Nuclear components responsible for the retention of steroid--receptor complexes, especially from the standpoint of the specifcity of hormonal responses

The ubiquitous nature of the progesterone receptor binding factor-1 (RBF-1) in avian tissues

The avian oviduct receptor binding factor-1 (RBF-1) is a 10 kDa nuclear matrix protein that was originally identified through its ability to effect high affinity interaction of activated progesterone receptor (PR) with chromatin. In the present study, the RBF-1 is shown to not be restricted to reproductive tissues (e.g., oviduct) but present in all avian tissues examined by Western blot analysis with a monoclonal antibody prepared against purified RBF-1. The heart and pancreas had the highest and lowest RBF-1 levels, respectively; the concentration ranging by approximately 50-fold in these tissues. The 10 kDa size of the RBF-1 detected in all tissues suggests no significant tissue-specific differences in the protein. This was consistent with the finding that purified hepatic and oviductal RBF-1 have identical amino-terminal sequence. Using a recently isolated cDNA to RBF-1, the levels of RBF-1 mRNA were found to correlate well with the ubiquitous presence of the protein as well as tissue-specific differences in concentration. The presence of RBF-1 in non-progesterone responsive tissues suggests the possibility that RBF-1 may not be specifically involved in PR-DNA interactions but may play a more diverse role, possibly involving other steroid receptors such as the glucocorticoid receptor.

Immunohistochemical localization of the avian progesterone receptor and its candidate receptor binding factor (RBF-1)

An avian oviduct nuclear matrix protein in the 6-10 kDa size range has been implicated to function in the cell-free nuclear binding of the avian oviduct progesterone receptor (PR). This protein, termed the receptor binding factor-1 (RBF-1), has been purified and partially characterized [Schuchard et al.: Biochemistry 30:4535-4542, 1991]. This paper describes the immunohistochemical co-localization of the RBF-1 and PR in the avian oviduct cell nuclei and rat reproductive cell nuclei using antibodies directed specifically against the RBF-1 and activated PR. In the undifferentiated oviduct, the immunoreactivities for both PR and RBF-1 were co-localized in the nuclei of only epithelial cells, but not the stromal cells or smooth muscle cells. In the partially differentiated oviduct of estrogen treated chicks, the immunoreactivity co-localized in the nuclei of not only epithelial but also glandular and stromal cells. Staining for the PR, but not RBF-1, was detected in the smooth muscle cells. The intensity of the PR but not the RBF-1 staining was markedly down-regulated in these cells at 2 and 6 h after treatment of the animals with progesterone (P). However, the band patterns for RBF-1 in the Western blots did show qualitative changes which may reflect P-induced posttranslational modifications which alter the epitope on the RBF-1. Interestingly, immunohistochemical analysis of several reproductive tissues of the rat showed that certain cell types in the uterus, ovary, and prostate displayed strong positive nuclear staining for an RBF-1-like antigen(s).(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of a nuclear protein (receptor binding factor-1) associated with the chromatin acceptor sites for the avian oviduct progesterone receptor

The specific high affinity binding of the avian oviduct progesterone receptor (PR) to target cell nuclei and chromatin has been shown to involve DNA complexed with specific chromatin acceptor proteins. One of these chromatin acceptor proteins has been partially purified and found to be a small hydrophobic protein with a broad pI of 5.0-6.0 [Goldberger and Spelsberg (1988), Biochem. 27, 2103-2109]. Using western immunoblots with anti-RBF-1 polyclonal antibodies to monitor the purification, a 10 kD candidate acceptor protein, termed the Receptor Binding Factor-1 (RBF-1), has been purified to apparent homogeneity. RBF-1 has an amino acid composition consistent with a hydrophobic protein having an acidic pI and a unique N-terminal sequence. Two-dimensional polyacrylamide gel electrophoresis and high-performance capillary electrophoresis support the purity of a protein congruent to 10 kD in size, having an acidic pI, but with evidence of several differently charged isoforms. Phosphatase treatment provides evidence that charge heterogeneity may result from variable phosphorylation states. A role of this factor as a candidate "acceptor protein" in the chromatin acceptor sites for the avian oviduct PR is proposed.

The binding of 3H-labelled androgen-receptor complexes to hypothalamic chromatin of neonatal mice: effect of sex and androgenization

The binding of 3H-labelled androgen-receptor complexes, prepared by (NH4)2SO4 precipitation from the 105,000 g supernatant of hypothalamic cytosol, to hypothalamic chromatin of neonatal mice covalently coupled to cellulose was measured in vitro. Saturation binding was also determined after extraction of histones and the masking of acidic proteins with high molarities of guanidine hydrochloride. This investigation showed the presence of high-affinity, low-capacity acceptor sites for [3H]-testosterone-receptor complexes in male hypothalamic chromatin (Kd value = 0.39 x 10(-10) M and binding sites of 41 fmol per mg of DNA). Acceptor activity seems to be associated with the acidic protein fraction of chromatin. No specific acceptor sites of similar nature were found in chromatin taken from the hypothalami of female mice. On the basis of these results, it is suggested that the androgen-unresponsiveness of female mice is related to the absence of acceptors for the androgen-receptor in female mice hypothalami.

Characteristics of the rat prostate androgen receptors analyzed by sucrose density gradient and high-performance liquid chromatofocusing

Rat prostate cytosolic androgen-receptor complexes were analyzed by sucrose density gradient (SDG) centrifugation and by high-performance liquid chromatofocusing (HPCF). Without protecting agents, these complexes were resolved by HPCF at basic (8.25-7.1), intermediary (7.0-5.0) and acidic (4.6-4.2) pH. Sodium molybdate stabilized labeled complexes which migrated in the 8-9S and 3.5-6S areas on SDG. These were further stabilized by the presence of sodium molybdate and four protease inhibitors: complexes then sedimented mainly in the 8-9S area with a shoulder at 6-7S. Forms eluting at acidic pH on HPCF were favored by the presence of sodium molybdate and further enhanced by the addition of inhibitors, to the detriment of basic ones. Furthermore, when chromatographed on phosphocellulose (P-c), unretained complexes sedimented as a symmetrical peak on SDG centrifugation in the 8-9S area, but were eluted from HPCF columns as two entities at pH 4.1 and 4.6. The P-c retained complexes subsequently detached by 0.6 M KCl, were resolved into three entities by HPCF with a major component at pH 8.2, which sedimented in the 4S areas. These results demonstrate that the gradual decrease in the negative net charge of androgen receptor correlates with the gradual reduction in mass of the androgen-receptor complex. Moreover, this can be interpreted as further evidence for a heterogeneity of androgen receptor population in rat prostate, suggesting the involvement of a multistep mechanism preceding the induction of specific gene transcription by the hormone.

Preferential binding of androgen to transcriptionally active chromatin in rat prostate

Rat ventral prostate chromatin was fractionated by the Micrococcal nuclease procedure of Berkowitz and Riggs (Biochemistry 20, 7284-7290, 1981). Four chromatin fractions were obtained and analyzed by gel electrophoresis for their protein and DNA components, by hybridization of their DNAs with an androgen regulated cDNA for transcribed DNA sequence, and by [3H]dihydrotestosterone exchange assay and steroid competition experiment for specific androgen binding. All four chromatin fractions contained a complete complement of histones and DNA in multiples of about 200 base pairs, indicating repeating nucleosome structure. The active chromatin fraction, which has the shortest nucleosome repeats of 186 basepairs, was enriched in the androgen regulated sequence and bound androgen. These results correlate androgen binding sites with transcribed DNA sequence and structurally distinctive domain of chromatin in the rat prostate.

DNA and protein components of nuclear acceptor sites for androgen receptors in the rat prostate

Androgen receptor-acceptor complexes in nuclei from rat ventral prostates were cross-linked in situ with formaldehyde and partially purified using affinity chromatography. To isolate acceptor DNA, the cross-linked receptor-acceptor complexes in formaldehyde-treated chromatin samples were adsorbed to dihydrotestosterone-17 beta-succinyl agarose, eluted with 75 microM dihydrotestosterone-1% SDS, digested with proteinase K and extracted with phenol-chloroform. After 32P end-labelling and PAGE, this DNA contained two distinct bands of DNA (about 300 and 400 base pairs respectively) which were unique relative to the total prostatic DNA. As an alternative approach for characterizing acceptor DNA, the DNA in prostatic nuclei and cross-linked chromatin was labelled with 32P by nick translation and analysed in glycerol density gradients for associations with cross-linked androgen receptors. A symmetrical 7s peak of 32P-DNA with a small amount of coincident receptor was observed in the gradients after mild trypsin treatment. In the absence of trypsin treatment, both the cross-linked receptors and the labelled DNA sedimented to the bottom of the gradients. Isolation of acceptor proteins involved iodination of cross-linked chromatin with 125I and androgen affinity chromatography. A comparison of the relative efficiency of retention and elution of 125I-proteins from different affinity columns revealed that testosterone-17 beta-succinyl agarose was potentially most suitable for purification of acceptor proteins. After electrophoresis on SDS-polyacrylamide gels, the eluates from this type of affinity matrix were found to contain two major peaks of 125I-labelled proteins--one corresponding to a protein with a similar molecular weight as the nuclear androgen receptor (33,000 Da); the other having a molecular weight of 20,000 Da. While the precise identity of this latter entity is unknown, its enrichment and retention by the affinity gel implies that it is closely associated with the androgen receptor and may be a component of the acceptor sites.

Nuclear protein matrix of seminal vesicle epithelium

Nuclear protein matrix was isolated from guinea pig seminal vesicle epithelium and liver. The two matrices were similar in fine structure as seen by transmission electron microscopy, in protein electropherograms, and in percent composition relative to protein, DNA, and RNA. Scanning electron microscopy was used to examine intact seminal vesicle nuclei, nuclei after treatment with Triton X-100 and DNAse I, and purified nuclear matrix. The matrix surface presented a 'porous' appearance by both scanning and transmission electron microscopy. The matrices of liver and seminal vesicle epithelium (SVE) and the intact nuclei of SVE were assayed for specific binding of free synthetic androgen, 17 alpha-methyltrienolone (R1881). Saturable specific binding was demonstrable for seminal vesicle matrix but not for liver matrix. Maximal binding of androgen occurred at a concentration of approximately 12 nM and was demonstrated to be 1.34 +/- 0.22 pmol of R1881 per mg of seminal vesicle matrix protein; the Kd was approximately 8 nM. The binding of labeled R1881 to matrix could be inhibited with low concentrations of unlabeled androgens, but not with estrogens or other steroids. Our data indicate that the binding of androgen to matrix could account for at least 21% of the binding to intact nuclei.

Differential effects of polyamines on the phosphorylation of chromatin-associated proteins

Studies are presented on the nature of chromatin-associated phosphoproteins whose phosphorylation is influenced by polyamines. After labelling with 32P, chromatin-associated proteins were separated into four fractions. Fraction I comprised neutral and basic non-histone phosphoproteins, including high-mobility-group non-histones; fraction II consisted mostly of histones; fraction III consisted of a class of (salt-soluble) acidic non-histone phosphoproteins; and fraction IV consisted of residual (salt-insoluble) acidic non-histone phosphoproteins. The average relative distribution of protein in the four fractions (I-IV) was about 1:4:2:1 for both liver and prostate. However, tissue-dependent differences were observed in the incorporation of 32P in various protein fractions. In the presence of polyamines (e.g. 1 mM-spermine or 2 mM-spermidine) maximal stimulation of phosphorylation was observed in non-histone proteins of fraction I (160-180%), followed by that in non-histone proteins of fraction III (80-110%). The phosphorylation of residual non-histone proteins in fraction IV, and the small extent of phosphorylation of histones in fraction II, remained unaltered in the presence of polyamines. Thus polyamines do not stimulate the phosphorylation of all non-histone proteins; their stimulative effect is most prominent in the phosphorylation of neutral and basic non-histone proteins and a class of salt-soluble acidic non-histone proteins. In accord with our hypothesis, these differential effects of polyamines on phosphorylation of endogenous non-histone proteins may relate to the conformation of these substrates rather than to endogenous kinases.

The role of androgen metabolism in the control of androgen action in the rat prostate

Recent results from a number of laboratories have led us to re-examine the role of 3 beta-androstanediol in the rat ventral prostate. Whereas previously 5 alpha-dihydrotestosterone and 3-beta androstanediol were thought to have distinctly separate effects on the prostate, we suggest that 3 beta-androstanediol serves only as an intermediate in the metabolism and removal of 5 alpha-dihydrotestosterone from the organ. In our view the action of androgens on the prostate are exerted exclusively through the binding of 5 alpha-dihydrotestosterone to the androgen receptor and its subsequent translocation to the nucleus. Differences in effects are related to the amount of 5 alpha-dihydrotestosterone available to the gland. 3 alpha-Hydroxysteroid dehydrogenase may play a critical role in modulating the level of 5 alpha-dihydrotestosterone available to the translocatable receptor.

Radioimmunoassay measurements of nuclear dihydrotestosterone in rat prostate. Relationship to androgen receptors and androgen-regulated responses

The concentration of dihydrotestosterone was measured by radioimmunoassay in nuclear and cytoplasmic extracts from rat ventral prostates. In the regenerating prostates of castrated rats treated with dihydrotestosterone for 4 days, the nuclear concentration of this steroid increased from approx. 70nM to 800nM as a linear function of the injected dose, whereas the cytoplasmic concentration remained relatively constant (70-130nM). Isotope-exchange measurements of nuclear androgen receptors by using [3H]methyltrienolone indicated that, although the concentration of nuclear dihydrotestosterone was several-fold higher than the concentration of androgen receptors, they were logarithmically related. The recruitment of prostatic cells into the growth fraction and the stimulation of 5 alpha-reductase activity were more directly correlated to the nuclear concentration of androgen receptors than to the total nuclear concentration of dihydrotestosterone. Maximal restoration of a specific isoenzyme of acid phosphatase ws achieved when approx. 2000 androgen receptors were present in the prostatic nuclei; higher concentrations of nuclear androgen receptors were associated with decreased amounts of this enzyme. Hence the results imply, first, that the total amount of dihydrotestosterone accumulated by nuclei is not a direct consequence of carrier-mediated transport by androgen receptors, and, secondly, that, whereas acid phosphatase may be differentially controlled by androgens in the regenerating prostate, increases in the amount of cell proliferation and 5 alpha-reductase activity directly parallel increases in the nuclear concentration of androgen receptors.

Problems associated with dose response in steroid-hormone activation of structural genes

A typical target cell for a sex steroid hormone contains 10 000--20 000 specific high-affinity receptors for that hormone. However full physiological responses can be achieved with only 2000 of these receptors involved in hormone--receptor complex interaction with the nucleus. The number of nuclear acceptor sites that must be filled before responses occur maybe even less. This implies that multiple occupation of nuclear acceptor sites by hormone--receptor may occur permitting co-operative induction of transcription of selected genes. The numbers of sites of initiation of RNA synthesis seem excessively high (about 70 000 per cell). Although this may be an artifact of the isolation procedures the proportion of initiation sites under hormonal control (equivalent to about 30 000 per cell) is still large. The numbers of mRNA species under hormonal control varies greatly depending on the particular hormone and target tissue. The extent to which these different observations can be incorporated into a unifying theory is discussed.

Antagonism of the estradiol-mediated repression of microsomal 3 beta-hydroxysteroid dehydrogenase activity in rat liver by antiestrogenic substances

5 alpha-Dihydrotestosterone, 17-hydroxyprogesterone caproate, 2-methoxyestrone and a number of nonsteroidal antiestrogens (clomiphene citrate, nafoxidine hydrochloride, tamoxifen, MER-25) were tested for their ability to block estradiol-mediated repression of the androgen-dependent 3 beta-hydroxysteroid dehydrogenase activity of male rat liver. With the exception of 5 alpha-dihydrotestosterone, which induced activity in females, none of these substances affected 3 beta-hydroxysteroid dehydrogenase activity when administered alone to otherwise untreated male and female rats. Tamoxifen (100 or 500 micrograms/day) was the only substance which prevented a decrease in enzyme activity when given simultaneously with estradiol (5 micrograms/day). The estradiol-mediated decrease in activity was not antagonized by a 100-fold higher dose of androgen (5 alpha-dihydrotestosterone, 0.5 mg/day), demonstrating the potent antiandrogenic effect of estradiol on this hepatic androgen-dependent enzyme activity.

Nuclear acceptor sites for androgen-receptor complexes in seminal-vesicle epithelium

An assay method in vitro was developed and applied to quantify acceptor binding of steroid-receptor complexes in nuclei from isolated epithelium of guinea-pig seminal vesicle. Steroid-receptor complex prepared from 1-day-castrated animals was incubated with purified nuclei from 1-28 day-castrated animals in a medium containing 0.15 M-KCl. Free and bound steroid-receptor complexes were measured and the data were submitted to Scatchard analysis. With nuclei from 1-day-castrated animals the Kd for binding of cytosolic [3H]dihydrotestosterone-receptor complexes was found to be 0.83 X 10(-10) M and the capacity for binding was 0.35 pmol/mg of nuclear DNA. Scatchard analysis consistently disclosed only a single line of constant slope and gave the same kinetic constants for nuclei obtained from animals castrated up to 28 days before assay. Administration of 2 mg of dihydrotestosterone, 3 alpha-androstanediol or androsterone or 100 microgram of oestradiol-17 beta 1 h before killing of the 1-day-castrated animals that provided the nuclei resulted in a significant decrease in nuclear acceptor binding of the steroid-receptor complex compared with untreated animals. Thus our assay method disclosed nuclear acceptor sites that may be involved in responses to androgens (and oestrogens) in vivo. We conclude that there is a class of nuclear accept or sites of high affinity and limited capacity that may be occupied by steroid-receptor complexes in vivo.

Circannual rhythms in progesterone receptor levels and functions

Studies in this laboratory on the nuclear binding sites (acceptors) for progesterone receptor in the developed chick oviduct have resulted in the detection of seasonal variations in the levels and functions of the receptor. Cytosol preparations obtained from the chick oviducts during the winter/spring period between January and May display reduced receptor levels as well as a loss of the capacity of the receptor to bind to nuclear "acceptor" sites in vitro. The binding of [3H]P-R to whole chromatin or purified acceptor proteins reannealed to DNA display the same rhythm. No such rhythm is detected for the binding of P-R to pure DNA. Computer analysis of the data, using least squares method to fit the data to cosine curves, shows a significant fit indicating a circannual rhythm in P-R binding to the acceptor protein-DNA complex but not to pure DNA. The nuclear binding in vivo, achieved by injecting [3H]progesterone into the wing vein and analyzing the radioactivity localized in the oviduct nuclei, also displays a similar rhythm. These results support that native nuclear acceptor sites for progesterone in the chick oviduct represent protein-DNA complexes and not pure DNA. The failure of P-R to bind the nuclear acceptor sites in vivo and in vitro during this period can be explained by the two subunit hypothesis of Schrader and O'Malley, whereby one of the two subunits is absent or inactive during this period.

Factors influencing association of glucocorticoid receptor-steroid complexes with nuclei, chromatin, and DNA: interpretation of binding data

Attempts to reconstruct, in a test tube, the steroid-hormone system of a responsive cell are fraught with enumerable difficulties. In this chapter I have attempted to point out some of the factors that affect receptor-steroid complexes and their interactions with acceptors. In most cases there is a quantitative influence of these factors on the level of steroid complex binding to acceptors. In some cases, selected experimental designs that neglect these factors and methods of presenting the observed data may lead to artifactual conclusions. Several of these problems should disappear when the prospect of pure receptor-steroid complexes [127, 147, 150, 181, 247, 248] becomes a common occurrence. Nevertheless much has already been learned about the interactions of complexes with acceptors, which in turn have been used to help formulate models of steroid-hormone action.

Effects of polyamines on prostatic chromatin- and non-histone-protein-associated protein kinase reactions

Studies are presented on the influence of polyamines on prostatic chromatin- and non-histone-protein-associated protein kinase reactions involving both exogenous and endogenous substrates. The activities toward the model acidic protein substrate, dephosphophosvitin, were maximal at 160--200mM-NaCl (or -KCl or -NH4Cl). Under these conditions, spermidine and spermine added in concentrations up to 2mM were essentially without effect. However, without addition of NaCl to the medium, marked stimulation of these reactions was elicited by these polyamines at 1--2mM concentrations. The stimulatory effects were not due to non-specific changes in the ionic strength or to substitution of spermine for Mg2+, as maximal stimulation by 1 mM-spermine was observed only at optimal (2--4mM) Mg2+ concentrations. Qualitatively similar effects of polyamines were observed with enzyme preparations from the prostates of castrated rats, and with chromatin and non-histone-protein preparations from other tissues besides ventral prostate. When phosphorylation of endogenous non-histone proteins of the chromatin was measured, spermine stimulated both the initial rates and the final extent of transphosphorylation, even in the presence of optimal concentration of NaCl. By contrast, spermine or spermidine had no effect on the chromatin- and non-histone-protein-associated protein kinase reactions determined with lysine-rich histones as substrates. Chemically NN-dimethylated dephosphophosvitin was a less active substrate for the chromatin-associated protein kinase, but its phosphorylation was more markedly stimulated by spermine in comparison with unmodified dephosphophosvitin. These observations hint that the polyamine stimulations of the various protein kinase reactions may be due to effects on the conformations of the non-histone protein substrates rather than on the kinases themselves.

Androgenic regulation of elongation of polyribonucleotide chains on rat ventral-prostate chromatin

The kinetics of polyribonucleotide-chain elongation by rat ventral-prostate RNA polymerase B with homologous chromatin as a template were investigated. Chain elongation was measured under conditions wherein all initiation had occurred, no reinitiation took place and the reaction rate was constant. The kinetic behaviour of prostate RNA polymerase B was consistent with a mathematical model formulated for the multisubstrate enzyme. The addition of each nucleoside triphosphate was independent of the other three. The overall rate of chain elongation was lower when prostate chromatin from castrated rats was used than with prostate chromatin from normal rats. The inclusion of dihydrotestosterone-receptor complexes stimulated the rate of elongation. Androgenic effects did not appear to be directed towards the addition of individual nucleoside triphosphates, but probably towards one of the other major events in RNA-chain elongation, i.e., unwinding of DNA or movement of the enzyme along the template.

Initiation and elongation of polyribonucleotide chains on rat ventral-prostate chromatin transcribed by homologous ribonucleic acid polymerase B

The characteristics of initiation of RNA synthesis and the elongation of RNA chains on rat ventral-prostate chromatin by RNA polymerase B were investigated by two methods. 1. Initiation was carried out under low-salt conditions with three ribonucleoside triphosphates, and elongation was begun in the absence of reinitiation by the addition of the fourth ribonucleoside triphosphate and increasing the salt concentration. 2. Stable initiation complexes were formed by preincubation of enzyme with template at 37 degrees C, elongation was started by the addition of all four ribonucleoside triphosphates and reinitiation or spurious RNA synthesis was prevented by rifamycin AF/013. The latter method gave more reliable results. The dependence of those parameters on the androgenic status of the animal was studied. During the first 24h after castration, elongation was mainly affected, whereas after 72h a smaller number of initiation sites for RNA polymerase B on chromatin was evident. Considerable diurnal variations in the various parameters were observed. Changes in the relative concentrations of the chromatin-associated proteins were also observed after castration. In the rat ventral-prostate gland androgenic steroids may not only influence one stage of the transcriptional process, but may affect many factors involved in the control of gene expression.

Binding of estradiol receptor complexes to isolated human breast chromatin

The interaction of estradiol-receptor complexes and isolated human breast tumor chromatin was studied under equilibrium conditions. The estradiol-receptor complexes bound specifically to the chromatin of hormone dependent tumors and showed a single class of binding sites with a Ka of 0.96 X 10(10) M-1 and a binding capacity of 1.5 pmoles/mg DNA. The binding was a temperature-dependent process and involved a "transformation" of the receptor protein. The heat-activated hormone-receptor complex was more active than the 8S form in the binding phenomenon. The specific interaction of estradiol-receptor complex with isolated chromatin was saturable and sensitive to conditions of temperature and ionic strength. Furthermore under optimal conditions no acceptor sites were detected in chromatin of hormone independent tumors.

Early events in the steroidal regulation of alpha2mu globulin in rat liver. Evidence for both androgenic and estrogenic induction

1. A double-antibody radioimmunoassay for alpha2mu globulin has been developed. With the help of this highly sensitive radoimmunoassay the early effects of both androgen and estrogen treatments on the hepatic synthesis of alpha2mu globulin in the rat have been investigated. 2. Results show that the earlier observation of the long lag period in the androgenic induction of alpha2mu globulin is more apparent than real. 3. Single injections of either 5alpha-dihydrotestosterone(5alpha-dihydro-17beta-hydroxy-4-androsten-3-one) or its physiological antagonist estradiol-17beta (1,3,5(10)-estratriene-3,17beta-idol) to castrated female rats resulted in the induction of alphamu globulin reaching maximum hepatic level of the protein between 6--9 h after the hormone administration. Administration of cycloheximide 15 prior to hormone treatment blocked both androgenic andestrogenic induction of alpha2mu globulin. 4. Daily pretreatments with 5alphamu-dihydrotestosterone increased the sensitivity of subsequent androgenic response to alpha2muglobulin synthesis. On the other hand, daily pretreatments with estradiol 17beta decreased and ultimately abolished the estrogenic induction of alpha2mu globulin. 5. The possible mechanism of both androgenic and estrogenic induction of alpha2mu globulin in rat liver mediated through a sex-hormone-binding protein with dual affinity for both dihydrotestosterone and estradiol has been suggested.