Up-regulation of miR-182
was suggested to exist in a large part of HCC tissues [15
]. In our HCC cases with complete clinical data, we also found the up-regulation of miR-182
and its up-regulation was significantly associated with intrahepatic metastasis (tumor number
2) and early recurrence, which is an important clinical determinant for the prognosis of HCC patients. Up-regulation of miR-182
was further suggested to correlate with reduced disease-free survival of HCC patients. Hence, determination of miR-182
expression level in HCC tissues may be a novel approach to predict and identify the prognosis of HCC patients.
Although miRNA profile did reveal very prospective features in cancer, the functions and real targets of miRNAs were largely unknown. The predicted targets of the majority of microRNAs based on sequence homology remained to be comprehensively validated by in vitro and in vivo experiments. Target scan and Pictar showed metastasis suppressor 1 (MTSS1) is one important target of miR-182 with a high context score. Meanwhile, we found its expression in HCC decreased significantly compared to that of adjacent normal tissue and negatively correlated with the expression of miR-182, which indicated MTSS1 maybe the regulation target of miR-182.
, also known as MIM
(missing in metastasis), was originally identified by Lee et al. [18
] as a potential metastasis suppressor gene that was present in non-metastatic bladder cancer cell lines, but was not expressed in a metastatic bladder cancer cell line [19
]. This gene, mapped to human chromosome 8q24.1, encodes a 5.3 kb mRNA and a polypeptide predicted to be an actin-binding protein of 356 amino acids with homology to the WASp (Wiscott-Aldrich Syndrome protein) family [20
]. Functional analyses of MTSS1
have shown that MTSS1
induced actin-rich protrusions resembling microspikes and lamellipodia at the plasma membrane and promoted disassembly of actin stress fibres [21
]. Actin filament assembly is associated with cytoskeletal structure organization and many forms of cell motility [22
]. These data have suggested that MTSS1
protein may be important in regulating cytoskeletal dynamics, and as a consequence it would play a potential role in the invasion and metastatic behavior of cancer cells. Therefore, the down-regulation of MTSS1
potentiated by the up-regulation of miR-182
may further aggravate the epigenetic changes in HCC. We then focused on the mechanisms that whether the up-regulation of miR-182
mediates the inhibition of MTSS1
and induced epigenetic alterations in HCC pathogenesis.
miR-182 can bind to MTSS1 at two conserved sites with a high context score. Our luciferase assay in HCC cell lines demonstrated MTSS1 can be regulated directly by miR-182. The interesting results in HCC cell lines is that cells with high invasive ability showed higher expression level of miR-182 than those with low invasive potential, which is inversely related with the expression of MTSS1. Analyses on human samples reinforced the relevance of miR-182 regulation on MTSS1 in HCC by revealing an inverse correlation between their expressions. Considering the characteristic heterogeneity of HCC and that MTSS1 is regulated by additional mechanisms, a statistically significant association with miR-182 is especially remarkable. The ability of MTSS1 over-expression to counteract miR-182’s pro-invasion effects unequivocally shows the importance of this inverse relationship in HCC metastasis. The functional analysis of miR-182 together with MTSS1 in animal models will particularly further evaluate their metastatic role and show us the clinical treatment value for patients with HCC. That would be our future research aim.
Concerning the target of miR-182
, Miguel and et al. also reported that the microRNA promotes melanoma metastasis by repressing FOXO3
and microphthalmia-associated transcription factor [13
]. Together with our study, it is consistent with current opinions that a single miRNA can target multiple mRNAs, named ‘targetome’, to post-transcriptionally regulate gene expression [23
]. Hence, it is probable that we are still far from unveiling the last target of miR-182
. According to this presumption, interesting future work may be carried out to identify the ‘targetome’ and the entire roles of miR-182
in cancer development. Another important issue is why miR-182
is up-regulated in HCC and other cancers [15
]. The current view suggests that miRNA expression is mainly controlled at the transcriptional level. A large number of transcription regulators that influence the transcription and production of miRNAs have been identified including Myc, E2F, p53, and STAT3 [25
]. Another possible mechanism for the up-regulation of miRNAs in cancer may result from the amplification of DNA copy number. Such as miR-182
is one member of a miRNA cluster in a chromosomal locus (7q31-34) frequently amplified in HCC [13
], the amplification may cause the up-regulation of miR- 182
. This is our future’s research field.