CRF and cAMP regulation of POMC gene expression in corticotrophic tumor cells

Nucleobindin-derived nesfatin-1 and nesfatin-1-like peptide stimulate pro-opiomelanocortin synthesis in murine AtT-20 corticotrophs through the cAMP/PKA/CREB signaling pathway

Nucleobindin (NUCB)-derived peptides, nesfatin-1 (NES-1) and nesfatin-1-like peptide (NLP) have several physiological roles in vertebrates. While NES-1 is implicated in stress, whether NUCB1/NLP and NUCB2/NES-1 have any effect on proopiomelanocortin (POMC) remains unknown. The main aim of this study was to determine if NES-1 and/or NLP affect POMC synthesis in mouse corticotrophs. Immunocytochemistry was employed to target NUCB colocalization with POMC in immortalized mouse tumoral corticotrophs (AtT-20 cells). The ability of NES-1 and NLP to modulate POMC mRNA and protein in AtT-20 cells was assessed by qPCR and Western blot, respectively. Moreover, cell-signaling molecules mediating the effect of NES-1 and NLP on POMC synthesis in mouse tumoral corticotrophs were studied using pharmacological blockers. Mouse tumoral corticotrophs showed immunoreactivity for both NUCB1/NLP and NUCB2/NES-1. Both NES-1 and NLP exerted a stimulatory effect on POMC transcript abundance and protein expression in a dose- and time-dependent manner. This effect was comparable to corticotropin-releasing factor (CRF, positive control) stimulation of POMC. Incubation of mouse tumoral corticotrophs with NES-1 or NLP upregulated the phosphorylation of protein kinase A (PKA) and cAMP-response element-binding protein (CREB). The stimulatory effect of these peptides on POMC transcript abundance and protein expression was blocked by the PKA inhibitor, H89, and an adenylate cyclase inhibitor, 2',3'-dideoxyadenosine (DDA). These pharmacological studies indicate that NES-1 and NLP act through the cAMP/PKA/CREB cellular pathway to stimulate POMC synthesis. Our results provide molecular evidence to support a stimulatory role for nucleobindin-derived peptides on POMC synthesis from corticotrophs. Collectively, this research indicates that corticotrophs produce NUCBs, and the encoded peptides NES-1 and NLP could elicit a direct action to stimulate the pituitary stress hormone. This stimulatory effect is mediated by an uncharacterized G protein-coupled receptor (GPCR) that utilizes the cAMP/PKA/CREB pathway.

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.

Production and release of proopiomelanocortin (POMC) derived peptides by human melanocytes and keratinocytes in culture: regulation by ultraviolet B

It is demonstrated that ultraviolet B (UVB) radiation stimulates increased expression of the proopiomelanocortin (POMC) gene which is accompanied by production and release of alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropin (ACTH) by both normal and malignant human melanocytes and keratinocytes. The production and release of both peptides are also stimulated by dibutyryl cyclic adenosine monophosphate (dbcAMP) and interleukin 1 alpha (IL-1 alpha) but not by endothelin-1 (ET-1) or tumor necrosis factor-alpha (TNF-alpha). N-acetyl-cysteine (NAC), a precursor of glutathione (GSH), an intracellular free radical scavenger, abolishes the UVB-stimulated POMC peptide production and secretion. Conclusions are as follows: (1) Cultured human cells of cutaneous origin, namely keratinocytes and melanocytes, can produce and express POMC; (2) POMC expression is enhanced by exposure to UVB, possibly through a cyclic AMP-dependent pathway; and (3) The action of UVB on POMC production may involve a cellular response to oxidative stress.

Characterization of a corticotropin releasing hormone responsive region in the murine proopiomelanocortin gene

The most potent, physiologic activator of proopiomelanocortin (POMC) gene transcription is corticotropin releasing hormone (CRH) and increased intracellular cAMP is critical for this effect. The 5'-flanking region of the murine POMC gene has several potential binding sites for regulatory proteins. To characterize the region between nucleotides -141 and -106, which includes a TRE-like site and an adjacent AP-2 consensus sequence, and to study its role in signal-transcription coupling, gel mobility shift assays and transient expression of CAT chimeras were performed. In transient transfections of AtT-20 cells with pCATp-141/-106, CRH treatment led to significant increases in CAT expression compared with CRH treatment of cells transfected with the enhancerless vector. However, no response to direct activation of cAMP dependent protein kinase or protein kinase C was detected. Despite the high homology of the sequence -137/-131 to the consensus AP-1 binding site (TRE), the nuclear factor(s) in AtT-20 cells binding to this region appears to be different than authentic AP-1 since neither a competitor oligonucleotide having the authentic TRE sequence nor antibodies against Jun or Fos affected the gel shift pattern of a probe having the -137/-131 sequence. We conclude that the -141 to -106 region of the murine POMC gene contains a functional CRH responsive element and that second messenger systems that transduce the CRH signal to this element do not exert their actions solely through activation of PKA or PKC.

A putative AP-2 binding site in the 5' flanking region of the mouse POMC gene

Using extracts of AtT-20 cell nuclei, protein binding sites on the POMC gene 5'-flanking region were examined with an exonuclease protection approach. One such binding site, located from -119 to -106 bp upstream from the mouse POMC gene transcription initiation site, which exhibited a close homology to the activator protein-2 (AP-2) site [1]. A double-stranded oligonucleotide containing this site was subsequently used in gel shift assays to demonstrate AP-2 consensus sequence binding activity in extracts of AtT-20 cell nuclei. Gel shift competition experiments using both homologous and heterologous competitor DNA sequences revealed that the AP-2 like factor(s) exhibited specific binding to the mouse AP-2 consensus sequence. Furthermore, AP-2 factor binding was also modulated by a CTF/NF1-like factor. Pretreatment of AtT-20 cell nuclear extracts with alkaline phosphatase prior to inclusion in gel shift assays led to a reduction in the intensities of AP-2 factor-specific bands, indicating a potential involvement of protein phosphorylation in AP-2 factor binding in AtT-20 cells.

Effects of protein kinase C down-regulation on secretory events and proopiomelanocortin gene expression in anterior pituitary tumor (AtT-20) cells

To elucidate the role of the diacylglycerol-protein kinase C (PKC) pathway in beta-endorphin synthesis and secretion in anterior pituitary corticotrope tumor cells (AtT-20), a procedure for down-regulating PKC activity in the cells was developed. Treatment of AtT-20 cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) led to an increase in [3H]phorbol 12,13-dibutyrate binding to PKC in the membrane fraction of these cells 30 s after its addition to the culture medium. Thereafter, a decrease in both [3H]phorbol 12,13-dibutyrate binding and PKC-specific phosphotransferase activity occurred in a time- and dose-dependent manner in both the cytosolic and membrane fractions. For example, treatment of the cells with 100 nM TPA for 24 h resulted in an almost complete depletion of PKC activity. Immunoreactive beta-endorphin secretion was found to be stimulated two- to fourfold in the control cells after incubation with corticotropin-releasing factor (10(-7) M), forskolin (10(-6) M), or TPA (10(-7) M) for 4 h. In cells rendered PKC deficient, TPA-stimulated immunoreactive beta-endorphin release was abolished, forskolin-stimulated release was unaffected, and corticotropin-releasing factor-stimulated release was depressed. Treatment of control cells with any one of the three stimulatory agents led to an increase in proopiomelanocortin mRNA levels, and these responses were also depressed after TPA pretreatment. The results suggest that physiological processes thought to be entirely cyclic AMP dependent, such as corticotropin-releasing factor-elicited secretion, may be partially dependent on PKC-mediated biochemical events.

Cyclic AMP-responsive region of the human proopiomelanocortin (POMC) gene

Transcription of the human proopiomelanocortin (POMC) gene is regulated by cAMP. To identify the region in the human POMC gene responsible for this regulation, we constructed chimeric genes containing different portions of the 5'-flanking region of the human POMC gene fused to the structural sequence encoding the bacterial reporter enzyme chloramphenicol acetyltransferase (CAT). The transcriptional activity of the fusion genes introduced into the rat glial cell line C6 was assayed by measuring CAT activity in the cell lysate. Forskolin, an adenylate cyclase-activating agent, stimulated the expression of POMC-CAT fusion genes. Deletion analysis demonstrated that the region between -417 and -97 bp from the transcriptional origin of the human POMC gene was responsible for regulation by cyclic AMP.

Nuclear protein kinase substrates in AtT-20 cells: translocation of a 14 kDa phosphoprotein

In this work we characterize some acidic nuclear substrates of protein kinase C (PKC) and cyclic AMP-dependent protein kinase (PKA), using intact anterior pituitary corticotrophic tumor cells (AtT-20/D16-16). It was found that, as in the cytosolic fraction, substrates for both PKC and PKA exist in the nucleus and that changes in the phosphorylation states of a few of these phosphoproteins are mediated by both kinases. One of the phosphoproteins examined, a 14 kDa phosphoprotein (pp14) described previously, exhibited a phorbol-ester induced translocation from nucleus to cytosol in pulse-chase experiments utilizing 35S-methionine labeling. These results suggest that pp14 may be involved in signal transduction in AtT-20 cells. Although its identity remains to be determined, a 14 kDa DNA-binding protein was also seen in nuclear extracts of AtT-20 cells.