Estramustine-binding protein (
EMBP) is a M(r) 46,000 heterodimeric
protein originally isolated from prostatic tissue. It has a demonstrated high affinity for, and selective binding of,
estramustine, which is a derivative of
17 beta-estradiol and
nornitrogen mustard with
antimitotic activity. In this study, we have analysed the expression of an
EMBP-like
protein in
astrocytoma specimens. Immunohistochemistry revealed a pronounced reactivity for
EMBP in
astrocytoma grades III-IV as well as in metastatic prostatic
adenocarcinoma used as positive control. In
astrocytoma grades I-II, the expression was weak. The
EMBP-like
protein was quantified by radioimmunoassay in
astrocytoma tumor tissue with higher concentrations in malignant
astrocytoma, grades III-IV, compared to grades I-II
tumors. Western immunoblotting of immunoaffinity purified
EMBP-like
protein under nonreducing conditions revealed an immunoreactivity corresponding to M(r) 138,000 and 200,000, indicating a different structure of
EMBP in
astrocytoma compared to prostatic tissue. Specific binding and the presence of saturable binding sites for 3H-labeled
estramustine were demonstrated in
astrocytoma tissues expressing
EMBP-like
protein. Scatchard plot analysis showed a Kd at approximately 30 nM, which suggests a binding affinity for
estramustine in the same range as previously reported for
EMBP in the prostate. Moreover, the number of
estramustine binding sites/g
tumor as calculated from the Scatchard plots was well correlated with the
EMBP levels determined in the radioimmunoassay. In conclusion, an
EMBP-like
protein is expressed in
astrocytoma. This
protein may be responsible for the specific binding of
estramustine in the
tumor tissue. Whether this specific binding of
estramustine is of importance for the cytotoxic effect in
glioma cells remains to be evaluated.