Charles C. Capen

Adrenal Gland: Structure, Function, and Mechanisms of Toxicity:

The adrenal gland is one of the most common endocrine organs affected by chemically induced lesions. In the adrenal cortex, lesions are more frequent in the zona fasciculata and reticularis than in the zona glomerulosa. The adrenal cortex produces steroid hormones with a 17-carbon nucleus following a series of hydroxylation reactions that occur in the mitochondria and endoplasmic reticulum. Toxic agents for the adrenal cortex include short-chain aliphatic compounds, lipidosis inducers, amphiphilic compounds, natural and synthetic steroids, and chemicals that affect hydroxylation. Morphologic evaluation of cortical lesions provides insight into the sites of inhibition of steroidogenesis. The adrenal cortex response to injury is varied. Degeneration (vacuolar and granular), necrosis, and hemorrhage are common findings of acute injury. In contrast, chronic reparative processes are typically atrophy, fibrosis, and nodular hyperplasia. Chemically induced proliferative lesions are uncommon in the adrenal cortex. The adrenal medulla contains chromaffin cells (that produce epinephrine, norepinephrine, chromogranin, and neuropeptides) and ganglion cells. Proliferative lesions of the medulla are common in the rat and include diffuse or nodular hyperplasia and benign and malignant pheochromocytoma. Mechanisms of chromaffin cell proliferation in rats include excess growth hormone or prolactin, stimulation of cholinergic nerves, and diet-induced hypercalcemia. There often are species specifi city and age dependence in the development of chemically induced adrenal lesions that should be considered when interpreting toxicity data.

Calcium-Regulating Hormones and Diseases of Abnormal Mineral (Calcium, Phosphorus, Magnesium) Metabolism

Publisher Summary Calcium (Ca) is a mineral that plays a central role in maintaining the homeostasis of vertebrate animals, including muscle contraction, blood coagulation, enzyme activity, neural excitability, hormone secretion, and cell adhesion in addition to being an essential structural component of the skeleton. Calcium is also involved in the pathogenesis of metabolic diseases that disrupt the normal regulation of Ca balance and may result in hypercalcemia or hypocalcemia. Phosphate in the mammalian body is present predominantly (90%) as hydroxyapatite in the mineralized matrix of bone, with most of the remaining 10% occurring intracellularly in soft tissues. Phosphate is the major intracellular anion existing in organic or inorganic forms and plays an integral role in many metabolic processes. Magnesium (Mg) is an essential dietary element for animals. The method of choice for determination of total Mg in biologic materials is atomic absorption spectrophotometry. However, disorders related to magnesium metabolism also are recognized in other animals, including cats, dogs, goats, and horses.

The Effects of Xenobiotics on the Structure and Function of Thyroid Follicular and C-Cells

The mammalian thyroid gland is composed of 2 distinct endocrine cell populations concerned with the synthesis of 2 different classes of hormones. Follicular cells secrete the metabolically active iodothyronines whereas the C- (parafollicular) cells are concerned with the production of calcitonin, a hormone that influences blood levels of calcium and phosphorus, and bone cell metabolism. The synthesis of metabolic thyroid hormones is different than in other endocrine glands because the final assembly of hormone occurs within the follicular lumen. This extracellular synthesis of thyroid hormones is made possible by thyroglobulin, a glycoprotein synthesized by follicular cells. The secretion of thyroid hormones under the influence of pituitary thyrotrophin (TSH) from stores in the luminal colloid is initiated by elongation of microvilli and formation of pseudopods. FD&C Red No. 3 is a tetraiodinated derivative of fluorescein which in lifetime studies increases the incidence of thyroid follicular cell adenomas in male Sprague-Dawley rats. The striking changes in circulating levels of thyroid hormones and morphologic evidence of follicular cell stimulation are the result of alterations in the peripheral metabolism of thyroxine. An inhibition by FD&C Red No. 3 of 5′-deiodinase in the liver and kidney would explain the lower serum triiodothyronine (T3) levels. The pituitary, sensing the lowered circulating levels of T3, increased the secretion of thyroid stimulating hormone which resulted in the morphologic evidence of follicular cell stimulation in the long-term studies. Other xenobiotics increase the incidence of thyroid tumors in rodents by a direct effect on the thyroid gland to disrupt 1 of 3 or more possible steps in the biosynthesis of thyroid hormones. Physiologic perturbations alone, such as iodine deficiency or partial thyroidectomy, can disrupt thyroid hormone economy in rodents and, if sustained, increase the development of thyroid tumors. The wide variety of drugs, chemicals, and physiologic perturbations which increase thyroid tumor development appear to act through a secondary (indirect) mechanism to promote tumor development by causing a long-standing hypersecretion of thyroid stimulating hormone. Nodular and/or diffuse hyperplasia of C-cells occurs with advancing age in many strains of laboratory rats and in response to long-term hypercalcemia in certain animal species and human beings. Focal or diffuse hyperplasia of ten precedes the development of C-cell neoplasms. Radiation and the feeding of diets high in vitamin D resulting in hypercalcemia have been reported to increase the incidence of C-cell tumors in rats.

Neoplastic Lesions of Questionable Significance to Humans

Many compounds giving a positive result in animal carcinogenicity studies through mechanisms involving secondary carcinogenesis pose little or no risk to humans. This article provides an overview of current understanding, with particular reference to renal tumors in male rats with α2μ-globulin nephropathy, urinary bladder neoplasia in rodents, mesovarian leiomyomas induced in rats by β2-receptor stimulants, carcinoid tumors in the rodent stomach induced by prolonged suppression of acid secretion, thyroid follicular cell tumors in rodents, canine mammary neoplasia due to administration of progestagens, rodent mammary neoplasia induced by estrogens, uterine endometrial carcinomas of rats induced by dopamine agonists, Leydig cell tumors in the testis of rats, and ovarian tubulostromal adenomas in mice. A positive result on a rodent carcinogenicity study should not automatically preclude further development of a compound; future progress in this field should increase the accuracy of the rodent carcinogenicity study as a tool in human safety assessment.

Loss of p53 Promotes Anaplasia and Local Invasion in ret/PTC1-Induced Thyroid Carcinomas

Papillary thyroid carcinomas in humans are associated with the ret/PTC oncogene and, following loss of p53 function, may progress to anaplastic carcinomas. Mice with thyroid-targeted expression of ret/PTC1 developed papillary thyroid carcinomas that were minimally invasive and did not metastasize. These mice were crossed with p53-/- mice to investigate whether loss of p53 would promote anaplasia and metastasis of ret/PTC1-induced thyroid tumors. The majority of p53-/- mice died or were euthanized by 17 weeks of age due to the development of thymic lymphomas, soft tissue sarcomas, and testicular teratomas. All ret/PTC1 mice developed thyroid carcinomas, but tumors in p53-/- mice were more anaplastic, larger in diameter, more invasive, and had a higher mitotic index than tumors in p53+/+ and p53+/- mice. Thyroid tumors did not metastasize in any of the experimental p53+/+ and p53+/- mice </=28 weeks of age or p53-/- mice </= 17 weeks of age; however, an older (170-day-old) male p53-/- mouse used to maintain the colony developed anaplastic thyroid carcinoma with liver metastases. These findings demonstrate that the lack of functional p53 in ret/PTC1 mice promotes anaplasia and invasiveness of thyroid carcinomas.

Hypercalcemia Associated with an Adenocarcinoma Derived from the Apocrine Glands of the Anal Sac

Clinical, gross, and light microscopic findings are described for 36 dogs, 33 females and three males, with adenocarcinomas arising from the apocrine glands of the anal sac. All tumors had light microscopic features of malignancy and 22 of 23 metastasized to iliac and lumbar lymph nodes. Nine dogs had disseminated metastases, but bone metastases were found in only one dog. Differentiated neoplasms formed secretory acini and tubules lined by tall columnar or cuboidal epithelium. Most neoplasms were histologically bimorphic, with glandular areas and solid nests. Parathyroid glands were atrophic. Hypercalcemia (mean = 16.1 mg/dl) was present in 20 of 22 dogs (90%) and hypophosphatemia (mean = 3.2 mg/dl) in 12 of 17 (71%). Remission of hypercalcemia by tumor ablation and recurrence of hypercalcemia with tumor regrowth suggested that the tumor produced a substance that caused hypercalcemia. This unique clinicopathologic syndrome is characterized by hypercalcemia in old. predominantly female, dogs with an adenocarcinoma arising from the apocrine glands of the anal sac.

The toxicological evaluation of the mycotoxins T-2 and T-2 tetraol using normal human fibroblasts in vitro

Abstract Normal human fibroblasts in vitro were used as a system to examine the cellular effects of potentially toxic compounds. In addition to tests for viability, indices of toxicity were included which would demonstrate functional and structural alterations associated with specific subcellular components. Fibroblast cultures derived from human neonatal foreskin were treated with various doses of the mycotoxins T-2 and T-2 tetraol. Methods for demonstrating viability showed T-2 to be 10-fold more toxic than its hydroxylated derivative, T-2 tetraol. Both compounds induced a dose-dependent reduction in protein and scheduled DNA synthesis, as well as an induction of unscheduled DNA synthesis (DNA repair). However, no qualitative alterations in the activity of cytoplasmic, lysosomal or membrane-associated enzymes were observed with T-2 or T-2 tetraol-treated cells. Transmission electron microscopy revealed structural alterations predominantly associated with the nucleus and the endoplasmic reticulum with associated ribosomes. Multiple biochemical parameters with ultrastructural and cytotoxicity studies were therefore effective in demonstrating the mechanisms as well as the degree of toxicity induced by these chemical compounds.

Chronic cardiotoxicity of doxorubicin (adriamycin) in the rat: morphologic and biochemical investigations.

Abstract A cardiotoxic response similar to that occurring with chronic doxorubicin (Adriamycin, ADR) administration in man and rabbits has been produced in the new Zealand Black (NZB) rat. ADR was given by the iv or ip route in groups of rats at 2 mg/kg/week for 10 weeks, 1 mg/kg/week for 10 weeks, and 1 mg/kg on Monday, Wednesday, and Friday (MWF) with a week of rest and then repeating to a total of 10 injections. Rats on the weekly ADR schedule developed gross (ascites, pleural effusion, cardiomegaly) and microscopic (focal myocytic degeneration, interstitial and myocytic vacuolization, loss and disorganization of sarcomeres) evidence of cardiotoxicity during the 150-day period of observation. A dosage of 2 mg/kg/week iv in rats produced high mortality (45%) and decreased mean body weight gains compared to controls. This dosage given sc resulted in significant elevations in serum creatine phosphokinase and lactic dehydrogenase. Preliminary evidence suggests that cardiotoxicity may be diminished by ADR administration on a MWF rest-repeat schedule. The sensitivity of this strain of rat to the cardiotoxic effects of ADR provides a model system in which to evaluate simultaneously the chemotherapeutic effects of ADR on an experimental osteosarcoma and new therapeutic measures to ameliorate cardiotoxicity.

Ultrastructural and functional alterations of the rat thyroid gland produced by polychlorinated biphenyls compared with iodide excess and deficiency, and thyrotropin and thyroxine administration

SummaryHistologic and ultrastructural lesions in thyroid follicular cells of Osborne-Mendel rats produced by feeding diets containing 5, 50, and 500 ppm polychlorinated biphenyls (PCB) for 4 weeks were compared with alterations produced by low (Remington diet) and high (1%KI) iodide diets, chronic thyroxine administration, and a single injection of thyrotropin.Exposure to PCB resulted in a dose-dependent hypertrophy and hyperplasia of follicular cells with an abnormal accumulation of large colloid droplets and irregular lysosomes. There was limited evidence of colloid droplet-lysosome interaction necessary for the secretion of thyroid hormones. Microvilli on luminal surfaces were decreased, abnormally shaped, and short. Serum thyroxine and triiodothyronine were decreased significantly after feeding PCB.Follicular cells in rats fed the iodine-deficient diet underwent hypertrophy and hyperplasia with increased development of rough endoplasmic reticulum. They had increased numbers of round lysosomes and colloid droplets. There were numerous uniformly long microvilli and occasional pseudopodia were present engulfing colloid. Serum thyroxine was significantly decreased but triiodothyronine levels were increased. The iodide-excess diet resulted in hypertrophy and hyperplasia of follicular cells which contained numerous colloid droplets and abnormally shaped lysosomes with little evidence of fusion. Numerous long microvilli extended into the lumen. Many follicular cells contained unique intracytoplasmic microfollicles lined by uniform microvilli. Serum thyroxine was significantly reduced.Follicular cells of rats chronically administered thyroxine underwent involution and had only a few short microvilli, scattered round lysosomes, and poorly developed organelles. Serum thyroxine and triiodothyronine were elevated significantly. Thyrotropin (TSH) produced hypertrophy of follicular cells by 30 min post-injection with increased large colloid droplets and small round lysosomes. Large pseudopod-like projections of follicular cells were involved in endocytosis of colloid. There was evidence of fusion of lysosomal membranes with colloid droplets. Circulating thyroid hormone levels were significantly elevated from 30 min to 8 h after administration of thyrotropin but normal by 24 h.The ultrastructural alterations produced by PCB in the luminal plasma membrane and structure of microvilli were distinct from changes observed with acute stimulation by TSH, long-term stimulation by iodide excess or deficient diets, and chronic suppression by thyroxine. The changes in lysosomal structure and the accumulation of colloid droplets with little evidence of fusion in follicular cells of rats fed PCB closely resembled the findings in rats fed the iodide-excess diet. The principal effect of PCB on follicular cells appeared to be on limiting colloid droplet-lysosome interaction thereby inhibiting the proteolysis of thyroglobulin necessary for the release of thyroid hormones.

Human parathyroid hormone‐related protein in ovarian small cell carcinoma. An immunohistochemical study

Background. Small cell carcinomas (SCC) are the most common ovarian tumors associated with hypercalcemia. Parathyroid hormone‐related protein (PTHrp) is the most frequent cause of hypercalcemia of malignancy.