Internalization of 125I-human choriogonadotropin in bovine luteal slices. A biochemical study

Nuclear volume and chromatin conformation of small and large bovine luteal cells: effect of gonadotropins and prostaglandins and dependence on luteal phase

Change in nuclear volume and chromatin conformation are generally considered to reflect altered gene expression in eukaryotic cells. The present studies were undertaken to investigate whether these nuclear parameters of luteal cells can be altered by hormone treatment in vitro or change during the estrous cycle. The nuclear volume of small luteal cells was significantly lower than that of large luteal cells during the cycle and pregnancy. The nuclear volumes of small and large luteal cells from pregnancy did not change during incubation without any hormone or with 10 nM prostaglandin (PG)F 2 alpha. However, incubation with 1 nM human chorionic gonadotropin (hCG) or 10 nM PGE1 resulted in a significant increase of nuclear volume of small luteal cells by 4 h and that of large luteal cells by 6 h. Small luteal cells were more responsive to hCG than large luteal cells. The nuclear volumes of small and large luteal cells also significantly increased from early to mid luteal phase with no further change in late luteal phase. hCG and PGE1, as well as PGF2 alpha, treatment resulted in a change of chromatin conformation of small and large luteal cells. Dibutyryl cyclic AMP (10 mM) mimicked the hormones by increasing nuclear volumes and changing the chromatin conformation of small and large luteal cells. Chromatin conformation of small and large luteal cells also changed from early to mid luteal phase and mid to late luteal phase. In conclusion, in vitro, hCG and PGs can regulate nuclear volume and/or chromatin conformation of small as well as large bovine luteal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of luteal cell nucleus in the expression of gonadotropin action

Gonadotropin receptors are not only present in cell membranes, but also in nuclei of bovine and human luteal cells. hCG/hLH can directly regulate several nuclear functions. To further investigate the role of luteal cell nucleus in the expression of gonadotropin action, the effect of enucleation of luteal cells on gonadotropin receptors and gonadotropin response was studied. Luteal cytoplasts were prepared by colchicine treatment of purified whole luteal cells followed by centrifugation at 37 C in a Percoll gradient. The cytoplasts were 85 to 90% pure with a recovery of about 57%. Cytoplasts were viable as determined by trypan blue exclusion (87%) and metabolically competent as determined by 3H-leucine incorporation into proteins. On the day of preparation, the viability and metabolic competency of cytoplasts were similar to control cells, i.e. untreated and colchicine treated whole luteal cells. In addition, cytoplasts and control cells showed a similar decline in number and viability during storage at 4 C. While control cells continue to be metabolically competent, cytoplasts showed a dramatic decline by 48 h of storage at 4 C. Neither the cytoplasts nor control cells degraded 125I-hCG. The kinetics of 125I-hCG association and dissociation, specificity and affinity of binding to cytoplasts were similar to control cells. However, the number of available gonadotropin receptors in cytoplasts was significantly lower than in control cells. Cytoplasts contained lower progesterone levels and more importantly, they could not be stimulated by 10 nM hCG or 10 mM dibutyryl cyclic AMP to produce more progesterone. Controls cells, on the other hand, contained higher progesterone levels and responded to hCG and dibutyryl cyclic AMP stimulation. In summary, removal of nuclei from luteal cells results in a partial loss of gonadotropin receptors and complete loss of steroidogenic response to hCG and dibutyryl cyclic AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Corpus luteum: animal models of possible relevance to reproductive toxicology

The presence of a normally functioning corpus luteum is an essential requirement for the maintenance of gestation in mammals. The chief function of the corpus luteum in all species is to synthesize the steroid hormone progesterone that is necessary for implantation and for the subsequent development of the fetoplacental unit. Activation and maintenance of luteal function involve pituitary, placental, and ovarian hormones. Perturbation in the secretion and/or action of any of these luteotropins by exogenous compounds can profoundly affect the steroidogenic capacity of the corpus luteum. Abnormal luteal function causes failure of implantation and embryonic wastage. Both in vitro and in vivo models for assessing luteal function are available; while the former are more convenient, the model of choice for toxicological studies is one in which in vitro findings can be easily confirmed in whole animals in vivo. This article is concerned primarily with the techniques used in basic research that might be useful for the evaluation of luteal function in reproductive toxicology.

Rat ovarian subcellular compartmentalization of luteinizing hormone

To determine if human luteinizing hormone (hLH) follows the same subcellular route in the pseudo-pregnant rat ovary as human chorionic gonadotropin (hCG), the distribution of immunoreactive and bioactive hLH within cytosol, lysosomes, and a combined plasma membrane/prelysosomal vesicle fraction was examined. Immunoreactive levels were determined using a specific radioimmunoassay and bioactive levels were determined in an in vitro Leydig cell bioassay. Low cytosolic hLH levels were apparent the first few hours after hLH administration and may reflect at least some contamination by serum and interstitial fluid. Plasma membrane/prelysosomal vesicles attained the highest hLH concentration 1 h after hLH injection, after which hLH levels declined rapidly until barely detectable levels were observed at 18 h. The hormone in this fraction was receptor bound and exhibited the highest bioactivity of the three fractions examined. Lysosomal hLH concentrations were highest at 12 after hLH injection and were maintained at high levels through 18 h. Substantial amounts of lysosomal hLH appeared to be receptor bound but this hormone was not bioactive. In situ degradation of the oligosaccharides may have occurred while lysosomal hLH was receptor bound. Lysosomal degradation is the major pathway for hLH inactivation as has been described for hCG. However, hLH may be degraded within lysosomes more quickly than hCG. The differential subcellular distribution of immunoreactive and bioactive hLH provides strong support for hLH internalization and degradation within ovarian luteal cells.

Divergent metabolism of human chorionic gonadotropin subunits within rat ovarian lysosomes

Pseudopregnant rats were injected with either native human chorionic gonadotropin or with (125I)-human chorionic gonadotropin and their ovarian homogenates fractionated on Percoll density gradients. The levels of alpha and beta subunits within subcellular fractions were measured using radioimmunoassays specific for each subunit. Radioactivity measurements of fractions obtained from rats injected with (125I)-human chorionic gonadotropin were used as a separate index of alpha subunit distribution. The alpha subunit was primarily restricted to a combined plasma membrane/prelysosomal vesicle fraction. Immunoreactive beta subunit was present at high concentrations within both this plasma membrane/prelysosomal vesicle fraction and within lysosomes. The striking difference in alpha and beta subcellular distribution may arise from differential sensitivities to lysosomal enzymes.

Cycle of VIP in the human transformed colonic epithelial cells (HT-29) in culture

The kinetics of VIP processing in different compartments (medium, plasma membrane and intracellular) by HT-29 cells was studied using direct biochemical methods and a homogeneous monoiodinated VIP. The compartmental radioactivity was characterized by HPLC fractionation and specific receptor binding. VIP once bound to the cell surface remains intact and is rapidly and maximally internalized (less than 10 min) at 37 degrees C. Then two different processes occur: (1) release of degradation products 125I and monoiodinated tyrosine in the medium; (2) VIP remains intact in the cells representing 67.2 +/- 4.7% of total radioactivity up to 90 min. The overall processing of VIP is time- and temperature-dependent and maximal internalization of VIP with minimal medium release is observed at 20 degrees C. Our results demonstrate a receptor mediated internalization of VIP and that at least two intracellular pathways may exist in the cycle of VIP. One is associated with a complete degradation of VIP detected in the extracellular medium and is optimal at 37 degrees C. The other results in the presence of intact intracellular VIP and is optimal at 20 degrees C.

A quantitative electron microscope autoradiographic study on [3H]prostaglandin E1 and F2 alpha internalization in bovine luteal slices

Very few silver grains were seen on the luteal cell surface and none intracellularly after incubation for 2 h at 4 degrees C with 10 nM [3H]prostaglandin (PG) E1 or [3H]PGF2 alpha. However, incubation at 22 degrees C or 38 degrees C for 2 h resulted in association of numerous grains on the luteal cell surface as well as in several intracellular organelles. Qualitatively, the grain distribution was similar at 22 degrees C and 38 degrees C, but quantitatively there were fewer grains at 22 degrees C than at 38 degrees C. In addition to luteal cells, the grains were also found on erythrocytes, platelets, fibroblasts and capillary endothelial cells in luteal tissue incubated with [3H]PGE1. Grain association with non-luteal cells was never seen with [3H]PGF2 alpha. Coincubation of [3H]PGE1 or [3H]PGF2 alpha with only corresponding excess unlabeled PGs resulted in complete disappearance of silver grains from all the subcellular organelles. In autoradiographs, grains were seen on plasma membranes, nuclei, lysosomes, mitochondria, rough and smooth endoplasmic reticulum and Golgi. Three-step mask analysis, which effectively corrects for radiation spread, revealed that the grains found on mitochondria and rough endoplasmic reticulum in the case of [3H]PGE1, and those found on mitochondria, rough and smooth endoplasmic reticulum and Golgi in the case of [3H]PGF2 alpha, were due to the radiation spread. For both [3H]PGE1 and [3H]PGF2 alpha the plasma membrane associated grains progressively decreased, while the intracellular organelle associated grains increased with time at 38 degrees C. There were no grains in nuclei at 5 min but they appeared at 10 min and increased until 60 or 120 min. The optimized source density following 2 h of incubation at 38 degrees C with [3H]PGE1 was Golgi greater than plasma membranes greater than lysosomes = nuclei, and for [3H]PGF2 alpha, lysosomes greater than plasma membranes greater than nuclei. In summary, the present studies demonstrate for the first time that exogenously added [3H]PGE1 and [3H]PGF2 alpha internalize in bovine luteal cells in a ligand-specific, time- and temperature-dependent manner. The observations that the internalized [3H]PGE1 does not associate with rough endoplasmic reticulum and that [3H]PGF2 alpha does not associate with rough endoplasmic reticulum and Golgi, even though they contain the binding sites, suggest the presence of mechanism(s) to direct the internalized [3H]PGs to only certain intracellular organelles of luteal cells.

A quantitative electron microscope autoradiographic study on 125I-human choriogonadotropin internalization in bovine luteal slices

Very few silver grains were seen on the cell surface and none intracellularly after incubation for 2 h at 4 degrees C. However, numerous grains were seen in various subcellular organelles when the tissues were incubated for 2 h at 22 degrees or 38 degrees C. The grain distribution was qualitatively similar, but quantitatively, there were fewer grains at 22 degrees than at 38 degrees C. Co-incubation of 125I-hCG with excess unlabelled hCG resulted in the virtual disappearance of silver grains from all the subcellular organelles. Excess unlabelled human luteinizing hormone (but not follicle-stimulating hormone or prolactin) inhibited the appearance of silver grains in luteal tissue. There were no silver grains in bovine liver slices incubated with 125I-hCG. The plasma membrane-associated grains progressively decreased, while intracellular organelle-associated grains increased with time at 38 degrees C. There were no grains in nuclei at 5 min, but they appeared at 10 min and increased until 120 min. After correction for radiation spread by three-step mask analysis, smooth endoplasmic reticulum and mitochondria did not contain any grains. The grain density was the highest in Golgi, followed by lysosomes, rough endoplasmic reticulum, nuclei, and plasma membranes after incubation for 2 h at 38 degrees C. Thus, the electron microscope autoradiography approach confirmed our biochemical data in the preceding paper (Chegini et al., Exp cell res 151 (1984) 466 [5]) on time, temperature dependency and specificity of 125I-hCG internalization, association of internalized hormone with a variety of intracellular organelles, and the highest uptake in Golgi.