Physical-chemical properties of the estrogen receptor solubilized by micrococcal nuclease

Effect of age on physico-chemical properties of the uterine nuclear estrogen receptors of albino rats

Estrogen receptors (ER) were extracted with high ionic strength buffer from the nuclei of uteri of young (21 weeks) and old (89 weeks) rats. Following the analysis of these receptors on a sucrose gradient and a Sephadex column, two peaks representing the two forms of receptors were obtained. The minor peak sedimented at 6.8 S with Stokes radius (RS) = 7.2 nm, molecular weight (Mr) = 204 K and frictional ratio (f/f0) = 1.71. On the other hand, the major peak sedimented at 4.1 S with RS = 3.3 nm, Mr = 57 K and f/f0 = 1.18. These properties of nuclear ER were similar in both ages. Also, the half life of ER complexes from both ages at 25 degrees C and 37 degrees C were 135 min and 30 min, respectively. However, these complexes were retained for longer periods in the nuclei of young than old rats. Furthermore, the dissociation constant of the binding of nuclear receptors to estrogen remained constant, but the number of binding sites decreased from 1.56 in young to 1.05 pmol/mg DNA in the old. In young rats, about 61% of nuclear receptors bound to DNA-cellulose. Out of this 2/5 was eluted with 0.15 M and the remaining 3/5 with 0.5 M KCl. On contrary, only 37% of total receptors bound to DNA-cellulose in the old. Out of this 3/5 was eluted with 0.15 M and the remaining 2/5 with 0.5 M KCl. These data suggested that despite the similarity in different physicochemical properties, the number of estrogen binding sites and the retention time of ER complexes in nuclei and the ability of these complexes to bind to DNA decrease in the uterus of old age.

Analysis of the 4-hydroxytamoxifen (4-OHTAM) bound nuclear estrogen receptor from MCF-7 cells by limited proteolysis

The assumption that a different conformational form was induced in the nuclear estrogen receptor following binding by antiestrogens compared to estrogens was studied by analysing the proteolytic fragments of the receptor following limited digestion with chymotrypsin and trypsin. Nuclei were isolated from MCF-7 cells previously exposed to [3H] 4-OHTAM. The proteolytic digestion was performed either on the micrococcal nuclease hydrolysate or on intact nuclei. The molecular weights (Mr) were calculated from the sedimentation coefficients (S) determined on a sucrose gradient and from the Stokes radii (Rs) estimated by gel filtration. Digestion of the nuclei with micrococcal nuclease solubilized a receptor form of Mr = 155,000. This receptor form was degraded by chymotrypsin to a receptor of Mr = 63,000 which could not be further dissociated by 0.4 M KCl and 3 M urea. A similar receptor molecule was released by chymotrypsin from intact nuclei. Digestion of the micrococcal nuclease hydrolysate with trypsin degraded the receptor to a form of a Mr = 67,000 which could not be further dissociated by 0.4 M KCl and 3 M urea. Digestion of intact nuclei with trypsin followed by micrococcal nuclease, solubilized a receptor form of Mr = 80,000 which could be further dissociated with 0.4 M KCl and 3 M urea to a receptor form of Mr = 67,000. This trypsin degraded receptor form seems to be similar in Mr to the chymotrypsin degraded form. On the other hand different receptor fragments of Mr = 33,000 and Mr = 60,000 were excised by chymotrypsin and trypsin respectively from the estradiol ligated estrogen receptor. (Geier et al., J. steroid Biochem. 26 [1987] 35-40.) These results support the assumption of a different conformational form for the antiestrogen ligated receptor, compared to the estrogen ligated receptor since they were differentially susceptible to proteolytic degradation by chymotrypsin.

Differences in the association of the progesterone receptor ligated by antiprogestin RU38486 or progestin ORG 2058 to chromatin components

We assessed the hypothesis that due to variations in the conformation of the progesterone receptor induced by the antiprogestin RU38486 compared to the progestin ORG 2058, differences may result in the association of the receptor with some of the chromatin components. The physical properties of the receptor-bound chromatin fragments released by micrococcal nuclease digestion were characterized by sucrose gradient sedimentation and by gel filtration on Agarose A-1.5m or Agarose A-5m columns. The nuclear fraction was isolated from T47D cells previously exposed to 0.1 microM [3H]RU38486 or 0.1 microM [3H]ORG 2058. Micrococcal nuclease digestion solubilized two receptor forms sedimenting at 4.4 S and 6.3 S for the antiprogestin bound receptor and only one receptor at 4.4 S for the progestin ligated receptor. High-salt buffer dissociated either the antiprogestin or the progestin-bound receptor to smaller receptor forms sedimenting at 3.5 S. Chemical cross-linking with the cross-linker 2-iminothiolane of the micrococcal nuclease solubilized receptor forms resulted in 6.7-S and 4.4-S forms sedimenting on 0.4 M KCl gradients for the antiprogestin and progestin ligated receptors, respectively. Stokes radii of 7.3 nm and 6.4 nm were determined by gel filtration in 0.4 M KCl for the 6.7-S and the 4.4-S receptor forms, respectively. Using the sedimentation coefficient and the Stokes radius, molecular weights of 202,000 and 116,000 were calculated for the antiprogestin and progestin ligated receptors. We conclude that the micrococcal nuclease solubilized antiprogestin ligated receptor is associated with additional or different chromatin components compared to the progestin bound receptor.

Analysis of the nuclear estrogen receptor from MCF-7 cells by limited proteolysis

The proteolytic fragments of the nuclear estrogen receptor in the MCF-7 cell line were characterized following limited digestion with chymotrypsin and trypsin. Nuclei were isolated from cells previously exposed to 10 nM [3H]estradiol. The proteolytic digestion was performed either on the micrococcal nuclease hydrolysate or on intact nuclei. The molecular weights (Mr) were calculated from the sedimentation coefficients determined on a sucrose gradient and from the Stokes radii estimated by gel filtration. Digestion of the nuclei with micrococcal nuclease solubilized a receptor form of Mr = 151,000. This receptor form was degraded by chymotrypsin to a receptor of Mr = 33,000 and by trypsin to a receptor of Mr = 60,000. Digestion of intact nuclei with chymotrypsin solubilized a receptor form of Mr = 62,000 which dissociated in 0.4 M KCl to a receptor of Mr = 32,000. Digestion of intact nuclei with trypsin followed by micrococcal nuclease solubilized a receptor form of Mr = 75,000 which was further dissociated by 0.4 M KCl to a receptor form of Mr = 60,000. The ability of the receptor forms to bind DNA was tested using DNA-cellulose column chromatography. About 40% of the micrococcal nuclease solubilized receptor form, compared to about 7% of the chymotrypsin degraded receptor and to about 13% of the trypsin degraded receptor forms, all bound to the column and could be eluted by high salt concentrated buffer. We conclude that the nuclear estrogen receptor in the MCF-7 cell line can be partially degraded either in the micrococcal nuclease hydrolysate or in intact nuclei by chymotrypsin or trypsin generating protein moieties, probably receptor fragments of Mr = 33,000 and 60,000 respectively. Both fragments retain their estradiol binding domain and it may be hypothesized that the heavier fragment retains its chromatin binding domain.

Progesterone-modulation of estrogen action: rapid down regulation of nuclear acceptor sites for the estrogen receptor

Our previous studies demonstrated that progesterone down regulates the occupied form of nuclear estrogen receptor (Re). Using the density shift method, we discovered that progestins stimulate the turnover of nuclear Re within 3 h of treatment, and Re synthesis is suppressed subsequently. Thus, the primary site of progestin action in down-regulating Re is the stimulation of nuclear Re turnover followed by the inhibition of Re replenishment. A major breakthrough in our understanding of how progestin controls Re turnover was made by studying nuclear acceptor sites for Re that were found to decrease markedly within 2 h of progestin treatment. These and other results indicate that progestin induces a factor called the Re regulatory factor (ReRF) which acts to block nuclear Re acceptor sites, and this in turn decreases nuclear Re retention on chromatin acceptor sites, leading to an enhanced turnover (or processing) of nuclear Re.

Characterization of the progesterone receptor solubilized by micrococcal nuclease and DNase I digestion

In order to investigate the functional organization of the progesterone receptor in chromatin we characterized the physical-chemical properties of the receptor bound chromatin fragments released by micrococcal nuclease and DNase I digestion. The crude nuclear fraction was isolated from T 47 D cells, previously exposed to 0.1 microM [3H]ORG 2058. The parameters determined in low and high salt concentrated buffers were: sedimentation coefficients (S) on a sucrose gradient, Stokes radii (Rs) by gel filtration on a Sephadex G-200 column and the binding abilities to a DNA-cellulose column. The molecular weights (Mr) and frictional ratios (f/fo) were calculated from the S and Rs values. Micrococcal nuclease digestion solubilized a receptor form sedimenting as a single peak at 4.4 S with a Rs = 7.78 nm and an estimated Mr = 144,000. About 53% of the applied receptor bound to a DNA-cellulose column could be eluted by high salt concentrated buffer. 0.4 M KCl dissociated this receptor form into a smaller receptor sedimenting at 3.3 S with Rs = 5.53 nm and a calculated Mr = 76,000. A similar receptor form was extracted by 0.6 M KCl from the undigested crude nuclear fraction. DNase I digestion solubilized a receptor form sedimenting at 3.3 S with a Rs = 6.87 nm and a calculated Mr = 94,000. About 26% of the applied receptor bound to a DNA-cellulose column could be eluted by high salt concentrated buffer. Dissociation of this receptor form by 0.4 M KCl resulted in a receptor sedimenting at 2.8 S with a Rs = 6.53 nm and an estimated Mr = 76,000. These results suggest: The progesterone receptor in chromatin is associated with several molecules probably proteins which complexed it to DNA. Some of these molecules still associated with the progesterone receptor could be released by nucleases digestion. Micrococcal nuclease releases a larger portion of these molecules than those release by DNase I.

Physical-chemical properties of the estrogen receptor released by deoxyribonuclease I

The physical-chemical properties of the nuclear estrogen receptor released by DNase I were characterized. Nuclei were isolated from MCF-7 cells previously exposed to 10-nM-[3H]estradiol. The parameters determined were: sedimentation coefficients (S) on a sucrose gradient, Stokes radii (Rs) by gel filtration on a Sephadex G-200 column and the binding ability to a DNA-cellulose column. The molecular weights (Mr) and frictional ratios (f/fo) were calculated from the S and Rs values. The properties of the receptor released by DNase I obtained from Worthington were compared to the properties of the receptor released by DNase I obtained from Sigma. Digestion with DNase I (Worthington) excised a receptor form which could be solubilized from nuclei by EDTA. This form sedimented at 5.2S with a Rs = 7.08 nm and a calculated Mr = 152.000. About 40% of this receptor form bound to a DNA-cellulose column. 0.4 M KCl dissociated this receptor form into a smaller form sedimenting at 4.2S with Rs = 4.64 nm and a calculated Mr = 80.000. The properties of the receptor solubilized by micrococcal nuclease followed by DNase I (Worthington) digestion were identical to the properties of the DNase I (Worthington) released receptor. Digestion with DNase I (Sigma) released a 3.2S receptor form, which diffused through the nuclear membrane and a 4-5S form which could be extracted from nuclei by EDTA. The 3.2S receptor had a Rs = 2.41 nm, a calculated Mr = 32.000 and less than 5% of it bound to a DNA-cellulose column. Digestion with micrococcal nuclease followed by DNase I (Sigma) solubilized a receptor form with identical properties to the 3.2S receptor. These results suggest that DNase I (Worthington) released a receptor form still associated with some molecules, probably chromatin proteins, which complexed it to DNA, while DNase I (Sigma) released the estradiol binding fragment of the receptor (meroreceptor) as a result of a proteolytic activity present in this preparation.

Characterization of the 4-hydroxytamoxifen (4-OHTAM) bound estrogen receptor of MCF-7 cells solubilized by micrococcal nuclease

In order to get an insight into the molecular mechanism of antiestrogen action at the chromatin level, we characterized the physical-chemical properties of the chromatin fragments released by micrococcal nuclease digestion of nuclei isolated from MCF-7 cells previously exposed to [3H]4-OHTAM. The [3H]4-OHTAM bound solubilized fragments were characterized in a low ionic strength buffer and in a high ionic strength buffer without and with urea. The following parameters were determined: sedimentation coefficients (S) on a sucrose gradient, Stokes radii (Rs) by gel filtration on a Sephadex G-200 column and the binding ability to a DNA-cellulose column. The molecular weights (Mr) and frictional ratios (f/fo) were calculated from the S and Rs values. Following mild nuclease digestion, the solubilized [3H]4-OHTAM bound receptor sedimented as an abundant 6-7 S form and a less abundant approximately 12 S species. Increasing the extensiveness of digestion resulted in one receptor form sedimenting at 5.2 S, Rs = 7.25 nm and Mr = 155,000. About 45% of the applied receptor bound to a DNA-cellulose column could be eluted by a high salt concentrated buffer. Dissociation of the micrococcal nuclease solubilized receptor in 0.4 M KCl resulted in a smaller receptor form with a 4.9 S, Rs = 5.87 nm and Mr = 119,000. Further dissociation in the presence of 3 M urea resulted in a receptor with a 3.5 S, Rs = 5.78 nm and Mr = 83,000. These results suggested that the antiestrogen bound estrogen receptor in chromatin, is associated with a tightly bound protein component and with an additional less tightly bound protein, complexes with DNA.