Ghrelin and a novel preproghrelin isoform are highly expressed in prostate cancer and ghrelin activates mitogen-activated protein kinase in prostate cancer

Purpose: There is evidence that the hormone ghrelin stimulates proliferation in the PC3 prostate cancer cell line although the underlying mechanism(s) remain to be determined. A novel, exon 3-deleted preproghrelin isoform has previously been detected in breast and prostate cancer cells; however, its characterization, expression, and potential function in prostate cancer tissues are unknown. Experimental Design: Expression of ghrelin and exon 3- deleted preproghrelin was investigated in prostate cancer cell lines and tissues by reverse transcription- PCR and immunohistochemistry. Proliferation and apoptosis assays were done in the LNCaP prostate cancer cell line to determine if ghrelin stimulates proliferation and/or cell survival. Stimulation of mitogen-activated protein kinase (MAPK) pathway activation by ghrelin was determined in PC3 and LNCaP cells by immunoblotting with antibodies specific for phosphorylated MAPKs. Results: Prostate cancer tissues display greater immuno reactivity for ghrelin and exon 3- deleted preproghrelin than normal prostate tissues, and prostate cancer cell lines secrete mature ghrelin into conditioned medium. Treatment with ghrelin (10 nmol/L), but not the unique COOH-terminal peptide derived from exon 3- deleted preproghrelin, stimulates proliferation in the LNCaP cells (45.0 +/- 1.7% above control, P < 0.01) and rapidly activates the extracellular signal-regulated kinase-1/2 MAPK pathway in both PC3 and LNCaP cell lines. Ghrelin, however, does not protect prostate cancer cells from apoptosis induced by actinomycin D (1 mu g/mL). The MAPK inhibitors PD98059 and U0126 blocked ghrelin-incluced MAPK activation, as well as proliferation, in both cell lines. Conclusions: These data suggest that these components of the ghrelin axis may have potential as novel biomarkers and/or adjunctive therapeutic targets for prostate cancer.