No abstract

Intercellular communication between malignant cells and neighboring nonmalignant cells is involved in carcinogenesis. In the progression of carcinogenesis, exosomes are messengers for intercellular communication. Circular RNAs (circRNAs) are noncoding RNAs with functions that include regulation of the cell cycle and proliferation. However, the functions of exosomal circRNAs are not clear. The present research aimed to determine whether circRNAs secreted from arsenite-transformed human hepatic epithelial (L-02) cells are transferred into normal L-02 cells and become functionally active in the normal cells. The results showed that circRNA_100284 is involved in the malignant transformation of L-02 cells induced by arsenite. The medium from transformed L-02 cells induced upregulation of circRNA_100284, accelerated the cell cycle, and promoted proliferation of normal L-02 cells. Transformed cells transferred circRNA_100284 into normal L-02 cells via exosomes and led to the malignant transformation of the non-transformed cells. Knockdown of circRNA_100284, which reduced circRNA_100284 levels in exosomes derived from transformed L-02 cells, blocked the accelerated cell cycle and reduced proliferation and malignancy. In addition, in normal L-02 cells, exosomal circRNA_100284 derived from arsenite-transformed L-02 cells induced acceleration of the cell cycle and promoted proliferation via acting as a sponge of microRNA-217. Further, exosomal circRNA_100284 was upregulated in the sera of people exposed to arsenite. Thus, exosomes derived from transformed L-02 cells transferred circRNA_100284 to surrounding cells, which induced an accelerated cell cycle and promoted proliferation of normal liver cells and led to the malignant transformation of the non-transformed cells. The findings support the concept that exosomal circRNAs are involved in cell–cell communication during carcinogenesis induced by arsenite.

Arsenic is well established as a human carcinogen, but the molecular mechanisms leading to arsenic-induced carcinogenesis are complex and elusive. It is also not known if lncRNAs are involved in arsenic-induced liver carcinogenesis. We have found that MALAT1, a non-coding RNA, is over-expressed in the sera of people exposed to arsenite and in hepatocellular carcinomas (HCCs), and MALAT1 has a close relation with the clinicopathological characteristics of HCC. In addition, hypoxia-inducible factor (HIF)-2α is up-regulated in HCCs, and MALAT1 and HIF-2α have a positive correlation in HCC tissues. During the malignant transformation of human hepatic epithelial (L-02) cells induced by a low concentration (2.0 μM) of arsenite, MALAT1 and HIF-2α are increased. In addition, arsenite-induced MALAT1 causes disassociation of the von Hippel-Lindau (VHL) protein from HIF-2α, therefore, alleviating VHL-mediated HIF-2α ubiquitination, which causes HIF-2α accumulation. In turn, HIF-2α transcriptionally regulates MALAT1, thus forming a positive feedback loop to ensure expression of arsenite-induced MALAT1 and HIF-2α, which are involved in malignant transformation. Moreover, MALAT1 and HIF-2α promote the invasive and metastatic capacities of arsenite-induced transformed L-02 cells and in HCC-LM3 cells. The capacities of MALAT1 and HIF-2α to promote tumor growth are validated in mouse xenograft models. In mice, arsenite induces an inflammatory response, and MALAT1 and HIF-2α are over-expressed. Together, these findings suggest that the MALAT1/HIF-2α feedback loop is involved in regulation of arsenite-induced malignant transformation. Our results not only confirm a novel mechanism involving reciprocal regulation between MALAT1 and HIF-2α, but also expand the understanding of the carcinogenic potential of arsenite.

No abstract

Accelerated glycolysis, a common process in tumor cells called the Warburg effect, is associated with various biological phenomena. However, the role of glycolysis induced by arsenite, a well-established human carcinogen, is unknown. Long non-coding RNAs (lncRNAs) act as regulators in various cancers, but how lncRNAs regulate glucose metabolism remains largely unexplored. We have found that, in human hepatic epithelial (L-02) cells, arsenite increases lactate production; glucose consumption; and expression of glycolysis-related genes, including HK-2, Eno-1, and Glut-4. In L-02 cells exposed to arsenite, the lncRNA, metastasis-associated lung adenocarcinoma transcript 1 (MALAT1), and hypoxia inducible factors (HIFs)-α, the transcriptional regulators of cellular response to hypoxia, are over-expressed. In addition, HIF-1α, not HIF-2α, is involved in arsenite-induced glycolysis, and MALAT1 enhances arsenite-induced glycolysis. Although MALAT1 regulates HIF-α and promotes arsenite-induced glycolysis, MALAT1 promotes glycolysis through HIF-1α, not HIF-2α. Moreover, arsenite-increased MALAT1 enhances the disassociation of Von Hippel-Lindau (VHL) tumor suppressor from HIF-1α, alleviating VHL-mediated ubiquitination of HIF-1α, which causes accumulation of HIF-1α. In sum, these findings indicate that MALAT1, acting through HIF-1α stabilization, is a mediator that enhances glycolysis induced by arsenite. These results provide a link between the induction of lncRNAs and the glycolysis in cells exposed to arsenite, and thus establish a previously unknown mechanism for arsenite-induced hepatotoxicity.