REPROGRAMMING THE CELL-CYCLE FOR ENDOREDUPLICATION IN RODENT TROPHOBLAST CELLS

Differentiation of trophoblast giant cells in the rodent placenta is a ccompanied by exit from the mitotic cell cycle and onset of endoredupl ication. Commitment to giant cell differentiation is under development al control, involving down-regulation of Idl and Id2, concomitant with up-regulation of the basic helix-loop-helix factor Hxt and acquisitio n of increased adhesiveness. Endoreduplication disrupts the alternatio n of DNA synthesis and mitosis that maintains euploid DNA content duri ng proliferation. To determine how the mammalian endocycle is regulate d, we examined the expression of the cyclins and cyclin-dependent kina ses during the transition from replication to endoreduplication in the Rcho-1 rat choriocarcinoma cell line. We cultured these cells under c onditions that gave relatively synchronous endoreduplication. This all owed us to study the events that occur during the transition from the mitotic cycle to the first endocycle. With giant cell differentiation, the cells switched cyclin D isoform expression from D3 to D1 and alte red several checkpoint functions, acquiring a relative insensitivity t o DNA-damaging agents and a coincident serum independence. The initiat ion of S phase during endocycles appeared to involve cycles of synthes is of cyclins E and A, and termination of S was associated with abrupt loss of cyclin A and E. Both cyclins were absent from gap phase cells , suggesting that their degradation may be necessary to allow reinitia tion of the endocycle. The arrest of the mitotic cycle at the onset of endoreduplication was associated with a failure to assemble cyclin B/ p34(cdk1) complexes during the first endocycle. Ln subsequent endocycl es, cyclin B expression was suppressed. Together these data suggest se veral points at which cell cycle regulation could be targeted to shift cells from a mitotic to an endoreduplicative cycle.