Macrophage Growth Arrest by Cyclic AMP Defines a Distinct Checkpoint in the Mid-G₁ Stage of the Cell Cycle and Overrides Constitutive c-myc Expression

Abstract

Proliferation of a murine macrophage cell line (BAC1.2F5) in response to colony-stimulating factor 1 (CSF-1) is inhibited by prostaglandin E₂ (PGE₂)-mediated elevation of intracellular cyclic AMP (cAMP). When BAC1.2F5 cells were growth arrested in early G₁ by CSF-1 starvation and stimulated to synchronously enter the cell cycle by readdition of growth factor, PGE₂ inhibited [³H] thymidine incorporation when added before mid-G₁, but its addition at later times did not block the onset of S phase. Reversible cell cycle arrest mediated by a cAMP analog required the presence of CSF-1 for cells to initiate DNA synthesis, whereas cells released from an aphidicolin block at the G₁/S boundary entered S phase in the absence of CSF-1. PGE₂ or cAMP analogs did not block the initial induction of c-myc mRNA by CSF-1 but abolished the CSF-1-dependent expression of c-myc mRNA in the mid-G₁ stage of the cell cycle. The cAMP-mediated reduction in c-myc RNA levels was due to decreased c-myc transcription. However, CSF-1-dependent BAC1.2F5 clones infected with a c-myc retrovirus were growth arrested by cAMP analogs despite constitutive c-myc expression. Therefore, the reduction of endogenous c-myc expression by cAMP is neither necessary nor sufficient for growth inhibition.