Novel relationships of growth factors to the G1/S transition in cultured astrocytes from rat forebrain

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Abstract
The cell cycle encompasses the sequential events regulating cell division. In mammalian brain, initiation of astrocyte cycling is critical during development and injury. To investigate the timing of growth factor requirements as they commit to passing through the G1 phase, primary and secondary rat astrocytes were stimulated to enter the cycle after serum or growth factor deprivation. Bromodeoxyuridine immunofluorescence was used to monitor S phase nuclei after growth factor re‐addition (at time 0). Cycle kinetics were identical whether quiescent cultures were exposed to 10% (vol/vol) calf serum, or to a defined medium containing fibroblast growth factor, insulin, and epidermal growth factor. The control point in late G1 that represents commitment to achieving the G1/S transition was identified by cycloheximide (CHX, 0.1 μg/ml) addition. Sensitivity to cycle arrest by CHX disappeared at 9–10 h. In contrast, shift‐down to growth factor‐deficient medium arrested cell cycling virtually until G1/S (12 h). With selective exposure during late G1 (9–12 h), no single agent permitted cycle progression. However, any two agents enabled cycling, and complementary or synergistic effects were apparent. These requirements were identical in astroglia from newborn and long‐term cultures.Thus, temporal dissociation exists between the processes of escape from CHX sensitivity and from requirements for growth factors, two recognized hallmarks of commitment to cycle progression. Furthermore, simultaneous presence of at least two growth factors is necessary at or near G1/S. Both findings distinguish astrocytes from several other cell types.