Knockdown of SR-BI inhibits xenograft tumor growth in vivo. (A) Knockdown of SR-BI results in decreased angiogenesis in MDA-MB-231 xenograft tumors. Immunohistochemistry was performed to determine expression of CD31. CD31 staining was quantified by averaging the number of vessels per field in five separate fields per sample. Columns represent the mean microvessel density; bars represent ± SD. Microvessel density was statistically greater in shCTL MDA-MB-231 tumors compared with shSRBI MDA-MB-231 cells (*P < 0.05 Student's t test; n = 4 per group). (B) Knockdown of SR-BI results in increased apoptosis in MDA-MB-231 xenograft tumors. Images shown are representative of TUNEL assay staining. Percentage of TUNEL-positive cells was calculated by dividing the number of TUNEL-positive cells by the total number of cells in four separate fields. Columns represent the percentage of TUNEL-positive cells; bars represent ± SD. TUNEL staining is statistically increased in shSRBI MDA-MB-231 tumors compared with shCTL MDA-MB-231 tumors (***P < 0.001, Student's t test, shCTL n = 3, and shSRBI n = 4).