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

Prostaglandin F(2 alpha) (PGF(2 alpha)) 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(2 alpha) 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(2 alpha) 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(2 alpha).