COX-2 expression is associated with an aggressive phenotype in ductal carcinoma in situ

Cyclooxygenase type-2 (COX-2) is overexpressed in malignant tumours including breast cancers, though the mechanism of upregulation is unclear. This study aimed to determine COX-2 expression in ductal carcinoma in situ (DCIS) in comparison to invasive breast cancer (IBC) and normal breast, and also to investigate the relationship of COX-2 expression with HER-2 expression, oestrogen receptor (ER), tumour grade and cellular proliferation (Ki67) in DCIS. Cyclooxygenase type-2, HER-2, ER and Ki67 expression were determined by immunohistochemistry on paraffin tissue sections of DCIS (n = 187), IBC (n = 65) and normal breast reduction tissue (n = 60). Cyclooxygenase type-2 expression in DCIS (67%, P <0.001) and IBC (63%, P <0.001) was significantly greater than in normal breast (23%). There was no difference in COX-2 expression level between DCIS and IBC (P = 0.87) or between normal breast from reduction mammoplasty tissue and normal breast ducts around DCIS (22%, P = 0.29). In DCIS, COX-2 expression was associated with higher cellular proliferation rates (P <0.0001), nuclear grade (P = 0.003), with ER negativity (P = 0.003) and with HER-2 positivity (P <0.0001). Cyclooxygenase type-2 expression is upregulated in in situ breast cancer and is associated with surrogate markers of an aggressive DCIS phenotype including nonoestrogen-regulated signalling pathways. Cyclooxygenase type-2 inhibition may potentially prevent the development of ER-positive and ER-negative breast cancers. © 2004 Cancer Research UK.