Abstract
The cell‐surface glycoprotein KAI1 suppresses tumor growth and metastasis in various animal models. Downregulation of KAI1 has been implicated in the progression of cancer. However, the mechanisms of KAI1 inactivation are poorly understood. This is the first study that investigates expression and regulation of KAI1 in human breast cancer. KAI1 expression was analyzed on custom‐made tissue microarrays comprising 209 well‐characterized breast cancers and normal mammary gland tissue. Strong KAI1 immunoreactivity was observed throughout the normal mammary gland epithelium. In breast cancer tissue, KAI1 immunoreactivity was lost in 161/209 (77%) cases. Strikingly, KAI1 was preferentially lost in estrogen receptor (ER)‐positive breast cancers (p < 0.001). This was validated by real‐time RT‐PCR analyses showing a 7.5‐fold downregulation of KAI1 mRNA in ER‐positive relative to ER‐negative tumors (p = 0.028). Notably, this was also corroborated by Affymetrix microarray expression data of an independent cohort of 49 breast cancers. Class comparison analysis identified KAI1 as downregulated in ER‐positive tumors. Subsequently, human breast cancer cell lines were employed to test a potential role of ER‐activity in the downregulation of KAI1, as suggested by our expression analyses. Exposure of ER‐positive breast cancer cells to fulvestrant, a clinically approved ER‐antagonist that reverses ER‐mediated gene repression, induced a significant upregulation of KAI1 and inhibited cell proliferation as well as migration. In summary, we demonstrate for the first time that KAI1 is a target of ER‐mediated gene‐repression, and thus, it is downregulated in ER‐positive breast cancer. Importantly, KAI1 might be reinducible by endocrine therapy with ER‐antagonists in patients suffering from ER‐positive breast cancer. © 2008 Wiley‐Liss, Inc.