Functional heterogeneity among cell types in the normal pituitary gland and in human and rat pituitary tumors.

Hormone secretion by the anterior pituitary gland is under control of hypothalamic regulatory factorsjhormones (see chapter I.l) and peripheral hormones. Apart from the direct effects of these hormones on anterior pituitary hormone secretion several fine- regulatory mechanisms may play a role in determining the ultimate response of hormone secretion by the anterior pituitary to the hypothalamic or peripheral feedback hormones. These fine- regulatory mechanisms include paracrine effects between anterior pituitary cell types (see chapter I. 1. 2. 1) and functional heterogeneity within anterior pituitary cell types (see chapter I .1. 2. 2) . Whether these fine- regulatory mechanisms may also occur within human pituitary adenomas is not known (see chapter I.2.3). It has been suggested that differences in the hypothalamic input, due to a different intrapituitary localization of cell types, may be a basis for normal rat mammotrope heterogeneity. In addition it is not known whether a similar "mechanism" may cause functional heterogeneity as has been demonstrated within the normal rat somatotrope population (see chapter I. 1. 2. 2) . One of the aims of this thesis was to investigate whether normal rat mammotropes and somatotropes are functionally heterogeneous with respect to basal hormone release and responsiveness to secretagogues when cultured in the absence of hypothalamic influence, in order to provide additional evidence for the above hypothesis. In chapter III the results of experiments are described in which rat mammotrope and somatotrope subpopulations, separated according to differences in their density, were cultured during 7 days in the absence of hypothalamic hormones. Subsequently, these subpopulations were tested for their responses to the hypothalamic hormones which normally regulate the secretion of PRL and GH. As has been mentioned in chapter I. 2 functional heterogeneity of cells (cell types) within experimental rat pituitary tumors and within human pituitary tumors has not been studied extensively yet. The second aim of this thesis was to investigate whether functional heterogeneity exists among cells of pituitary adenomas. We have used for our studies two types of pituitary adenomas, an experimental prolactin-secreting rat pituitary tumor and human GHsecreting pituitary adenomas. In chapters IV, V and VI the results of experiments concerning intratumor heterogeneity of an experimental rat pituitary tumor {chapter IV) and of GHsecreting human pituitary adenomas {chapters v and VI) are described. In chapter VII the results of a study concerning heterogeneity between GH- secreting pituitary adenomas from acromegalic patients are presente

The pathophysiological consequences of somatostatin receptor internalization and resistance

Somatostatin receptors expressed on tumor cells form the rationale for somatostatin analog treatment of patients with somatostatin receptor-positive neuroendocrine tumors. Nevertheless, although somatostatin analogs effectively control hormonal hypersecretion by GH-secreting pituitary adenomas, islet cell tumors, and carcinoid tumors, significant differences are observed among patients with respect to the efficacy of treatment. This may be related to a differential expression of somatostatin receptor subtypes among tumors. In addition, the property of somatostatin receptor subtypes to undergo agonist-induced internalization has important consequences for visualizing, as well as for therapy, of receptor-positive tumors using radioisotope- or chemotherapeutic-compound-coupled somatostatin analogs. This review covers the pathophysiological role of somatostatin receptor subtypes in determining the efficacy of treatment of patients with somatostatin receptor-positive tumors using somatostatin analogs, as well as the preclinical and clinical consequences of agonist-induced receptor internalization for somatostatin receptor-targeted radio- and chemotherapy. Herein, the development and potential role of novel somatostatin analogs is discussed

Role of somatostatin receptors in normal and tumoral pituitary corticotropic cells

Normal and tumoral pituitary corticotropic cells express sst2and sst5, of which sst5is the predominantly expressed receptor subtype. Somatostatin (SS) inhibits pituitary adrenocorticotropin hormone (ACTH) secretion in vitro, but the sensitivity to SS is strongly regulated by glucocorticoids. In pathological conditions of a low endogenous cortisol level, i.e. in patients with adrenal insufficiency and in patients with Nelson's syndrome, SS and sst2-preferring SS analogs (SSA), such as octreotide, are able to lower circulating ACTH and cortisol levels. On the other hand, sst2-preferring SSA seem not effective in lowering ACTH and cortisol levels in patients with untreated Cushing's disease (CD), in which circulating cortisol levels are high. This is likely due to the downregulation of sst2receptors by glucocorticoids. sst5receptor expression is more resistant to the inhibitory effect of glucocorticoids. In recent years, novel sst subtype-selective and universal SSA have been developed. In particular, SSA with a high sst5-binding affinity are potent inhibitors of ACTH secretion by pituitary corticotropic adenoma cells. This knowledge has initiated clinical trials evaluating the efficacy of these novel SSA in patients with CD, with the aim to lower circulating ACTH and cortisol levels by targeting multiple ssts on the corticotropic adenoma cells. In this minireview, the effects of SS in the regulation of normal and tumoral ACTH secretion, the role of sst subtypes involved herein, as well as the potentials of novel SSA in the treatment of patients with recurrent or persisting CD are discussed

Medical treatment of Cushing's syndrome: Adrenal-blocking drugs and ketaconazole

Cushing's syndrome is associated with serious morbidity and increased mortality. Irrespective of its cause, i.e. a pituitary adenoma, ectopic ACTH production or an adrenal neoplasia, Cushing's syndrome is primarily treated surgically. However, when surgery is unsuccessful or contraindicated, medical therapy is needed to treat hypercortisolism. The spectrum of available drugs includes adrenal-blocking agents, neuromodulatory drugs and glucocorticoid receptor antagonists. Adrenal blocking drugs suppress adrenal cortisol production via inhibition of steroidogenic enzymes. Ketoconazole and metyrapone are most frequently used for this purpose, but chronic treatment with these drugs can be limited by side effects like hepatotoxicity (ketoconazole) and increased androgen and mineralocorticoid production (metyrapone). Etomidate can be used to rapidly reverse cortisol excess in patients with acute complications of (severe) hypercortisolism like psychosis. In Cushing's disease, combination therapy with drugs that target the corticotropic adenoma, i.e. the universal somatostatin analogue pasireotide and/or the dopamine agonist cabergoline, and low-dose ketoconazole seems a rational approach to achieve biochemical control

The effect of clozapine on prolactin secretion at the level of the lactotroph

Abstract Clozapine is an antipsychotic drug which is unusual in that it has no dopamine receptor-blocking activity. Previous studies gave conflicting results whether administration of clozapine induces hyperprolactinemia. In the present study it was shown that a wide concentration range of clozapine does not interfere with dopamine-mediated inhibition of prolactin (PRL) secretion by normal cultured rat pituitary cells. This in contrast to other neuroleptics, like haloperidol and trifluoperazine. Clozapine does also not antagonize norepinephrine-mediated inhibition of PRL secretion. Clozapine exerts at micromolar concentrations a direct inhibitory action on PRL release by cultured normal rat pituitary cells. In cultured rat pituitary tumor cells, these high concentrations of clozapine directly inhibit PRL release as well as the DNA content of the cells, suggesting a direct antimitotic action. In this model clozapine was about 5–10 times less potent than trifluoperazine. Clozapine and trifluoperazine exert an additive inhibitory action both on PRL release and on the DNA content of the pituitary tumor cells. It is concluded that clozapine does not interfere at the pituitary level with dopamine-mediated inhibition of PRL release. At micromolar concentrations clozapine may act on lactotrophs as a calmodulin-inhibitor. These observations suggest that the transient PRL-releasing effects which have been observed in both animal and human studies after clozapine administration are mediated via supra-pituitary actions of the drug

Percoll density gradient centrifugation of rat pituitary tumor cells

Abstract Tumor cells prepared from PRL-secreting rat pituitary 7315b tumors of increasing weight were separated on continuous Percoll density gradients, according to differences in their density. Whether the cell subpopulations obtained by density gradient separation showed differences in protein content per cell, PRL production per cell, growth rates and responsiveness to the somatostatin analog SMS 201–995 in vitro was investigated. In addition, we studied PRL release by individual 7315b tumor cells, using the reverse hemolytic plaque assay (RHPA). The tumor cells from tumors of increasing weight were recovered within a narrow density range (1.060–1.070 g/ml) and showed a normal distribution profile. There were no differences between the subpopulations with respect to the parameters mentioned above. Moreover, no differences were found with respect to these parameters between tumor cells derived from tumors of increasing weight. In agreement with the above data we found no evidence for subtypes of adenoma cells being preferentially responsive to SMS 201–995, using the RHPA. Conclusions: (1) the transplantable PRL-secreting rat pituitary tumor 7315b consists of a functionally homogeneous cell population; (2) growth of this tumor in vivo does not lead to the induction of functionally heterogeneous cell subpopulations within this tumor; (3) the escape of this tumor from the tumor growth-inhibitory effect of SMS 201–995, which has previously been demonstrated in vivo, may not have been the result of clonal selection of somatostatin-unresponsive cells