Multiple sources of sn-1,2-diacylglycerol in mitogen-stimulated Swiss 3T3 cells; evidence for activation of phosphoinositidase C and PtdChophospholipase D

Abstract

A number of growth factors, exemplified by bombesin, bind to receptors and stimulate the hydrolysis of PtdIns(4,5)P₂ to generate Ins(1,4,5)P₃ and sn-1,2-diacylglycerol (DG) (reviewed by Berridge, 1987). These two molecules are potent second messengers: Ins(1,4,5)P₃ binds to a specific receptor, probably on the E.R., stimulating the release of sequestered calcium into the cytosol (reviewed by Berridge & Irvine, 1989) whilst DG remains within the inner leaflet of the plasma membrane where it binds to and activates protein kinase C (reviewed by Parker et al., 1989). This pathway is implicated in the regulation of a number of the early events associated with entry into the cell cycle including increases in pH_i (Hesketh et al., 1986), induction of the expression of c-fos and c-myc proto-oncogenes (Rozengurt & Sinnet-Smith, 1987) and activation of the ribosomal protein S6 kinase (Susa et al., 1989). However PtdIns(4,5)P₂ hydrolysis is not a pre-requisite for stimulation of cell proliferation by all growth factors, e.g. EGF in Swiss 3T3 cells (Whitman & Cantley, 1988). In addition, it should be noted that the increase in Ins(1,4,5)P₃ is essentially transient, as is the associated increase in [Ca²⁺]_i, declining towards control within 1–2 minutes whereas DNA synthesis requires the continued presence of the mitogen for up to 8–10 hours.