Introduction: Gout is an important inflammatory disease with high prevalence in developed countries. Since the most researchers mainly focus on inflammation due to the MSU crystal depositions, there are only a few studies beeing published on how soluble uric acid modulates gouty inflammation. In the present study we hypothesize that urate modulates IGF-1R gene expression after 24 hours incubation with different concentrations of uric acid. Moreover, we were also interested if the IGF1 pathway mediates uric acid priming based on in vitro cytokine production while inducing or inhibiting its role. Therefore, considering IGF-1R gene as having a role in the inflammatory response, we suppose that IGF-1R rs6598541 polymorphism may affect the risk of gout development. In the present paper, we conducted a study using gout patients, hyperuricemic controls and normouricemic controls to search for possible genetic associations between IGF-1R rs6598541 polymorphism and gout in a European population.
Materials and methods: PBMCs from healthy donors were cultured for 24 h with RPMI for control and uric acid or IGF1 binding protein solubilized in RPMI with 10% serum. After 24h the cells were restimulated with LPS with or without MSU crystals. In parallel, the transcription rate for IGF-1R was investigated in cells treated with increasing doses of uric acid. The capacity of the cells to be primed with urate was evaluated using qPCR and ELISA for IL-1β, IL-6 or IL-1Ra. Moreover, samples from gout patients were compared with hyperuricemic controls. 140 HU controls, 177 gout patients and 366 healthy volunteers were analyzed by Taqman Assay genotyping. Data analysis was carried out using the dominant or recessive risk models for the obtained genotypes. The association for this SNP to disease status or markers of inflammation has been also tested. Cytokine production in response to MSU in the presence or absence of palmitate (C16) or Pam3Cys was assessed and linked to the SNP.
Results: The in vitro data shows that uric acid does not modulate IGF-1R gene expression in cells treated with increasing doses of uric acid. The IGF1 pathway seems to be involved in uric acid priming in vitro, as a tendency of synergism between uric acid and IGF1 in the production of IL-1b and IL-6 is observed, but the effect is not major. Moreover, IGF1 enhanced IL-1Ra, but the uric acid dependent downregulation of IL-1Ra is not modified. In the given population the distribution of the alleles and genotypes shows a similar pattern between the gout and hyperuricemic subjects. There were statistically significant differences between the genotypes of gout and normouricemic subjects, and hyperuricemic and normouricemic subjects. The SNP was not found to be statistically correlated with gout or markers of inflammation in the functional genetics testing.
Conclusion: IGF1R does not seem to be involved in uric acid induced proinflammatory responses. Moreover, the IGF1 pathway seems unlikely to modulate uric acid induced inflammation, but further investigations are required. The genetic data showed that the polymorphism in the IGF1R gene is not associated to gout susceptibility, the SNP is more likely to be a urate locus. In addition, in a functional genetics assay no correlation was found between the SNP and the cytokine production. However, further studies in larger cohorts are needed in order to draw more relevant conclusions for the general population.
Keywords: IGF1R, gout, SNP