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Gastrin-releasing peptide (GRP) is typically viewed as a growth factor in cancer. However. we have suggested that in colon cancer, GRP acts primarily as a morphogen when it and its receptor (GRP-R) are aberrantly upregulated. As such. GRP/GRP-R act(s) primarily to modulate processes contributing to the assumption or maintenance of tumor differentiation. One of the most important such processes is the ability of tumor cells to achieve directed motility in the context of tissue remodeling. Yet the cellular conditions affecting GRP/GRP-R expression, and the biochemical pathways involved in mediating its morphogenic properties, remain to be established. To study this, we evaluated the human colon cancer cell lines Caco-2 and HT-29 cells. We found that confluent cells do not express GRP/GRP-R. In contrast, disaggreation and plating at subconfluent densities results in rapid GRP/GRP-R upregulation followed by their progressive decrease as confluence is achieved. GRP/ GRP-R coexpression correlated with that of focal adhesion kinase (FAK) phosphorylation of [Tyr.sup.397], [Tyr.sup.407], [Tyr.sup.861], and [Tyr.sup.925] but not [Tyr.sup.576] or [Tyr.sup.577]. To more specifically evaluate the kinetics of GRP/ GRP-R upregulation, we wounded confluent cell monolayers. At t = 0 h GRP/GRP-R were not expressed, yet cells immediately began migrating into the gap created by the wound. GRP/GRP-R were first detected at ~2 h. and maximal levels were observed at ~6 h postwounding. The GRP-specific antagonist [D-[Phe.sup.6]]-labeled bombesin methyl ester had no effect on cell motility before GRP-R expression. In contrast, this agent increasingly attenuated cell motility with increasing GRP-R expression such that from t = 6 h onward no further cell migration into the gap was observed. Overall, these findings indicate the existence of GRP-independent and -dependent phases of tumor cell remodeling with the latter mediating colon cancer cell motility during remodeling via FAK. bombesin: morphogen; motogen