Inhibition of cell death by lens-specific overexpression of bcl-2 in transgenic mice.

Previous studies on cell cycle regulation in the ocular lens using transgenic mice have shown that inactivation of the retinoblastoma tumor suppressor protein (pRb) can cause postmitotic lens fiber cells to enter the cell cycle. However, when the p53 gene and protein are intact, inactivation of pRb in this terminally differentiated cell type results in cell death, rather than continued proliferation. Since bcl-2 has been shown to act as a cell death repressor, the ability of this gene to block p53-dependent apoptosis in lenses was examined. Transgenic mice were generated that overexpress bcl-2 in a lens-specific fashion. Surprisingly, overexpression of bcl-2 was sufficient to interfere with normal fiber cell differentiation, inducing cataracts, microphakia, vacuolization, fiber cell disorganization, and inhibition of fiber cell denucleation. The bcl-2 mice were mated to mice exhibiting lens-specific expression of the N-terminal region of simian virus 40 large T antigen (termed truncT). The resulting double transgenic mice showed a marked reduction in the truncT-induced fiber cell death. Apoptosis in the truncT mice could also be suppressed by crossing these mice into a p53-deficient background. Either overexpression of bcl-2 or loss of p53 in truncT mice resulted in proliferation of fiber cells around the cortex of the lens. These proliferating fiber cells continue to express beta- and gamma-crystallin proteins, which are normally only expressed following withdrawal from the cell cycle. The p53 protein is known to upregulate expression of certain target genes, including p21, a protein that can block cell cycle progression by inhibition of cyclin-dependent kinases. In order to assess whether bcl-2 interferes with the transcriptional activation activity of p53, transgenic lenses were assayed by in situ hybridization for levels of p21 expression. Lenses that expressed both truncT and bcl-2 showed elevated p21, implying that bcl-2 does not inhibit apoptosis by directly inhibiting p53, but instead may block a later step in the apoptosis pathway. In addition, overexpression of p21 is not sufficient to cause apoptosis. These experiments show that the lenses of transgenic mice represent a valuable in vivo setting for studies of both induction and inhibition of programmed cell death.