Applications of hormones in the metabolic regulation of growth and lactation in ruminants

Exogenous natural and synthetic estrogenic and androgenic steroid hormones are used commercially to stimulate metabolic processes associated with increased rate and efficiency of body growth in ruminants. However, mechanisms of action of steroid hormone-induced effects on metabolism are relatively unknown. Application of peptide hormones to muscle growth, fat deposition, and lactation has lagged because of lack of sufficient quantities of the hormones. However, with recombinant DNA technology synthesis of large quantities of peptide hormones is now feasible. Most efforts have focused on growth hormone (GH), growth hormone-releasing factor (GRF), and prolactin (PRL) effects on lactation. For example, administration of GH or GRF stimulates yields of milk, milk fat, protein, and lactose as much as 41% in cattle. The mechanism of GH action probably involves somatomedin C acting at extramammary sites and (or) directly at the mammary cell. PRL is lactogenic but has no significant effect on established lactation in cattle. Daily exposure of cattle to 16 h light and 8 h of darkness stimulates milk yield and body growth and reduces fat accretion in the carcass, but the hormonal signals responsible for these photoperiod-induced responses are unknown. Photoperiod manipulations are relatively easy to apply to ruminants, but development of suitable delivery systems for animals will greatly enhance application of peptide hormones to further studies of metabolism as well as commercial livestock production systems.     

INSECT HORMONES IN DEVELOPMENT

Advances in studies of prothoracicotropic hormone (ecdysiotropin), ecdysteroids and juvenile hormones in the past decade are considered:
1. Until recently there has been little progress with prothoracicotropic hormone. The development of a sensitive bioassay for the hormone promises to produce rapid advances.
2. Current methods of hormone analysis are described, with detection limits. The application of these methods in studies of hormones at different stages and in different tissues of insects have revealed a far greater complexity in hormone titres than was predicted from classical studies.
3. Very few studies employ chemical characterization of hormones and some assays do not distinguish biologically inactive metabolites of the hormones from the active hormones. Many studies have thus failed to reveal the numerous rapid fluctuations in hormone titre necessary for insect development.
4. While ecdysteroids act, via a receptor, on specific chromosome sites, the cellular mode of action of juvenile hormone in larval development is still unknown. Recent evidence suggests that juvenile hormone acts prior to the time at which its effects are realized by ecdysteroids.
5. Insect hormones produce dramatic changes in gene activity and their co-ordinate control of specific protein synthesis has been the basis for a number of 'model systems' of gene control in higher eukaryotes.     

Regulation of low-density lipoprotein receptors in cultured bovine adrenocortical cells

Bovine adrenocortical cells in monolayer culture produce cortisol and respond to corticotropin (ACTH) by an increase in cortisol secretion. Several lines of evidence are indicative that much of the cholesterol that serves as precursor for steroid hormone biosynthesis by these cells is derived from low-density lipoprotein (LDL) cholesterol that is taken up endocytotically by means of specific receptors localized in bovine adrenocortical plasma membranes. ACTH stimulated this process concomitant with an increase in steroid production. In the absence of LDL, ACTH had no effect on steroid biosynthesis. ACTH action in bovine adrenocortical cells resulted in an increase in the number of LDL receptor sites in the membrane fractions, whereas the dissociation constant for LDL binding was not changed. Chloroquine and NH₄Cl, considered to be inhibitors of lysosomal degradative activity, caused an increase in the number of [¹²⁵I]iodoLDL binding sites in the plasma membrane but the effect of ACTH was still apparent in the presence of these agents. These results are suggestive that the lifetime of the LDL receptor is increased when lysosomal activity is inhibited. When aminoglutethimide was added to block cholesterol side-chain cleavage activity and inhibit steroid production, the number of [¹²⁵I]iodoLDL binding sites in the membrane fractions prepared from bovine adrenocortical cells cultured in the presence of ACTH was reduced to 50% of that in cells maintained in aminoglutethimide-free medium. However, under these conditions the number of binding sites was still significantly greater than in cells maintained in the absence of ACTH. The effects of aminoglutethimide on uptake and degradation of [¹²⁵I]iodoLDL were similar to the effects on the number of [¹²⁵I]iodoLDL binding sites. Based on these results, we conclude that the action of ACTH to stimulate LDL metabolism in bovine adrenocortical cells results from an increase in the number of LDL binding sites in the plasma membranes. This action of ACTH appears to be, at least in part, independent of cholesterol utilization for cortisol biosynthesis. However, the effect of aminoglutethimide is indicative that changes in the intracellular cholesterol concentration might modulate the action of ACTH to increase the number of LDL binding sites and therefore to stimulate LDL degradation.     

Plant hormones: a fungal point of view

Most classical plant hormones are also produced by pathogenic and symbiotic fungi. The way in which these molecules favour the invasion of plant tissues and the development of fungi inside plant tissues is still largely unknown. In this review, we examine the different roles of such hormone production by pathogenic fungi. Converging evidence suggests that these fungal‐derived molecules have potentially two modes of action: (i) they may perturb plant processes, either positively or negatively, to favour invasion and nutrient uptake; and (ii) they may also act as signals for the fungi themselves to engage appropriate developmental and physiological processes adapted to their environment. Indirect evidence suggests that abscisic acid, gibberellic acid and ethylene produced by fungi participate in pathogenicity. There is now evidence that auxin and cytokinins could be positive regulators required for virulence. Further research should establish whether or not fungal‐derived hormones act like other fungal effectors.     

L to D amino acid isomerization in a peptide hormone is a late post-translational event occurring in specialized neurosecretory cells

Modification of the chirality of a single amino acid residue within a peptide chain appears to be novel additional mechanism leading to structural and functional diversification of eukaryotic bioactive peptides. This phenomenon has been studied at the cellular level in a neuroendocrine organ which elaborates a mixture of diastereoisomers of a 72-residue neuropeptide, crustacean hyperglycemic hormone. For the first time, amino acid isomerization has been shown to occur in the perikarya of fully specialized neurosecretory cells, as a late step of the maturation of the hyperglycemic hormone precursor and after propeptide cleavage. The specificity and efficiency of this phenomenon indicates the existence of a new enzyme family involved in the biogenesis of peptide hormones.     

Steroid hormone radioautography in cultured cells: Studies with Shionogi carcinoma 115

A radioautographical technique for the localization of soluble compounds in cultured cells is described. It has been used to investigate the distribution of steroid hormones in target cells, and an example is given with the Shionogi carcinoma SC-115, an androgen-sensitive mammary tumor in mice. Experiments at 37 °C have given direct evidence for the specific binding of [³H]-androstanolone (5α-dihydrotestosterone) in the cytoplasm. This result is based on the labeling after incubation with 0.5 nM radioactive androgen, on the isotope dilution-test showing a limited capacity, and on competition assays with other hormones. Results also show the transfer to and the specific binding of the hormone in the nucleus. No cellular labeling has been observed at 0 °C, even when using 10 nM of high specific activity [³H]androstanolone. The technique appears promising for the study of various aspects of the interactions between hormones and isolated cells.     

Animal cell cultures as a source of hormones

Current methodology now makes it possible to establish in culture a variety of mammalian cells which perform organ-specific functions during serial propagation for periods of months or years. This report describes the results of experiments with animal and human cells that produce growth hormone, adrenocortical steroid hormones, thyrocalcitonin, and parathyroid hormone. Within the next decade it should be possible to use cell culture methods for manufacturing purposes to produce hormones and other valuable cellular products which are difficult to obtain in other ways. At first it may be necessary to use neoplastic cells for this purpose; but evidence is accumulating to suggest that it may eventually be possible to establish in culture normal, functional animal and human cells.     

From the structure of steroid receptors to their assessment by immunocytochemistry in target cells

Immunohistochemical studies with antibodies to steroid hormone receptors provide new insight in the mechanism of action of steroid hormones. Immunologically reactive estrogen and progesterone receptors are found exclusively in cell nuclei of target cells even in the absence of the hormonal ligand. A hormonal treatment inducing receptor transformation and "translocation" to the nucleus does not modify the intracellular distribution of the receptor. This result is in contradiction with most biochemical studies which show a displacement of receptor from the cytosolic fraction to the nuclear fraction after hormone-receptor complex formation. We propose that different affinity levels of the non-transformed and hormone-complexed receptor molecules for nuclear structure produce unequal losses of nuclear receptor during homogenization. A lesser loss appears as an increase in nuclear binding sites or immunologically reactive receptor. The glucocorticosteroid receptor differs from the others in that it shows an increase of nuclear immunoreactive receptor after hormone administration. This result was accepted as evidence for a nuclear translocation in the sense initially proposed for all steroid hormones. Alternatively, one may propose another explanation based on the same experimental artefact as invoked for the estrogen and progesterone cytosol receptors. A higher affinity of the hormone-complexed receptor entails a lesser loss from the nucleus during tissue processing, and consequently an apparent increase in nuclear staining. Such a possibility is currently tested in parallel with the progesterone receptor.     

Thyroid Hormone Receptors in the Developing Rat Brain

It is widely believed that the adult mammalian brain is insensitiveto thyroid hormones unlike the neonatal brain which is criticallydependent on these hormones for the development of normal structureand function. Recent studies have demonstrated the presenceof limited capacity, high affinity, triiodothyronine (T3) bindingnuclear sites in tissues that are considered responsive to thyroidhormones. Furthermore, there is evidence from studies on peripheraltissues that these T3 binding sites act as true receptors ininitiating thyroid hormone action. This report examines whetherthe higher sensitivity of neonatal brain to thyroid hormonesand the purported decline in sensitivity in adulthood are relatedto changes in the concentration and affinity characteristicsof thyroid hormone receptors in rat cerebral nuclei. Analysisof Scatchard plots of in vitro T3 binding data indicate thatcerebral nuclei from adult rats contain T3 specific nuclearbinding sites at a concentration comparable to that presentduring the period when the brain is critically dependent onthe presence of thyroid hormones and exceed that in the liver,a tissue generally considered thyroid sensitive. Neonatal thyroidectomysignificantly increased the number of binding sites. The resultsshow that the apparent unresponsiveness of the cerebral cortexof adult rats to thyroid hormones is not due to the absenceor a low density of T3 nuclear binding sites. The significanceof these results is discussed.     

The honey-bee “dance language” hypothesis and the foundations of biology and behavior

We have carried out a detailed analysis of the region of the hormone, which can be called an “active site”, that is essential for receptor binding and/or agonist activity, on the basis of structure-activity data of peptide hormones published in the literature. We find that one or more aromatic residues, often in a cluster, is present at the active sites of insulin, glucagon, adrenocorticotropin, gastrin, endorphins and angiotensin. Recognition of the functional importance of aromatic residues, combined with sequence comparisons and secondary structural predictions, enable us to identif active sites of nerve growth factor, somatostatin, calcitonin, parathyroid hormone and luteinizing hormone releasing hormone. Aromatic residues also appear to be essential for the function of nonpeptide hormones that act at the plasma membrane, and opiates. The apparently ubiquitous presence of aromatic groups at the active site of peptide hormones may facilitate understanding of the mechanism of action of hormones and provide insights into the design of hormone analogues.