Immunohistochemical expression of protein kinase C type III in human pituitary adenomas

Matrix metalloproteinase-9 is differentially expressed in nonfunctioning invasive and noninvasive pituitary adenomas and increases invasion in human pituitary adenoma cell line

The complete resection of pituitary adenomas (PAs) is unlikely when there is an extensive local dural invasion and given that the molecular mechanisms remain primarily unknown. DNA microarray analysis was performed to identify differentially expressed genes between nonfunctioning invasive and noninvasive PAs. Gene clustering revealed a robust eightfold increase in matrix metalloproteinase (MMP)-9 expression in surgically resected human invasive PAs and in the (nonfunctioning) HP75 human pituitary tumor-derived cell line treated with phorbol-12-myristate-13-acetate; these results were confirmed by real-time polymerase chain reaction, gelatin zymography, reverse transcriptase-polymerase chain reaction, Western blot, immunohistochemistry, and Northern blot analyses. The activation of protein kinase C (PKC) increased both MMP-9 activity and expression, which were blocked by some PKC inhibitors (Gö6976, bisindolylmaleimide, and Rottlerin), PKC-alpha, and PKC-delta small interfering (si)RNAs but not by hispidin (PKC-beta inhibitor). In a transmembrane invasion assay, phorbol-12-myristate-13-acetate (100 nmol/L) increased the number of invaded HP75 cells, a process that was attenuated by PKC inhibitors, MMP-9 antibody, PKC-alpha siRNA, or PKC-delta siRNA. These results demonstrate that MMP-9 and PKC-alpha or PKC-delta may provide putative therapeutic targets for the control of PA dural invasion.

Molecular defects in the pathogenesis of pituitary tumours

The majority of pituitary adenomas are trophically stable and change relatively little in size over many years. A comparatively small proportion behave more aggressively and come to clinical attention through inappropriate hormone secretion or adverse effects on surrounding structures. True malignant behaviour with metastatic spread is very atypical. Pituitary adenomas that come to surgery are predominantly monoclonal in origin and roughly half are aneuploid, indicating either ongoing genetic instability or transition through a period of genetic instability at some time during their development. Few are associated with the classical mechanisms of tumour formation but it is generally believed that the majority harbour quantitative if not qualitative differences in molecular composition compared to the normal pituitary. Despite their prevalence and the ready availability of biopsy material, at the present time, the precise molecular pathogenesis of the majority of pituitary adenomas remains unclear. This review summarizes current thinking.

Tamoxifen-induced growth arrest and apoptosis in pituitary tumor cells in vitro via a protein kinase C-independent pathway

Protein kinase C (PKC), a kinase family involved in cell signal transduction, is overexpressed in most pituitary adenoma cells. We studied the effect of tamoxifen, an estrogen receptor antagonist and also a protein kinase inhibitor, on pituitary tumor cell proliferation and the induction of apoptosis; and we compared its effects with those of another PKC inhibitor, staurosporine. Tamoxifen induced growth arrest and apoptosis in a mouse pituitary adenoma cell line, AtT20, and in low-passage human primary pituitary tumor cell cultures. Staurosporine also inhibited pituitary tumor cell growth. PKC activity in AtT20 cells was inhibited by staurosporine and by prolonged treatment with phorbol myristate acetate, which down-regulates PKC activity, but not by tamoxifen, at the dosages used to induce apoptosis. Our findings suggest that tamoxifen induces apoptosis in AtT20 cells independent of a classical PKC isozyme pathway.

Pituitary expression of protein kinase C isotypes during early development

Protein kinase C (PKC) is a critical regulator of signal transduction and cell function in many tissues, including pituitary. Although PKC influences pituitary hormone secretion in adults, its role in determining characteristic perinatal patterns of hormone secretion and synthesis is not known, and the expression of major PKC isotypes in perinatal pituitary is poorly defined. We therefore determined the developmental, cell-specific expression of the major PKC isotypes, using Western analysis and double label immunohistochemistry, in pituitaries of perinatal and mature rats. Expression of specific PKC isotypes was strikingly age-dependent. Pituitary expression of PKC alpha was particularly high in neonates and declined significantly with age, with levels in adult rats approximately half those of neonates as assessed by Western analysis. Similarly, immunohistochemistry indicated that PKC alpha was less abundant in adult than in neonatal pituitaries; the most intensely staining cells of both age groups were identified as somatotrophs and gonadotrophs. In contrast to PKC alpha, pituitary expression of PKC epsilon increased approximately two-fold with advancing age as assessed by Western analysis; this age-dependent pattern was confirmed by immunohistochemistry. Perinatal pituitaries expressed PKC epsilon in some somatotrophs and in all gonadotrophs, whereas PKC epsilon expression was limited to gonadotrophs in the mature pituitary. Pituitary expression of PKC betaII, delta, and zeta did not differ with age, and PKC gamma was not detected in pituitaries of any age group. These results indicate that expression of PKC isotypes within the pituitary is developmentally regulated in a cell-specific and isotype-specific manner, and are consistent with the concept that PKC contributes to the regulation of pituitary function during early development.

Hormone secretion by cell culture of human GH-PRL secreting pituitary adenomas: effects of bromocriptine

Dopamine agonists effectively reduce the secretion of prolactin (PRL) in the great majority of prolactinomas and reduce the bulk of the adenomas, as well as have partial therapeutic effect on some patients with acromegaly. The inhibitory effect of bromocriptine (BC), a dopamine agonist, on growth hormone (GH) and PRL secretion of dispersed cells from the pituitary adenomas of 16 cases of acromegaly, which secret GH and PRL simultaneously, were evaluated in vitro. The significant inhibitory effects of BC on PRL secretion were found in 12 cases. It was also found that PRL secretion was strongly inhibited when GH was suppressed; on the contrary, when GH secretion was not suppressed, the production of PRL was not or weakly inhibited. The exact mechanism of the effects is unclear so far. It is necessary to investigate, at molecular level, the etiology of GH-PRL adenomas and its response to therapeutic agents.

Relationship between protein kinase C and adenylyl cyclase activity in the regulation of growth hormone secretion by human pituitary somatotrophinomas

OBJECTIVE

To determine the potential role of protein kinase C (PKC) and its relationship to adenylyl cyclase activity in controlling growth hormone (GH) secretion by human pituitary somatotrophinomas.

METHODS

Twenty-eight somatotrophinomas were placed into cell culture, and the in vitro effects of the PKC activator 12-O-tetradecanoylphorbol 13-acetate (TPA) and the PKC inhibitor staurosporine on basal and GH-releasing hormone (GHRH)-stimulated GH secretion were examined. In addition, the influence of chronic exposure of cultured somatotrophinoma cells to TPA on the rate of inositol 1,4,5-trisphosphate production was determined. Each tumor was assessed for the presence of gsp oncogenes, and thus constitutive adenylyl cyclase activity, by direct sequence analysis of polymerase chain reaction-generated deoxyribonucleic acid. GH secretory responses of tumors with and without these oncogenes were compared.

RESULTS

TPA consistently stimulated GH secretion by cultured somatotrophinoma cells. There was no difference in response between somatotrophinomas with and without gsp oncogenes, and the effects did not correlate with the variable stimulation exerted by GHRH. Tumors in which GHRH had no significant effect nevertheless responded to TPA. In combination, TPA and GHRH exerted additive stimulation. TPA treatment of cultured somatotrophinoma cells eventually resulted in suppression of inositol 1,4,5-trisphosphate production, probably reflecting down-regulation of membrane phosphatidylinositol hydrolysis, a second messenger system that also generates the endogenous PKC activator diacylglycerol. GHRH had no effect on phosphatidylinositol hydrolysis. In contrast to the effects of TPA, the PKC inhibitor staurosporine tended to reduce GH secretion, although this effect was not observed in all tumors examined. As with TPA, the effects of staurosporine did not correlate with presence or absence of gsp oncogenes. Furthermore, staurosporine did not reduce the stimulatory effects exerted by GHRH on GH secretion.

CONCLUSION

These results demonstrate a role for the phosphatidylinositol-PKC second messenger cascade in controlling GH secretion by human pituitary somatotrophinomas. The results also show that the system operates relatively independent of intracellular adenylyl cyclase and, thus, protein kinase A.

Inhibition of cellular growth and induction of apoptosis in pituitary adenoma cell lines by the protein kinase C inhibitor hypericin: potential therapeutic application

Protein kinase C (PKC) is an enzyme involved in the regulation of cellular growth, proliferation, and differentiation in a number of tissues including the anterior pituitary, in which it is also believed to play a role in hormone secretion. Protein kinase C activity and expression have been found to be greater in adenomatous pituitary cells than in normal human and rat pituitary cells and higher in invasive pituitary tumor cells than in noninvasive ones. Inhibition of PKC activity has been shown in a variety of tumor cells to inhibit growth in a dose-related fashion. The purpose of the current study was to determine whether hypericin, a potent inhibitor of PKC activity that may be administered clinically, alters the growth and proliferation in established pituitary adenoma lines and to determine if inhibition of PKC activity induces apoptosis, as reported in some other tumor cell types. Two established pituitary adenoma cell lines, AtT-20 and GH4C1, were treated with hypericin in tissue culture for defined periods following passage. Inhibition of growth was found to be dose dependent in all three cell lines in low micromolar concentrations of hypericin, as determined by viable cell counts, methylthiotetrazole assay, and [3H]thymidine uptake studies. Concentrations of hypericin as low as 100 nM also induced apoptosis in these established lines, whereas treatment of normal human fibroblasts with a concentration of 10 microM failed to induce apoptosis. The potential use of hypericin in the therapy of pituitary adenomas warrants additional in vitro investigations with the aim of later moving toward therapeutic trials in selected patients in whom surgical or medical therapy has failed.

Protein kinase C and growth regulation of pituitary adenomas

The present study was undertaken to explore the role of the Protein Kinase C (PKC) signal transduction system in growth regulation of pituitary adenomas. Primary tumor cultures were plated from fresh surgical tumor specimens. The PKC inhibitors Staurosporine and Tamoxifen were added at varying dosages to the cell cultures. Measurements of cell proliferation were performed by [3H]-thymidine uptake and the [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H tetrazolium bromide] (MTT) assay. After a 48 h treatment period, both [3H]-thymidine uptake and absorbance on the MTT assay decreased in a dose-related manner in both the staurosporine and tamoxifen-treated cultures (IC50 of 10 nM and 30 microM respectively). Direct measurement of PKC activity using an in vitro assay revealed very high activity (range of 1465-5708 pmol/min/mg protein; within the range previously published for malignant glioma specimens) in 12 frozen specimens of pituitary adenomas (9 nonfunctional adenomas, 1 prolactinoma, 1 gonadotrophin-secreting and 1 corticotroph-secreting adenoma). In contrast, PKC activity measured in normal adenohypophysis was comparatively very low. These data indicate that pituitary adenoma cells display high PKC activity and are sensitive to growth inhibition by PKC inhibitors. These data suggest a role for the PKC system in regulating pituitary tumor growth, which may have implications for future therapy of these tumors.

Involvement of protein kinase C in growth regulation of human meningioma cells

In order to investigate the possible role of protein kinase C (PKC)-mediated signal pathways in growth regulation of meningiomas, we examined the effect of two PKC-activating phorbol esters, 12-O-tetradecanoyl-13-phorbol acetate (TPA) and phorbol 12, 13-dibutyrate (PDBu), and PKC inhibitor, staurosporine, on cell proliferation using low-passage human meningioma cells in culture. TPA (0.1 to 100 ng/ml) caused a dose-dependent stimulation of cell proliferation in six of eight meningioma cultures. At optimal concentrations of TPA, the cell growth ranged from 113% to 251% versus control. In contrast, PDBu (0.1 to 100 ng/ml) caused a significant inhibition of cell proliferation in three of five meningioma cultures. At optimal concentrations of PDBu, the cell growth ranged from 52% to 79% of control. Staurosporine exhibited a stimulation of cell proliferation (135% to 178%) in three of four meningioma cultures studied at a concentration of 10(-10) to 10(-9)M, although a tendency of growth inhibition was observed at a lower concentration. A time course of DNA synthesis in response to TPA, assessed by [3H] thymidine incorporation studies, revealed a time- and dose-dependent stimulation and/or inhibition which further depended on the serum concentration of the growth medium used. The overall results indicate that PKC-mediated signal pathways are closely involved in growth regulation of human meningioma cells. The results further suggest that the signalling processes consist of complex mechanisms which await to be elucidated.