Synthesis of human placental lactogen and human chorionic gonadotropin by polyribosomes and messenger RNA's from early and full term placentas

Double-stranded RNA-stimulated enzyme activities isolated from human placental extracts

A (2'-5')An synthetase activity was isolated from human placental extracts by affinity chromatography on poly(rI) . poly(rC)-agarose. The oligonucleotide (2'-5')An was identified by (1) chromatography on PEI-cellulose and DEAE-cellulose, (2) inhibition of polypeptide synthesis in lysed rabbit reticulocytes (3) competition of the binding of pppA(pA)3,3'-[32P]pCp to rabbit reticulocyte lysates, and (4) alkaline phosphatase digestion. The synthetase activity in most placental preparations is activated by natural or synthetic dsRNA. However, in a few placental synthetase preparations, dsRNA is only marginally stimulatory and only becomes effective by prior treatment of the enzyme preparations with the calcium-dependent micrococcal nuclease. This suggests that there is an endogenous placental dsRNA contaminant in the enzyme preparations. In some synthetase preparations, a second dsRNA-stimulated product, tentatively identified as the nucleotide 5'-IMP, is also observed. Because the specific AMP deaminase inhibitor coformycin (10 microM) blocks the formation of IMP from ATP and causes a quantitative accumulation of AMP, and because the formation of IMp becomes independent of dsRNA when ADP or AMP is used in place of ATP, the presence of a dsRNA-stimulated ATP phosphohydrolase (ATPase) activity in human placenta is suggested.

Translation in a cell-free system of mRNA from term placenta extracted by guanidine hydrochloride

Human term placenta RNA and polyadenylated mRNA were prepared using guanidine HCl and oligo (dT)-cellulose affinity chromatography. Both fractions translated in a wheat germ cell-free system showed, under optimal condition of K+ and Mg++ ions and spermidine, about 9 times activity for RNA and 15-25 times for poly(A+) mRNA greater than the control. Homogenization of the tissue at high speed compared to that at low speed improved 4-fold activity. Analysis of tritiated products by SDS-Polyacrylamide gel electrophoresis and detected by fluorography showed more than ten different intensity bands ranging between 12 and 66 kD. According to the results obtained, guanidine HCl is an advantageous procedure for the extraction of RNA from this nuclease-rich tissue compared with that obtained with phenol extraction, in both activity and in larger translation products.

In vitro translation of human placental pregnancy-specific beta 1-glycoprotein (SP1)

Synthesis of SP1-glycoprotein by the human placenta was directly demonstrated, by in vitro translation of RNA extracted from full term and from early placentas in a cell-free wheat germ system followed by specific immunoprecipitation of the radioactively labeled nascent peptides. De novo synthesized SP1-glycoprotein in both RNA preparations accounted for 1-1.3% of total protein synthesis.

Distribution, quantification and biological activity of messenger RNA coding for human chorionic somatomammotropin during normal pregnancy

Synthesis of hCS by RNA fractions from human placentas obtained at different stages of pregnancy was estimated either by immunological or electrophoretical methods in wheat-germ and reticulocyte cell-free systems. hCS synthesis is preferentially associated with polyribosomes bound to the membrane of the endoplasmic reticulum, as is assumed for a secreted protein. In both translational systems we determined only one precursor form of this hormone of a molecular weight near 24 000. Full-term placentas synthesize hCS as the major protein. This conveniently allowed us to isolate the messenger RNA coding for the hormone and to synthesize a specific hCS complementary DNA which we used as a probe for quantifying sequences of RNA coding for hCS during pregnancy. In placentas from first-trimester pregnancy, the concentration of hCS mRNA was 4 times less than in the full-term organs, and the hCS synthesis per microgram of RNA added into the translational medium was diminished in the same order of magnitude. In placentas from second-trimester pregnancy, the concentration of hCS mRNA was similar to that obtained at term, and in vitro the hCS synthesis per microgram of translated RNA was also similar to that observed at the end of pregnancy. However, the hCS mRNA content per placenta from mid-term pregnancy was much lower than from full-term gestation. We established a good parallelism, as pregnancy progressed, between the hCS mRNA content, its capacity of hCS synthesis in vitro and the maternal plasma hCS level, indicating that hCS production is controlled essentially by the biological active mass of the placenta.

Interrelationships of human chorionic gonadotropin, human placental lactogen, and pregnancy-specific beta 1-glycoprotein throughout normal human gestation

Human chorionic gonadotropin (hCG), human placental lactogen (hPL), and pregnancy-specific beta 1-glycoprotein (PSBG) were measured by radioimmunoassay in 270 samples of serum from women with uncomplicated pregnancies. All three proteins were significantly correlated with each other in individual samples of serum and with the estimated trophoblastic mass during the first trimester. No significant correlation could be demonstrated between the concentrations of hCG and PSBG in maternal serum during the second or third trimesters or between the concentrations of hCG and hPL during the second trimester. Levels of PSBG and hPL in serum were significantly correlated throughout all three trimesters. These findings suggest that the secretion of hCG, hPL, and PSBG may be regulated by similar control mechanisms during the first trimester of pregnancy. However, after this period, the factors that modulate the production of hCG differ from those that regulate the production of hPL and PSBG.

Isolation of ribonuclease-free polysomes from human placenta

Human placenta is known to have a high level of polysome-bound ribonuclease which has hindered the isolation of intact polyribosomes from this tissue. We describe conditions for preparing polysomes devoid of apparent ribonuclease activity from both first trimester and term placenta. This stable preparation was achieved by utilizing buffer at low pH containing 300 mM LiCl and precipitating the polysomes chemically with 200 mM MgCl2. The yield of polysomes obtained by this procedure is 2-2.5 fold greater than that obtained by the conventional method of preparing placental polysomes. The polysomes are considered pure as judged by the ratios of absorbance 260/280 and 260/235. Moreover, the yield of polysomes obtained is greater than 95% of the tissue content and the profile of the polysomes is probably representative of the in vivo population. This is concluded from experiments in which a known amount of labelled chick polysomes was added to fresh placental tissue and the recovery of label and its distribution was analyzed.

The cDNA for the beta-subunit of human chorionic gonadotropin suggests evolution of a gene by readthrough into the 3'-untranslated region

A 579-base pair approximately full-length cDNA which codes for the 145-amino acid long beta-subunit of human chorionic gonadotropin (HCG) has been cloned in the plasmid vector pBR322 and its complete nucleotide sequence determined. A hydrophobic presequence of 20 amino acids can be identified from the nucleotide sequence. The amino acid sequence of the beta-subunit is known to be related to those of the beta-subunits of the other glycoprotein hormones LH, FSH and TSH, but the beta-subunit of HCG is unique in that it contains a C-terminal extension of about 30 amino acids which has no homologous counterpart in the other three hormones. Analysis of the beta HCG cDNA nucleotide sequence suggests that this extension may have arisen by the loss of the termination codon of an ancestral beta-like gene so that most of what was previously the 3'-untranslated region now codes for protein. The beta-subunit of HCG terminates with the codon UAA located 16 bases before the poly(A) in the sequence AAUAAA. This sequence is believed to be a recognition signal involved in either polyadenylation or processing and therefore has a dual role in this gene, serving both a coding and a regulatory function.

Synthesis and processing of RNA by isolated human placental nuclei

Conditions for isolating intact and active nuclei from human term pacenta and for studying their transcription products are described. The isolated nuclei can synthesize cell-free RNA for a prolonged period at 29 degree C in a medium containing 100 mM KCl and 5 mM MgCl2. Actinomycin D inhibited 92 per cent of RNA synthesis, whereas approximately 60 per cent of RNA synthesis was sensitive to alpha-amanitin. When nuclei were incubated at 29 degrees C for 1 h, about 27 per cent of the newly synthesized RNA was released into the medium outside the nucleus. Analysis of this released material by affinity chromatography on an oligo(dT)-cellulose column revealed that 2.4 per cent of the total released RNA was adsorbed at high salt concentration. Most of this fraction was eluted with a low-salt buffer at 45 degrees C and the remainder by 50 per cent formamide, conditions that are necessary for elution of poly(A)-containing mRNP particles from oligo(dT)-cellulose. These results show that placental nuclei incubated in vitro synthesize poly(A)-containing RNA, which is released as a protein-bound complex. This procedure allows exploration of changes in mRNA release during placental development.

Control of RNA content of developing human placenta

Human placentae obtained early in pregnancy or at full term were examined for RNA content per cell, RNA polymerase types and activities, and chromatin template availability. The RNA:DNA ratio fell from 0.7 at 15 to 20 weeks to 0.4 at 40 weeks of pregnancy. Since RNase activities were similar at both times, the reduction in RNA content was attributed not to increased degradation, but to reduced synthesis. At both stages of pregnancy, about 55 to 60 per cent of the RNA polymerase activity in isolated placental nuclei was accounted for by RNA polymerase II, as judged by suppression of activity with alpha-amanitin and by separation of the extracted polymerases on DEAE-Sephadex. The relative roles of changes in polymerase activity and template availability were measured in nuclei from 20- and 40-week placentae. Nuclei showed 20 per cent greater polymerase activity in full-term than in early placentae, but the template availability of isolated chromatin for transcription by RNA polymerase II was 70 per cent less at full term. We conclude that the reduced amount of RNA per cell in the full-term placenta is due to reduced template availability that more than offsets the slight increase in polymerase activity.

Isolation, cloning and sequence analysis of the cDNA for the alpha-subunit of human chorionic gonadotropin

A 621-base pair fragment of the cDNA for the alpha-subunit of human chorionic gonadotropin has been isolated by cloning in a plasmid vector, and the complete nucleotide sequence determined. The entire coding region, including the 24-amino acid pre-sequence and most of the untranslated regions, are present in the fragment.

Translation of biologically active messenger RNA from human placenta in Xenopus oocytes

Polysomal RNA was extracted from human term placenta and total poly(A)-containing RNA purified by affinity chromatography on oligo(dT)-cellulose. Poly(A)-containing RNA constituted approximately 1.2% of the total polysomal RNA and 8% of this purified preparation was able to anneal with [3H]poly(U). When injected into Xenopus oocytes, this poly(A)-rich RNA directed the synthesis of a polypeptide which is immunoprecipitable with a specific antiserum to human placental lactogen. The identity of authentic human placental lactogen and the immunoreactive polypeptide synthesized in the oocytes is suggested by their identical behaviour in dodecylsulfate gel electrophoresis and by the formation of identical cyanogen bromide peptides. No precursor of human placental lactogen can be detected in the oocytes. The messenger RNA for human placental lactogen is very stable in oocytes; it is translated efficiently for a period of at least 7 days.

The wheat germ cell-free system possesses processing activity for the precursor of human placental lactogen

1. Total RNA was extracted from human term placenta and mRNA purified by chromatography on oligo(dT)-cellulose. The poly(A)-containing fraction stimulated amino acid incorporation 5- to 10-fold in the wheat germ cell-free system. Immunoprecipitation with an anti-lactogen serum indicated that 14-27% of the peptides synthesized in vitro contained antigenic determinants of this hormone. 2. Analysis of the [3H]leucine labelled product in the immunoprecipitate on sodium dodecyl sulfate-polyacrylamide gels revealed a complex mixture of polypeptides. Two heavily labelled bands (I and III) were seen corresponding in mobility with pre-lactogen (Mr = 25 000) and native lactogen (Mr = 22 200), each accounting for about 30% of the immunoprecipitable radioactivity. Two additional bands with an intermediate mobility were also observed. 3. Synthesis of the hormone was inhibited by 7-methylguanosine-5'-monophosphate suggesting the presence of a 7-methylguanosine 'cap' on the 5'-end of the mRNA for lactogen. 4. Peptide analysis of the cyanogen bromide cleavage products of band I, band III and authentic lactogen showed marked similarities in their primary structure. The precursor molecule, however, was lacking the N-terminal peptide present in authentic hormone indicating the presence of an extension of 25 amino acids at this side of the molecule. 5. The presence of one or several processing enzymes in the wheat germ cell-free system was indicated by the effect of Triton X-100. Low concentrations of this detergent (0.04%) while inhibiting the protein synthesizing activity for only 15%, completely abolished the precursor cleavage activity. Under these conditions only pre-lactogen was detected in the immunoprecipitate.

Clinical relevance of research on the structure of human chorionic gonadotropin

Human chorionic gonadotropin (hCG) has been studied extensively over the past 10 years, and some structural correlates of its chemical, biological, and immunologic properties have been defined. A review of these observations and their relevance to clinical applications of methods for detecting hCG is presented.

Serum levels of human placental lactogen and human chorionic gonadotropin in early pregnancy: a maturational index of the placenta

Between weeks 10 and 20 of gestation, there are rapid and opposing changes in blood levels of two hormones produced by the syncytiotrophoblast; hPL increases and hCG decreases. In this study, the relationship between these two changing hormone levels was examined, rather than the serum concentrations themselves, as a potential test for placental maturation. The relationship between the hCG and hPL concentrations and GA determined in a selected group of patients was described by the following equation: GA (in days)=97.3 + 24.8 ln [hPL]-14.5 LN [HCG]. The endocrine evaluation of placental maturation by comparison of [hCG] and [hPL] in early pregnancy shows promise as a simple, safe, and inexpensive clinical tool for GA assessment.