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 tra nsgenic mice have shown that inactivation of the retinoblastoma tumor suppressor protein (pRb) can cause postmitotic lens fiber cells to ent er the cell cycle. However, when the p53 gene and protein are intact, inactivation of pRb in this terminally differentiated cell type result s 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 mi ce were generated that overexpress bcl-2 in a lens-specific fashion. S urprisingly, overexpression of bcl-2 was sufficient to interfere with normal fiber cell differentiation, inducing cataracts, microphakia, va cuolization, fiber cell disorganization, and inhibition of Fiber cell denucleation. The bcl-2 mice were mated to mice exhibiting lens-specif ic expression of the N-terminal region of simian virus 40 large T anti gen (termed truncT). The resulting double transgenic mice showed a mar ked reduction in the truncT-induced fiber cell death. Apoptosis in the truncT mice could also be suppressed by crossing these mice into a p5 3-deficient background. Either overexpression of bcl-2 or loss of p53 in truncT mice resulted in proliferation of fiber cells around the cor tex of the lens. These proliferating fiber cells continue to express b eta-and gamma-crystallin proteins, which are normally only expressed f ollowing withdrawal from the cell cycle. The p53 protein is known to u pregulate expression of certain target genes, including p21, a protein that can block cell cycle progression by inhibition of cyclin-depende nt kinases. In order to assess whether bcl-2 interferes with the trans criptional activation activity of p53, transgenic lenses were assayed by in situ hybridization For levels of p21 expression. lenses that exp ressed 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, overexpress ion of p21 is not sufficient to cause apoptosis. These experiments sho w that the lenses of transgenic mice represent a valuable in vivo sett ing for studies of both induction and inhibition of programmed cell de ath. (C) 1997 Wiley-Liss, Inc.