The expression of PCAT6 is found to be aberrantly elevated in various human tumor tissues and cell lines compared with matched normal ones, including bladder cancer (BC) [21,24,25], breast cancer (BrCa) [26,27], cervical cancer (CC) [28,29], colorectal cancer (CRC) [30,31], gastrointestinal stromal tumor (GIST) [32], gastric cancer (GC) [33,34], glioblastoma (GBM) [35], hepatocellular carcinoma (HCC) [36,37,38], lung cancer (LC) [20,39,40,41,42,43], osteosarcoma (Osa) [44,45,46], ovarian cancer (OvCa) [47,48], cholangiocarcinoma (CCA) [49], pituitary adenoma (PA) [50], pancreatic ductal adenocarcinoma (PDAC) [51], and prostate cancer (PCa) [52,53]. In subsequent experiments on the biological functions of tumor cells, it has been revealed that a high level of PCAT6 has strong cancer-promoting effects, mainly including the promotion of cell proliferation, enhancement of migration, invasion and EMT process, as well as the inhibition of cell apoptosis. Meanwhile, PCAT6 has been shown to promote tumor growth and metastasis in xenograft mouse models (Table 1) [26,29,30,38,39,40,43,44,46,50,52,53,54]. However, Amelia et al. reported that the expression level of PCAT6 was opposite in lung tumor tissues and lung cancer cell lines compared with the normal control group [55]. Compared with paired normal tissue, PCAT6 expression level is higher in lung tumors, while its level is lower in non-small cell lung cancer (NSCLC) cell lines compared to the normal human fetal lung fibroblast cell line (IMR-90) [55]. Interestingly, Tu et al. found that, compared to T cells, B cells, dendritics, and neutrophils, PCAT6 expression was the highest in macrophages which derived from patients of CCA, especially M2 macrophages [54]. Furthermoer, PCAT6 expression level is also significantly higher in the blood samples of some cancer patients, including BC [24] and LC [41,56]. Contradictorily, Siddique et al., testified that PCAT6 level had no significant difference in the blood between Saudi CRC patients and healthy donors [57]. It is speculated that the cause of this result may be ethnically related, and the expression level of PCAT6 in CRC patients of different races might be different.

LncRNAs play diverse functions depending on different subcellular or extracellular compartmental localizations. Most studies indicate that PCAT6 is primarily located in the cytoplasm of BC [25], GIST [32], GBM [35], Osa [44,45], PA [50], and Pca [52] cells. Cytoplasmic lncRNAs regulate genes at the translational and post-transcriptional levels, such as interaction with cytoplasmic proteins [58], and interaction with microRNAs to regulate downstream mRNA levels [59,60,61,62,63]. Shi et al. determined that PCAT6 was principally distributed in the nucleus of NSCLC cells [40]. Nucleic lncRNAs regulate genes at the epigenetic and transcriptional levels, including histone modifications [64,65], DNA methylation [66], and chromatin remodeling [67]. Furthermore, Dong and Lang et al. demonstrated that PCAT6 was located in both the cytoplasm and nucleus of BrCa and PCa cells by fluorescence in situ hybridization (FISH) and subcellular fraction assays, which was different from most studies [26,53]. This is similar to the lncRNA HOTAIR, which regulates genes at both the epigenetic and transcriptional levels, as well as at the post-transcriptional level [68].