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- Worms in space |
The levels of E2F-4 increase in human colorectal tumors and the protein is differentially modified. Researchers in the Department of Biological Sciences working in the Pipas laboratory investigate protein dynamics in cancer cells with promising results. The E2F-family of transcription factors, which comprises the products of at least 6 different genes, mediates the regulation of cell proliferation by differential association with the retinoblastoma tumor suppressor proteins. In contrast with other members of the group, E2F-4 is constitutively expressed in all cell culture systems tested so far. Because E2F-4 levels peak around mid-G1 phase it has been suggested that this product may contribute to the regulation of the G0/G1 transition, and that its main function "in vivo" would be to maintain the quiescent and/or differentiated status in cells from adult tissues. Nevertheless, the E2F-4 protein is able to induce cell proliferation and transformation of certain cell types, both "in vivo" and "in vitro". Accordingly, E2F-4 transcripts have been found in proliferating cells through embryonic development, contradicting the previous hypothesis. In addition, we have demonstrated that E2F-4 plays a pivotal role in the correct formation and/or maintenance of the stem cells generating the intestinal epithelium. Therefore, at, at least in the intestine, the E2F-4 function is required to ensure proper functioning of actively proliferating cells. We have investigated the presence of different E2F- factors in samples from human patients diagnosed with tumors of the colon. We have found that the protein levels of E2F-4 increase selectively and significantly in cells from the tumors, while they remain within normal levels in the non-altered adjacent and distant colonic tissue. In addition, the E2F-4 products expressed in tumors show abnormal mobility when analyzed by SDS-polyacrilamide electrophoresis and we present evidence that such modification is due, at least in part, to protein phosphorylation. In clear contrast, the overall levels and protein modifications of other members of the retinoblastoma pathway and controllers of general cell proliferation and apoptosis remain unchanged. These results challenge the predominant view of E2F-4 as a keeper of the quiescent, differentiated status in all cell types, and suggest that an increased expression of E2F-4 may be associated with the progression of colon human cancers. We are presently investigating the connections to other regulators of the cell cycle, or the possible effects that observed modifications may have on the E2F-4 function.
The figure above shows the immunoblot analysis of human colorectal tissue: Protein extracts from tumor (T) or adjacent mucosa samples (M) were resolved in denaturing acrylamide gels and tested for the presence of E2F-4. Each pair corresponds to a different patient. The figure below shows that the alterations in electrophoretic mobility are mostly due to phosphorylation effects on the E2F-4 product, which are removed by treatment with alkaline phosphatase.
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