Expression of p15^Ink4b in response to treatment with growth arrest stimuli. (A) M1 cells were cultivated in the presence of IL-6, IFNβ, IFNγ, and LPS, all of which are known to induce growth arrest. Twenty-four hours following treatment with these agents, cells were harvested and total RNA was isolated. Northern analysis was carried out using a p15^Ink4b cDNA probe. To control for sample loading, the same blot was rehybridized with a β-actin probe. (B) RT-PCR analysis of p15^Ink4b expression in M1 cells cultivated in the presence of IFNβ for 0, 3, 7, 14, 24, and 48 hours. To control for sample loading, β-actin cDNA was amplified from the same RNA samples. (C) Western blot analysis of M1 cells that were treated with IFNβ for 0, 3, 7, 14, 24, and 48 hours, using a monoclonal antibody against p15^Ink4b and an antibody against β-actin.