Extrapituitary Effects of the Growth Hormone-Releasing Hormone

Hippokratis Kiaris∗; Andrew V. Schally†; Anastasios Kalofoutis∗    ∗ Department of Biological Chemistry, Medical School, University of Athens 115 27 Athens, Greece
† Endocrine Polypeptide and Cancer Institute, VA Medical Center & Section of Experimental Medicine, Tulane University Medical Center New Orleans, Louisiana 70112, USA

I Introduction

Growth hormone-releasing hormone (GHRH) is secreted by the hypothalamus; after binding to specific GHRH receptors in the pituitary, GHRH induces the secretion and release of growth hormone (GH) (Krulich et al., 1968; Schally and Comaru-Schally, 1998; Schally et al., 1965). GH in turn stimulates the production of insulin-like growth factor-1 (IGF-1 predominantly in the liver, which is a mitogen-stimulating cell cycle progression and a survival factor/inhibitor of apoptosis for various cell types (Koutsilieris et al., 2001, 2000, 1999; Mitsiades and Koutsilieris, 2001; Reyes-Moreno et al., 1998; Schally and Varga, 1999). Several molecular forms of biologically active GHRH have been isolated from various sources, including pancreatic tumors and the hypothalamus in humans and animals (Guillemin et al., 1982, 1984; Ling et al., 1984; Rivier et al., 1982). Most of the biologically active forms of GHRH isolated from hypothalami and extrapituitary tissues and cancers contain 40 to 44 amino acids, corresponding to peptides of about 5.2 kD. These forms of GHRH have been shown to correspond to peptides resulting from the extensive and complex proteolytic cleavage to which pre-pro-GHRH (12.3 kD) is subjected following translation of its mRNA (Dey et al., 2004). This proteolytic processing has been shown to be mediated predominantly by furin at the N-terminal cleavage site followed by PC1/3 at the C terminus (Dey et al., 2004). The precise biological function of each pre-pro-GHRH–derived peptide as well as potential differences in the various peptide properties remain unclear. Whereas mature GHRH represents the peptide to which the well-characterized biological function (i.e., the stimulation of GH release) has been attributed, other products of pre-pro-GHRH processing are thought to possess some, yet clear, biological activity. One of these peptides, which has been designated GHRH-related peptide (GHRH-RP), has been shown to exert some effects in nonpituitary tissues by a mechanism involving the stem cell factor (SCF), with a potential role in blood cell development (Fang et al., 2000). Virtually full intrinsic biological activity is present in the 29 N-terminal amino acid residues [GHRH(1–29)NH2] which serves as the core for the development of potent agonistic and antagonistic analogs of GHRH (Rivier et al., 1982).

Despite the fact that the source for the initial isolation and characterization of GHRH was nonhypothalamic tumor tissue, namely pancreatic islet tumors with neuroendocrine properties (Frohman and Szabo, 1981; Guillemin et al., 1982; Rivier et al., 1982), and not the hypothalamus, little attention has been given to the potential role of this neuropeptide in carcinogenesis, probably with the exception of the role of GHRH in the development of pituitary tumors. An avalanche of research has instead focused on the endocrine system, including systemic effects of GHRH in the regulation of GH production and secretion and regulation of IGF-1, which together constitute the GHRH/GH/IGF-1 axis. Until research in the 1990, the direct effects of GHRH in carcinogenesis were ignored. Two lines of evidence point to a direct role of GHRH in the initiation, growth, as well as maintenance of nonpituitary malignant tumors. The first line of evidence is the pattern of expression of GHRH: besides the identification of the hypothalamus as the predominant source for the expression of GHRH, this neuropeptide has been recently detected in many types of nonpituitary tumors as well as some normal tissues (Bagnato et al., 1992, 1991; Berry and Pescovitz, 1998; Ciampani et al., 1992; Gaylinn, 1999; Kahan et al., 2000; Khorram et al., 2001a,b; Margioris et al., 1990; Moretti et al., 1990a,b; Stephanou et al., 1991). GHRH's relatively wide pattern of expression, wider than initially thought, implies functions for GHRH independent of the regulation of GH production because the pituitary is the only tissue in which a causative association between GH release and GHRH stimulation has been demonstrated. The second and equally important line of evidence is provided by the inhibition of GHRH action by antagonists of GHRH in vitro in cancer cells, in which by definition the endocrine GHRH/GH/IGF-1 axis is not operational (Csernus et al., 1999; Kiaris et al., 1999; Rekasi et al., 2000b). These GHRH antagonists strongly inhibit cell proliferation, suggesting that GH release is not necessary for manifestation of biological activities of GHRH. It has to be mentioned, however, that a causative relationship between the proliferation of the pituitary somatotrophs and the expression of GHRH has been established in experiments using transgenic and mutant animals. For example, overexpression of GHRH in mice results in pituitary hyperplasia due to the hyperproliferation of the somatotrophs, whereas the inhibition of the expression (GSH-1 deficient) or normal action (in little mice or dwarf rats) of GHRH produces the opposite phenotype exemplified by reduced proliferation of the pituitary somatotrophs (Downs and Frohman, 1991; Frohman and Kineman, 2002a, 2000b; Hammer et al., 1985; Mayo et al., 2000; Mutsuga et al., 2001; Tierney and Robinson, 2002). In those studies, however, the effects of GHRH were closely linked with the stimulation of the pituitary GHRH receptor.

The authors of this review have tried to summarize the available information associating GHRH with carcinogenesis in nonpituitary tissues with special emphasis on the autocrine//paracrine stimulatory signals that appear to operate in tumors. This review will not discuss the role of GHRH in pituitary tumors, although well described in both human specimens as well as in transgenic animals (Hammer et al., 1985; Losa and Werder, 1997), because the topic remains outside the discussion's scope. Instead, this chapter focuses on the extrapituitary effects of the neuropeptide.

II Endocrine Role of GHRH in Carcinogenesis

Considering that IGF-1 has been causatively associated with carcinogenesis (Furstenberger and Senn, 2002; Hadsell and Abdel-Fattah, 2001) and that GHRH affects IGF-1 production through stimulation of GH secretion, it is conceivable for GHRH to induce the growth of IGF-1-dependent tumors. Indeed, this hypothesis has been tested by early studies in mice more than a decade ago, which indicated that hypophysectomy, which surgically disrupts the normal secretion of GH and subsequently the production of IGF-1 inhibits the growth of osteosarcomas that are highly dependent on IGF-1 (Pollak et al., 1992). Subsequent studies, which involved little (lit/lit) mice that are deficient in GH production due to a missense mutation in the GHRH receptor, confirmed these findings and underlined the role of GHRH in tumor development through secondary regulation of IGF-1 (Bugni et al., 2001; Deitel et al., 2002). Additional experiments using these animals demonstrated that genetic ablation of GHRH action results in reduced sensitivity to chemically induced liver carcinogenesis. Furthermore, the same type of animal was shown to provide a nonpermissive environment for the growth of established breast tumors, as compared to wild-type animals (Yang et al., 1996). Inoculation of...