Proteasome targeted therapy in arthritis models : effects on pain and inflammation

Abstract

The ubiquitin proteasome system (UPS) regulates numerous cellular functions by selectively degrading cellular proteins. It controls activation of the transcription factor NF-ĸB which is involved in the expression of pro-inflammatory mediators. The UPS regulates neuronal signalling by controlling levels of synaptic proteins during chronic pain. Pain and inflammation are the major components of inflammatory joint disorders. The focus of this thesis has been to further investigate mediators of pain and inflammation and to explore the role of UPS in pathogenesis of chronic arthritic conditions such as the rat models of rheumatoid arthritis (RA) and osteoarthritis (OA).

Opioid peptides dynorphins are involved in the maintenance of pathological pain. The localization and distribution pattern of dynorphins precursor protein prodynorphin (PDYN) was studied in the brain. We could demonstrate that high levels of PDYN were present in the amygdala, hippocampus and stratum and lower amounts in the cerebral cortex. Furthermore, PDYN was detected in the ventral trigeminal area and in the hypocampal CA3 regions that do not have cell bodies of PDYN-producing neurons but contain axons and axon terminals. PDYN is thus transported to and stored in axon terminals prior to release from secretary granules as mature peptides. Furthermore, we could demonstrate that the depolarization of neuronal cells stimulates processing of PDYN into mature dynorphins which may represent the local regulation of synaptic transmission.

The UPS regulates the processing and secretion of dynorphins and CGRP during neuropathic pain. We investigated the role of UPS in pain and inflammation in adjuvant arthritis by the using proteasome inhibitor MG132. We demonstrated an increased expression of p50 (a subunit of NF-κB) and NF-κB activity in arthritic joints. A decrease in p50 expression and in NF-κB activity was observed and coincided with reduced arthritic severity when MG132 was administered. Furthermore, an increased expression of SP and CGRP was observed in arthritic joints to correspond with increased pain. MG132 therapy reversed the up-regulated expression of SP and CGRP in the arthritic joints and of SP and PDYN in both the dorsal root ganglia (DRG) and in the spinal cord (SC). These results suggest that the UPS regulates pain and inflammation at the peripheral tissues. UPS-mediated protein regulation in the peripheral and central nervous system most likely also regulates inflammatory and nociceptive mediators. In addition, the role of UPS in the mediation of pain and disease progression was studied in OA. Pain mechanisms in OA are unclear but the sensitization of nociceptors in the synovium and bone probably contributes to the initiation and maintenance of pain. We demonstrated that the decreased pain thresholds were related to the increased expression of SP and CGRP in the knee joint and in corresponding DRG. We observed an enhanced expression of matrix metalloproteinase-3 (MMP-3) in the knee joints that coincided with pathological changes in the OA cartilage. MG132 administration caused a significant reversal of pain behavior, attenuated cartilage and bone destruction and resulted in a decrease in SP, CGRP and MMP-3 expression in knee joint.

In conclusion, the UPS represents a major intracellular pathway that critically regulates the development of both joint inflammation and inflammatory pain. In future perspectives, novel safe proteasome inhibitors with limited adverse side-effects would be available with the benefit of targeting both pain and inflammation.