We have recently started a new avenue of investigative study to determine if the soluble factors produced during embryogenesis, in particular at the implantations stage, offer an alternative model to study the cellular events underlying carcinogenesis (promotion/invasion). Several reports have implicated growth factor expression in early embryo development to parallel that observed for neoplasm expression and that fetal trophoblast attachment during implantation mimics the invasive properties of malignant cells. Evaluating different embryonic stages of mouse/rat fetal development by molecular (northern blot/RT-PCR, in situ hybridization and in situ PCR) and immunohistochemical techniques have identified adrenomedullin (AM)/adrenomedullin receptor (AM-R) expression during implantation and organogenesis. We have previously demonstrated AM/AM-R functions as an autocrine growth loop for a variety of human tumor cell lines. High levels of these markers were identified at the site of fetal trophoblast attachment to placental tissue. Studies are now be preformed using a neutralizing anti-AM monoclonal antibody (MoAb-G6) to examine the in vitro/in vivo effects on murine fetal development. The analysis of soluble factor regulating embryogenesis may offer an innovative approach to study the carcinogenesis pathway and define appropriate markers of tumor expression. Finally, we have utilized a second approach to identify trophic factors of tumor cells by examining the post-translational processing events involved in peptide hormone generation. Although there are several enzymatic modifications that can occur during prohormone processing only two such events consistently track with biological activity, these being proteolytic cleavage and alpha-amidation. Since there are know amino acid motifs which code for such enzymatic processing steps, we have used the presence of such consensus sequences to identify alternatively processed mitogenic factors cryptically expressed in the precursor molecule of established growth factors. Following this course of study we have previously identified an unknown peptide amide (IBE1), encoded within the IGF-IB prohormone, which initiates a proliferative response in both normal and malignant bronchial epithelial cells through a novel receptor system. Applying a similar analysis to the amino acid sequence of human Tf we have identified nine candidate peptide amides implicating additional bioactive potential for Tf beyond that of iron transport. This same approach was used to predict the existence of an additional bioactive peptide, proadrenomedulln N-terminal 20 peptide (PAMP), encoded within the AM precursor molecule. We have recently evaluated the effect of synthetic PAMP on human tumor cell lines in culture and have identified growth suppressive activity. Such findings underlie the importance of alternative peptide processing from a common precursor molecule and implicate a potentially new negative growth regulator of tumor cell proliferation. Our collective studies have identified potential target for the early detection and intervention of malignant disease and offer a rational approach in the investigative avenues to define new biological markers of neoplasm expression.
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