The p53-inducible gene PC3 (TIS21, BTG2) is endowed with antiproliferative activity. Here we report that expression of PC3 in cycling cells induced accumulation of hypophosphorylated, growth-inhibitory forms of pRb and led to G 1 arrest. This latter was not observed in cells with genetic disruption of the Rb gene, indicating that the PC3-mediated G 1 arrest was Rb dependent. Furthermore, (i) the arrest of G 1 -S transition exerted by PC3 was completely rescued by coexpression of cyclin D1 but not by that of cyclin A or E; (ii) expression of PC3 caused a significant down-regulation of cyclin D1 protein levels, also in Rb-defective cells, accompanied by inhibition of CDK4 activity in vivo; and (iii) the removal from the PC3 molecule of residues 50 to 68, a conserved domain of the PC3/BTG/Tob gene family, which we term GR, led to a loss of the inhibition of proliferation as well as of the down-regulation of cyclin D1 levels. These data point to cyclin D1 downregulation as the main factor responsible for the growth inhibition by PC3. Such an effect was associated with a decrease of cyclin D1 transcript and of cyclin D1 promoter activity, whereas no effect of PC3 was observed on cyclin D1 protein stability. Taken together, these findings indicate that PC3 impairs G 1 -S transition by inhibiting pRb function in consequence of a reduction of cyclin D1 levels and that PC3 acts, either directly or indirectly, as a transcriptional regulator of cyclin D1.The control of the cell cycle plays an essential role in cell growth and in the activation of important cellular processes such as differentiation and apoptosis. pRb (retinoblastoma protein) and p53 are two molecules identified as key regulators of the cell cycle.pRb is a nuclear phosphoprotein whose phosphorylation state oscillates regularly during the cell cycle. Its underphosphorylated forms predominate in G 0 and G 1 , while highly phosphorylated forms exist in S, G 2 , and M phases (13,16,21). The primary biological function of underphosphorylated pRb is to inhibit progression toward S phase by controlling a checkpoint in late G 1 (for reviews, see references 8, 22, and 51). In fact, underphosphorylated pRb associates with members of the E2F family of transcription factors, impairing their activity and leading to a cell cycle block in G 1 . Conversely, the phosphorylation of pRb inactivates its growth suppression activity by freeing E2F molecules, thus enabling them to transactivate genes required for the progression of the cell into S phase and the remainder of the cell cycle (52, 97, 114).Cyclin-dependent kinases (CDKs) are the molecules responsible for pRb phosphorylation and its consequent inactivation (reviewed in references 70 and 102). Each CDK has its own functional specificity, based on the period of its activity during the cell cycle and on the specific cyclin partner. CDK4, CDK5, and CDK6 form complexes with D-type cyclins during the G 1 phase (65,69,116). CDK2, when bound to cyclin A or E, is instead essential for G 1 -to-S transition (28, 78), while the c...

Chronic hepatitis C virus (HCV) infection is a leading cause of liver cirrhosis and hepatocellular carcinoma (HCC) worldwide. The HCV capside core is a multifunctional protein with regulatory functions that a ects transcription and cell growth in vitro and in vivo. Here, we show that both HCV genotype 1a and 3 core proteins activate MEK1 and Erk1/2 MAP kinases and that the costitutive expression of the HCV core results in a high basal activity of Raf1 and MAP/kinase/ kinase, as determined by endogenous Raf1 in vitro kinase assay and immunodetection of hyperphosphorylated Erk1 and Erk2 even after a serum starvation. Moreover, the activation of both Erk1/2 and the downstream transcription factor Elk-1 in response to the mitogenic stimulus EGF is signi®cantly prolonged. The sustained response to EGF in cells expressing the HCV core occurs despite a normal induction of the MAP phosphatases MKP regulatory feedback and is likely due to the costitutive activation of Raf-1 activity. The ability of HCV core proteins to directly activate the MAP kinase cascade and to prolong its activity in response to mitogenic stimuli may contribute to the neoplastic transformation of HCV infected liver cells.