Involvement of SUMO-1 in the regulation of apoptosis in prosthesis loosening fibroblasts

Background

Prosthesis loosening fibroblasts (PLF) contribute significantly to the pathogenesis of aseptic prosthesis loosening (APL) and show similarities to rheumatoid arthritis synovial fibroblasts (RA-SF). Based on data that have linked the increased expression of the small ubiquitin-like modifier SUMO-1 to the resistance of RA-SF against apoptosis, we investigated the expression of SUMO-1 in APL and used gene transfer of the SUMO protease SENP1 to study consequences of SUMOylation in PLF.

Methods

We used in situ hybridization (ISH) to study the expression of SUMO-1 in APL interface tissues. Quantitative PCR and Western blot were applied to measure the expression of SUMO-1 in PLF in comparison with RA-SF. Confocal microscopy of PLF transfected with GFP-labeled wild type (wt) or mutant (mt) gene constructs of SENP1 was performed to investigate the cellular localization of SUMO-1. To study the functional contribution of SUMO-1 to the apoptosis of PLF, SENP1wt-, SENP1mt- and mock-transfected PLF were stimulated with FasL, and apoptosis was determined by a histon fragmentation assay.

Results

ISH revealed marked expression of SUMO-1 in all tissues, with most prominent staining at sites attached to bone. The expression of SUMO-1 in PLF was comparable with that in RA-SF. Confocal microscopy revealed localization of SUMO-1 mainly in nuclear PML-bodies. After transfection of PLF with SENP1, high levels of SENP1 mRNA were measured (up to 472-fold versus endogenous SENP-1). Transfection of PLF with SENP1wt decreased the nuclear staining for SUMO-1, and increased significantly FasL-induced apoptosis (155% versus SENP-1mt). Spontaneous cell death remained unaffected.

Conclusion

The data suggest that SUMO-1 is involved in the activation of PLF and RA-SF by preventing these cells from Fas-induced apoptosis. The modification of nuclear proteins by SUMO-1 appears to contribute to the antiapoptotic effects of SUMO-1. By de-conjugating SUMO-1 from its nuclear substrates, SENP1 can regulate the apoptotic response of PLF.