Transforming Growth Factor- s Inhibit Somatostatin Messenger Ribonucleic Acid Levels and Somatostatin Secretion in Hypothalamic Cells in Culture

Abstract

Treatment of hypothalamic cells in monolayer culture with transforming growth factor-beta1 (TGFbeta1) significantly reduced both basal and cAMP-induced somatostatin messenger RNA (mRNA) levels and somatostatin secretion. This inhibitory effect was dose- and time-dependent and not mediated by glial cells, as it was also observed in glial-free hypothalamic cell cultures treated with cytosine arabinonucleoside. TGFbeta2 and -beta3 mimicked the actions of TGFbeta1, which indicated that the three isoforms of the TGFbeta family expressed in the central nervous system displayed similar effects on the somatostatinergic neurons. The blockade of synthesis of proteins with either cycloheximide or puromycin for 24 h prevented the inhibitory effect of TGFbeta1 on somatostatin mRNA. This implied that the reduction of this mRNA by TGFbeta1 required de novo protein synthesis. We next studied whether TGFbeta1 acted at the transcriptional or posttranscriptional level by altering the stability of somatostatin mRNA. Examination of the rate of disappearance of somatostatin mRNA by Northern blot, after inhibition of mRNA transcription with either actinomycin D (AcD) or 5,6-dichloro-1beta-ribofuranosyl benzimidazole revealed that TGFbeta1 did reduce the stability of somatostatin mRNA. This effect was observed when we pretreated the cultures with TGFbeta1 4 h before the addition of AcD, but not when we administered TGFbeta1 simultaneously with AcD or 5,6-dichloro-1beta-ribofuranosyl benzimidazole. Altogether these results demonstrated that the treatment of hypothalamic cells in culture with TGFbeta1, TGFbeta2, or TGFbeta3 resulted in a decrease in somatostatin mRNA levels and somatostatin secretion. TGFbeta1 reduced the steady state levels of somatostatin mRNA by inducing the synthesis of a protein (s), that appears to accelerate the degradation of the mRNA of somatostatin. Whether TGFbeta1 has additional effects on the transcription of the somatostatin gene will require further study.

Induction of transforming growth factor‐β1 by basic fibroblast growth factor in rat C6 glioma cells and astrocytes is mediated by MEK/ERK signaling and AP‐1 activation

“…As a whole, the present studies have demonstrated a previously unrecognized regulation of TGF‐β isoforms by bFGF in glioma cells and hypothalamic astrocytes. Potentially, bFGF‐induced TGF‐β1 release may regulate the secretion of hormones such as GnRH (Melcangi et al, 1995; Galbiati et al, 1996; Messi et al, 1999; Buchanan et al, 2000; Prevot et al, 2000; Zwain et al, 2002; Bouret et al, 2004), somatostatin (Quintela et al, 1997; Raber et al, 1997), and CRH (Raber et al, 1997). Additionally, astrocyte‐derived TGF‐β may contribute to synaptic plasticity and neuronal survival in the hypothalamus (Dhandapani et al, 2003a, b).…”

“…Astrocytes modulate neuronal activity, function and survival in the brain through the release of various factors, including TGF‐β and bFGF (Dhandapani et al, 2003a, b; Garcia‐Segura and McCarthy, 2004). In particular, astrocyte‐derived TGF‐β superfamily members and bFGF can influence neuroendocrine function by exerting diverse actions on hypothalamic neurons, including the regulation of releasing factors, including gonadotropin‐releasing hormone (GnRH; Melcangi et al, 1995; Galbiati et al, 1996; Messi et al, 1999; Buchanan et al, 2000; Prevot et al, 2000; Zwain et al, 2002; Bouret et al, 2004), somatostatin (Quintela et al, 1997), and corticotropin‐releasing hormone (CRH) release (Raber et al, 1997). Additionally, hypothalamic astrocyte‐derived TGF‐β promotes neuroplasticity and neuroprotection (Dhandapani et al, 2003a, b) and regulates proopiomelanocortin (POMC) expression in the arcuate nucleus (Bouret et al, 2001), an area of the brain important for feeding homeostasis, stress responses, and regulation of reproductive function.…”

Somatostatin and Somatostatin Receptors

“…With regard to the hormones investigated, thyroid, GH, IGF-I, and insulin augment SST release from the hypothalamus (Patel 1992;Patel et al 2001;Barnett 2003); insulin, leptin, and epinephrine inhibits its release from the pancreas and hypothalamus respectively (Patel 1992;Patel et al 2001;Barnett 2003). Inflammatory mediators have also shown differential effects on SST secretion: IL-1, IL-6, IL-10, INF-g, and TNF-a stimulate SST release while TGF-b inhibits it (Scarborough et al 1989;Quintela et al 1997;Elliott 2004).…”

Fas/CD95 Ligation Induces Proliferation of Primary Fetal Astrocytes Through a Mechanism Involving Caspase 8-Mediated ERK Activation

“…The PI-3K/Akt pathway is another prominent signalling mechanism involved in some of the non-apoptotic functions of Fas/CD95 [see [43] for review]; and PI-3K has been recently reported as an important mediator of Fas/CD95 effects on glioma cells [36]. Reportedly, Fas/CD95 stimulation induces a strong phosphorylation of the catalytic unit of PI-3K in both Jurkat and apoptosis-sensitive glioma cells [11].…”

“…The cultures were maintained at 37°C in a humidified atmosphere of 5% CO 2 for one week and thereafter cells were trypsinized and subcultured for the different experiments. Astrocytes were characterized in culture by immunofluorescence using a monoclonal antibody against glial fibrillary acidic protein (GFAP) (Dako Diagnosticos, Barcelona, Spain) [43]. Hamster anti-Fas/CD95 antibody (Jo2) and its isotype control (IgG 2 λ 2 ) were purchased from BD Biosciences.…”