The link between secretion and mitosis in the endocrine glands

Subchronic and Chronic Toxicity/Carcinogenicity Feeding Studies with Lactitol in Rats

The toxicity and possible carcinogenicity of lactitol, a lactose-derived reduced calorie sugar substitute, were examined in Wistar Cpb:WU rats. In a first 13-week study, groups of 10 weanling rats of each sex were fed diets containing 0, 5, 10, or 20% lactitol, or 25% lactose. In a second 13-week study, groups of 20 rats of each sex received diets with 0, 10, or 20% lactitol, or 25% lactose starting from an age of 3 months. In a lifetime toxicity/cancerogenicity study with in utero exposure, groups of 50 rats of each sex consumed diets with 0, 2, 5, or 10% lactitol, or 20% lactose for 130 weeks. Satellite groups of 10 rats/group received the same diet but were sacrificed after 53 weeks. Subchronic and chronic ingestion of lactitol and lactose was generally well tolerated and an effect on mortality rate was not seen. Body weight gains were slightly decreased in a dose-related manner in all experiments. Dose-related cecal enlargement was observed consistently with lactitol and lactose. Examinations for hematological parameters and urine composition did not reveal any treatment-related abnormalities. The clinical chemistry examinations revealed increased plasma alkaline phosphatase (AP) levels in rats fed 5 or 10% lactitol, or 20% lactose. Slight (microscopic) cytoplasmic alterations in the staining pattern of hepatocytes were observed in the first but not the second 13-week study in a number of male rats of the 10 and 20% lactitol groups. In the chronic study, a statistically significant increase in bile duct hyperplasia was noted in female rats fed 5 or 10% lactitol, or 20% lactose. Measurement of femur calcium content in terminated female rats revealed a dose-related, yet statistically nonsignificant decrease in the lactitol groups and lowest levels in the 20% lactose group. The histopathological examination of rats treated with lactitol and lactose for their lifetime showed an increased incidence of (1) pelvic nephrocalcinosis (5% lactitol females, and 10% lactitol and 20% lactose males and females), (2) adrenomedullary proliferative changes (10% lactitol and 20% lactose males), and (3) hyperplastic and neoplastic changes of Leydig cells (10% lactitol and 20% lactose males). Results from other studies suggest that nephrocalcinosis and adrenal medullary proliferative disease represent sequelae of a chronic, treatment-related hyperabsorption of calcium which does not occur in man. The mechanism of the testicular effect is not yet identified. However, lactitol gave completely negative results in standard tests for mutagenicity and clastogenicity, and no testicular effects or other neoplastic response was observed in lactitol-fed mice. Since Leydig cell tumors occur in humans at an extremely low rate despite the ingestion of substantial amounts of lactose with ordinary meals, the observation made in rats lacks apparent relevance for man.

Cell kinetics in parathyroid adenomas: evidence for decline in rates of cell birth and tumour growth, assuming clonal origin

OBJECTIVE

We aimed to provide a cell kinetic explanation for the demonstrated lack of disease progression in most patients with mild, asymptomatic primary hyperparathyroidism.

DESIGN

We compared cell birth rates, estimated at the time of adenoma excision, with the lowest birth rates needed to grow tumours of the observed size.

PATIENTS

Sixty-three patients with primary hyperparathyroidism due to a single chief cell adenoma who had normal renal function were followed up for long enough to demonstrate cure after surgical excision.

MEASUREMENTS

Fresh adenoma tissue was incubated with tritiated thymidine. The proportion of cells synthesizing DNA was determined directly by radioautography in 18 cases, and indirectly from the regression of label index on rate of DNA synthesis in 45 cases. The birth rate of new cells was calculated assuming the duration of S phase to be 12 hours. The number of cells in each adenoma was estimated both from parenchymal weight and from total DNA content, and the minimum birth rate needed to produce this number of cells from a single cell, beginning in utero, was calculated on an exponential model.

RESULTS

The mean observed birth rate of new cells (mean (SD)) was 17.3 (11.1)%/year, and the minimum needed birth rate was 42.8 (21.6)%/year, significantly, (P less than 0.001) higher than the observed birth rate.

CONCLUSIONS

The rate of mitosis had fallen substantially during the life span of most parathyroid adenomas. To account for this, we propose that the mutation implied by a clonal origin increases the secretory setpoint. Because proliferation, as well as hormone secretion, is influenced by calcium in parathyroid cells, the expected result would be rapid initial growth, slowing down as tumour size reached an asymptotic value corresponding to the total rate of hormone secretion needed to raise the plasma calcium to the new setpoint.

An update on the cellular and molecular biology of brain tumors

Patients with tumors of the central nervous system (CNS) remain difficult to treat despite recent advances in surgical, chemotherapeutic and radiotherapeutic techniques. A better understanding of the molecular and cellular biology of neoplasia is providing neuroscientists with a framework on which to devise novel therapies for these patients. It thus becomes imperative that neurologists and neurosurgeons be aware of these advances in basic science that may eventually have a positive impact on patient management. This paper reviews our present knowledge of the process of CNS oncogenesis and the roles that chemicals, viruses, oncogenes, growth inhibitor genes, and growth factors play in the process.

Comparative aspects of contraceptive steroids: effects observed in rats

Current regulatory guidelines for testing contraceptive drugs in long-term rodent studies have established dosages based on multiples of the proposed human usage level. These multiples in rodents are 1-2, 10, and 50. The estrogen/progestogen ratio for most human contraceptive drugs ranges from 1/5 to 1/80. One of the biological endpoints in arriving at the human estrogen/progestogen ratio is the development of an endometrial decidualization response. The ratio necessary to achieve a similar uterine response in the rat is 1:10,000 to 1:20,000. Thus, dosages in the rodent, when based only on a multiple of the proposed human usage level, result in a highly estrogenic combination with estrogen being completely dominant. Continuously elevated estrogen in the rat is toxic to dopaminergic neurons in the hypothalamus which secrete prolactin inhibiting factor (PIF). Hyperplasia of pituitary lactotrophs occurs from both the direct stimulatory effect of estrogen and the uninhibited secretory activity of lactotrophs related to depressed PIF secretions. Prolactinomas result. Increased levels of prolactin lead to mammary gland stimulation and tumor development. Dosage levels for future rodent studies of contraceptive drugs should be based on pharmacokinetics, endocrine profiles, and biological endpoints rather than on multiples of the human usage level.

Inhibitory effect of calcium channel blockers on proliferation of human glioma cells in vitro

The effects of 2 specific calcium channel blockers, verapamil and nimodipine, on the proliferation of human glioma tumour cells were investigated in vitro. Tumour tissues for primary cell cultures were obtained bioptically from 3 patients with the histopathological diagnosis of glioblastoma. The [3H]-thymidine incorporation into glioma tumour cells DNA was used as a sensitive index of the cell proliferation. It was found that verapamil (10(-4)-10(-5) M) and nimodipine (10(-4)-10(-6) M) significantly inhibited the [3H]-thymidine uptake in a dose-related manner. The inhibitory effect of both calcium channel antagonists was reversed by simultaneous addition of calcium chloride (5 x 10(-3) M). These results indicate that verapamil and nimodipine may exert an antiproliferative effect on glioma cells growth acting through a blockade of specific voltage-dependent calcium channels.

Suprasellar craniopharyngioma associated with hyperprolactinemia, pituitary lactotroph hyperplasia, and microprolactinoma. Case report

A case with the coexistence of a microprolactinoma and suprasellar craniopharyngioma lactotroph hyperplasia is presented. Loss of inhibitory hypothalamic dopaminergic input may have played a role in hyperprolactinemia and the proliferation of pituitary lactotrophs. This is believed to be the first published case of suprasellar craniopharyngioma with this association.

Influence of cell cycle phases on the electrical activity and hormone release in a transformed line of anterior pituitary cells

Electrophysiological experiments have shown that about 50% of cultured GH3 cells (tumoral cell line, from the anterior pituitary gland) are inexcitable i.e. they do not display action potentials either spontaneously or when depolarized by a current pulse. We report here this inexcitability may be related to cellular kinetics. Thus we have studied the relationship between the various phases of the cell cycle, the electrophysiological properties of GH3/B6 cells and spontaneous or induced Prolactin and Growth Hormone (GH) release rates. Asynchronous populations of viable cells were stained with Hoechst 33 342 DNA fluorescent dye, and sorted using a flow cytometer into G1 and S phases. After selection intracellular potentials were recorded using a single glass micro-electrode; the basal or TRH stimulated rates of PRL and GH secretions were determined by RIA. Electrical properties of the cells i.e. resting potentials, input membrane resistance and excitability, reached a maximum for cells in G2+M phases. Only cells in G2+M displayed action potentials and TRH increased their secretion by 5 times for GH and by 6 times for PRL. In G1 and S phases the cells were electrically inactive and secretion rates remained at their basal levels. These findings demonstrate that the mechanism of stimulus secretion coupling is dependent upon the phases of the cell cycle.

The pharmacological mediation of epithelial cell proliferation

In the past, it has been necessary for pharmacological intervention of epithelial proliferation to mostly be limited to non-specifically arresting or killing actively proliferating cells. As our understanding of the mechanisms involved in mediating the processes of growth and differentiation increases, we can hope to see the development of a new pharmacology, in which proliferation of individual systems may be regulated in a less drastic manner.

Bromocriptine inhibits incorporation of [3H]thymidine into rat pituitary tumor cells

The effects of bromocriptine on GH3 pituitary tumor cell [3H]thymidine incorporation were studied. Cells were grown in the presence of bromocriptine, then exposed to a short-term pulse of [3H]thymidine in serum-free medium containing deoxycytidine (10 microM) to prevent deoxythymidine triphosphate (dTTP) pooling. After 48 h exposure to bromocriptine, basal prolactin (PRL)-secretion during 45 min was inhibited by 50% by 10 microM bromocriptine and thyroid releasing hormone-induced PRL stimulation was suppressed. Incorporation of radiolabelled thymidine into acid-precipitable DNA increased progressively from 15 to 60 min and was abolished by simultaneous incubation with excess unlabelled thymidine (100 microM). Bromocriptine (10 microM) inhibited incorporation of 5-50 microM [3H]thymidine, but this was not reversed by simultaneous incubation with metoclopramide (10 microM). Aminopterin, an inhibitor of endogenous de novo DNA synthesis, stimulated [3H]thymidine incorporation twofold and this increased DNA salvage pathway activity was also blocked by bromocriptine. As incorporation of [3H]thymidine into acid-soluble cell nucleotides was also inhibited by bromocriptine, the data suggest that in these cells the drug inhibits thymidine kinase activity, a salvage pathway of DNA synthesis.

Enzootic and epizootic adrenal medullary proliferative disease of rats: influence of dietary factors which affect calcium absorption

Adrenal medullary hyperplasia and neoplasia occur both enzootically and epizootically in untreated laboratory rats. The lesions are typically chromaffin-negative and are found incidentally in animals that have died from unrelated causes or have been killed at the end of long-term toxicity/carcinogenicity tests. Urinary excretion of catecholamines is not usually increased. Environmental, particularly dietary, factors are seemingly much more important than genetic ones as determinants of the incidence of proliferative lesions. Recent observations of enhancement of adrenal medullary proliferative disease in rats by the feeding, in high dietary concentration, of certain polyols (sorbitol, mannitol, xylitol, lactitol), or of lactose, suggested that increased absorption of calcium from the gastrointestinal tract may be a risk factor. This evidence is reported and discussed in the light of other evidence linking disturbed calcium homeostasis with adrenal medullary function in the rat. In man, adrenal medullary proliferative disease is relatively rare and there is no evidence of any relation between the hypercalcaemia associated with hyperparathyroidism and increased risk of phaeochromocytoma. Adrenal medullary proliferative disease in rats is usually seen against a background of multiple endocrine neoplasia, with the pituitary gland, the pancreatic islets and the thyroid C-cells being most commonly affected in addition to the adrenal medulla. A parallel between this situation and Sipple's disease in humans has previously been suggested. We now stress the possible importance of three factors as determinants of enzootic and epizootic adrenal medullary proliferative disease in rats: excessive food intake, excessive dietary levels of calcium and phosphate and excessive intake of other food components, such as vitamin D and poorly absorbable carbohydrates, which predispose to increased calcium absorption.

Sympathetic denervation impairs agonist-stimulated phosphatidylinositol metabolism in rat parotid glands

Substance P, muscarinic and alpha-adrenoceptor agonists stimulated the incorporation of [3H]inositol into phosphatidylinositol in rat parotid gland slices. Surgical denervation of the sympathetic input to the rat parotid gland by superior cervical ganglionectomy produced marked reductions in these responses. The stimulated incorporation of radiolabelled precursors into phosphatidylinositol is a measure of its resynthesis after receptor-mediated breakdown of inositol phospholipids. We therefore examined the enzymic site of the lesion induced by sympathetic denervation using parotid gland slices labelled with either [3H]inositol or [32P]phosphate and stimulated with substance P. Receptor-activated phospholipase C attack upon [3H]inositol phospholipids was assayed by measuring the formation of [3H]inositol 1-phosphate in the presence of 10 mM-Li+ to inhibit further breakdown. It was not affected by denervation. Substance P elicited a rapid breakdown of phosphatidylinositol 4,5-bisphosphate and this response was reduced in the denervated gland. The second step in stimulated phosphatidylinositol turnover, phosphorylation of diacylglycerol to phosphatidate was not affected by denervation. Sympathetic denervation appears to induce a specific enzymic lesion in the parotid gland that impairs receptor-stimulated resynthesis of phosphatidylinositol from phosphatidate. This change in membrane lipid metabolism may be related to a number of the effects of sympathetic denervation, such as agonist supersensitivity, reduced gland cell proliferation and induction of new surface receptors.

The antimitotic effect of bromocriptine on human fibroblasts

The effect of bromocriptine on tritiated thymidine incorporation into nuclei of human fibroblasts has been investigated. A dose--dependent decrease of the labelling index has been found. This observation suggests that the antimitotic action of bromocriptine is a more general phenomenon not restricted to the adenohypophysial cells.

Damage to hypothalamic dopaminergic neurons is associated with development of prolactin-secreting pituitary tumors

Old female rats with spontaneous prolactin-secreting pituitary tumors (prolactinomas) and young females with prolactinomas produced by prolonged estrogen treatment had damaged tuberoinfundibular dopaminergic neurons. Since these neurons inhibit the function of pituitary prolactin-secreting cells, their destruction may lead to development of prolactinomas.

Effects of cyclic AMP on DNA replication and protein biosynthesis in fetal rat islets of Langerhans maintained in tissue culture

The regulatory role of cyclic AMP (cAMP) in the growth and insulin production of the islet organ in vitro has been investigated. The effects of dibutyryl cyclic AMP (dbcAMP), theophylline, and 3-isobutyl-1-methylxanthine (IBMX) on DNA replication and on the biosynthesis of RNA and insulin in fetal rat islets of Langerhans maintained in tissue culture have been studied. Raising the glucose concentration from 2.7 mM to 16.7 mM caused a two-fold increase in DNA replication. Both dbcAMP and theophylline markedly inhibited the DNA replication at all glucose concentrations studied. Low concentrations of IBMX stimulated DNA synthesis. However, at higher concentrations of this drug, known to considerably increase the islet cAMP levels, a marked inhibition of islet DNA replication was observed. Both (pro)insulin and total protein biosynthesis were stimulated by glucose, whereas dbcAMP stimulated only the (pro)insulin biosynthesis. Since glucose is known to raise islet intracellular levels of cAMP, which is known to be an inhibitor of cellular proliferation, the observed glucose stimulation of both islet-cell DNA replication and insulin production appeared conflicting. It is suggested that this dual effect of glucose may depend on a stimulation of proliferation in a limited pool of islet cells which may not exhibit an increase in cAMP.