GHRH proliferative action on somatotrophs is cell‐type specific and dependent on Pit‐1/GHF‐1 expression

Abstract

To investigate the mechanisms by which the hypothalamic peptide GHRH influences cell division, we analyzed its effects on the proliferation of two different cell lines: CHO-4, an ovary-derived cell line, and GH3, a pituitary-derived cell line. We found that GHRH induces the proliferation of pituitary-derived cells but inhibits the proliferation of ovary-derived cells. We further characterized this dual effect of GHRH to find that the cytoplasmic signals induced by this hormone are similar in both cell lines. Moreover, in CHO-4 cells GHRH stimulates two well-known positive cell cycle regulators, c-myc and cyclin D1, but is unable to induce the degradation of the negative cell cycle regulator p27(Kip1). Significantly, when the Pit-1/GHF-1 gene is exogenously expressed in CHO-4 cells, the negative effect of GHRH on the proliferation of these cells is attenuated. Furthermore, when the levels of Pit-1 are downregulated by siRNA in GH3-GHRHR cells, the positive effects of GHRH on the proliferation of these cells are diminished. These findings add to our understanding of the molecules involved in the regulation of cell proliferation by GHRH, as we demonstrate for the first time that Pit-1 is not only required to drive the expression of the GHRH receptor, as previously described, but is also needed for the downstream effects that occur after its activation to modulate cell proliferation. These data suggest that the regulation of cell proliferation in response to a specific growth factor depends in certain cell populations on the presence of a tissue-specific transcription factor.

IGF-1 Induces GHRH Neuronal Axon Elongation during Early Postnatal Life in Mice

“…Indeed, we previously observed that the heterozygous invalidation of IGF-1R alters the development of the somatotropic axis, which is associated with a decrease of GHRH immune-reactivity in the median eminence at 10 days of age and a specific permanent pituitary hypoplasia of GH-producing somatotroph cells by 20 days of age. These findings are in agreement with previous studies that highlighted a strong implication of GHRH in the stimulation of proliferation and differentiation of somatotroph cells during the early postnatal period through an induction of Pit-1in developing pituitary [8]. Moreover, our previous report indicates that nutritional restriction during lactation in wild type pups, which decrease circulating levels of IGF-I, similarly induces a permanent pituitary hypoplasia in somatotroph cells by 20 days of age that is preceded by a transient decrease of GHRH expression level at 10 days of age.…”

Direct promoter induction of p19Arf by Pit-1 explains the dependence receptor RET/Pit-1/p53-induced apoptosis in the pituitary somatotroph cells

“…In general CDKN2A/p19Arf expression is regulated by two main pathways: oncogenes such as Ras and Myc in the so-called oncogene-induced senescence (OIS) or apoptosis (OIA), and DNA damage (Evan and d'Adda di Fagagna, 2009). We propose that the direct induction of p19Arf promoter by Pit-1 could be classified as OIA although Pit-1 is not an oncogene; but in a somatotroph Pit-1 is required to proliferate (Castrillo et al, 1991;Solloso et al, 2008). Thus, somatotrophs are protected against excessive proliferation through this RET/Pit-1/p19Arf pathway.…”

“…This induces the binding of cEBPa/CREB to an element of the Pit-1 promoter conserved in mouse, rat and humans with a huge induction of Pit-1 (Canibano et al, 2007;Garcia-Lavandeira et al, 2010). Pit-1 is an essential transcription factor for embryonic differentiation of pituitary precursors into somatotrophs and postnatal maintenance of the somatotroph phenotype (Castrillo et al, 1991;Pfaffle et al, 1992;Lin et al, 1993;Ward et al, 2006;Solloso et al, 2008). In a mature somatotroph, an over-the-level increase in the Pit-1 protein expression induces accumulation of p53 and apoptosis.…”

Essential role of p21/waf1 in the mediation of the anti-proliferative effects of GHRH antagonist JMR-132

“…The major receptor that mediates the effects of GHRH in the extrapituitary tissues is SV1, a splice variant of the pituitary GHRH receptor that also has ligand-independent activity (Rekasi et al 2000, Kiaris et al 2002, Barabutis et al 2007. With regard to the effects of GHRH on proliferation, at the level of the signaling cascades activated upon binding of GHRH to its receptor(s), a role for mitogen-activated protein kinase (MAPK), and recently Pit-1 transcription factor has been recognized in pituitary cells (Pombo et al 2000, Solloso et al 2008. In breast cancer cells, Ras, Raf, and MAPK have been shown to play an essential role in the mediation of the proliferative effects of GHRH (Siriwardana et al 2006).…”