4 Discussion
OA is a common clinical degenerative joint disease in elderly
population, which is one of the main causes of chronic pain and joint
disability. OA can severely reduce the quality of life of affected
individuals. Current therapeutic goal of OA in clinical practice is to
relieve the symptoms but cannot slow down the disease progression.
Non-steroidal anti-inflammatory drugs (NSAIDs) are generally considered
to be the first-line pharmacological treatments for OA. However, NSAIDs
cannot achieve the long-term pain relief for OA patients, and they are
associated with significant side efforts such as gastric ulceration,
renal impairment, and cardiovascular accidents 14. At
the end stage of OA, a replacement surgery remains to be the only
treatment option. Thus, identification of emerging pharmaceutical
therapies is urgently needed in the treatment of OA.
Empagliflozin,
a selective inhibitor of sodium-glucose cotransporter-2 (SGLT2), has
been reported to show the anti-inflammatory properties in several
conditions such as cardiovascular diseases, Alzheimer’s disease, liver
injury, and chronic kidney disease 15-19. In this
study, we discovered the protective effect of empagliflozin on cartilage
degeneration via inhibition of NF-κB pathway.
Previously thought to simply be damaged from ”wear and tear”, OA is now
understood as a complex cell-mediated process. Current research has
shown that OA is characterized by progressive cartilage degradation, but
also underlying bone remodeling, osteophyte formation, and synovial
inflammation 3,20. A series of pathological factors
such as ECM degradation, chondrocyte inflammation, oxidative stress,
mitochondrial dysfunction, abnormal mechanical load, and
senescent molecules contribute to the
degeneration of chondrocytes, ultimately causing cartilage damage and OA
development 21,22. Chondrocytes, as the only cellular
component of cartilage, play a central role in the balance of ECM
metabolism through the synthesis of cartilage matrix. However, the
inflammatory factors like IL-1β can disrupt the balance of ECM
metabolism by increasing the expression of catabolic enzymes including
MMPs, causing the gradual loss of ECM. MMPs inhibitors could be
promising agents for the treatment of OA 23. Our data
revealed that IL-1β upregulated the expression of matrix-degrading
enzyme (MMP9 and MMP13) and activated the inflammatory cascade reaction
by the secretion of inflammatory cytokines (NO, PGE2, IL-6, COX2, and
INOS) in mouse chondrocytes, whereas these effects could be offset by
the administration of empagliflozin. These founds indicated that
empagliflozin could protect mouse chondrocytes from IL-1β-induced ECM
degradation and inflammatory reaction. What’s more, chondrocyte
senescence is also involved in the pathological process of OA. Jeon et
al found that senescent cells accumulated in the articular cartilage and
synovium after anterior cruciate ligament transection, and local
clearance of these senescent cells attenuated the development of
post-traumatic OA 24. Therefore, the senescence
analysis of chondrocytes was conducted in this
study. Cellular senescence is
generally regarded as a cell state characterized by an irreversible
cell-cycle arrest 25. P21 and P53 are the key factors
for promoting senescence. We found that empagliflozin could counteract
the elevated expression of senescence markers (P21 and P53) induced by
IL-1β. Empagliflozin could also decrease the SA-β-Gal-positive cells.
Thus, empagliflozin could alleviate IL-1β-induced senescence in mouse
chondrocytes in vitro. All these results displayed the antidegradation
role of empagliflozin on chondrocytes in vitro. The effect of
empagliflozin on cartilage damage in vivo was also observed in
DMM-induced OA mice model. We found that empagliflozin could also
protect mouse knee cartilage from wear and matrix degeneration.
Multiple signaling pathways are involved in OA such as NF-κB,
Wnt/β-catenin, HIFs, TGFβ/ΒΜP and so on 26. NF-κB
signaling participates in many OA-associated events, including
chondrocyte catabolism, survival, and inflammation 27.
NF-κB transcription factor has been considered as a disease-contributing
factor of OA for a long time 27. When NF-κB exists in
the cytoplasm, it is in an inactive state and cannot enter the nucleus
to play its function because it binds to the inhibitory protein IκB.
Once cells are stimulated by certain inductive factors, IκB is
phosphorylated and the molecular conformation of IκB changes, resulting
in the activation and nuclear translocation of NF-κB28,29. Activation of NF-κB ,on the one hand, can
directly bind to the promoters of MMPs genes to promote the expression
of matrix-degrading enzyme 30. On the other hand,
activation of NF-κB regulates the transcription of many nuclear genes
related to inflammation, including cyclooxygenase (COX2) and inducible
nitric oxide synthase (iNOS) genes 31,32. COX2 and
iNOS can promote the production and secretion of induce prostaglandin
(PGE2) and nitric oxide (NO). Increased PGE2 and NO levels can
upregulate the expression of MMPs, ultimately causing cartilage
degeneration. NF-κB signaling pathway was focused in this study. Our
results showed that empagliflozin inhibited the phosphorylation of IκB
and P65, displaying the suppressive role on NF-κB signaling. Thus, the
underlying mechanism of empagliflozin in mouse cartilage protective
roles is related to the inhibition of NF-κB pathway.
However, there are still some limitations in our study. First at all, we
confirmed the protective effect of empagliflozin on preventing cartilage
degeneration via inhibiting the activation of NF-κB pathway. But whether
NF-κB is the direct target of empagliflozin needs to be further
identified. Secondly, we have only demonstrated the effects of
empagliflozin on cellular inflammation and senescence in chondrocytes.
The effects of empagliflozin on other aspects involving the onset and
progression of OA deserve further investigation.