Regulation of cellular alpha-MSH and beta-endorphin during stimulated secretion from intermediate pituitary cells: involvement of aspartyl and cysteine proteases in the control of cellular levels of alpha-MSH and beta-endorphin

Differentiation of hypothalamic-like neurons from human pluripotent stem cells

The hypothalamus is the central regulator of systemic energy homeostasis, and its dysfunction can result in extreme body weight alterations. Insights into the complex cellular physiology of this region are critical to the understanding of obesity pathogenesis; however, human hypothalamic cells are largely inaccessible for direct study. Here, we developed a protocol for efficient generation of hypothalamic neurons from human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) obtained from patients with monogenetic forms of obesity. Combined early activation of sonic hedgehog signaling followed by timed NOTCH inhibition in human ESCs/iPSCs resulted in efficient conversion into hypothalamic NKX2.1+ precursors. Application of a NOTCH inhibitor and brain-derived neurotrophic factor (BDNF) further directed the cells into arcuate nucleus hypothalamic-like neurons that express hypothalamic neuron markers proopiomelanocortin (POMC), neuropeptide Y (NPY), agouti-related peptide (AGRP), somatostatin, and dopamine. These hypothalamic-like neurons accounted for over 90% of differentiated cells and exhibited transcriptional profiles defined by a hypothalamic-specific gene expression signature that lacked pituitary markers. Importantly, these cells displayed hypothalamic neuron characteristics, including production and secretion of neuropeptides and increased p-AKT and p-STAT3 in response to insulin and leptin. Our results suggest that these hypothalamic-like neurons have potential for further investigation of the neurophysiology of body weight regulation and evaluation of therapeutic targets for obesity.

Continuous stress promotes expression of VGF in melanotroph via suppression of dopamine

Prolonged exposure to stress elicits profound effects on homeostasis that may lead to cryptogenic disorders such as chronic fatigue syndrome. To investigate the pathophysiology associated with the syndrome, we used a rat continuous stress (CS) model where the pituitary represents one of the most affected organs. Here we found that mRNA for VGF (non-acronymic), a member of the granin family, was induced specifically in the intermediate lobe (IL). This was matched by a concomitant increase at the peptide/protein level assessed by C-terminal antibody. Furthermore, the up-regulation of VGF was confirmed by immunohistochemistry in a subset of melanotrophs. VGF expression was altered in the IL of rats receivingthe dopamine D2 receptor agonist bromocriptine or the antagonist sulpiride. In vitro, dopamine dose-dependently decreased the mRNA levels in cultured melanotrophs. These findings suggest that VGF expression under CS is negatively regulated by dopaminergic neurons projecting from the hypothalamus.

MicroRNA 375 mediates the signaling pathway of corticotropin-releasing factor (CRF) regulating pro-opiomelanocortin (POMC) expression by targeting mitogen-activated protein kinase 8

Pro-opiomelanocortin (POMC) is a common precursor of melanocortin-related peptides in the pituitary and primarily regulated by corticotropin- releasing factor (CRF). Our results show that miR-375 is highly expressed in the mouse pituitary gland and located specifically in the intermediate lobe of pituitary. The functional studies show that the forced inhibition of endogenous miR-375 in AtT-20 mouse pituitary tumor cells and in the intermediate lobe of the pituitary gland significantly increases POMC expression, whereas miR-375 overexpression down-regulates POMC expression and ACTH secretion stimulated by CRF. This function of miR-375 is accomplished by its binding to the 3'-UTR of mitogen-activated protein kinase kinase kinase-8. Our results here have demonstrated that miR-375 acts as a negative regulating molecule mediating the signaling pathway of CRF and affecting POMC expression by targeting mitogen-activated protein kinase kinase kinase-8, which subsequently down-regulates ERK1/2 phosphorylation and nerve growth factor-induced clone B (NGFI-B) transcription activity. Taken together, our results show that miR-375 is a novel negative regulator of POMC expression and related hormone secretion.

Id1, Id2 and Id3 are induced in rat melanotrophs of the pituitary gland by dopamine suppression under continuous stress

In rats under continuous stress (CS) there is decreased hypothalamic dopaminergic innervation to the intermediate lobe (IL) of the pituitary gland, which causes hyperactivation and subsequent degeneration of melanotrophs in the IL. In this study, we investigated the molecular basis for the changes that occur in melanotrophs during CS. Using microarray analysis, we identified several genes differentially expressed in the IL under CS conditions. Among the genes up-regulated under CS conditions, we focused on the inhibitor of DNA binding/differentiation (Id) family of dominant negative basic helix-loop-helix (bHLH) transcription factors. RT-PCR, Western blotting and in situ hybridization confirmed the significant inductions of Id1, Id2 and Id3 in the IL of CS rats. Administration of the dopamine D2 receptor agonist bromocriptine prevented the inductions of Id1-3 in the IL of CS rats, whereas application of the dopamine D2 antagonist sulpiride induced significant expressions of Id1-3 in the IL of normal rats. Moreover, an in vitro study using primary cultured melanotrophs demonstrated a direct effect on Id1-3 inductions by dopamine suppression. These results suggest that the decreased dopamine levels in the IL during CS induce Id1-3 expressions in melanotrophs. Because Id family members inhibit various bHLH transcription factors, it is conceivable that the induced Id1-3 would cooperatively modulate gene expressions in melanotrophs under CS conditions to induce hormone secretion.

Major role of cathepsin L for producing the peptide hormones ACTH, beta-endorphin, and alpha-MSH, illustrated by protease gene knockout and expression

The pituitary hormones adrenocorticotropic hormone (ACTH), beta-endorphin, and alpha-melanocyte stimulating hormone (alpha-MSH) are synthesized by proteolytic processing of their common proopiomelanocortin (POMC) precursor. Key findings from this study show that cathepsin L functions as a major proteolytic enzyme for the production of POMC-derived peptide hormones in secretory vesicles. Specifically, cathepsin L knock-out mice showed major decreases in ACTH, beta-endorphin, and alpha-MSH that were reduced to 23, 18, and 7% of wild-type controls (100%) in pituitary. These decreased peptide levels were accompanied by increased levels of POMC consistent with proteolysis of POMC by cathepsin L. Immunofluorescence microscopy showed colocalization of cathepsin L with beta-endorphin and alpha-MSH in the intermediate pituitary and with ACTH in the anterior pituitary. In contrast, cathepsin L was only partially colocalized with the lysosomal marker Lamp-1 in pituitary, consistent with its extralysosomal function in secretory vesicles. Expression of cathepsin L in pituitary AtT-20 cells resulted in increased ACTH and beta-endorphin in the regulated secretory pathway. Furthermore, treatment of AtT-20 cells with CLIK-148, a specific inhibitor of cathepsin L, resulted in reduced production of ACTH and accumulation of POMC. These findings demonstrate a prominent role for cathepsin L in the production of ACTH, beta-endorphin, and alpha-MSH peptide hormones in the regulated secretory pathway.

Regulatory elements of the melanocortin 1 receptor

Among more than 120 genes that are now known to regulate mammalian pigmentation, one of the key genes is MC1R, which encodes the melanocortin 1 receptor, a seven transmembrane G protein-coupled receptor expressed on the surface of melanocytes. Since the monoexonic sequence of the gene was cloned and characterized more than a decade ago, tremendous efforts have been dedicated to the extensive genotyping of mostly red-haired populations all around the world, thus providing allelic variants that may or may not account for melanoma susceptibility in the presence or absence of ultraviolet (UV) exposure. Soluble factors, such as proopiomelanocortin (POMC) derivatives, agouti signal protein (ASP) and others, regulate MC1R expression, leading to improved photoprotection via increased eumelanin synthesis or in contrast, inducing the switch to pheomelanin. However, there is an obvious lack of knowledge regarding the numerous and complex regulatory mechanisms that govern the expression of MC1R at the intra-cellular level, from gene transcription in response to an external stimulus to the expression of the mature receptor on the melanocyte surface.

Regulation of ACTH levels in anterior pituitary cells during stimulated secretion: evidence for aspartyl and cysteine proteases in the cellular metabolism of ACTH

The regulation of cellular levels of adrenocorticotropin hormone (ACTH) in response to stimulated secretion was investigated to define the extent of cellular depletion of ACTH and subsequent increases to replenish ACTH levels in anterior pituitary cells (in primary culture). Treatment of cells with secretagogues for short-term incubation times (hours) resulted in extensive depletion of cellular ACTH. Corticotropin releasing factor (CRF) induced depletion of cellular levels of ACTH by 60-70% of control levels. The CRF-induced reduction of cellular ACTH was inhibited by the glucocorticoid dexamethasone. Phorbol myristate acetate (PMA), which stimulates protein kinase C (PKC), reduced ACTH levels by 50-60%. Forskolin, a stimulator of cAMP production, produced a moderate reduction in cellular ACTH. During prolonged incubation of cells (2 days) with these secretagogues, further reduction of ACTH levels by 70-80% was observed. However, increased cellular levels of ACTH occurred with continued treatment of cells with secretagogues, which provided nearly complete replenishment of cellular ACTH after 5 days treatment with secretagogues. Notably, the rising levels of cellular ACTH were inhibited by the aspartyl protease inhibitor acetyl-pepstatin A, and by the cysteine protease inhibitor E64d. These results demonstrate that depletion and recovery of ACTH levels are coordinately regulated, and that the increases in cellular levels of ACTH during the recovery phase involves participation of aspartyl and cysteine proteases. Thus, aspartyl and cysteine proteases may be involved in the cellular metabolism of ACTH.