Mounting studies have highlighted the critical roles of lncRNAs in CRC carcinogenesis and cancer cells metastasis. They could serve as a multifaceted regulator in a broad range of cancer gene modulation in transcriptional, post-transcriptional and epigenetic levels. Here, we firstly reported lncRNA LINC00115 as a novel cancer-promoted regulator in CRC. LINC00115 was originally reported in lung cancer as a potential prognostic biomarker[20]. In another study, LINC00115 was identified to be a critical regulator of glioma stem-like cell tumorigenicity[21]. In our study, qRT-PCR assay was conducted to estimate the expression levels of LINC00115 in 100 paired CRC tissues and adjacent normal tissues. The results showed a significantly increased LINC00115 expression in CRC tissues compared to adjacent normal tissues. We also observed that upregulation of LINC00115 was strongly related to advanced TNM stage, larger tumor size, lymphatic metastasis of CRC patients, suggesting that LINC00115 could be a prognostic factor of CRC.
There is increasing evidence that a high lncRNA expression is significantly correlated with unfavorable CRC prognosis. The elevated expressions of lncRNA TTN-AS1, XIST, TUG1 and SNHG12 indicate poorer prognoses of CRC patients[17, 22–24]. Our multivariate survival study proved that LINC00115 are independent prognostic factors for poor relapse-survival in CRC patients. Importantly, functional assays further identified that knockdown of LINC00115 could inhibited cell proliferation, migration and invasion and facilitated cell apoptosis in vitro. These findings indicated that LINC00115 was associated with the carcinogenesis and progression of CRC, but the exact regulatory mechanism is needed to illuminated.
Increasing amount of multiscale omics data showed that lncRNAs exert their biological effects by working together with different molecules, such as miRNAs, mRNAs and proteins. For instance, lncRNAs might interact with miRNAs by binding them onto theirs 3′-UTR region and therefore making themselves acting as a miRNA sponge. Currently, the functional pattern of LINC00115 in CRC cells remains to be established. Hence, we searched several databases and found that miR-489-3p was a target gene for LINC00115. Previous research has shown that miR-489-3p was sponged by LINC01446 and targeted TPT1 to regulate glioblastoma progression[25]. In osteosarcoma, miR-489-3p significantly suppressed cell invasion and metastasis both in vitro and in vivo[26]. In the present study, we firstly proposed that miR-489-3p might be a target of LINC00115. Through the luciferase reporter gene assay, we confirmed that LINC00115 could directly bind to miR-489-3p. We also found a negative correlation between LINC00115 and miR-489-3p expressions in CRC tissues. Therefore, we predicted that LINC00115 acts as a ceRNA to sequester miR-489-3p, but further experiment is needed.
Numerous studies collectively indicated the importance of intricate crosstalk existing between lncRNAs and PI3K/AKT/mTOR signaling[27, 28]. mTOR frequently exerts as an oncogenic signaling cascade in human malignancies[29]. It is a serine/threonine protein kinase belonging to PI3K-related kinase family and forms the catalytic subunit of two kinds of protein complexes including mTOR complex 1 (mTORC1) and 2 (mTORC2)[30]. While mTORC1 controls gene transcription and protein translation in growth-related processed, mTORC2 promotes cell proliferation and survival. Accumulating studies have revealed that PI3K/AKT/mTOR acted as a key driver of cellular growth, adhesion, migration and survival in human carcinogenesis, including CRC, in which activation of PI3K/AKT/mTOR signaling supports cancer cell growth, metastasis, and drug-resistance[31, 32]. PI3K/AKT/mTOR signaling also serves as an integration mediator in the crosstalk of oncogenic signaling pathways[31]. Phosphorylation of PI3K and AKT is the key step for their activation. Once phosphorylation, PI3K activates and phosphorylates its downstream AKT and mTOR to cause a cascade reaction. In recent study, miR-489-3p was shown to interact with PI3K/AKT/mTOR signaling in various cancers, such as glioma, breast cancer and melanoma[33–35]. Interestingly, it was found that the phosphorylated levels of PI3K (p-PI3K), AKT (p-AKT), and mTOR (p-mTOR) were decreased after LINC00115 depletion, while their total protein levels remained unchanged, suggesting that LINC00115 positively regulates the PI3K/AKT/mTOR pathway. On finding on the implication of LINC00115 in mediating cell proliferation and migration in CRC via modulating PI3K/AKT/mTOR pathway exemplified the idea that anti-LINC00115 compound or agent that consequently targeting PI3K/AKT/mTOR pathway might serve as a novel therapeutic tactics for the treatment of CRC.
In conclusion, our study has delineated the unique role of LINC00115 in CRC and specifically described underlying molecular mechanisms. We confirmed that LINC00115 is upregulated in CRC and function as an independent predictor of progression-free survival, leading to tumor progression and aggressiveness. Importantly, we are the first to demonstrate that the LINC00115/miR-489-3p axis is markedly linked to CRC cell proliferation, migration, and invasion, which was achieved by interacting with PI3K/Akt/mTOR signaling pathway. The findings from our study have opened up a new understanding of CRC and identified a promising biomarker and target for enhancing anti-CRC therapy.