1 Introduction
Osteoarthritis (OA) is a degenerative joint disorder characterized by
progressive destruction of articular cartilage, synovial hyperplasia and
subchondral osteosclerosis [1]. It is one of the main causes of
disability in middle-aged and elderly people. With the increase in the
aging population and its high medical costs, osteoarthritis and
disability caused by it have become an urgent public health issue
[2]. The prevalence of diagnosed symptomatic knee osteoarthritis was
4.2%~15.5%, which increased with age. About 80% of
the population above 65 years reveals radiographic evidence of KOA
[3]. Among the 291 disabilities listed by global burden of disease
(GBD), hip and knee OA ranked 11th globally in 2010 and 38th in
disability affects life year (DALY) [4]. Although osteoarthritis is
considered a disease of the elderly, disabling osteoarthritis can still
have a significant impact on work. It is reported that 44.4% of people
who are unemployed due to illness are related to OA in Canada, and the
influence is expected to increase over time [5]. In addition to
aging, many other factors play a role in the pathogenesis of OA. Both
metabolic syndrome and dietary habit play a role in the susceptibility
and development of OA [6]. Recently, more and more researches have
been conducted on the molecular mechanism of OA to find target
treatments for OA. More than 25 miRNAs involved in the occurrence of
chondrogenesis and OA have been revealed. In particular, it is regulated
by more than one miRNA in chondrocyte formation, chondrocyte
differentiation, chondrocyte proliferation, chondrocyte hypertrophy,
endochondral ossification and proteolytic enzyme regulation [7].
After knocking down RNA binding-protein zinc finger protein 36 type-like
1 (ZFP36L1) gene, the mRNA expression of two heat shock protein 70
(HSP70) family members increased, the two genes inhibited apoptosis of
cartilage, so RNA-binding protein ZFP36L1 plays a role in the
pathogenesis of osteoarthritis by modulating members of the HSP70
family, inhibiting chondrocytes apoptosis [8]. However, the specific
mechanism of RNA-binding protein in the pathogenesis of osteoarthritis
still needs to be clarified. So, the role of RNA-binding protein GNL3
was studied in this research in association with the pathogenesis of OA,
which can provide new target treatments for OA.
RNA-binding proteins play a key role in post-transcriptional regulation
and can affect different stages of RNA metabolism, and process of cell
proliferation and apoptosis [9-11]. GNL3 ( Nucleolar GTP-binding
Protein 3, Nucleostemin), also known as nucleostem factor, is an
RNA-binding protein which plays an important role in cell proliferation,
cell cycle regulation, differentiation inhibition, ribosome
biosynthesis, stem cell characteristic maintenance, genome stability and
telomere integrity [12-14]. It is abundantly expressed in bone
marrow mesenchymal stem cells, and correlates with chondrocytes
differentiation. GNL3 levels were found to be significantly increased in
synovial tissue and fluid of osteoarthritis patients in the clinical
experiment [15]. Furthermore, the relationship between GNL3 gene and
osteoarthritis in Han Chinese population has also been verified
[16]. Further study found that cis-acting regulatory polymorphisms
acting on GNL3 contributed to the OA association signal at chromosome
3p21, and the osteoarthritis susceptibility locus had been mapped to
3p21 chromosome [17]. These results proved that GNL3 is closely
related to the pathogenesis of osteoarthritis. Therefore, it was
speculated that GNL3 is involved in the pathogenesis of osteoarthritis
as an RNA-binding protein. So, in this study we explored the role of
RNA-binding protein GNL3 in the pathogenesis of osteoarthritis, which
can provide a new target for the treatment of osteoarthritis.
In this study, unbiased transcriptome analysis has been utilized to
investigate how GNL3 regulates gene transcription in Hela cells. We
successfully knocked down the GNL3 gene by shRNA Hela. High-throughput
RNA sequencing (RNA-seq) were performed for the experimental group and
the control group to obtain the gene expression profiles regulated by
GNL3. The results showed evident changes in the transcription profile of
specific genes after knocking down the GNL3 gene, some of which were
correlated with the incidence of osteoarthritis, including IL24 gene and
PTN gene.
IL24 inhibits cell proliferation and angiogenesis, and induces apoptosis
in tumor cells [18]. It is determined that IL24 plays an important
role in the classical apoptotic pathway p38 [19], and participates
in inducing apoptosis in a variety of diseases. It is reported that IL24
reduces the production of dickkopf-1 by fibroblast-like synovial cells
in the pathogenesis of spinal arthritis, and also induces osteogenic
mineralization, playing a role in aggravating the progression of
osteoarthritis [20]. PTN is a secretory growth factor that
contributes to the skeletal development of human embryos [21]. Bone
epiphysis in the state of ischemia and hypoxia leads to extensive bone
cell and tissue death [22], while it is suggested that PTN directly
participates in angiogenesis and promotes bone repair [23, 24].
Angiogenesis is essential in the pathogenesis of OA, facilitating the
invasion of inflammatory cells and increase in local pain receptors that
contribute to structural damage and pain. So, PTN can aggravate the
development of osteoarthritis by promoting angiogenesis [25]. In
addition, the PTN promotes original bone cell chemotaxis, which can
promote the total colony formation of osteoblast, alkaline phosphatase
positive colony formation and specificity of alkaline phosphatase
activity [26].
Based on previous studies, it was speculated that IL24 mainly aggravates
the progression of osteoarthritis by inducing bone cells apoptosis at
the joint and PTN may contribute to the progression of osteoarthritis by
promoting angiogenesis [19, 24]. While, IL24 and PTN contribute to
the progression of osteoarthritis directly, GNL3 promotes the
development of osteoarthritis by acting on both of these two downstream
genes.