Figure 5. Low-dose bortezomib inhibits proliferation and induces cell cycle arrest in HPV-positive, but not HPV-negative, HNSCC cells. (A) HPV-negative (top panels) and HPV-positive (bottom panels) cells were plated in triplicate in 96-well plates, allowed to recover overnight, then treated for varying lengths of time with either 0.1% DMSO or 5 nM bortezomib. Following treatment, viable cell numbers were determined in trypan blue exclusion assays. p values were determined by comparing DMSO- vs. bortezomib-treated cells on days 5–8. (B) HPV-negative (top panels) and HPV-positive (bottom panels) cells were treated for 48 h with 0.1% DMSO or 5 nM bortezomib, followed by flow cytometric determination of cell cycle status, as described in Materials and Methods. Columns represent the means from three independent experiments and error bars represent the standard error of the means.