Do “normal” and “tumorigenic” cells use the same mechanisms to mediate cell proliferation and maintain homeostasis? The small intestine constitutes an excellent material on which to analyze the properties of actively growing and differentiating cells, as well as to study the mechanisms governing tumor induction. Within the intestine, the stem and differentiated cell populations are in physically distinct compartments (crypts and villi), which can be separated and isolated to determine protein activity or gene expression. We use these techniques in wild type and mutant/transgenic mice to characterize the molecular properties distinguishing "resting" and "proliferating" cells, both during normal development and in cases of altered morphology and/or proliferation of the intestine. To induce tumorigenesis in the intestine, we use the large T antigen (TAg) encoded by polyomavirus SV40, which associates with key cellular proteins like pRBs and p53 and thus disrupts normal cellular regulation and proliferation in vivo and in vitro. The use of transgenic model systems expressing TAg and mutant derivatives in the intestine allowed us to show a prominent role of the pRB family and E2F2 -one of the E2F transcription factors they control- in mediating tumorigenic proliferation in this tissue, while we have also discovered instances in which cell proliferation occurs in the absence of the E2Fs (normal crypts) thought to be required for cell cycle entry and progression. We will continue to characterize the mechanisms and differences between normal and tumorigenic proliferation in vivo and in vitro.
Although the two best characterized cellular targets of TAg are the pRb (retinoblastoma) family, and the tumor suppressor p53, many other TAg targets have been described and await full study. We are also extending our studies to the oncoproteins encoded by other polyomavirus, by simultaneous use of biological assays, genetics and proteomics. Some of these viruses have already been associated with human diseases including cancer. However, although SV40 and other polyomavirus transform cells, the mechanisms used in each instance are not yet understood and different members of the same DNA tumor virus might have evolved distinct mechanisms for controlling cell biology.