Centriole ciliation is related to quiescence and DNA synthesis in 3T3 cells

Both DNA and the centriole pairs are replicated once in each cell generation. The cyclic changes in both must be coordinated so that the two centriole pairs can participate in mitosis when the genetic material is to be partitioned to the two daughter cells. One of the centriole pairs also forms a primary ("9 + 0") cilium sometime during the cell cycle. In this study, we asked whether some aspects of the coordination of the DNA and centriole cycles occur in G₁, a part of the cell cycle when non-neoplastic cells become irreversibly committed to DNA synthesis. We used indirect immunofluorescence with antitubulin antibody to reveal the centriole pairs as a microtubule organizing center with or without a cilium. Quiescent Balb/c and Swiss 3T3 cells in low serum or at high cell density stopped in G₁ with ciliated, probably unduplicated centrioles. When these quiescent 3T3 cells were stimulated to enter DNA synthesis, the centriole's ciliation changed in three phases: first, an initial but transient deciliation within 1-2 hr; second, a return of the cilium by 6-8 hr; and third, a subsequent final deciliation of the centriole coincident with the initiation of DNA synthesis at 12-24 hr. The deciliated and duplicated centrioles subsequently separated in preparation for mitosis. Together with other information, these results imply that centrioles in growing mammalian cells are primarily ciliated in a part of G₁ during which the cells can arrest in suboptimal environmental conditions. Arrests in low serum or at high cell density also occur before centriole replication. These results suggest that deciliation and duplication of the centriole may occur near the time that quiescent cells become irreversibly committed to DNA synthesis. Certain centriole events may therefore be necessary before DNA synthesis can be initiated in 3T3 cells.