Background/Purpose: Ankylosing spondylitis (AS) is a predominantly male disease and spinal ankylosis more severely affect male AS¹ suggesting role of testosterone on spinal ankylosis. In previous studies, dihydrotestosterone (DHT) exhibited promoting osteoblast differentiation in in vitro experiment². However, testosterone suppressed IL-17 expression³ and exogenous IL-17A to primary bone-driven cells from AS promoted osteoblast activity and differentiation⁴. But the effect of testosterone on bony ankylosis in AS is still debated. This study aims to investigate the effect of testosterone on spinal ankylosis and osteoblast differentiation with curdlan-induced SKG mice and human primary osteoprogenitors.

Methods: Eight-week-old male SKG mice were intraperitoneally injected 3mg of curdlan (curdlan group, n=10) or PBS alone as control (n=6) at 0 and 2 weeks. Experimental diet with dutasteride (41.2mg/kg/day), 5α reductase inhibitor that block the conversion of testosterone into DHT, was begun at 9 weeks after the first curdlan injection (dutasteride group, n=10). Clinical scores of peripheral arthritis were monitored weekly, and at 15 weeks after first curdlan injection, whole-body imaging of mice using fluorescent in vivo bisphosphonate agent was performed using OsteoSense^® 680 EX. At 17 weeks after first curdlan injection, mice were sacrificed and sera and splenocytes were collected. Bone metabolism-related molecules (RANKL, osteoprotegerin, DKK1, and sclerostin) were analyzed using luminex multiplex assay and IL-17A by single molecule arrays. T cell population in the spleen was measured using flow cytometry. Additionally, human bone tissues were obtained at surgery from facet joints of 10 patients with noninflammatory spinal disease from traffic trauma or spinal compression disease. Primary osteoprogenitor cells were cultured to assess osteoblastic activity. the effect of DHT on primary preosteoblasts was assessed by alkaline phosphatase (ALP) activity and staining, alizarin red staining (ARS) for calcium deposit, and qPCR

Results: The accumulation of hydroxyapatite that suggested spinal mineralization and osteoblast activity was increased in dutasteride group compared with curdlan group, and the osteoblast activities were correlated with levels of serum IL-17A. Among bone metabolism-related molecules, level of sclerostin was decreased in dutasteride group compared with curdlan group. Interestingly, increased frequency of Th17 frequency and increased IL-17A secretory Treg population was evident from splenocytes in dutasteride group. In in vitro human experiments, continuous exposure of DHT to osteoprogenitors resulted in low calcium deposit with ARS during osteoblasts differentiation, but not intercellular ALP activity and ALP staining. In terms of gene expressions, DHT treated cells showed decreased osteocalcin and increased DKK1 and SOST1.

Conclusion: Conclusively, treatment with dutasteride results in more aggressive mineralization of spine in curdlan-induced SKG mice and DHT treatment attenuates osteoblasts differentiation in in vitro model. Therefore, anti-androgen treatment in AS patients may be undertaken with caution when considering the progression of spinal ankylosis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/effect-of-testosterone-on-spinal-ankylosis-in-ankylosing-spondylitis-in-vivo-and-in-vitro/