Delayed reversal of shape change in cells expressing FP _B prostanoid receptors. Possible role of receptor resensitization

Prostaglandin F _2α (PGF _2α) receptors are G-protein-coupled receptors consisting of two alternative mRNA splice variants, named FP _A and FP _B. As compared with the FP _A isoform, the FP _B isoform lacks the last 46 amino acids of the carboxyl terminus and, therefore, represents a truncated version of the FP _A. We recently found (Pierce, K. L., Fujino, H., Srinivasan, D., and Regan, J. W. (1999) J. Biol. Chem. 274, 35944-35949) that stimulation of both isoforms with PGF _2α leads to activation of a Rho signaling pathway, resulting in tyrosine phosphorylation of p125 focal adhesion kinase, formation of actin stress fibers, and cell rounding. Although the activation of Rho and subsequent cell rounding occur at a similar rate for both isoforms, we now report that following the removal of PGF _2α the reversal of cell rounding is much slower for cells expressing the FP _B isoform as compared with the FP _A isoform. Thus, in HEK-293 cells that stably express the FP _A isoform, the reversal of cell rounding appears to be complete after 1 h, whereas for FP _B-expressing cells there is essentially no reversal even after 2 h. Similarly, the disappearance of stress fibers and dephosphorylation of p125 focal adhesion kinase following removal of agonist are much slower in FP _B-expressing cells than in FP _A-expressing cells. The mechanism of this differential reversal appears to involve a difference in receptor resensitization following the removal of agonist. Based upon whole cell radioligand binding, agonist-induced stimulation of inositol phosphate formation, and mobilization of intracellular Ca ²⁺, the FP _B isoform resensitizes more slowly than the FP _A isoform. These findings suggest that the carboxyl terminus of the FP _A is critical for resensitization and that the slower resensitization of the FP _B isoform leads to prolonged signaling. This differential signaling distinguishes the FP _A and FP _B receptor isoforms and could be important toward understanding the physiological actions of PGF _2α.