Cell-free synthesis of a prolactin precursor directed by mRNA from cultured rat pituitary cells

Novel prolactin related mRNAs in rat pituitary cells

From cytoplasm of rat pituitary GH4C1 tumour cells, anti prolactin anti-serum precipitates a polypeptide with apparent molecular weight of 75.000 in addition to prolactin. In vitro translation of size fractionated RNA shows that a 82.000 molecular weight PRL-like polypeptide is encoded by a mRNA larger than the 1 kb prolactin mRNA. Northern blot analysis shows that a rat prolactin cDNA probe hybridize to a 3.2 kb RNA and a 1.5 kb RNA in addition to the 1 kb PRL mRNA. The 82.000 molecular weight translation product and the 3.2 kb mRNA is also detected in rat anterior pituitary cytoplasm. We conclude that at least one high molecular weight mRNA which code for a prolactin-like polypeptide, is present in normal rat anterior pituitary gland and in GH4C1 cells.

Electrophoretic separation of prolactin secreted from mouse pituitary glands in vitro

Prolactin secreted from the mouse pituitary gland in organ culture was characterized after disc or SDS-polyacrylamide gel electrophoresis. Some pituitary glands were cultured with 3H-labelled leucine for 24 h to obtain radioactive prolactin. In disc electrophoresis, immunoreactive, receptor-bindable and 3H-incorporated prolactins formed a single band with the same relative mobility (0.5). Prolactin migrating at Rf = 0.5 was extracted and re-analysed by SDS electrophoresis. A single stained and radioactive band was observed at the same position with an apparent molecular weight of 23 000 regardless of denaturing in the presence of absence of dithiothreitol. No other band was detected. These results indicate that mouse prolactin synthesized and secreted in organ culture is homogeneous and that mouse prolactin is a single-chain molecule.

Translation of mRNA for Limulus polyphemus haemocyanin polypeptides in vitro: studies on subunit heterogeneity

The haemocyanin of Limulus polyphemus is composed of a number (possibly 10-15) of polypeptides and is believed to be synthesised in cells called cyanoblasts. In vitro translation in the rabbit reticulocyte haemolysate system and in Xenopus oocytes, of mRNA isolated from cyanoblast-containing tissue, allowed the detection of several haemocyanin polypeptides amongst the products of translation. At least seven polypeptides with molecular weights in the range 68 000-71 000 were identified by an immunological method followed by electrophoretic characterisation on two-dimensional polyacrylamide gels. Comparison of the polypeptide patterns of authentic haemocyanin, reticulocyte lysate translation products and Xenopus oocyte translation products led to the conclusion that the polypeptides are unlikely to undergo significant post-translational modification or to possess cleavable signal sequences. It is proposed that release of haemocyanin into the haemolymph in vivo may involve bursting of the cyanoblasts.

Analysis of brain and pituitary RNA metabolism: a review of recent methodologies

Recent characterization of brain and pituitary RNA metabolism is reviewed. Relative to other tissues, the brain transcribes more of the unique, single-copy DNA. This transcriptional diversity reflects the inherent heterogeneity in organization and development of the brain. The end product of transcriptional regulation in the brain is a population of functional cytoplasmic mRNAs with multiple components, differing in complexity and abundance. Analysis of nuclear and cytoplasmic RNA provides evidence that both brain-specific synthesis and processing may determine the final mRNA population. Both polyadenylated and non-polyadenylated RNA classes contribute significantly to the total brain polysomal mRNA fraction. Characterizations of individual species of mRNA from both brain and pituitary are described. One possible transcriptional modulator in both the pituitary and brain is the presence of steroid hormone at responsive sites. Functional consequences of steroid accumulation within the brain may be (1) interactions with neurotransmitter, especially catecholamine, metabolism and function, (2) developmental interactions with neuronal systems, and (3) differentiation of glial cell function. The pleiotropic nature of steroid hormone effects (both transcriptional and non-transcriptional) within one brain region is considered by examining the biochemical effects of glucocorticoids in the hippocampus.

A homologous radioimmunoassay for turkey prolactin: changes during the reproductive and ovulatory cycle

A rabbit antiserum to turkey prolactin (tPRL) was used in a homologous radioimmunoassay with 125I-tPRL. This assay did not cross-react with turkey luteinizing hormone (tLH), chicken LH (cLH), turkey follicle stimulating hormone (tFSH), or turkey growth hormone (tGH). The within- and between-assay coefficients of variation were 6.2 and 14.0%, respectively, and the useful range of the standard curve extended from .5 to 10.0 ng/tube. In two separate experiments, the plasma concentrations of prolactin were estimated throughout the reproductive cycle of the turkey hen using a heterologous assay previously described and the current homologous assay. The correlation coefficients between the two estimates of prolactin in nonbroody hens were .84 and .83. In both experiments, the plasma concentrations of prolactin were low when egg production was initiated, rose to maximum concentrations 5 to 7 weeks later, and thereafter declined. In the first experiment, the hens were selected because their plasma contained large amounts of immunoreactive PRL during the first 2 weeks after photostimulation in the heterologous tPRL assay. This immunoactivity was not evident when using the homologous assay. Using the heterologous assay, no differences in the plasma prolactin concentrations were detected between broody and nonbroody hens during the reproductive cycle. However, using the homologous assay, a sustained increase in the concentration of prolactin was noted for several weeks after broody behavior was detected. The maximum levels observed in broody hens were equal to those observed when the rate of egg production was maximal. There were no statistically significant changes in the concentrations of prolactin during the ovulatory cycle.

RNA processing regulation of neuroendorcrine gene expression

Alternative RNA processing events account for biological polymorphism in the endocrine, immune, and other systems; these may have possible widespread significance in the regulation of eukaryotic gene expression. The selective use of alternative hormone-encoding exons can produce multiple mRNAs generating different protein products. The resultant "peptide switching" is a previously unrecognized mechanism allowing a single gene to encode different hormones in different tissues and thus increases the diversity of endocrine gene expression.

Inhibition by tamoxifen of estrogen-stimulated accumulation of preprolactin messenger ribonucleic acid

Estrogens are involved in the stimulation of prolactin synthesis in the rat anterior pituitary. After 5 days of treatment with 17beta-estradiol, strong enhancement of [3H]leucine incorporation into prolactin an stimulation of translatable preprolactin mRNA, respectively, were noted. Increase in prolactin synthesis was found following daily application with estriol and the synthetic estrogen 1,3-dibenzoyloxy-17 beta-methyl-1,3,5(10)-estratrien-17 beta-ol (DB-EE2). The antiestrogen tamoxifen (trans-1-(p-beta-dimethylaminoethoxyphenyl)-1,2-diphenylbut-1-ene) was demonstrated to inhibit estradiol-stimulated prolactin syntheses. Antagonistic effects of tamoxifen were dose dependent. Low loses of the antiestrogen were already sufficient to suppress 17 beta-estradiol-enhanced levels of prolactin mRNA. On the other hand, estriol potentiated 17 beta-estradiol-stimulated levels of serum prolactin and prolactin synthesis. Our results add further information about the transcriptional control by estrogens and demonstrate inhibitory actions of antiestrogens on distinct regulatory levels of protein synthesis in the rat pituitary.

Translational analysis of the killer-associated virus-like particle dsRNA genome of S. cerevisiae: M dsRNA encodes toxin

The M species (medium sized) dsRNA (1.1-1.4 x 10(6) daltons) isolated from a toxin-producing yeast killer strain (K+R+) and three related, defective interfering (suppressive) S species dsRNAs of the yeast killer-associated cytoplasmic multicomponent viral-like particle system were analyzed by in vitro translation in a wheat germ cell-free protein synthesis system. Heat-denatured M species dsRNA programmed the synthesis of two major polypeptides, M-P1 (32,000 daltons) and M-P2 (30,000 daltons). M-P1 has been shown by the criteria of proteolytic peptide mapping and cross-antigenicity to contain ihe 12,000 dalton polypeptide corresponding to the in vivo produced killer toxin, thus establishing thiat it is the M species dsRNA which carries the toxin gene. An M species dsRNA obtained from a neutral strain (K-R+) also programmed the in vitro synthesis of a polypeptide identical in molecular weight to M-P1, thus indicating that the cytoplasmic determinant of the mutant neutral phenotype is either a simple point mutation in the dsRNA toxin gene or a mutation in a dsRNA gene which is required for functional toxin production. In vitro translation of each of the three different suppressive S dsRNAs resulted in the production of a polypeptide (S-P1) of approximately 8000 daltons instead of the 32,000 dalton M-P1 polypeptide programmed by M dsRNA. This result is consistent with the heteroduplex analysis of these dsRNAs by Fried and Fink (1978), which shows retention of M dsRNA ends, accompanied by large internal deletions in each of the S dsRNAs translated.

Identification and characterization of a biosynthetic precursor (preprolactin) of ovine pituitary prolactin

Poly(A)-containing mRNA prepared from ovine anterior pituitary glands stimulated the incorporation of radioactivity into protein in a wheat germ cell-free system from 7- to 25-fold over background. Approximately 30% of the total protein synthesized could be precipitated by antiserum to prolactin, indicating that a molecule which shares antigenic determinants with prolactin was being synthesized. Analysis of the immunoprecipitable cell-free translation product, by electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulphate demonstrated the presence of a prolactin-like molecule with a molecular weight greater than that of prolactin itself. Partial N-terminal sequence analysis of the immunoprecipitable translation product showed that it contained one methionine residue at position 1 and several leucine residues within the first 30 amino acids. Experiments with labelled initiator and internal met tRNAMet indicated that the product being synthesized contained the initiating methionine and was probably the initial translation product of prolactin mRNA.

Quenching of intrinsic tryptophan fluorescence in membranes of rat pituitary cells

The GH4C1 strain of hormone-producing rat pituitary cells has specific receptors for the tripeptide thyrotropin-releasing hormone (TRH). Membranes prepared from GH4C1 cells show intrinsic tryptophan fluorescence which was quenched by low concentrations (10--100 nM) of TRH and Ntau-methyl TRH but not by biologically inactive analogs of TRH. Membranes from GH4C1 cells were subjected to thermal denaturation. A conformational transition was noted above 40 degrees C and an irreversible denaturation was observed at 52 degrees C. TRH-induced quenching of intrinsic fluorescence was lost completely in membranes previously incubated for 10 min at 30 degrees C while loss of [3H]-TRH binding was only about 20% at this temperature. Collisional quenching by iodide revealed that about 38% of the tryptophanyl residues in GH4C1 membranes were exposed to solvent. Quenching by TRH occurred with a shift in wavelength maximum from 336 to 342 nm suggesting that few of the tryptophanyl residues quenched by the tripeptide are totally exposed. Membranes prepared from cells preincubated with 20 nM TRH for 48 h, in which TRH receptors were decreased to 30% of control values, showed no quenching of tryptophan fluorescence in response to freshly added TRH. We conclude that the TRH-receptor interaction in GH4C1 cells is associated with a change in membrane conformation that can be measured by differential spectrofluorometry of intrinsic tryptophan fluorescence.

Regulation of growth hormone messenger RNA

In cultured rat pituitary cells, glucocorticoids regulate growth hormone production by modulating the number of growth hormone messenger RNA molecules. The effect is quite specific, since only a few other mRNAs are affected by the hormones. This response is demonstrated by assays involving cell-free mRNA translation and cDNA-RNA hybridization. Furthermore, the inducibility by the glucocorticoids is regulated by at least one other class of hormones, thyroid hormone. Thus, this system serves as a model for studying not only the glucocorticoid regulation of specific mRNA, but also the control of this regulation by other factors in the target tissue.

The plastid membranes of barley (Hordeum vulgare). Light-induced appearance of mRNA coding for the apoprotein of the light-harvesting chlorophyll a/b protein

Illumination of dark-grown barley plants induces a massive insertion of the light-harvesting chlorophyll a/b protein into the developing thylakoid membrane. In addition to the onset of chlorophyll synthesis, light induces specifically the appearance of a prominent mRNA species which codes for a polypeptide of Mr 29500. This component was identified as a precursor of the apoprotein of the light-harvesting chlorophyll a/b protein. The precursor has an Mr larger than the authentic protein by approximately 4000. Studies of the chlorophyll-b-less mutant chlorina f2 of barley offer the first clue to the mechanism which controls the light-dependent mRNA formation. The induction of the mRNA coding for the aproprotein of the light-harvesting chlorophyll a/b protein does not seem to be linked directly to the assembly process of the light-harvesting structure and does not require chlorophyll b. It is proposed that light exerts its influence on the mRNA formation by a reaction which is different from the phototransformation of protochlorophyll(ide) to chlorophyll(ide).

Membrane-dependent cleavage of the human placental lactogen precursor to its native form in ascites cell-free extracts

Messenger RNA derived from term placenta directs the synthesis of a precursor to human placental lactogen (prelactogen, molecular weight 25000) in an ascites cell-free system containing ribosome free supernatant and preincubated purified ribosomes. The processing of prelactogen to native lactogen (molecular weight 22200) was only observed when a microsomal membrane preparation was added prior to the synthesis of complete protein, i.e. before release. Placental mRNA directed the synthesis of prelactogen in asystem containing free polysomes, whereas in a comparable system containing membrane-bound polysomes both prelactogen and lactogen were synthesized. The prelactogen synthesized in the latter system could be cleaved by the addition of membranes at the start of incubation. Preprotein cleavage activity was inhibited 100% by 0.04% Tritonx-100, while protein synthesis was inhibited only about 30%. Using Triton to block cleavage specifically at intervals after mRNA and membrane additions, it was determined that the overall cleavage reaction required about 15 min. When the ascites system was incubated with charged initiator [35S]methionyl-tRNAfMet prelactogen was formed. The labeled prelactogen was processed when membranes were added after 60 min of incubation. The results indicate that prelactogen is the primary gene product, and cleavage activity is apparently associated only with the membrane-bound ribosomal fraction of the cell.