HMGA2 induces pituitary tumorigenesis by enhancing E2F1 activity

HMGA2 gene amplification and overexpression in human prolactinomas and the development of pituitary adenomas in HMGA2 transgenic mice showed that HMGA2 plays a crucial role in pituitary tumorigenesis. We have explored the pRB/E2F1 pathway to investigate the mechanism by which HMGA2 acts. Here we show that HMGA2 interacts with pRB and induces E2F1 activity in mouse pituitary adenomas by displacing HDAC1 from the pRB/E2F1 complex-a process that results in E2F1 acetylation. We found that loss of E2F1 function (obtained by mating HMGA2 and E2F1(-/-) mice) suppressed pituitary tumorigenesis in HMGA2 mice. Thus, HMGA2-mediated E2F1 activation is a crucial event in the onset of these tumors in transgenic mice and probably also in human prolactinomas.     

Identification of E2F-3B, an alternative form of E2F-3 lacking a conserved N-terminal region

We have identified a novel form of the full-length E2F-3 protein that we term E2F-3B. In contrast to full-length E2F-3, which is expressed only at the G1/S boundary, E2F-3B is detected throughout the cell cycle with peak levels in GO where it is associated with Rb. Transfection and in vitro translation experiments demonstrate that a protein identical to E2F-3B in size and iso-electric point is produced from the E2F-3 mRNA via the use of an alternative translational start site. This alternative initiation codon was mapped by mutagenesis to codon 102, an ACG codon. Mutation of the ACG codon at position 102 abolished E2F-3B expression, whereas the conversion of ACG 102 to a consensus ATG led to the expression of a protein indistinguishable from E2F-3B. Given these results, E2F-3B is missing 101 N-terminal amino acids relative to full-length E2F-3. This region includes a moderately conserved sequence of unknown function that is present only in the growth-promoting E2F family members, including E2F-1, 2 and full-length E2F-3. These observations make E2F-3B the first example of an E2F gene giving rise to two different protein species and also suggest that E2F-3 and E2F-3B may have opposing roles in cell cycle control.     

Regulation of the Cdc25A gene by the human papillomavirus Type 16 E7 oncogene

Cdc25A is a tyrosine phosphatase that is involved in the regulation of the G1/S phase transition by activating cyclin E/Cdk2 and cyclin A/Cdk2 complexes through removal of inhibitory phosphorylations. The E6 and E7 oncoproteins of the high-risk human papillomaviruses (HPV) interact with and functionally abrogate the p53 and pRB proteins, respectively. In the present study we have investigated the regulation of the Cdc25A promoter during G1 and S-phases of the cell cycle and by the HPV-16 E7 oncoprotein. Serum induction leads to a derepression of the Cdc25A promoter and can be mediated through two E2F binding sites, E2F-A and E2F-C. While E2F-A is by both E2F1 and E2F4, E2F-C is regulated only by E2F1. The Cdc25A promoter is transactivated by the E7 oncogene of HPV-16. Furthermore, Cdc25A levels are highly increased in E7-expressing cell lines. Inducible expression of E7 leads to an immediate increase in Cdc25A protein levels. These data suggest that Cdc25A may be a critical target of HPV-16 E7 in the disruption of the G1/S phase transition.