Growth factors, oncogenes and inositol lipids.

Normal cells require growth factors to multiply. One group of growth factors such as platelet-derived growth factor, bombesin and vasopressin in fibroblasts or antigen in lymphocytes uses a specific inositol lipid as part of a transduction mechanism for generating intracellular mitogenic signals. These growth factors stimulate the hydrolysis of phosphatidylinositol 4,5-bisphosphate to give diacylglycerol (DG) and inositol 1,4,5-trisphosphate (Ins1,4,5P3). The DG remains within the plane of the membrane to activate protein kinase C, one function of which is to increase intracellular pH by switching on a Na+/H+ exchanger. The other product, Ins1,4,5P3, functions as a second messenger to mobilize calcium from intracellular stores. These two ionic events, the increase in pH and calcium, contribute to the onset of DNA synthesis. The hydrolysis of an inositol lipid is a key event in this signal pathway which mediates the action of competence factors. A separate signal pathway, perhaps based on tyrosine phosphorylation, carries out the effects of progression growth factors such as epidermal growth factor (EGF) and insulin. It is argued that oncogenes may be arranged into groups associated with specific signal pathways. For example, the sis oncogene encodes platelet-derived growth factor which might use the src gene product as part of its transduction mechanism to generate the second messengers DG, Ins1,4,5P3 and calcium. These last then act to stimulate the transcription of myc and fos. On the other hand, the erbB gene encodes a protein which resembles the receptor for EGF. The function of the ras protein remains a major unsolved problem but there is indirect evidence for proposing that it may mediate the action of progression factors such as EGF or insulin.