TRANSLATIONAL REGULATION OF THE MAMMALIAN GROWTH-RELATED PROTEIN P23 - INVOLVEMENT OF EIF-4E

Synthesis of the mammalian growth-related protein P23 is rapidly induc ed after serum stimulation of mouse fibroblasts and Ehrlich ascites tu mour cells., This induction occurs at the translational level. Growth- induction leads also to an increase in phosphorylation of the rate-lim iting initiation factor eIF-4E. Here, we present the following evidenc e indicating the involvement of eIF-4E in the regulation of P23 synthe sis: 1) P23 synthesis is induced by the same mitogenic stimuli which l ead to enhanced eIF-4E phosphorylation, 2) Upon heat shock treatment o f Ehrlich ascites cells (which results in immediate dephosphorylation and concomitant inactivation of eIF-4E), P23 synthesis is rapidly shut off. 3) In control NIH 3T3 cells, P23 synthesis is readily induced by growth stimulation. This response is strongly diminished in cells ove rexpressing eIF-4E, and the basal level of P23 synthesis is elevated i n these cells. Overexpression of a nonfunctional mutant of eIF-4E dimi nishes the basal level of P23 synthesis as well as the serum-response of the cells with respect to P23 induction. 4) Cells transformed by ov erexpression of the ras or src genes in which eIF-4E is highly phospho rylated do not show any inducibility of P23 synthesis. 5) HeLa cells e xpressing antisense RNA of eIF-4E, have reduced levels of eIF-4E/F and show reduced rates of growth and protein synthesis. In these cells th e total amount of P23 protein is about 50% compared with control cells . The results suggest that P23 is one of the gene products, the synthe sis of which is regulated by eIF-4E activity.