Delayed reversal of shape change in cells expressing FPB prostanoid receptors - Possible role of receptor resensitization

Prostaglandin F-2 alpha (PGF(2 alpha)) receptors are G-protein-coupled rece ptors consisting of two alternative mRNA splice variants, named FPA and FPB . As compared with the FPA isoform, the FPB isoform lacks the last 46 amino acids of the carboxyl terminus and, therefore, represents a truncated vers ion of the FPA. We recently found (Pierce, K. L,, Fujino, H., Srinivasan, D ., and Began, J. W. (1999) J, Biol; Chem. 274, 35944-35949) that stimulatio n of both isoforms with PGF(2 alpha) leads to activation of a Rho signaling pathway, resulting in tyrosine phosphorylation of p125 focal adhesion kina se, formation of actin stress fibers, and cell rounding, Although the activ ation 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 reve rsal of cell rounding is much slower for cells expressing the FPB isoform a s compared with the FPA isoform, Thus, in HEK-293 cells that stably express the FPA isoform, the reversal of cell rounding appears to be complete afte r 1 h, whereas for FPB-expressing cells there is essentially no reversal ev en after 2 h, Similarly, the disappearance of stress fibers and dephosphory lation of p125 focal adhesion kinase following removal of agonist are much slower in FPB-expressing cells than in FPA-expressing cells. The mechanism of this differential reversal appears to involve a difference in receptor r esensitization following the removal of agonist, Based upon whole cell radi oligand binding, agonist-induced stimulation of inositol phosphate formatio n, and mobilization. of intracellular Ca2+, the FPB isoform resensitizes mo re slowly than the FPA isoform, These findings suggest that the carboxyl te rminus of the FPA is critical for resensitization and that the slower resen sitization of the FPA isoform leads to prolonged signaling. This differenti al signaling distinguishes the FPA and FPB receptor isoforms and could be i mportant toward understanding the physiological actions of PGF(2 alpha).