Cell cycle kinetics and commitment in newborn, adult, and tumoral astrocytes

Abstract

In terms of cell cycle phases, mammalian astrocytes maintain the capacity to leave G0G1 and enter S phase in response to brain injury or due to neoplastic transformation. This report compares proliferative behavior in vitro, particularly departure from G0, in three types of rat astroglial cells — newborn astrocytes, astrocytes from gelatin implants into the traumatized striata of adults, and astrocytoma cells (C6 glioma). Newborn and adult astrocytes demonstrated nearly identical proliferation kinetics as determined by peaks in cell number and rates of DNA synthesis. C6 glioma (C6G) proliferated more rapidly. Exit from G0 was examined by shift-down of serum from 10 to 0.1% for 48 h, followed by return to 10% at time 0. Synchronization of newborn and adult astrocytes in this way resulted in a 12 h lag phase (G0G1) followed by a 6–10-fold surge in DNA synthesis and a corresponding increase in S-phase nuclei from < 15% to > 70%. Timing of S-phase commitment was established in late G1 by resistance to the inhibitors cycloheximide and mevinolin. Decay of commitment was assessed by addition of hydroxyurea (HU) at 10 h to cause accumulation at the G1S boundary. Removal of HU after an additional 14, 16, and 20 h resulted respectively in these percentages of maximal S-phase DNA synthesis in newborn and adult astrocytes: 75 ± 9, 60 ± 8, 23 ± 3, and 87 ± 20, 62 ± 7, 34 ± 5. In contrast, synchronization of C6G resulted in a 6 h lag before a surge in DNA synthesis and an increase in S-phase nuclei from < 20% to 100%. Cell cycle commitment occurred earlier with C6G, and decay of commitment was not observed, even after 20 h of HU treatment. Thus, these in vitro techniques for cell cycle analysis are applicable to astrocytes obtained from developing and adult brain, and to at least some astroglioma cells. Furthermore, this comparative study showed that important cell cycle parameters differ markedly in the non-tumoral astrocytes and glioma cells. These differences could lead to strategies for selective targeting of the proliferation of neoplastic astroglia.