Background: Adipose tissue is a large endocrine organ known to secret adiponectin, which has anti-diabetic, anti-atherogenic, and anti-inflammatory properties. Adiponectin is widely involved in systemic disease, diabetes mellitus, and cardiac infraction. This study aimed to investigate the involvement of adiponectin in intervertebral disc (IVD) degeneration.
Methods: Adipose and IVD tissues were obtained from human patients undergoing surgery (n = 4) and from skeletally mature Sprague-Dawley rats (n = 21). Tissues were stained immunohistochemically for adiponectin and adiponectin receptors AdipoR1 and AdipoR2. Changes in adiponectin receptor expression with IVD degeneration severity were then investigated using a rat tail temporary compression model. Rat IVD tissues were stained immunohistochemically with AdipoR1 or AdipoR2, and immunopositive cell percentages were calculated. Rat nucleus pulposus (NP) and annulus fibrosus (AF) tissues were isolated separately and treated with recombinant adiponectin (Ad 0.1 or 1.0 μg/ml) and/or interleukin-1 beta (IL-1β) (0.2 μg/ml) for 24 h. The four groups were as follows: control group (no treatment), IL-1β group (IL-1β-only treatment), IL-1β+Ad (0.1) group (IL-1β and adiponectin [0.1 μg/ml] treatment), and IL-1β+Ad (1.0) group (IL-1β and adiponectin [1.0 μg/ml]). Real-time reverse transcription-polymerase chain reaction was performed to evaluate messenger-RNA (mRNA) expression of tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6).
Results: Adiponectin was widely expressed in human subcutaneous and epidural adipose tissue. In rat IVD tissue, adiponectin was not observed in NP and AF. However, both AdipoR1 and AdipoR2 were widely expressed in both human and rat IVD tissues, with no significant differences in expression levels between receptors. Furthermore, expression levels of AdipoR1 and AdipoR2 were gradually decreased with increased IVD degeneration severity. Interestingly, mRNA expression levels of TNF-α and IL-6 were significantly upregulated by IL-1β stimulation. TNF-α expression in the IL-1β+Ad 1.0 group was significantly lower than that in the IL-1β group in both NP and AF cells (P < 0.05). Finally, IL-6 expression was not affected by adiponectin treatment in IVD cells.
Conclusions: This study investigated for the first time the expression of adiponectin receptors in human and rat IVD cells. The findings indicate that adiponectin produced by the systemic or epidural adipose tissue may be involved in the pathomechanism of IVD degeneration.
Keywords: Adiponectin; Adiponectin receptor; Animal model; Anti-inflammatory effect; Degeneration; Intervertebral disc.