HSP27 is overexpressed in angiogenic human breast cancer cells compared with nonangiogenic human breast cancer cells. Differences in HSP27 expression between the angiogenic and nonangiogenic variants of the MDA-MB-436 cell line were validated. (A) Western blot analysis of HSP90, HSP70, and HSP27 protein expression in angiogenic (A) and nonangiogenic (NA) variants of the human breast cancer cell line. (B and C) Immunohistochemical staining for HSP27 protein in cell pellets containing angiogenic (B) or nonangiogenic (C) cells, grown under in vitro conditions. (D–F) Immunohistochemical staining of HSP27 protein in tumor xenografts was quantified using a staining index (D) and was significantly increased in angiogenic tumors (E) compared with nonangiogenic tumors (F). The sample shown in F is negative for HSP27 expression in tumor cells, representing one extreme end of the expression spectrum. In F, note the strong expression of HSP27 protein in some tumor-associated stromal (including endothelial) cells.