Background

Transfection is a specialized technique for introducing exogenous genes into cells. With the deepening of research on gene and protein functions, transfection has become a common basic method in laboratory work. Transfection can be roughly divided into three methods: physical, chemical, and biological mediation. Electroporation, microinjection, and gene gun are very promising physical methods. There are also many chemical methods, such as the classical calcium phosphate coprecipitation method and the lipofection method. Biological methods include primitive protoplast transfection and various virus-mediated transfection techniques (Kim and Eberwine, 2010).

Lipofection refers to the cationic liposome with a positive charge on the surface, which can interact with the phosphate group of nucleic acid through electrostatic interactions to encapsulate DNA or RNA molecules, forming lipid complexes. It can also be adsorbed by negatively charged cell membranes on the surface and then enter cells through fusion or endocytosis. Lipofection is suitable for transfect DNA or RNA into suspension or adherent cultured cells, and is currently one of the most convenient and efficient transfection methods in the laboratory (Felgner et al., 1987).

MicroRNAs (miRNAs) are a class of non-coding small RNAs composed of 18–25 nucleotides. Mostly, they regulate the expression of various protein cod genes at the post-transcriptional level, by inhibiting translation or inducing mRNA degradation, and have a significant impact on physiological processes such as cell differentiation, proliferation, apoptosis, and hematopoiesis. Hence, abnormal expression of miRNA is pathological and can result in the development and progression of multiple diseases (Bartel, 2004).

Some studies have found multiple miRNA expression disorders in rheumatoid arthritis (RA) patients, suggesting that an abnormal miRNA expression may be the molecular mechanism of the disease. These studies involve various cellular biological behaviors, as well as the regulation of signaling pathways and the study of target genes. With the expansion of research fields, the important role of miRNA in RA is gradually receiving increased attention. For example, miR-18a was found to be an inhibitor of nuclear factor kappa-B (NF-κB) in rheumatoid arthritis synovial fibroblasts (RASF) (Trenkmann et al., 2013); miR-19a is associated with the expression of Toll-like receptors in RASF (Philippe et al., 2012); and miR-23b is related to the expression of various inflammatory cytokines in RASF (Zhu et al., 2012).

As a key cell type in the research of pathogenesis of rheumatoid arthritis, the excessive proliferation and limited apoptosis of RASFs are considered to be the pathological basis of RA, which can directly promote joint destruction and cartilage damage by enhancing the production of matrix degrading enzymes (van der Helm-van Mil and Huizinga, 2008; Bartok and Firestein, 2010). Mimics and inhibitors of miRNAs were transfected into the regular SW982 cell line and the primary RASF cells from patients to study the molecular mechanism of these miRNAs during gain and loss of their function.