LEUKOTRIENE-D-4-INDUCED MOBILIZATION OF INTRACELLULAR CA2-CELLS IS CRITICALLY DEPENDENT ON ACTIVATION OF THE SMALL GTP-BINDING PROTEIN-RHO(IN EPITHELIAL)

We have previously shown that the leukotriene D-4 (LTD(4))-induced mob ilization of intracellular Ca2+ in epithelial cells is mediated by a G -protein that is distinctly different from the pertussis toxin-sensiti ve G-protein that regulates the subsequent flux of Ca2+. In the presen t study, we attempted to gain further knowledge about the mechanisms i nvolved in the LTD(4)-induced mobilization of intracellular Ca2+ in ep ithelial cells by investigating the effects of compactin, an inhibitor of the isoprenylation pathway, on this signalling event. In cells pre incubated with 10 mu M compactin for 48 h, the LTD(4)-induced mobiliza tion of intracellular Ca2+ was reduced by 75% in comparison with contr ol cells. This reduction was reversed by co-administration of mevalona te (1 mM). The effect of compactin occurred regardless of whether or n ot Ca2+ was present in the extracellular medium, suggesting that isopr enylation must occur before Ca2+ is released from intracellular stores . In accordance with this, we also found that both the LTD(4)-induced formation of inositol 1,4,5-trisphosphate and the LTD(4)-induced phosp horylation of phospholipase C gamma 1 (PLC gamma 1) on tyrosine residu es were significantly reduced in compactin-pretreated cells. These res ults open up the possibility that the activation of PLC gamma 1 is rel ated to a molecule that is sensitive to impaired activity of the isopr enylation pathway, such as a small monomeric G-protein. This idea was supported by the observation that Clostridium botulinum C3 exoenzyme-i nduced inhibition of Rho proteins abolished the LTD(4)-induced intrace llular mobilization of Ca2+. A regulatory role of Rho proteins in the LTD(4)-induced activation of PLC gamma 1 is unlikely to be indirectly mediated via an effect on the cytoskeleton, since cytochalasin D had n o major effect on the LTD(4)-induced mobilization of Ca2+. Although th e mechanism of interaction remains to be elucidated, the present findi ngs indicate an important role of an isoprenylated protein such as Rho in the LTD(4)-induced Ca2+ signal.