INDUCTION OF CYCLIN D1 BY SIMIAN-VIRUS-40 SMALL TUMOR-ANTIGEN

Cell-cycle progression is mediated by a coordinated interaction betwee n cyclin-dependent kinases and their target proteins including the pRB and E2F/DP-1 complexes. Immunoneutralization and antisense experiment s have established that the abundance of cyclin D1, a regulatory subun it of the cyclin-dependent kinases, may be rate-limiting for G(1) phas e progression of the cell cycle. Simian virus 40 (SV40) small tumor (t ) antigen is capable of promoting G(1) phase progression and augments substantially the efficiency of SV40 transformation through several di stinct domains, In these studies, small t antigen stimulated cyclin D1 promoter activity 7-fold, primarily through an AP-1 binding site at - 954 with additional contributions from a CRE site at -57. The cyclin D 1 AP-I and CRE sites were sufficient for activation by small t antigen when linked to an heterologous promoter, Point mutations of small t a ntigen between residues 97-103 that reduced PP2A binding were partiall y defective in the induction of the cyclin DI promoter, These mutation s also reduced activation of MEK1 and two distinct members of the mito gen-activated protein kinase family, the ERKs (extracellular signal re gulated kinases) and the SAPKs (stress activated protein kinases), in transfected cells, Dominant negative mutants of either MEK1, ERK or SE K1, reduced small t-dependent induction of the cyclin D1 promoter, SV4 0 small t induction of the cyclin D1 promoter involves both the ERK an d SAPK pathways that together may contribute to the proliferative and transformation enhancing activity of small t antigen.