Correction of hyperglycemia in diabetic mice transplanted with reversibly immortalized pancreatic beta cells controlled by the tet-on regulatory system.

Pancreatic beta cell lines may offer an abundant source of cells for beta-cell replacement in type I diabetes. Using regulatory elements of the bacterial tetracycline (tet) operon for conditional expression of SV40 T antigen oncoprotein in transgenic mouse beta cells, we have shown that reversible immortalization is an efficient approach for regulated beta-cell expansion, accompanied by enhanced cell differentiation upon growth arrest. The original system employed the tet-off approach, in which the cells proliferate in the absence of tet ligands and undergo growth arrest in their presence. The disadvantage of this system is the need for continuous treatment with the ligand in vivo for maintaining growth arrest. Here we utilized the tet-on regulatory system to generate beta cell lines in which proliferation is regulated in reverse: these cells divide in the presence of tet ligands, and undergo growth arrest in their absence, as judged by [3H]thymidine and BrdU incorporation assays. These cell lines were derived from insulinomas, which heritably developed in transgenic mice continuously treated with the tet derivative doxycycline (dox). The cells produce and secrete high amounts of insulin, and can restore and maintain euglycemia in syngeneic streptozotocin-induced diabetic mice in the absence of dox. Such a system is more suitable for transplantation, compared with cells regulated by the tet-off approach, because ligand treatment is limited to cell expansion in culture and is not required for long-term maintenance of growth arrest in vivo.

Inducible and reversible beta-cell autoimmunity and hyperplasia in transgenic mice expressing a conditional oncogene.

Expression of the SV40 T antigen (Tag) in pancreatic beta-cells in transgenic mice has been shown to induce beta-cell tumorigenesis. We generated transgenic mice in which Tag expression is inducible and reversible by the tet-on gene regulation system. These mice develop beta-cell tumors only when treated with the inducer doxycycline (dox). Tag expression in vivo is reversible upon dox withdrawal. As a result, beta-cell proliferation is greatly reduced, indicating that genetic changes, which may occur in the transformed cells, do not allow Tag-independent proliferation. Induction of Tag expression after immune recognition of self-antigens has been established triggers an autoimmune response against beta-cells, as evidenced by insulitis. Shut-off of Tag expression results in elimination of insulitis, suggesting that this process depends on continuous expression of the target antigen. In addition, the reversibility of autoimmunity suggests that beta-cell damage caused by the anti-Tag immune response does not elicit secondary responses to other newly exposed beta-cell antigens, which would have persisted after Tag elimination. beta-Cell proliferation in this model is accompanied by cell apoptosis. Apoptosis persisted for several weeks in the islets after dox removal. In close to 40% of the mice analyzed, this process reduced the islet size back to normal, suggesting the existence of a homeostatic mechanism that maintains beta-cell mass within the normal range.