At the present, we clearly demonstrate that mutant MEG3 promotes the growth of human liver cancer stem cells in vivo and in vitro.Mechanistically,our results shows that mutant MEG3 enhances acetylation modification of HistoneH4K16.Then, mutant MEG3 enhances the expression of SETD2 dependent on H4K16Ac.Furthermore, mutant MEG3 increases the DNA damage repair through SETD2.Ultimately, mutant MEG3 increases the telomeras activity dependent on DNA damage repair.In particular,TERT determines the cancerous function of mutant MEG3 in liver cancer stem cells(Fig. 6H). These observations will play an important role in finding effective tumor treatment targets.
First, our results indicate that mutant MEG3 promotes the growth of human liver cancer stem cells in vivo and in vitro. LncRNA-MEG3 inhibits cell proliferation and invasion by modulating Bmi1/RNF2 in cholangiocarcinoma(21). Also, MEG3 inhibits HMEC-1 cells growth, migration and tube formation via sponging miR-147(22).
Secondly, our results suggest that mutant MEG3 enhances acetylation modification of HistoneH4K16 dependent on Sirt1 in human liver cancer stem cells. Sirtuin-1 (SIRT1) is a class-III histone deacetylase (HDAC), an NAD+-dependent enzyme deeply involved in gene regulation, genome stability maintenance, apoptosis, autophagy, senescence, proliferation, aging, and tumorigenesis(23). Sirt1 also appears to be important for the turnover of defective mitochondria by mitophagy(24).SIRT1 regulates macrophage self-renewal(25) and regulates lipid metabolism, oxidative stress and inflammation in the liver (26).Furthermore,SIRT1 has recently garnered tremendous attention because of its various regulatory effects in several pathological conditions(27).In additional, ATGL promotes autophagy/lipophagy via SIRT1 to control hepatic lipid droplet catabolism(28).H4K16Ac acts as epigenetic signatures of diffuse intrinsic pontine glioma(29).Selective binding of the PHD6 finger of MLL4 to histone H4K16Ac links MLL4 and MOF(30).JMJD6 modulates DNA damage response through downregulating H4K16Ac independently of its enzymatic activity(31).Acetylation of hMOF modulates H4K16Ac to regulate DNA repair genes (32, 33).
Moreover, our results demonstrate mutant MEG3 enhances the expression of SETD2 dependent on H4K16Ac. SETD2 restricts prostate cancer metastasis by integrating EZH2 and AMPK signaling pathways(34). SETD2-dependent histone H3K36 trimethylation is required for homologous recombination repair and genome stability(35).The H3 lysine 36 histone methyltransferase SETD2 is mutated across a range of human cancers (36). Loss of SETD2 promotes K-ras-induced acinar-to-ductal metaplasia and epithelia-mesenchymal transition during pancreatic carcinogenesis(37). Also, SETD2 acts as a regulator of N6-methyladenosine RNA methylation and modifiers in cancer(38).SETD2 mutations confer chemoresistance in acute myeloid leukemia partly through altered cell cycle checkpoints(39). Furthermore, SETD2 regulates cancer development(40).Dual chromatin and cytoskeletal was remodeled by SETD2(41).Interestingly, SETD2 mutation suppress autophagy via regulation of ATG12(42).
Notably, our results suggest that mutant MEG3 increases the DNA damage repair through H3K36me3 dependent on SETD2. There are diverse clues showing H3K36me3 participates in DNA damage response by directly recruiting DNA repair machinery to set the chromatin at a "ready" status (43). Histone H3 trimethylation at lysine 36 guides m6A RNA modification co-transcriptionally(44, 45). Chromosome 3P loss of heterozygosity reduces expression of H3K36me3 in sacral conventional chordoma (46). Furthermore, gene body DNA methylation conspires with H3K36me3 to preclude aberrant transcription(47).DNA damage is related to the balance between survival and death in cancer biology (48).As exemplified in diverse cancers,disruption or deregulation of DNA repair pathways results in genome instability(49).Moreover, the DNA mismatch repair triggers cell cycle arrest in some cases(50).The DNA damage respons makes it safe to play with knives(51). Cell fate regulation is associated with upon DNA damage(52, 53).
Intriguingly, we clearly identity that mutant MEG3 increases the telomeras activity dependent on DNA damage repair. Furthermore, our results indicate that TERT determines the cancerous function of mutant MEG3. Telomerase, an RNA-dependent DNA polymerase with telomerase reverse transcriptase (TERT), regulates cancer formation(54, 55).A particular attention is given to the putative connections between TERT transcriptional reactivation and signalling pathways frequently altered in cancer, such as c-MYC, NF-κB and β-Catenin(56). TERT promoter mutations are associated with poor prognosis and cell immortalization in meningioma(57).DNA methylation of the TERT promoter is associated with human cancer(58).TERT and TERC mutations suppress telomerase activity(59).TERT C228T mutation is associated with intravesical recurrence for patients with non-muscle invasive bladder cancer(60).TERC is an RNA component of telomerase and TERC promotes cellular inflammatory response independent of telomerase(61). HuR regulates telomerase activity through TERC methylation(62). Mitochondrion-processed TERC regulates senescence without affecting telomerase activities(63).C-MYC drives overexpression of telomerase RNA (hTR/TERC) (64). In particular,the TERC haploinsufficiency affcts on the inheritance of telomere length(65) and is involved in the process of genetic instability leading to tumorgenesis (66).
In conclusions, the present study will focus on studying the effective mechanism of mutant MEG3 in carcinogenesis. These studies will play an important role in finding effective tumor treatment targets.