Np95 is regulated by E1A during mitotic reactivation of terminally differentiated cells and is essential for S phase entry.
Terminal differentiation exerts a remarkably tight control on cell proliferation. However, the oncogenic products of DNA tumor viruses, such as adenovirus E1A, can force postmitotic cells to proliferate, thus representing a powerful tool to study progression into S phase. In this study, we identified the gene encoding Np95, a murine nuclear phosphoprotein, as an early target of E1A-induced transcriptional events. In terminally differentiated (TD) cells, the activation of Np95 was specifically induced by E1A, but not by overexpression of E2F-1 or of the cyclin E (cycE)-cyclin-dependent kinase 2 (cdk2) complex. In addition, the concomitant expression of Np95 and of cycE-cdk2 was alone sufficient to induce S phase in TD cells. In NIH-3T3 cells, the expression of Np95 was tightly regulated during the cell cycle, and its functional ablation resulted in abrogation of DNA synthesis. Thus, expression of Np95 is essential for S phase entry. Previous evidence suggested that E1A, in addition to its well characterized effects on the pRb/E2F-1 pathway, activates a parallel and complementary pathway that is also required for the reentry in S phase of TD cells (Tiainen, M., D. Spitkousky, P. Jansen-Dürr, A. Sacchi, and M. Crescenzi. 1996. Mol. Cell. Biol. 16:5302-5312). From our results, Np95 appears to possess all the characteristics to represent the first molecular determinant identified in this pathway.
Bonapace
Ian Marc
IM
The FIRC Institute for Molecular Oncology, 20134 Milan, Italy.
Latella
Lucia
L
Papait
Roberto
R
Nicassio
Francesco
F
Sacco
Alessandra
A
Muto
Masahiro
M
Crescenzi
Marco
M
Di Fiore
Pier Paolo
PP
D016475:3T3 Cells; D017500:Adenovirus E1A Proteins / Q000502:physiology*; D000818:Animals; D022762:CCAAT-Enhancer-Binding Proteins; D042846:CDC2-CDC28 Kinases; D002453:Cell Cycle; D002454:Cell Differentiation; D002455:Cell Division; D002460:Cell Line; D002467:Cell Nucleus / Q000737:chemistry; D019927:Cyclin E / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D004279:DNA, Viral / Q000502:physiology; D004789:Enzyme Activation; D005786:Gene Expression Regulation; D007700:Kinetics; D051379:Mice; D018482:Muscle, Skeletal / Q000166:cytology; D009687:Nuclear Proteins / Q000737:chemistry / Q000502:physiology*; D010750:Phosphoproteins / Q000737:chemistry; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D016196:S Phase / Q000502:physiology*; D044767:Ubiquitin-Protein Ligases
D017500:Adenovirus E1A Proteins; D022762:CCAAT-Enhancer-Binding Proteins; D019927:Cyclin E; D004279:DNA, Viral; D009687:Nuclear Proteins; D010750:Phosphoproteins; D044767:Ubiquitin-Protein Ligases; C117054:Uhrf1 protein, mouse; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495893:Cdk2 protein, mouse; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases