Regulation of adenohypophyseal pyroglutamyl aminopeptidase II activity by thyrotropin-releasing hormone and phorbol esters

Food-restricted and dehydrated-induced anorexic rats present differential TRH expression in anterior and caudal PVN. Role of type 2 deiodinase and pyroglutamyl aminopeptidase II

TRH synthesized in hypothalamic paraventricular nucleus (PVN) regulates thyroid axis function and is also implicated in anorexigenic effects. Under energy deficit, animals present decreased PVN TRH expression and release, low TSH levels, and increased appetite. Dehydration-induced anorexia (DIA) model allows insight into underlying mechanisms of feeding regulation. Animals drinking a 2.5% NaCl solution for 7 d present body weight reduction; despite their negative energy balance, they avoid food and have increased PVN TRH expression and TSH serum levels. These findings support an inhibiting role of PVN TRH in feeding control. We compared TRH expression by in situ hybridization in PVN subdivisions of 7-d dehydrated male rats to those of a pair-fed group (forced food-restricted) with similar metabolic changes than DIA, but motivated to eat, and to controls. We measured peripheral deiodinase activities, and expression and activity of medial basal hypothalamic type 2 deiodinase and pyroglutamyl-aminopeptidase II, to understand their regulating role in PVN TRH changes between food restriction and anorexia. TRH mRNA levels increased in anterior (aPVN) and medial-caudal subdivisions in DIA rats, whereas it decreased in medial PVN in both experimental groups. We confirmed the nonhypophysiotropic nature of aPVN TRHergic cells by injecting ip fluorogold tracer. Findings support a subspecialization of TRHergic hypophysiotrophic cells that responded differently between anorexic and food-restricted animals; also, that aPVN TRH participates in food intake regulation. Increased type 2 deiodinase activity seemed responsible for low medial PVN TRH synthesis, whereas increased medial basal hypothalamic pyroglutamyl-aminopeptidase II activity in DIA rats might counteract their high TRH release.

Tanycyte pyroglutamyl peptidase II contributes to regulation of the hypothalamic-pituitary-thyroid axis through glial-axonal associations in the median eminence

Pyroglutamyl peptidase II (PPII), a highly specific membrane-bound metallopeptidase that inactivates TRH in the extracellular space, is tightly regulated by thyroid hormone in cells of the anterior pituitary. Whether PPII has any role in the region where axons containing hypophysiotropic TRH terminate, the median eminence, is unknown. For this purpose, we analyzed the cellular localization and regulation of PPII mRNA in the mediobasal hypothalamus in adult, male rats. PPII mRNA was localized in cells lining the floor and infralateral walls of the third ventricle and coexpressed with vimentin, establishing these cells as tanycytes. PPII mRNA extended in a linear fashion from the tanycyte cell bodies in the base of the third ventricle to its cytoplasmic and end-feet processes in the external zone of the median eminence in close apposition to pro-TRH-containing axon terminals. Compared with vehicle-treated, euthyroid controls, animals made thyrotoxic by the i.p. administration of 10 microg L-T(4) daily for 1-3 d, showed dramatically increased accumulation of silver grains in the mediobasal hypothalamus and an approximately 80% increase in enzymatic activity. PPII inhibition in mediobasal hypothalamic explants increased TRH secretion, whereas i.p. injection of a specific PPII inhibitor increased cold stress- and TRH-induced TSH levels in plasma. We propose that an increase in circulating thyroid hormone up-regulates PPII activity in tanycytes and enhances degradation of extracellular TRH in the median eminence through glial-axonal associations, contributing to the feedback regulation of thyroid hormone on anterior pituitary TSH secretion.

Anterior pituitary pyroglutamyl peptidase II activity controls TRH-induced prolactin release

Ecto-peptidases modulate the action of peptides in the extracellular space. The relationship between peptide receptor and ecto-peptidase localization, and the physiological role of peptidases is poorly understood. Current evidence suggests that pyroglutamyl peptidase II (PPII) inactivates neuronally released thyrotropin-releasing hormone (TRH). The impact of PPII localization in the anterior pituitary on the endocrine activities of TRH is unknown. We have studied whether PPII influences TRH signaling in anterior pituitary cells in primary culture. In situ hybridization (ISH) experiments showed that PPII mRNA was expressed only in 5-6% of cells. ISH for PPII mRNA combined with immunocytochemistry for prolactin, beta-thyrotropin, or growth hormone, showed that 66% of PPII mRNA expressing cells are lactotrophs, 34% somatotrophs while none are thyrotrophs. PPII activity was reduced using a specific phosphorothioate antisense oligodeoxynucleotide or inhibitors. Compared with mock or scrambled oligodeoxynucleotide-treated controls, knock-down of PPII expression by antisense targeting increased TRH-induced release of prolactin, but not of thyrotropin. Similar data were obtained with either a transition-state or a tight binding inhibitor. These results demonstrate that PPII expression in lactotrophs coincides with its ability to control prolactin release. It may play a specialized role in TRH signaling in the anterior pituitary. Anterior pituitary ecto-peptidases may fulfill unique functions associated with their restricted cell-specific expression.

Thyrotropin-releasing hormone-induced down-regulation of pyroglutamyl aminopeptidase II activity involves L-type calcium channels and cam kinase activities in cultures of adenohypophyseal cells

Released thyrotropin-releasing hormone (TRH) is inactivated by a narrow specificity ectopeptidase, pyroglutamyl aminopeptidase II (PPII), present in brain and lactotrophs. Various hypothalamic/paracrine factors, including TRH, slowly (in hours) regulate the activity of PPII on the surface of adenohypophyseal cells. TRH-induced down-regulation was mimicked by protein kinase C (PKC) activation but was not affected by inhibition of PKC. Adenylate cyclase activation can also down-regulate PPII. The purpose of this study was to identify elements of the transduction pathway used by TRH to regulate PPII activity. In primary cultures of female adenohypophyseal cells, activation of the stimulatory G protein or adenylate cyclase produced an effect additive to that of TRH; inhibition of protein kinase A activity did not interfere with TRH action. However, regulation of PPII activity by TRH was inhibited by a phospholipase C beta inhibitor or chelation of intracellular calcium. L-type calcium channels (LCC) agonists mimicked TRH action and their effect was not additive with that of TRH. Antagonists of LCC channels and inhibitors of calmodulin or calcium/calmodulin-dependent protein kinase blocked TRH action. Therefore, TRH-induced calcium entry through L-type calcium channels and the activity of calcium/calmodulin-dependent protein kinase are required for TRH effect on PPII activity in primary cultures of adenohypophyseal cells. This pathway may coregulate PPII and prolactin biosynthesis in response to TRH.