Osteoarthritis (OA) is the most prevalent type of arthritis, affecting one in three people over 65 [1]. OA is an inflammatory disease that causes pain and inflammation in the joints and limits joint movement. This disease causes osteophyte formation, subchondral bone sclerosis, tendon inflammation, degeneration of ligaments in the knee, joint capsule hypertrophy, and synovial inflammation. OA causes abnormal regeneration of joint tissues by inflammatory mediators [2,3]. In this disease, cartilage homeostasis is lost through the loss of collagen and proteoglycans [4]. Moreover, cartilage and synovial cells become inflamed by producing inflammatory mediators such as IL-1 and TNF- α and chemokines through the activation of the NF-κB pathway. Intrinsic inflammatory signals are also involved in OA. Catabolic factors such as IL-1, IL-6, TNF-α, PGE2, FGF-2 and PKCδ play a role in cartilage destruction [5].

Metformin is an antidiabetic drug with limited adverse drug reactions which may be repurposed in many illnesses [[6], [7], [8]]. Metformin improves chronic inflammation by improving metabolic parameters such as hyperglycemia, insulin resistance, and atherogenic dyslipidemia, and has a direct anti-inflammatory effect [9]. It inhibits p65 nuclear translocation and prevents the activation of NF-κB pathway by inhibiting AMPK shutting down high mobility group box 1 (HMGB1) leading to decreased inflammation [10].

Interleukin-1 (IL-1) is a cytokine that has an important function as well as paracrine and autocrine inflammation [4]. The interleukin-1 family includes seven agonistic ligands, including IL- 1 (β and α) and IL-18, IL-33 (γ and β) and IL-36 α, and three ligands with antagonistic activity including (IL-1Ra, IL-36Ra and IL-38) and an anti-inflammatory cytokine including (IL-37) [11]. Interleukin-1 in OA causes destruction of the extracellular matrix, increases the destruction of collagen by metalloproteinase and nitric oxide activity and finally induces cartilage cell apoptosis [4]. As a member of the IL-1 family, IL-33 plays an important role in diseases such as asthma, rheumatism, ulcerative colitis, and OA. The release of IL-33 in cartilage is mediated by TLR3 signaling leading to a shift in cartilage tissue homeostasis to a catabolic state. IL-33 causes the expression of MMPs and decreases collagen type 2 synthesis. TLR3 induces IL-33 synthesis via p38 MAPK, which is upstream of p65 [12] [2].

The gene encoding IL-33 in humans and mice contains 8 exons. The IL-33 mRNA promoter is located upstream of the untranslated exon (exon 1 or exon 1a). More than 20 kb upstream of exon 2 contains the AUG initiation code. There are several studies on polymorphisms of the IL-33 (rs1929992) SNP in different inflammatory diseases such as ankylosing spondylitis, systemic lupus erythematosus (SLE), and chronic urticarial. It has been reported the GG and GA genotypes of rs1929992 were correlated with the risk of autoimmune diseases and higher serum levels of IL-33 [[13], [14], [15], [16], [17]]. To date, no study has evaluated the association of IL-33 (rs1929992) SNP with knee OA. In this study, we aimed to investigate the role of the genetic diversity of il-33 with the serum levels of il-33 and susceptibility to OA in OA patients under treatment by metformin and placebo.