Is In2-ghrelin variant, a ghrelin gene derived transcript, a novel player in breast cancer tumors?
Manuel D Gahete1, Jose Córdoba-Chacon1, Marta Hergueta2, Francisco Gracia-Navarro1, Rhonda D Kineman3, Gema Moreno-Bueno2, Raul M Luque1 & Justo P Castaño1
Ghrelin has been classically known as a GH- and metabolism-regulating hormone, mainly produced by stomach. However, it also acts as a paracrine or autocrine factor in several tissues, where it can regulate tissue growth and neoplastic cell proliferation. Of note, ghrelin needs to be acylated at Ser3 by the ghrelin-O-acyltransferase (GOAT) enzyme to bind to its receptor GHS-R1a. Interestingly, the ghrelin gene can give rise to distinct additional peptides, generated by alternative splicing and/or from post-translational modifications, with some ghrelin-derived variants being up-regulated in breast cancer. In this study, we have identified an alternative human ghrelin variant transcript containing Ex0, Ex1, In2, and Ex2 but lacking Ex3 and 4, named In2-ghrelin variant, which was strongly up-regulated in a series of breast cancer samples as compared with normal mammary gland (P<0.01). Translation of this splice variant would originate a new pre-propeptide of 117 amino acids that conserves only the signal peptide and the first 12 amino acids of native-ghrelin, but retains the acylation site on Ser3. This novel In2-ghrelin variant was thoroughly expressed in the 22 human tissues analyzed. Interestingly, the expression of In2-ghrelin variant but not that of native-ghrelin parallels the expression of the GOAT enzyme in both, normal (R2=0.921) and breast cancer (R2=0.655) samples, suggesting that In2-ghrelin variant could be a primary substrate for GOAT in normal and tumoral tissues. Moreover, In2-ghrelin variant expression in breast cancer was highly correlated with GHSR-type1b but not with GHSR-type1a. In the MDA-MB-231 cell line, an in vitro breast cancer cell model, the expression level of In2-ghrelin variant was inhibited by both acyl- and desacyl-ghrelin, whereas it was increased by tamoxifen. In summary, overexpression of In2-ghrelin variant, GOAT and GHSR1b in breast tumor samples coupled to the regulation observed in in vitro models, suggests that this novel ghrelin variant may play a relevant role in breast cancer. Support: RYC-2007-00186, BIO-0139, CTS-01705, BFU2007-60180/BFI, BFU2008-01136/BFI, NIDDK 30677, VA Merit Award.