Activation of extracellular signal-regulated kinase mediates bombesin-induced mitogenic responses in prostate cancer cells

Abstract

Bombesin and its mammalian homologue gastrin-releasing peptide have been shown to be highly expressed and secreted by neuroendocrine cells in prostate cancer, and are thought to be related to the carcinogenesis and progression of this disease. We found, in this study, bombesin specifically induced mitogen-activated protein (MAP) kinase activation as shown by increased extracellular regulated kinase (ERK) phosphorylation and epidermal growth factor (EGF) receptor transactivation in prostate cancer cells, which express functional gastrin-releasing peptide receptor. The transactivation of EGF receptor was required for bombesin-induced ERK phosphorylation. Furthermore, non-receptor tyrosine kinase Src and cellular Ca²⁺ were shown to be involved in bombesin-induced EGF receptor transactivation and ERK phosphorylation. Inhibition of either EGF receptor transactivation or ERK activation blocked bombesin-induced DNA synthesis in these cells. Taken together, these data suggest bombesin may act as a mitogen in prostate cancer by activating MAP kinase pathway via EGFR transactivation.

Introduction

The amphibian tetradecapeptide bombesin and its mammalian homologue gastrin-releasing peptide are regulatory peptides involved in numerous physiological processes such as exocrine pancreas secretion, smooth muscle contraction, neuromodulation and cell growth [1]. They have also been shown to act as potent mitogens for Swiss 3T3 cells and autocrine growth factors for small cell lung carcinoma cells [1], [2].

Both bombesin and gastrin-releasing peptide exert their functions by binding to their specific gastrin-releasing peptide receptor, a member of the large superfamily of G protein-coupled receptors (GPCRs). This triggers the activation of multiple signal transduction pathways that may act in a synergistic and combinatorial fashion to relay the mitogenic signal to the nucleus and promote cell proliferation. For instance, bombesin stimulated inositol phosphatidyl turn over, Ca²⁺ mobilization, activation of protein kinase C and enhancement of cAMP accumulation [3], [4]. In addition, bombesin induced the activation of both mitogen-activated protein (MAP) kinase pathway and epidermal growth factor (EGF) receptor [5], [6], [7], [8], [9], [10].

A number of studies have shown that neuroendocrine differentiation is a common feature of prostatic adenocarcinoma, which correlates with poor prognosis. The regulatory peptides secreted by those differentiated neuroendocrine cells and their paracrine interaction with their specific receptors have been associated with an aggressive course of this disease [11], [12], [13], [14], [15]. Bombesin and gastrin-releasing peptide are two of these peptides which have been shown to stimulate proliferation of androgen-independent prostate cancer cells [11], [12], [16]. Furthermore, abundant aberrant gastrin-releasing peptide receptor expression was found not only in virtually all invasive prostate cancers but also in the earliest phase of neoplastic transformation of the prostate, the prostatic intraepithelial neoplasia (PIN), as is in contrast to the absence or the low-level expression of gastrin-releasing peptide receptor in non-neoplastic prostatic tissue [17], [18], [19]. Antagonists of gastrin-releasing peptide receptor inhibited the growth of a number of prostate carcinoma models, either cancer cells in vitro or xenografts in syngeneic rats or nude mice [20], [21], [22], [23], [24]. These data indicate bombesin and its gastrin-releasing peptide receptor may be involved in the carcinogenesis and progression of prostate cancer; however, the underlying molecular mechanisms of prostate cancer cell proliferation in response to bombesin have not been elucidated.

Therefore, this study investigated the effect of bombesin on prostate cancer cells and the underlying molecular mechanisms that are responsible for cell proliferation. We found bombesin induced extracellular regulated kinase (ERK) phosphorylation by transactivating EGF receptor, and this was required for bombesin-stimulated DNA synthesis in prostate cancer cells. Furthermore, protein kinase Src and intracellular Ca²⁺ were required for bombesin-induced EGF receptor transactivation in prostate cancer cells. These results indicate bombesin may act as a mitogen in the carcinogenesis and progression of prostate cancer by activating MAP kinase pathway through transactivation of EGF receptor.