A. Kretzrommel et Ua. Boelsterli, SELECTIVE PROTEIN ADDUCTS TO MEMBRANE-PROTEINS IN CULTURED RAT HEPATOCYTES EXPOSED TO DICLOFENAC - RADIOCHEMICAL AND IMMUNOCHEMICAL ANALYSIS, Molecular pharmacology, 45(2), 1994, pp. 237-244
The nonsteroidal anti-inflammatory drug diclofenac can be bioactivated
to the reactive acyl glucuronide, which covalently binds to hepatocel
lular proteins in rat hepatocytes. Short term cultured rat hepatocytes
were used to further study the formation and nature of protein adduct
s after exposure to diclofenac. Incubation of cells with [C-14]diclofe
nac (30 mu M) for up to 24 hr was associated with a time-dependent inc
rease in radioactivity bound to proteins. Upon subcellular fractionati
on of hepatocytes exposed to diclofenac for 2 hr, the majority of the
radiolabel appeared in the microsomal fraction. By 24 hr, the specific
binding had decreased by 50% in this cell compartment. In contrast, t
he hepatocellular plasma membrane fraction, which also was associated
with high specific binding of diclofenac-derived radioactivity by 2 hr
, exhibited a similar to 3-fold increase in adduct formation by 24 hr.
Lesser amounts of radioactivity were associated with cytosolic protei
ns. After resolution of the proteins by sodium dodecyl sulfate-polyacr
ylamide gel electrophoresis and fluorography, the radioactivity was as
sociated with a major protein band with an apparent molecular mass of
60 kDa that was present in both microsomes and plasma membranes. Furth
er, we developed an antidiclofenac antibody against diclofenac-protein
adducts by Protein A chromatography of a polyclonal antiserum raised
in rabbits against a diclofenac-keyhole limpet hemocyanin adduct. The
antidiclofenac antibody did recognize diclofenac-protein adducts on We
stern blots of homogenates of cultured rat hepatocytes exposed to dicl
ofenac. The major detected adducts included the 60-kDa protein, which
was present at all diclofenac concentrations used. In addition, the an
tibody recognized proteins with apparent molecular masses of 50, 80, a
nd 126 kDa that were not evident in the radiochemical assay, There wer
e no detectable cross-reactive epitopes of proteins recognized by the
antibody on Western blots of cultured hepatocytes not treated with dic
lofenac. Moreover, immunoblots of liver homogenates from rats treated
with diclofenac (30 mg/ kg/day, intraperitoneally, for 4 days) also ex
hibited adducts with the 60- and 80-kDa proteins. Collectively, these
results suggest that binding of diclofenac to rat hepatocyte proteins
is selective and that a 60-kDa microsomal membrane protein (or protein
subunit) that accumulates in the plasma membrane fraction appears to
be the major target for alkylation both in cultured hepatocytes expose
d to diclofenac and in vivo.