Peroxisome proliferator-activated receptor-gamma1 is dephosphorylated and degraded during BAY 11-7085-induced synovial fibroblast apoptosis

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) plays a central
role in whole body metabolism by regulating adipocyte differentiation and energy
storage. Recently, however, PPAR-gamma has also been demonstrated to affect
proliferation, differentiation, and apoptosis of different cell types. As we have
previously shown that BAY 11-7085-induced synovial fibroblast apoptosis is
prevented by PPAR-gamma agonist 15d-PGJ2; the expression of PPAR-gamma in these
cells was studied. Both PPAR-gamma1 and PPAR-gamma2 isoforms were cloned from
synovial fibroblast RNA, but only PPAR-gamma1 was detected by Western blot,
showing constitutive nuclear expression. Within minutes of BAY 11-7085 treatment,
a PPAR-gamma1-specific band was shifted into a form of higher mobility,
suggesting dephosphorylation, as confirmed by phosphatase treatment of cell
extracts. Of interest, BAY 11-7085-induced PPAR-gamma1 dephosphorylation was
followed by PARP and caspase-8 cleavage as well as by PPAR-gamma1 protein
degradation. PPAR-gamma1 dephosphorylation was followed by the loss of PPAR-DNA
binding activity ubiquitously present in synovial fibroblast nuclear extracts.
Unlike the phosphorylated form, dephosphorylated PPAR-gamma1 was found in
insoluble membrane cell fraction and was not ubiquitinated before degradation.
PPAR-gamma1 dephosphorylation coincided with ERK1/2 phosphorylation that
accompanies BAY 11-7085-induced synovial fibroblasts apoptosis. 15d-PGJ2, PGD2,
and partially UO126, down-regulated ERK1/2 phosphorylation, protected cells from
BAY 11-7085-induced apoptosis, and reversed both PPAR-gamma dephosphorylation and
degradation. Furthermore, PPAR-gamma antagonist BADGE induced PPAR-gamma1
degradation, ERK1/2 phosphorylation, and synovial fibroblasts apoptosis. The
results presented suggest an anti-apoptotic role for PPAR-gamma1 in synovial
fibroblasts. Since apoptotic marker PARP is cleaved after PPAR-gamma1
dephosphorylation but before PPAR-gamma1 degradation, dephosphorylation event
might be enough to mediate BAY 11-7085-induced apoptosis in synovial fibroblasts.