Date of Award

1-2014

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Percio Gulko

Abstract

Rheumatoid arthritis (RA) is a relatively common autoimmune disease, affecting about 1% in most populations (1). Both genetic and environmental factors have been associated with disease development (2, 3), but the etiology is largely unknown. The list of risk-associated loci continues to grow but identification of environmental factors has not been as successful, although environmental factors are expected to contribute to 40-50% of the risk. Smoking is the best characterized environmental risk factor (4). Other factors include silica dust and mineral oil (5). Pristane induced arthritis (PIA) is induced by an intradermal injection of mineral oil and lead to severe arthritis in susceptible rats, mimicking human RA (6). Previous work done by the Gulko lab identified Cia25 from the ACI strain as a loci that regulated arthritis severity (7). By means of a genotype-eguided breeding strategy, this loci was transferred into the PIA susceptible DA background. This resulted in a congenic strain, DA.ACI(Cia25), that developed significantly less severe PIA than the original DA strain (8).

Using microarray technique, synovial tissue gene expression were compared between congenic DA.ACI(Cia25) and DA rats, induced with PIA, in order to characterize the disease modifying effect of the Cia25 loci. Evidence from the microarray analysis showed that the Cia25 loci mediated reduced levels of pro-inflammatory cytokines, chemokines and MMPs. Combined with signs of increased expression of genes related to lipid metabolism, this indicated influence from nuclear receptors. Several nuclear receptors with anti-inflammatory functions have been described, but little is known about their role in synovial tissue. Interestingly, the congenic strain expressed significantly higher levels (ranging from 2.5 – 8.8 fold), of the nuclear receptors: Pparg, Lxra, Rora, Rxrγ and Rev-erba. Most of these receptors have known anti-inflammatory properties, which suggested that they might contribute to the arthritis resistance.

Identification of a PPARγ, LXRα and perhaps a RORα target gene signatures in the congenic strain further supported this hypothesis. The identified nuclear receptors are regulated locally in an autocrine/paracrine manner. Interestingly, we found several genes involved in the generation of LXR and RXR ligands to be expressed at higher level in the congenic strain. Furthermore, isolation of synovial fibroblasts (FLS) from these rats revealed positive expression of several of these receptors and that IL-1β induced IL-6 production could be reduced by either LXRα or Rev-erbα activation in vitro. This nuclear receptor cluster and expression of genes involved in local activation all pointed towards a suppressive effect on DA.ACI(Cia25) synovial inflammation. The initiating factor activating this pathway in the congenic strain could not be determined, although some evidence point towards a key role for RXRγ. The successful LXR and Rev-erbα mediated suppression of IL-1β induced cytokines and MMPs in FLS suggest that these receptors can become new drug targets, but further studies are needed to fully clarify the role of these receptors in synovial tissue and RA.

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