Inflammation-dependent downregulation of miR-194-5p contributes to human intervertebral disc degeneration by targeting CUL4A and CUL4B.

Inflammation is one of the major causes of intervertebral disc degeneration (IDD). Emerging evidence has revealed that increase in the levels of pro-inflammatory cytokines, such as interleukin 6 (IL-6) and tumor necrosis factor alpha (TNF-α), can activate a variety of signaling pathways, eventually resulting in IDD. Here, we show that the two cullin family genes, CUL4A and CUL4B, but not other cullins, are specifically overexpressed in IDD samples compared with healthy controls, and the CUL4A and CUL4B levels are positively correlated with the severity of IDD. In vitro analyses in human osteoblast cells (hFOB1.19), nucleus pulposus cells (hNPCs), and annulus fibrosus cells (hAFCs) indicated that treatment with IL-6 and TNF-α can increase CUL4A and CUL4B levels. By performing a microRNA-based microarray analysis, we found a set of microRNAs (miRNAs) that were differentially expressed in IDD samples compared with samples from healthy controls. Of these miRNAs, miR-194-5p, was significantly downregulated in IDD samples and could bind to the three prime untranslated regions (3'-UTRs) of both CUL4A and CUL4B, thereby downregulating their expression. The in vitro overexpression or downregulation of miR-194-5p, with a miR-194-5p-mimic or with anti-miR-194-5p, can cause the repression or induction of both CUL4A and CUL4B, respectively. Interestingly, treatment with IL-6 and TNF-α inhibitors in primary hNPCs and hAFCs that were isolated from patients with IDD led to the downregulation of CUL4A and CUL4B. Together, these findings provide insight into how the inflammation-dependent downregulation of miR-194-5p contributes to the pathogenesis of IDD, which may aid in the development of new therapeutic approaches for IDD by directly targeting miR-194-5p or CUL4A and CUL4B.