Gastric cancer represents an important burden on human health, with hundreds of thousands of newly diagnosed cases as well as related deaths worldwide per year. Improved understanding of this deadly cancer at the basic molecular level is greatly needed. The current study provides insight into a potential novel regulatory mechanism involved in gastric carcinogenesis. Our data suggests that miR-21, which is known to be overexpressed in gastric cancer, downregulates Serpini1, which in turn releases cells from the G1-S transition checkpoint.
Our data shows that transfecting Serpini1 into MKN28 cells induces slower growth, however, these effects are not obvious until Day 6. A potential explanation is that MKN28 cells already express Serpini1 at baseline and adding Serpini1 may only mildly contribute to the growth arrest effects of Serpini1, at least in basal conditions. In accord with this hypothesis, as mentioned in the Cell Cycle section within Results, at baseline, transfecting Serpini1 into MKN28 cells does not induce G1 arrest. However, upon synchronization of cells with Nocodazole, which can be interpreted (due to its effects on the mitotic spindle) as a stressor on these cells, Serpini1 induces a vigorous G1 arrest, as demonstrated in . However, the growth curve in is performed with cells at baseline, and it is possible that the growth suppressive effects of Serpini1 are not obvious until cells reach a certain confluence, and become stressed either by the lack of nutrients or by paracrine signals secreted in the media by themselves.
MiR-21 was previously reported to be upregulated in a number of malignancies, including lymphoma, glioblastoma multiforme, osteosarcoma, cholangiocarcinoma and gastric cancer 41, 44, 45, 46, 47, 48
. Our data confirmed its upregulation on a large cohort of GC vs.
NS subjects. In addition, the fold difference in miR-21 levels between GCs and NSs in our study is similar to other reports 44
. To understand the involvement of miR-21 in GC, in the current study, its downstream targets were searched for in an unbiased fashion, by employing mRNA arrays. This approach offers the benefit of performing an impartial and comprehensive search for downstream targets and is based on growing evidence suggesting that the majority of miR functions are exerted through mRNA downregulation, rather than by protein translation inhibition 31
. This search identified as a putative miR-21 target the gene Serpini1, a previously unstudied gene in gastric cancer.
Serpini1, also known as neuroserpin, is an inhibitor of tissue plasminogen activator that is normally expressed during development and in the adult nervous systems 49
. Serpini1 was also recently identified in myeloid lineage cells 50
. Its expression is downregulated in glioblastomas and pheochromocytomas 49
, but, to our knowledge, it has never been reported to be downregulated in gastric cancer. Our report also suggests, for the first time, that Serpini1 appears to play a tumour-suppressive role in GC.
The data presented herein strongly suggest that miR-21 and Serpini1 are inversely expressed in a subgroup of gastric cancers, suggesting a regulatory network that includes both of these molecules. Further work is needed to characterize this subgroup of gastric cancers, with the purpose of advancing current molecular taxonomies, as well as in pursuit of potential novel therapeutics based on the interaction between miR-21 and Serpini1. Discoveries regarding the role of microRNAs in cancer have pointed to a broad array of potential applications regarding diagnostics and prognostics. The current manuscript describes, to our knowledge for the first time, the involvement of microRNA-21 and its downstream target, Serpini-1, in gastric adenocarcinoma.