Institutionen för molekylär medicin och kirurgi / Department of Molecular Medicine and Surgery
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