱导表达将以 FOXO3a 依赖的方式导致细胞凋亡。使用自发永生化的人角质形成细胞（HaCaT 细胞），我们观察到暴露于 UVB 和雷帕霉素靶激酶抑制剂 Torin 2 的细胞中 TRAIL 和 NOXA 的表达均增加。与 FOXO3a 的敲低相似，NOXA 的敲低逆转了 mTORC2 抑制引起的对 UVB 诱导的细胞凋亡的敏感性。相比之下，TRAIL 的敲除或敲除损失实际上增强了核 FOXO3a 的表达，从而维持了细胞凋亡。这些令人惊讶的结果并不是由于 HaCaT 细胞中死亡受体信号传导的错误，因为我们发现细胞在外源凋亡中经历凋亡反应对重组 TRAIL 的治疗。更引人注目的是，与野生型 HaCaT 细胞相比，TRAIL 敲除细胞对重组 TRAIL 诱导的细胞凋亡更加敏感，并且在存在 mTORC2 抑制时增加最大。总之，这些研究为 mTORC2 通过调节 FOXO3a 下游的 NOXA 表达来控制 UVB 诱导的细胞凋亡提供了有力证据。此外，mTORC2 抑制剂对 FOXO3a 的转录激活可能是预防或治疗 NMSC 的有价值的靶点，特别是在 TRAIL 内源性水平较低的情况下。
