Posttranslational modifications of the lutropin receptor: mass spectrometric analysis.

Our present knowledge of the lutropin (LH/hCG) receptor structure derives from deductions made from its amino acid sequence as established by studying the cDNA. To obtain direct experimental information, luteinizing hormone (LH) receptor expressed in L cells was immunopurified in sufficient amounts to warrant analysis by mass spectrometry and microsequencing. The mature receptor, complexed to human chorionic gonadotropin (hCG), was purified by using monoclonal antibodies recognizing the hormone, whereas the mannose-rich non-hormone-binding precursor was purified by use of antireceptor antibodies. Determination of the N-terminus showed that (2)/(3) of protein molecules started at Thr24 whereas (1)/(3) started at Ala28. All these molecules bound hCG, suggesting that the most N-terminal region of the receptor does not participate in hormone binding. Six N-glycosylation sites have been predicted from the amino acid sequence. One of them (Asn299) was found to be nonglycosylated in both the precursor and the mature protein. The most heavily glycosylated residue was Asn291, followed by Asn195 and Asn99. These three sites accounted for 82% and 97% of carbohydrate moieties in the mature receptor and in the mannose-rich precursor, respectively. The presence of some receptor molecules nonglycosylated at sites 99, 174, and 195 in hormone-receptor complexes dismisses a direct role of these glycosylation sites in hormone binding or in the correct folding of the protein. The mature carbohydrate chains were homogeneous at position 174, 195, and 313 (absence of Golgi mannosidase II activity at positions 174 and 313, absence of GlcNAc tranferases III and IV activity at position 195). Heterologous carbohydrates were present at sites 99 and 291. The latter, which is highly variable in carbohydrate chains, is unlikely to participate in a direct interaction with hormone. Site 313 thus remains as the main candidate for a role in hormone binding.

Basolateral localization and transcytosis of gonadotropin and thyrotropin receptors expressed in Madin-Darby canine kidney cells.

The thyrotropin (TSH) and follicle-stimulating hormone (FSH) receptors are present mainly on the basolateral cell surface in the thyroid gland and in Sertoli cells, whereas in ovarian and in testicular cells, the luteinizing hormone (LH) receptors are distributed throughout the cell surface. When expressed in Madin-Darby canine kidney (MDCK) cells, all three receptors accumulated at the basolateral cell surface showing that they carry the corresponding targeting signals. The receptors were directly delivered to the basolateral surface of the MDCK cells. A minor fraction of the gonadotropin receptors but not of TSH receptors was secondarily targeted to the apical surface through transcytosis. The mechanisms of basolateral targeting and transcytosis were analyzed using the FSH receptor as a model. Both were insensitive to brefeldin A and pertussis toxin. Gs activation by AlF4- and cholera toxin provoked a marked enhancement of FSH receptor transcytosis. The population of Gs proteins involved in this mechanism was different from that involved in signal transduction since neither FSH nor forskolin mimicked the effects of AlF4- and cholera toxin. Gs activation provoked a similar effect on LH receptor distribution in MDCK cells, whereas it did not modify the compartmentalization of the TSH receptor. Hormone-specific transcytosis was observed in MDCK cells expressing the gonadotropin (FSH and LH) receptors and was increased after cholera toxin administration.

The LH/CG and FSH receptors: different molecular forms and intracellular traffic.

Monoclonal antibodies have been raised against the LH/CG receptor [1] and have allowed to perform immunochemical studies of the receptor in target cells. Three different forms of the LH/CG receptor are physiologically expressed: a mature approximately 85 kDa transmembrane species corresponding to the full length receptor, a approximately 68 kDa high mannose containing species corresponding to a precursor which accumulates inside the cells, and truncated soluble approximately 45-48 kDa molecular weight species corresponding to the variant messanger RNAs generated by alternative splicing. Monoclonal antibodies against the human FSH receptor were also prepared. They allow to observe the existence of two forms of the FSH receptor in the ovaries: a major approximately 87 kDa species corresponding to the mature receptor and a minor approximately 81 kDa species corresponding to a high mannose rich precursor. No variant forms of the receptor corresponding to alternative mRNA transcripts were detected. The transport of hCG was examined in rat testicular microvasculature by electron microscopy and by analyzing the transfer of radiolabeled hormone and antireceptor antibodies. LH/CG receptors were present in endothelial cells and were involved in hormone transcytosis through these cells. Immunocytochemical experiments have shown that the FSH receptor has a polarized expression in the Sertoli cells of the testes whereas the LH/Cg receptor is spread on the surface of thecal granulosa and luteal cells in the ovary and Leydig cells in the testes. To study the mechanism of this polarization FSH, LH and TSH receptors were expressed in polarized MDCK cells. The mechanism of basolateral localization and of transcytosis of the receptors was studied using this model. The effect of hormone, cAMP and agents acting on G proteins was examined.

Comparison of cellular distribution of LH receptors and steroidogenic enzymes in the porcine ovary.

Previous studies have shown a heterogeneous expression of LH receptors in various structures of the porcine ovary. Specially striking was the existence in the preovulatory follicle of inner layers of theca interna cells devoid of LH receptor and the confinement in the corpus luteum of the LH receptor to the external cellular layers. In the present study, we have compared the steroidogenic capabilities of LH receptor-positive and -negative cells using immunocytochemistry for side-chain cleavage P450, 3 beta-hydroxysteroid-dehydrogenase, 17 alpha-hydroxylase P450 and aromatase P450. We have also examined, using the same methods, the evolution of the various cell types after ovulation and during the development of the corpus luteum. In preovulatory follicles the inner layers of theca cells which were not labelled with anti-LH receptor antibodies appeared to express the steroidogenic enzymes in a way similar to that of the outer LH receptor-positive cell layers. Ovulation per se did not change the distribution of LH receptors (present in the outer luteal cells and in the granulosa) or of steroidogenic enzymes. However, 48 h after follicular rupture there as a marked decrease in overall labelling with anti-LH receptor antibody, and especially a disappearance of immunostaining in the luteal cells of granulosa origin. In the mid-luteal phase (6 days after ovulation), the receptor content seemed to increase in the peripheral luteal cells derived from the theca but the receptor did not reappear in the granulosa-derived luteal cells. Thus the down-regulation of LH receptor appeared to be reversible in the external thecal layers but irreversible in the granulosa cells. Furthermore, the distribution of the various steroidogenic enzymes in the corpora lutea delineated granulosa-derived from theca-derived cells and showed that only the external layers of the latter expressed the LH receptor. These results showed the existence in the preovulatory follicle of two theca interna regions expressing the same steroidogenic enzymes but possibly submitted to a different hormonal control. Furthermore, the cells derived from these two regions as well as the cells of granulosa origin showed a distinct pattern of variation of LH receptivity during the development of the corpus luteum. During these studies we also observed that, in the interstitial tissue, only a minority of cells which derived from remnants of atretic follicles expressed both the LH receptor and the steroidogenic enzymes.

Shedding of human thyrotropin receptor ectodomain. Involvement of a matrix metalloprotease.

The thyrotropin (TSH) receptor in human thyroid glands has been shown to be cleaved into an extracellular alpha subunit and a transmembrane beta subunit held together by disulfide bridges. An excess of the latter component relative to the former suggested the shedding of the ectodomain. Indeed we observed such a shedding in cultures of human thyrocytes and permanently transfected L or Chinese hamster ovary cells. The shedding was increased by inhibitors of endocytosis, recycling, and lysosomal degradation, suggesting that it was dependent on receptor residency at the cell surface. It was slightly increased by TSH and phorbol esters, whereas forskolin and 8-bromo-cyclic AMP were without effect. Decreasing the serum concentration in cell culture medium enhanced the shedding by an unknown mechanism. The shedding of the TSH receptor alpha domain is the consequence of two events: cleavage of the receptor into alpha and beta subunits and reduction of the disulfide bridge(s). The complete inhibition of soluble TSH receptor shedding by the specific inhibitor BB-2116 indicated that the cleavage reaction is catalyzed probably at the cell surface by a matrix metalloprotease. This shedding mechanism may be responsible for the presence of soluble TSH receptor alpha subunit in human circulation.

[Pituitary glycoprotein hormone receptors]. / Les récepteurs des hormones glycoprotéiques hypophysaires.

Monoclonal antibodies have been raised against the porcine LH receptor and have allowed to clone the corresponding messenger RNA from testicular cells. The stricture of the LH receptor has been determined. It shows similarities but also differences with other G protein coupled receptors. Specially a large extracellular domain is specific of that new family of receptors. Variant forms of the LH receptor generated by alternative splicing and lacking transmembrane domain have been isolated. Immunochemical and immunocytochemical studies have been performed. Three different forms of the LH receptor are physiologically expressed: a mature 85kDa transmembrane species, a 68 kDa high mannose containing species corresponding to a precursor which accumulates inside the cells, and truncated 45-48kDa molecular weight species corresponding to the variant messenger RNAs identified during the cloning of the receptor. A novel zonation of the ovary has been described by immunocytochemical studies. Cross hybridization with the LH receptor clone allowed to isolate the related TSH receptor from human thyroid tissue. The human LH and FSH receptor genes have been localized to chromosome 2p21 and the TSH receptor gene to chromosome 14q31. The genes are very large (> 60 kbp) and have introns only within the 5' part encoding the extracellular domain of the receptor. Immunoelectron microscopic studies performed in Leydig cells and in stably transfected L cells have allowed to study intracellular traffic of the LH receptor. The same approach was used to study the transendothelial transfer of hCG in testicular microvasculature.

[LH receptors. A new family of G-protein receptors]. / Récepteur de la LH. Une nouvelle famille de récepteurs couplés à des protéines G.

Monoclonal antibodies have been raised against porcine LH receptor and allowed to clone the corresponding messenger RNA from testicular cells. The structure of the LH receptor have been determined. It shows similarities but also differences to other G protein coupled receptors. In particular a large extracellular domain is specific for that family of receptors. Variants forms of the LH receptor generated by alternative splicing and lacking transmembrane domains have been isolated. Immunochemical and immunocytochemical studies have been performed. Three different forms of the LH receptor are physiologically expressed: a mature 85 kDa transmembrane species, a 68 kDa high mannose containing species corresponding to a precursor which accumulate inside the cells, and truncated 45-48 kDa molecular weight species corresponding to the variant messenger RNAs identified during the cloning of the receptor. A novel zonation of the ovary has been described by immunocytochemical studies. Cross hybridisation with the LH receptor clone allowed to isolate the related human TSH receptor from thyroïds. The human LH and FSH receptor genes have been localized to chromosome 2p21 and the TSH receptor gene to chromosome 14q31. The genes are very large and have introns only within their 5' part corresponding to the extracellular domain of the receptor.

Pathways of internalization of the hCG/LH receptor: immunoelectron microscopic studies in Leydig cells and transfected L-cells.

Monoclonal anti-receptor antibodies were used to study the cellular traffic of the hCG/LH receptor by immunoelectron microscopy. The LHR38 antibody was shown to bind to the extracellular domain of the receptor but not to interfere with hormone binding, adenylate cyclase activation or with the rate of internalization of the receptor. Pig Leydig cells and a permanent L-cell line expressing the LH receptor were used for the study. Incubation with LHR38-gold complexes showed the LH receptors to be randomly distributed over the cell surface including the clathrin coated pits. The LH receptors were internalized via a route including coated pits, coated vesicles and multivesicular bodies to lysosomes. This route is different from that observed for beta-adrenergic, muscarinic, and yeast mating factor receptors and considered previously as possibly general for G-protein-coupled receptors. The use of [125I]LHR38 allowed precise measurement of the rate of internalization, showing the existence of a constitutive pathway which was increased 11-fold by hormone administration. Double labeling experiments suggested that the hormone (hCG-Au15nm) and the receptor (labeled with LHR38-Au5nm) have similar routes of endocytosis, both of them being degraded in lysosomes. Studies of the reappearance of LHR38-Au5nm on the surface of the cells and the use of monensin indicated that only a very small proportion of the receptor molecules were recycled to the cell surface. The distribution and the intracellular pathways of LH receptors are very similar in Leydig cells and transfected L-cells. This opens the possibility of using the latter to study, by in vitro mutagenesis, the molecular mechanisms involved in the cellular traffic of LH receptors.

[LH and TSH receptors. A new family of G protein-coupled receptors]. / Récepteurs de la LH et de la TSH. Une nouvelle famille de récepteurs couplés à des protéines G.

Monoclonal antibodies have been raised against porcine LH receptor and allowed to clone the corresponding messenger RNA from testicular cells. Cross hybridisation with the LH receptor clone allowed to isolate a clone corresponding to the human TSH receptor from thyroids. The structure of both receptors have been determined. They show similarities but also differences to other G protein coupled receptors. In particular a large extracellular domain is specific of that new family of receptors. Variant forms of the LH receptor lacking transmembrane domains have been isolated. The obtention of monoclonal antibodies against both receptors allowed immunochemical and immunocytochemical studies to be performed. The human LH receptor gene have been localized to chromosome 2p21 and TSH receptor gene to chromosome 14q31. The complete organisation of the human TSH receptor gene has been determined.

Monoclonal antibodies for immunocytochemistry of progesterone receptors (PR) in various laboratory rodents, livestock, humans, and chickens: identification of two epitopes conserved in PR of all these species.

Over 90 mouse monoclonal antibodies have been raised against rabbit and human uterine progesterone receptor (PR). These antibodies, because of their specificity, are powerful tools with which to examine the localization, structure, and function of PR. A selection of 22 well characterized mABs was made to test their ability to give the best immunocytochemical staining of PR in various species. Comparative analysis of the antibodies led to the following conclusions. Li 417 (and, to a lesser extent, Let 126) was the best monoclonal in humans; Let 126 and Mi 60 were the most sensitive monoclonals in guinea pigs, rabbits, and monkeys. In sheep, sows, cows and mares as well as in rats and chickens Let 81 or Let 548 gave the best results (Let 126 was also effective in sows and mares, while Li 169 was also effective in sheep and cows). Two antibodies (Li 169 and Let 548) cross-reacted with PR in all of the species tested, including mammals and birds, and appeared to recognize two highly conserved antigenic sites. Remarkably, these conserved sequences are located in the highly variable N-terminal part of the receptor; they may, thus, be related to the still poorly understood function of this domain of the receptor.

Immunocytochemical staining of progesterone receptor in paraffin sections of human breast cancers.

Monoclonal antibodies (mAB) against progesterone receptor (PR) and the peroxidase-antiperoxidase (PAP) method to visualize PR in paraffin sections from 68 human breast cancers were used. Ten mAB, which recognize human PR on frozen sections, were tested. Six could detect PR in paraffin sections, with Li 417, LET 456, and LET 126 giving the best results. LET 126 antibody was used for most further studies. The effects of fixation with picric acid-formaldehyde (PAF), buffered-formalin, or with Bouin solution were investigated; all fixation methods allowed PR immunolabeling, although PAF or buffered formalin usually gave the best results. Positive staining was seen in the nucleus of carcinoma cells. Variations in intensity and extent of immunoreactivity were observed in all sections and among different regions of the same specimen. These were probably related to the heterogeneity of the tumor cell population. Results were compared with the PR content in the respective tumor tissues, determined by steroid-binding assay, and with immunocytochemistry on frozen sections. It was shown that there were correlations between the immunocytochemical staining (positive or negative) and the steroid binding assay (80%) and between the immunocytochemical staining on paraffin sections and on frozen sections (78%). Weaker intensity and fewer number of PR-positive cells were found for paraffin-embedded tumors. Estrogen receptors were also detected on adjacent sections from the same paraffin-embedded tissues by use of monoclonal anti-ER antibodies (ERICA-kit[Abbott Laboratories, Chicago, IL]) and DNase pretreatment. In conclusion, this immunocytochemical method for detection of PR and ER on paraffin sections offers an alternative to frozen tissue. It allows histologic and immunocytochemical studies on the same sample and retrospective studies on stored tissue blocks.

Novel monoclonal antibodies against human uterine progesterone receptor. Mapping of receptor immunogenic domains.

Progesterone receptor was purified in a single step from human uteri using immunoaffinity chromatography with monoclonal antibodies raised against the rabbit receptor. A total of 39 monoclonal antibodies were prepared against the human receptor and characterized. Immunoblot experiments using crude uterine cytosol or purified receptor showed that the antibodies belonged to three groups: they recognized either a single receptor species (apparent molecular mass 110 kDa), two species (110 and 79 kDa) or three species (110, 79 and 65 kDa). The species specificity of the antibodies was very variable; some recognized only the human receptor, others interacted with several mammalian receptors (rabbit, guinea pig and rat), while a single one also cross-reacted with the chick receptor. The epitopes recognized by 15 of the antibodies showing the strongest affinity for the human receptor were mapped using a method recently described [Lorenzo, Jolivet & Milgrom (1988) Eur. J. Biochem. 176, 53-60] which involves immunoprecipitation of C-terminally truncated proteins obtained by transcription and translation of cloned cDNA in vitro. The antibodies recognized five different regions of the receptor, all localized on the N-terminal half of the protein. None of the antibodies interacted with an epitope present in the DNA-binding or steroid-binding regions of the receptor. Comparison of the pattern of receptor species recognized by the antibodies and the localization of their epitopes showed that the 79 and 65 kDa receptor species were derived from the 110 kDa form by deletion of its N-terminal part. The N-terminus of the 79 kDa species was found to lie between amino acids 121 and 208, and that of the 65 kDa species between amino acids 208 and 296.

Immunocytochemical study with monoclonal antibodies to progesterone receptor in human breast tumors.

Mouse hybridomas secreting monoclonal antibodies against rabbit uterine progesterone receptor (PR) have been prepared. Several of these immunoglobulins exhibited high affinity towards human progesterone receptor and two (LET 126 and LET 64) were selected as giving the best immunoperoxidase staining of human progesterone target organs. Using the indirect peroxidase-antiperoxidase method of Sternberger, optimal conditions for demonstrating PR involved brief fixation of frozen sections with formaldehyde-containing fixatives, among them picric acid-paraformaldehyde. This method allowed us to detect the receptor in breast carcinoma epithelial cells, T47D cell line, and uterine endometrium and myometrium. No staining was observed in intestine and muscle. Specific staining for PR was confined to the nucleus, irrespective of the concentration of progesterone in the blood of the patient. In a preliminary study of 27 human breast cancers by the immunocytochemical method, the presence or absence of nuclear staining for PR correlated well with the concentration of cytosolic progesterone receptor determined by a steroid-binding assay on tumor extracts. Differences in the intensity and distribution of staining within a section were observed, suggesting heterogeneity of the PR content of breast cancer cells. In 19 tumors, the immunocytochemical method for PR localization was also used in combination with a slightly modified Abbott ER-ICA staining for estrogen receptor to compare the distribution of both receptors within the same biopsy on adjacent frozen sections. Various combinations of estrogen receptor and PR contents that have been determined by steroid-binding assay have also been detected by the double immunocytochemical assay.

The rabbit progesterone receptor. Evidence for a single steroid-binding subunit and characterization of receptor mRNA.

Monoclonal antibodies were used to study the structure and the biosynthesis of the rabbit progesterone receptor. Proteins in nonfractionated uterine cytosol were submitted to gel electrophoresis in denaturing conditions, transferred onto nitrocellulose, and reacted with monoclonal antireceptor antibodies and 125I-protein A. A single 110,000-dalton protein was observed when precautions were taken during homogenization of the uteri and protease inhibitors used. Smaller forms of receptor (essentially of 79,000 daltons but also of 72,000 and in some experiments of 64,000 daltons) were present when these precautions were not observed and thus probably arose from artifactual proteolysis of receptor. When poly(A)+ RNA from rabbit uterus was translated in a reticulocyte lysate and the radioactive proteins precipitated by the antireceptor monoclonal antibodies, a radioactive protein of 110,000 daltons was also observed. Further evidence that this protein was the product of the translation of progesterone receptor mRNA was obtained by precipitation and immunoaffinity purification with several antireceptor monoclonal and polyclonal antibodies, inhibition of immunoprecipitation by purified receptor and its absence in a receptor-poor tissue (liver). Estrogen treatment is known to increase the concentration of progesterone receptor. RNA translation experiments showed that this effect is due to an increase in the concentration of receptor mRNA. The size of this messenger RNA was studied by sucrose gradient ultracentrifugation, followed by mRNA translation, and specific immunoprecipitation: progesterone receptor mRNA was found by this method to sediment at 20 S.

Monoclonal antibodies to rabbit progesterone receptor: crossreaction with other mammalian progesterone receptors.

A mouse was immunized with purified rabbit uterine cytosolic progesterone receptor (specific activity: 3 nmol of steroid bound per mg of protein). After fusion of its spleen cells with Sp2-OAg myeloma cells, supernatants of 11 hybrid cultures were found to react in both an immunoenzymatic test and a double-immunoprecipitation test with the progesterone receptor. Clones were obtained from the five hybrid cells that gave the strongest response in both tests. Antibodies from cell culture supernatants and ascitic fluids were characterized. Three are of the IgG1 and two of the IgG2a isotype. Their apparent affinity for the progesterone receptor was measured by immunoprecipitation in physiological salt conditions. The equilibrium dissociation constants were between 0.1 and 4 nM. All five monoclonal antibodies crossreacted with the rabbit nuclear receptor, the human cytosolic receptor, and other mammalian (rat, guinea pig) but not avian (chicken) cytosolic progesterone receptors. There was no interaction with the glucocorticoid receptor and corticosteroid binding globulin.

The rabbit uteroglobin gene. Structure and interaction with the progesterone receptor.

The study of the regulation of uteroglobin gene in the rabbit endometrium constitutes a model for analyzing the mechanism of action of progesterone in mammals. The gene has been cloned into lambda phage and sequenced. Comparison of the sequence of the gene with the amino acid sequence of preuteroglobin and the three-dimensional structure of uteroglobin established by crystal x-ray diffraction showed that the 3 exons correspond to different functional domains of the protein and that at least one of the splice junctions does not map at the surface of the protein. S1 mapping allowed us to define the RNA polymerase initiation site. No difference was observed when analyzing premessengers from the endometrium, where the gene is controlled by progesterone and estradiol, and from lung where the gene is constitutively expressed and not controlled by these hormones. In addition, S1 mapping revealed the existence of several minor transcription initiation sites. In the 5' flanking region between positions -33 and -24 there is the sequence AATACAAAAA which may correspond to a Goldberg-Hogness box. Two other A- and T-rich sequences were found further upstream from the gene, one of these preceding by about 30 nucleotides a minor start of transcription. No obvious feature, possibly related to steroid regulation, was observed in the nucleotide sequence. A fragment of the gene containing the "promoter" region (from nucleotide +10 to nucleotide -394) was preferentially retained on nitrocellulose filters after incubation with purified rabbit uterine receptor. A competitive binding assay was used to compare the affinity for the receptor of various DNA fragments. Labeled "promoter" region DNA was incubated with receptor and various concentrations of nonlabeled competing DNA, and the nitrocellulose-bound radioactivity was measured. This method showed the existence of several high affinity binding sites in the 5' part of the gene and in adjacent regions. However, no high affinity binding sites were observed in the 3' part of the gene. Also, within the "promoter" region there were at least two high affinity binding sites for the receptor.

Antibodies to rabbit progesterone receptor: crossreaction with human receptor.

Progesterone receptor from rabbit uterine cytosol was purified to a specific activity of approximately 2 nmol of bound hormone per mg of protein. A goat was immunized with this preparation and, after two injections of 0.7-0.8 nmol, yielded antireceptor antibodies. The antiserum reacted with both cytosolic and nuclear rabbit progesterone receptor and also with progesterone receptor from other rabbit tissues (vagina and pituitary). A crossreaction was observed with progesterone receptors from other mammalian, especially human, tissues (cytosolic receptor from rat and guinea pig uterus, cytosolic receptor from human breast cancer, and nuclear receptor from human endometrium). On the contrary, there was no interaction with a nonmammalian receptor (chicken oviduct progesterone receptor). The antibodies did not crossreact with other rabbit steroid receptors (uterine estradiol receptor and liver glucocorticoid receptor) or with nonreceptor progesterone-binding proteins (transcortin from plasma and uteroglobin from uterine fluid).

Local effect of the blastocyst on estrogen and progesterone receptors in the rat endometrium.

Nuclear receptors for both estradiol and progesterone were present in twofold higher concentrations in implantation sites than in nonimplantation regions of the endometrium of 6-day pregnant rats. Decidualization in the absence of an embryo was not accompanied by a similar increase in the concentration of nuclear receptors. Moreover, this difference in receptor distribution between the implantation and nonimplantation areas persisted when a major part of the maternal supply of sex steroids was suppressed by ovariectomy on day 5 of pregnancy. These results support the hypothesis that steroids originating from the embryo affect the endometrial implantation site.