Interleukin-1 stimulates diglyceride accumulation in the absence of protein kinase C activation

Abstract

Despite advances in the knowledge of the intracellular signalling in response to extracellular messengers, the mechanism of action of interleukin-1 (IL-1) has remained an enigma. In the present study, we have employed human dermal fibroblasts (Detroit 532 cells) to investigate IL-1β-induced changes in intracellular signals. Both recombinant human IL-1β and a native preparation purified from human placental tissue were employed. Cyclic AMP levels in cell monolayers were unaltered by IL-1β. Also, IL-1β did not influence significantly the levels of phosphatidylinositol, phosphatidylinositol 4-monophosphate, and phosphatidylinositol 4,5-bisphosphate in the membrane, nor the water-soluble inositol phosphates, inositol monophosphate, inositol bisphosphate and inositol trisphosphate, in cells prelabelled with myo-[ 3H]inositol. In addition, intracellular calcium as measured by Quin2 was unaffected by interleukin-1. However, in cells labelled with [ 3H]glycerol or [ 3H]arachidonic acid, IL-1β caused an immediate rise in diglyceride (DG) accumulation. As the effects of IL-1β have been reported to be mimicked by tumour-promoting phorbol esters, this rise in DG suggested the involvement of protein kinase C (PKC). However, repeated experiments failed to reveal any acute effect of IL-1β on the activity of this enzyme. Furthermore, IL-1β did not cause the translocation of PKC between the membrane and the cytosol as has been found in response to other extracellular signals. Rather, IL-1β appeared to increase the synthesis of PKC in both membrane and cytosol preparations, an effect which could be prevented by coincubation with cycloheximide. These findings suggest that the diglyceride formed in response to IL-1β does not activate protein kinase C.