Key Laboratory of Laboratory Medical Diagnostics Designated By Chinese Ministry of Education, Chongqing Medical University, Chongqing, 400016, China.
Experimental Teaching Center of Basic Medicine Science, Chongqing Medical University, Chongqing, 400016, China.
Cancer Lett. 2020 May 28;478:8-21. doi: 10.1016/j.canlet.2020.02.040. Epub 2020 Mar 4.
Drosha-dependent canonical microRNAs (miRNAs) play a crucial role in the biological functions and development of cancer. However, the effects of Drosha-independent non-canonical miRNAs remain poorly understood. In our previous work, we found a set of aberrant miRNAs, including some upregulated miRNAs, called Drosha-independent noncanonical miRNAs, in Drosha-knockdown gastric cancer (GC) cells. Surprisingly, Drosha-silenced GC cells still retained strong malignant properties (e.g., proliferation ability and cancer stem cell (CSC) characteristics), indicating that aberrantly upregulated non-canonical miRNAs may play an important role in the maintenance of the malignant properties in GC cells that express low Drosha levels. Here, we report that miR-6778-5p, a noncanonical miRNA, acts as a crucial regulator for maintenance of CSC stemness in Drosha-silenced GC cells. MiR-6778-5p belongs to the 5'-tail mirtron type of non-canonical miRNAs and is transcript splice-derived from intron 5 of SHMT1 (coding cytoplasmic serine hydroxymethyltransferase). It positively regulates expression of its host gene, SHMT1, via targeting YWHAE in Drosha-knockdown GC cells. Similar to its family member SHMT2, SHMT1 plays a crucial role in folate-dependent serine/glycine inter-conversion in one-carbon metabolism. In Drosha wild type GC cells, SHMT2 mediates a mitochondrial-carbon metabolic pathway, which is a major pathway of one-carbon metabolism in normal cells and most cancer cells. However, in Drosha-silenced or Drosha low-expressing GC cells, miR-6778-5p positively regulates SHMT1, instead of SHMT2, thus mediating a compensatory activation of cytoplasmic carbon metabolism that plays an essential role in the maintenance of CSCs in gastric cancer (GCSCs). Drosha wild type GCSCs with SHMT2 are sensitive to 5-fluorouracil; however, Drosha low-expressing GCSCs with SHMT1 are 5-FU-resistant. The loss of miR-6778-5p or SHMT1 notably mitigates GCSC sphere formation and increases sensitivity to 5-fluorouracil in Drosha-knockdown gastric cancer cells. Thus, our study reveals a novel function of Drosha-independent noncanonical miRNAs in maintaining the stemness of GCSCs.
依赖 Drosha 的经典 microRNAs（miRNAs）在癌症的生物学功能和发展中发挥着关键作用。然而，Drosha 非依赖性非经典 miRNAs 的作用仍知之甚少。在我们之前的工作中，我们在 Drosha 敲低的胃癌（GC）细胞中发现了一组异常的 miRNAs，包括一些上调的 miRNAs，称为 Drosha 非依赖性非经典 miRNAs。令人惊讶的是，Drosha 沉默的 GC 细胞仍然保持着强烈的恶性特性（例如，增殖能力和癌症干细胞（CSC）特征），这表明异常上调的非经典 miRNAs 可能在维持低 Drosha 水平表达的 GC 细胞的恶性特性中发挥重要作用。在这里，我们报告 miR-6778-5p，一种非经典 miRNA，作为 Drosha 沉默的 GC 细胞中 CSC 干性维持的关键调节剂。miR-6778-5p 属于 5'-尾 mirtron 型非经典 miRNAs，由 SHMT1（编码细胞质丝氨酸羟甲基转移酶）内含子 5 转录剪接而来。它通过靶向 Drosha 敲低 GC 细胞中的 YWHAE 来正向调节其宿主基因 SHMT1 的表达。与家族成员 SHMT2 类似，SHMT1 在叶酸依赖性丝氨酸/甘氨酸相互转化中发挥关键作用，参与一碳代谢。在 Drosha 野生型 GC 细胞中，SHMT2 介导线粒体碳代谢途径，这是正常细胞和大多数癌细胞中一碳代谢的主要途径。然而，在 Drosha 沉默或低表达的 GC 细胞中，miR-6778-5p 正向调节 SHMT1，而不是 SHMT2，从而介导细胞质碳代谢的代偿性激活，这对于维持胃癌（GCSCs）中的 CSCs 至关重要。具有 SHMT2 的 Drosha 野生型 GCSCs 对 5-氟尿嘧啶敏感；然而，具有 SHMT1 的 Drosha 低表达 GCSCs 对 5-氟尿嘧啶具有耐药性。miR-6778-5p 或 SHMT1 的缺失显著减轻了 Drosha 敲低胃癌细胞的 GCSC 球体形成，并增加了对 5-氟尿嘧啶的敏感性。因此，我们的研究揭示了 Drosha 非依赖性非经典 miRNAs 在维持 GCSC 干性中的新功能。
