MiRNA expression in the cartilage of patients with osteoarthritis

Micro RNAs regulate diverse aspects of normal physiologic conditions. On the other hand, miRNA levels change in response to pathologic alterations, possibly contributing to the pathogenesis of degenerative diseases, such as OA [5]. Therefore, it is important to determine how miRNA expression is regulated in OA. Here, we found that the expression of four miRNAs (hsa-miR-138-5p, hsa-miR-146a-5p, hsa-miR-335-5p, and hsa-miR-9-5p) was upregulated in OA patients compared to that in controls (Fig. 3).

miR-146a is one of the first miRNAs reported to be differentially expressed in OA cartilage. mir-146a is a crucial determinant in knee joint homeostasis and pain symptoms associated with OA, maintaining the balance between the inflammatory response and expression of pain-associated factors in cartilage glial cells and synovium [10]. Yamasaki et al. [11] reported that miR-146a is highly expressed in cartilage with a low Mankin grade, and its expression is decreased in proportion to the level of matrix metalloproteinase-13 expression. miR-146a is also expressed in all chondrocyte layers, especially in the superficial layers, while miR-146a-expressing cells are lightly distributed in the deep zone, where the matrix appears normal. Interleukin-1 beta induces the expression of miR-146a when the first degenerative changes appear and may play a key role in the inhibition of catabolic factors through negative feedback, including downregulation of the adaptor proteins IRAK1 and TRAF6, in early-stage OA cartilage. In late-stage OA, expression of miR-146a is low, which leads to progressive degradation of the cartilage due to decreased suppression of catabolic signals [9].

In our study, expression of miR-138-5p was upregulated which is consistent with the results of Yuan et al. They suggested that miR-138-5p in interleukin-1 beta (IL-1β) induced extracellular matrix (ECM) degradation of OA cartilage. The expression levels of miR-138-5p are significantly increased in OA cartilage and chondrocytes, as well as in response to IL-1β stimulation. The results indicate that miR-138-5p might be involved in the pathogenesis of OA. miR-138-5p directly modulates the expression of FOXC1, contributing to IL-1β-induced ECM degradation in chondrocytes [12].

Patients with OA exhibit early signs of cartilage degradation, synovial inflammation, and altered bone structure even before the disease manifests clinically [13]. Circulating miRNAs are easily accessible and stable [14, 15], making them convenient for daily clinical monitoring. A study of a large cohort representative of the general community by Beyer et al. [16] provided evidence that specific miRNAs, such as let-7e, can serve as valuable biomarkers for OA.

Expression of miR-146a is also reported in OA chondrocytes; mechanistically, miR-146a is apparently responsive to IL-1b signalling and may be involved in the pain-related pathophysiology of OA [10, 11, 17]. This miRNA was detected in our screening, but was not differentially expressed between cartilage regions. The main difference between miR-146a and miR-146b is the presence of two nucleotides located on different chromosomes. Thus, it is possible that the miRNA homologues mentioned above are differentially regulated during chondrogenesis and have distinct functions associated with the control of developmental processes and maintenance of homeostasis in mature tissue.

In the present study, we reviled increased expression of miR-9. According to Gu et al. results, targeting of miR-9 to NF-κβ1 may enhance proliferation and suppress apoptosis of knee OA chondrocytes through modification of IL-6 and MMP-13. Moreover, miR-9 and NF-κβ1 could potentially serve as diagnostic biomarkers and therapeutic targets for patients with knee OA [18]. Song et al. in their study found that protogenin (PRTG) is regulated by miR-9, resulting in an inhibition of cell proliferation and survival in chondrogenic progenitors and articular chondrocytes. Reduction of miR-9 induction, which results in increased PRTG levels in OA pathogenesis, may be responsible for chondrocyte apoptosis, a typical hallmark of OA [19].

We detected significant changes in the differential expression patterns of miR-335-5p and the less abundant 3p strand (miR-335*). We have observed a sharp decline in miR-335-5p expression during transforming growth factor β3-induced chondrocyte differentiation of hMSCs (unpublished data), suggesting a potential role for the 5p strand of miR-335 in regulating genes to maintain a more progenitor-like phenotype. Supporting this view, Tome et al. [20] reported that miR-335-5p downregulation is required for MSC differentiation toward the osteogenic or adipogenic lineage; overexpression of this miRNA inhibits MSC differentiation. On the other hand, Zhang et al. [21] reported opposite effects of miR-335-5p in regulating osteogenesis, but this may be explained by the fact that they used cell lines as opposed to primary MSCs. The present study is the first to report the presence of miR-138 in the cartilage and that higher expression of this miRNA is associated with differentiated and hypertrophic chondrocytes compared with precursor cells. These expression patterns suggest a potential role of miR-138 in regulating specific phases of chondrocyte differentiation. Other studies indicate that overexpression of miR-138 inhibits osteogenic and adipogenic differentiation [22, 23]. Interestingly, it has also been demonstrated that miR-138 promotes induced pluripotent stem cell generation via regulation of p53 [24], indicating that miR-138 can control cellular differentiation and may function through different mechanisms depending on the tissue microenvironment. How this miRNA regulates chondrogenesis and the mechanisms involved are important questions for future studies.