Effects of serial hormone treatments on egg white protein synthesis. Further evidence of translational regulation

The administration of either progesterone or estrogen to withdrawn chicks several hours after a first dose of estrogen affected ovalbumin synthesis differently than its mRNA levels [S. S. Seaver (1981) J. steroid Biochem. 14, 949-957]. This suggested that the hormones were regulating the translation of ovalbumin directly. In this paper we report that serial hormone treatments also affect the rates of synthesis of two other egg white proteins, conalbumin and ovomucoid. When progesterone was administered 4 h after estrogen, conalbumin synthesis decreased. When either progesterone or a second dose of estrogen was administered 12 h after the first dose of estrogen, conalbumin synthesis increased. Serial hormone treatments did not always affect all three proteins similarly. At later times, administering progesterone after estrogen decreased ovomucoid synthesis but did not affect conalbumin or ovalbumin synthesis. To determine if the serial hormone treatments affect egg white protein mRNA's in a similar way, changes in ovalbumin and conalbumin mRNA levels were quantified in a rabbit reticulocyte cell-free translation system and were compared to changes in ovalbumin and conalbumin synthesis as measured in chick oviduct tissue minces. When serial hormone treatments were 12 h apart, ovalbumin and conalbumin synthesis was 50-300% higher than that predicted by the changes in ovalbumin or conalbumin mRNA levels. This is further evidence that translation of both conalbumin mRNA and ovalbumin mRNA is directly regulated by steroid hormones.     

The induction of ovalbumin and conalbumin mRNA by estrogen and progesterone in chick oviduct explant cultures

Estrogen pretreated chick oviduct tissue can be restimulated in vitro by physiological concentrations of estrogen and progesterone. The rates of synthesis of the major egg white proteins, ovalbumin and conalbumin, as well as the cellular levels of their respective mRNAs, increase after characteristic lag periods; this confirms previously reported results in vivo and demonstrates that both the lag phenomena and the mRNA induction are a function of the direct interaction of steroids with oviduct cells.The antagonistic action of progesterone on an estrogen-mediated induction of conalbumin mRNA also occurs in vitro, and the kinetics of this response are examined. Progesterone terminates the estradiol-induced accumulation of conalbumin mRNA within 30 min after addition to the medium; progesterone alone or in combination with estrogen, however, is capable of inducing conalbumin mRNA after a 4 hr lag period. The temporary nature of this antagonism and the fact that it does not occur with ovalbumin induction indicate the complexity of the oviduct's response to steroids.The role of protein synthesis in the induction of both ovalbumin and conalbumin was examined by including protein synthesis inhibitors in the culture medium. Puromycin, cycloheximide, emetine, pactamycin and high salt all block the induction of both ovalbumin and conalbumin mRNA when added together with either estrogen or progesterone. The effect of puromycin is reversible. After the drug is removed from the medium, the mRNA accumulation begins with the same characteristic lag period seen when no inhibitors are added. When given 2 hr after estrogen, puromycin stops the accumulation of conalbumin mRNA within 30 min, whereas cycloheximide and emetine allow the mRNA to accumulate for another 2 hr before causing complete inhibition. There is no effect of protein synthesis inhibitors on the number of estrogen receptors localized in the nucleus. The data suggest a direct link between protein synthesis and the steroid-induced accumulation of specific mRNAs in this system.     

A significant lag in the induction of ovalbumin messenger RNA by steroid hormones: a receptor translocation hypothesis.

Although ovalbumin and conalbumin mRNA accumulate in the same tubular gland cells of the chick oviduct in response to estrogen or progesterone treatment, the kinetics of induction are markedly different. Conalbumin mRNA begins to accumulate within 30 min after estrogen administration, whereas there is a lag of approximately 3 hr before ovalbumin mRNA begins to accumulate, as measured by three independent assays. The kinetics of estrogen-receptor binding to chromatin indicate that these sites are saturated within 15 min of estrogen administration to the chicks, demonstrating that the lag is not due to slow uptake of the steroid. Suboptimal doses of estrogen produce the same lag, but the resultant rate of ovalbumin mRNA accumulation is lower than with an optimal dose. Partial induction of ovalbumin mRNA by a low dose of estrogen does not shorten the lag with an optimal dose. With progesteone, there is a lag of about 2 hr before either ovalbumin or conalbumin mRNA begins to accumulate. Treatment of chicks with hydroxyurea shortens the lag for ovalbumin induction with either hormone. Inhibition of protein synthesis with emetine does not prevent the accumulation of either ovalbumin or conalbumin mRNA. With cycloheximide, however, ovalbumin mRNA accumulation can be prevented. The existence of a lag suggests that there are intermediate steps between the binding of steroid receptors to chromatin and the induction of ovalbumin mRNA. There are basically two models to explain these delays in response: one involving the accumulation of an essential intermediate, and the other involving a rate-limiting translocation of steroid receptors from initial nonproductive chromatin-binding sites to productive sites. Several aspects of the kinetics of ovalbumin mRNA induction are more consistent with the latter model.     

Isolation of a nuclear ribonucleoprotein fraction from chick oviduct containing ovalbumin messenger RNA sequences

A method was developed for the isolation of a ribonucleoprotein fraction from chick oviduct nuclei that contains 70% of the pulse-labeled RNA. These fractions also contain about 1% of the nuclear DNA and have an average RNA to DNA ratio of about 4:1. The major nuclear RNP proteins of 32,000 M_r are present along with many additional proteins including histories. However, polysomal proteins and major oviduct cytoplasmic proteins are absent. Nuclei from fully stimulated chick oviduct contain about 3000 copies of ovalbumin messenger RNA sequences of which about 200 are in the RNP complexes: these complexes have sedimentation coefficients of 30 to 350 S and are resistant to disruption by EDTA.The level of ovalbumin mRNA sequences in these complexes reflects the overall rate of synthesis of this RNA. Withdrawal of estrogen leads to a parallel decline of nuclear estrogen receptors and ovalbumin mRNA sequences in the RNP complexes and a subsequent loss of cytoplasmic ovalbumin mRNA about three hours later. The 300-fold decrease in the level of ovalbumin mRNA sequences in these complexes and the eightfold decrease in stability of cytoplasmic ovalbumin mRNA account for the 2500-fold decrease in the level of cytoplasmic ovalbumin mRNA observed during withdrawal. Upon stimulation with estrogen, the kinetics of reappearance of ovalbumin mRNA sequences in the RNP complexes apparently accounts for the accumulation of cytoplasmic ovalbumin mRNA. Thus the nuclear RNP has some of the properties expected of nascent RNP complexes.The levels of ovalbumin and conalbumin mRNA sequences increase in the nuclear RNP with markedly different kinetics: conalbumin mRNA sequences reach half maximum by 1.5 hours, whereas ovalbumin mRNA sequences in these complexes reach half maximum at about eight hours. In the analysis in the accompanying Appendix, we show that the immediate increase of conalbumin mRNA sequences in the nuclear RNP may be accounted for by interaction of the hormone receptor complex with a single regulatory site, whereas the delayed increase of ovalbumin mRNA sequences in the RNP may be due to a requirement for interaction of the hormone receptor complex with multiple regulatory sites.     

Commitment of chick oviduct tubular gland cells to produce ovalbumin mRNA during hormonal withdrawal and restimulation

Acute withdrawal of estrogen from chicks leads to a precipitous decline in egg white protein synthesis and egg white mRNAs in the oviduct. In this paper we explore the biochemical basis of this phenomenon as well as the capacity of the "withdrawn" tubular gland cells to be restimulated with steroid hormones. During withdrawal, the decline in ovalbumin mRNA was closely correlated with the decline in nuclear estrogen receptors. Within 2-3 d of estrogen removal a withdrawn state was established and then maintained, as defined by a 1,000-fold-lower level of ovalbumin mRNA and a 20-fold-lower level of nuclear estrogen receptors, relative to the estrogen-stimulated state. The number of active forms I and II RNA polymerases declined by 50% during this time. Histological examination of oviduct sections and cell suspensions, combined with measurements of DNA content, revealed that tubular gland cells persisted as a constant proportion of the cell population for 3 d after estrogen removal. Despite a 1,000-fold decrease in the content of ovalbumin mRNA, the ovalbumin gene remained preferentially sensitive to digestion by DNase I. When 3-d-withdrawn oviducts were restimulated with either estrogen or progesterone, in situ hybridization revealed that greater than or equal to 98% of the tubular gland cells contained ovalbumin mRNA. Induction by a suboptimal concentration of estrogen was correlated with a lower concentration of ovalbumin mRNA in all cells rather than fewer responsive cells.     

Opposite effects of glucocorticoid on estrogen-induced growth and differentiation of quail oviduct: demonstration by sequential treatments

Control of the development and functions of avian oviduct is monitored by four classes of steroid hormones, including glucocorticoids. The effects of dexamethasone (DEX), a synthetic glucocorticoid, were studied via sequential treatments with estradiol benzoate, paying special attention to changes in estrogenic oviduct responses involving DNA synthesis and cell proliferation, ovalbumin accumulation and cell differentiation. DEX exerted an antagonistic effect upon estrogen stimulation when administered separately before or after estradiol benzoate (EB). Given before EB, DEX was more strongly antagonistic for DNA synthesis than when given simultaneously with EB. Administered after EB, DEX reversed EB-induced cell proliferation: the DNA content declined and the oviduct regressed. In the same way, protein and ovalbumin synthesis was inhibited and delayed by first intervention of DEX, and accelerated catabolism of ovalbumin and proteins was observed when DEX followed EB. DEX, which was ineffective alone, but synergistic on ovalbumin synthesis when given concomitantly with EB, prevented or dissipated the estrogenic effects, cell proliferation and secretory process when administered in sequential treatments.     

Estrogen-like effects of combined dexamethasone and tamoxifen in the chick oviduct

The effects of dexamethasone alone on withdrawn chick oviduct weight, DNA, protein content and progesterone receptor concentration were barely detectable, whereas ovalbumin and conalbumin synthesis were increased. When dexamethasone and tamoxifen were combined, a marked increase of total proteins, including egg white proteins, DNA and wet weight occurred. Progesterone receptor also was increased. The most striking result was the stimulation of DNA polymerase-α activity by combined dexamethasone and tamoxifen, whereas either compound was completely ineffective.     

Multihormonal regulation of insulin-like growth factor-I-related protein in MCF-7 human breast cancer cells

MCF-7 human breast cancer cells have been studied for hormonal regulation of secretion of an insulin growth factor-I (IGF-I)-related growth factor. 17 beta-Estradiol, which is required for tumorigenesis of the cell line in the nude mouse and which stimulates proliferation in vitro, was able to significantly induce IGF-I secretion at 10(-13) M, with maximal induction at 10(-11) M. Under optimal conditions IGF-I could be induced 4-fold after 4 days. Demonstration of estrogenic stimulations required removal of phenol red, a weak estrogen, from the cell culture medium. In addition to estrogen, insulin, epidermal growth factor, and transforming growth factor alpha induce both cellular proliferation and IGF-I secretion, while growth inhibitory antiestrogens, transforming growth factor beta, and glucocorticoids have the opposite effect. In each case, modulations in IGF-I secretion preceeded effects on cellular proliferation. IGF-I was not regulated by human GH, basic fibroblast growth factor, platelet-derived growth factor, or PRL, none of which affected proliferation rate. Thus, regulation of IGF-I secretion in human breast cancer is controlled by different hormones from those previously reported in human fibroblasts. Regulation of IGF-I by neither estrogen nor antiestrogen was associated with changes in steady-state mRNA levels; thus regulation may occur at a step beyond mRNA. We conclude that IGF-I production is tightly coupled to growth regulation by estrogens, antiestrogens, and other hormones and may contribute to autocrine and/or paracrine growth regulation by these agents in breast cancer.     

Estrogen Receptor Is Not Primarily Responsible for Altered Responsiveness of Ovalbumin mRNA Induction in the Oviduct From Genetically Selected High- and Low-Albumen Chicken Lines

The role of estrogen receptor on ovalbumin mRNA induction by steroid hormones was investigated in primary cultures of oviduct cells from estrogen-stimulated immature chicks of genetically selected high- and low-albumen egg laying lines (H- and L-lines). In experiment 1,the extent of ovalbumin mRNA induction and changes in estrogen and progesterone receptors were compared between the oviduct cells from H- and L-lines with or without steroid hormones in the culture medium. In experiment 2, the effect of estrogen receptor gene transfection on the induction of ovalbumin mRNA was studied in the oviduct cells from the L-line chicks. The results showed a close correlation of the changes in ovalbumin mRNA with the numbers of nuclear and total estrogen receptors in the oviduct cells but not with the numbers of nuclear and total progesterone receptors. Estrogen receptor gene transfection induced ovalbumin mRNA to a moderate extent in the absence of the steroid hormones. To our surprise, however, estrogen receptor gene transfection apparently suppressed the ovalbumin mRNA responsiveness to estrogen to a considerable extent. It was concluded, therefore, that the extent of estrogen receptor expression might not be primarily responsible for the differences in responsiveness to steroid hormones of oviduct cells from genetically selected H- and L-line chickens.     

Purification and translation of ovalbumin,conalbumin, ovomucoid and lysozyme messenger RNA

Messenger RNAs for 4 egg white proteins (ovalbumin, conalbumin, ovomucoid and lysozyme) were assayed in a cell-free, protein-synthesizing system derived from rabbit reticulocytes. Each of these messengers was purified about 33-fold from hen oviduct polysomal RNA using oligo(dT) cellulose. The apparent minimal translational efficiencies varied from 22 translations for each ovalbumin mRNA to less than 1 for lysozyme mRNA. These messengers are not polycistronic with each other since they have distinct sedimentation values of: lysozyme, 8.5S; ovomucoid, 11S; ovalbumin, 15S; and conalbumin 18S. Several aspects of this system indicate that it will be valuable for dissecting the fine control of mRNA metabolism.     

Failure of rabbit reticulocytes to incorporate conalbumin or lactoferrin iron.

Despite the remarkable molecular similarity of human lactoferrin and human transferrin, the results of this investigation indicate that human lactoferrin was unable to furnish rabbit reticulocytes with iron for heme synthesis. Although conalbumin closely resembles transferrin in many of its properties, conalbumin iron-binding differs from human transferrin iron-binding. There are conflicting reports in the literature regarding conalbumin's ability to furnish iron to reticulocytes. In this study, small amounts of lactoferrin or conalbumin were adsorbed to mature and immature cell surfaces but neither of these iron-binding proteins surrendered iron intracellularly to reticulocytes for heme synthesis.     

Steroid hormones induce cell proliferation and specific protein synthesis in primary chick oviduct cultures

A rapid method to obtain large amounts of tubular gland cells from chick oviduct was developed. Combined collagenase and trypsin treatment allowed within 1.5 h complete dissociation of the magnum portion of the oviduct. By differential attachment of cells, fibroblasts were separated from tubular gland- and ciliated cells. Tubular gland cells attached within 18 h to plastic Petri dishes, had large secretory granules and grew very actively. The responsiveness of cells to hormones and/or antihormone was tested by measurement of cell proliferation and specific protein synthesis. After 7 days of culture in the presence of estradiol (50 nM) or progesterone (100 nM), cell growth was increased by approximately 50 and 35% respectively. Tamoxifen (100 nM) inhibited the estradiol induced growth stimulation, but had also negative effects of its own. The anti-progesterone (in mammals) RU 486, inactive per se, did not antagonize progesterone induced growth. Ovalbumin- and conalbumin synthesis after 4-5 days of cultures under different hormonal conditions was assessed after immunoprecipitation of newly synthesized [35S]methionine labelled proteins. In the presence of estradiol (50 and 100 nM), progesterone (50 nM), and both estradiol and progesterone together (50 nM of each), ovalbumin and conalbumin synthesis was increased, when compared to control cultures without hormones, or to oviduct fibroblasts. Hormonal stimulation of ovalbumin synthesis was also shown in cell supernatant and culture medium after gel electrophoresis.     

Effects of dexamethasone on the production of insulin-like growth factor-I and insulin-like growth factor binding proteins in primary cultures of rat hepatocytes.

The effects of dexamethasone (Dex) on insulin-like growth factor (IGF)-I and IGF binding protein (IGFBP)-1 production were investigated in primary cultures of rat hepatocytes. Dex enhanced the secretion of IGFBP-1 as measured by ligand blot analysis but did not show any prominent effect on immunoreactive IGF-I secretion. EC50 of Dex on IGFBP-1 secretion was calculated to be 3 x 10(-8) M. The content of IGFBP-1 mRNA in the cells increased greatly in the presence of Dex but the IGF-I mRNA content did not change significantly under the same conditions. Insulin showed the opposite effect of Dex by decreasing the production of IGFBP-1 and the cellular content of IGFBP-1 mRNA. This effect of insulin was observed also with Dex in the medium. These results show that the gene expression of IGF-I and IGFBP-1 is differently regulated by glucocorticoids and insulin in primary cultures of rat hepatocytes. The results most possibly explain the in vivo effects of glucocorticoids and insulin in regulation of IGF-I and IGFBP-1 production by liver.