Stable COX Vb shRNA expression in A549 cells reduces the outgrowth of A549 xenografts and inhibits multiple histopathological indicators of poor differentiation in athymic mice. 5x106 A549 cells stably transfected with control or COX Vb-specific shRNA were subjected to Western blot analysis for COX Vb or β-actin expression (A) and then injected subcutaneously into athymic mice. After 4 weeks, tumors were measured weekly for a total of 3 additional weeks (timepoints 0, 7, 14 and 21) using microcalipers. Tumor mass was calculated based on bidimensional measurements and data are expressed as the mean ± SEM of two experiments (B). After 7 weeks of growth, mice were euthanized and tumors were excised, fixed in formalin, paraffin-embedded, sectioned and stained with hematoxylin/eosin. Light micrographs demonstrate that, relative to the COX Vb1 shRNA-transfected A549 tumors (F-H), the control A549 tumors were higher grade, poorly differentiated and invasive (C-E), contain large areas of tumor necrosis (D; white arrows), pleomorphic nuclei (E, black arrows), and numerous mitotic figures (E, white arrows).