Interleukin-1 induces growth arrest by hypophosphorylation of the retinoblastoma susceptibility gene product RB

Interleukin-1 (IL-1) causes G₀/G₁ phase growth arrest in human melanoma cells, A375-C6. Because hypophosphorylation of the retinoblastoma susceptibility gene product, RB, is one of the key events responsible for G₀/G₁ phase growth arrest, we investigated whether IL-1 altered the phosphorylation status of RB protein in these cells. Exposure to IL-1 caused a time-dependent increase in hypophosphorylated RB that correlated with an accumulation of cells arrested in the G₀/G₁ phase. The ability of IL-1 to cause hypophosphorylation of RB and growth arrest was abrogated by the SV40 large T antigen, which binds preferentially to hypophosphorylated RB, but not by the K1 mutant of the T antigen, which is defective in binding to RB. Furthermore, the cells were protected from IL-1-inducible growth inhibition by ectopic expression of dominant-negative mutants of the Rb gene, or the transcription factor E2F-1, which is a downstream target of RB. These results suggest that hypophosphorylated RB mediates the growth arrest induced by IL- 1.