Abstract
BackgroundActivated leukocyte cell adhesion molecule (ALCAM) is implicated in the prognosis of multiple cancers with low level expression associated with metastasis and early death in breast cancer. Despite this significance, mechanisms that regulate ALCAM gene expression and ALCAM's role in adhesion of pre-metastatic circulating tumor cells have not been defined. We studied ALCAM expression in 20 tumor cell lines by real-time PCR, western blot and immunochemistry. Epigenetic alterations of the ALCAM promoter were assessed using methylation-specific PCR and bisulfite sequencing. ALCAM's role in adhesion of tumor cells to the vascular wall was studied in isolated perfused lungs.ResultsA common site for transcription initiation of the ALCAM gene was identified and the ALCAM promoter sequenced. The promoter contains multiple cis-active elements including a functional p65 NF-κB motif, and it harbors an extensive array of CpG residues highly methylated exclusively in ALCAM-negative tumor cells. These CpG residues were modestly demethylated after 5-aza-2-deoxycytidine treatment. Restoration of high-level ALCAM expression using an ALCAM cDNA increased clustering of MDA-MB-435 tumor cells perfused through the pulmonary vasculature of ventilated rat lungs. Anti-ALCAM antibodies reduced the number of intravascular tumor cell clusters.ConclusionOur data suggests that loss of ALCAM expression, due in part to DNA methylation of extensive segments of the promoter, significantly impairs the ability of circulating tumor cells to adhere to each other, and may therefore promote metastasis. These findings offer insight into the mechanisms for down-regulation of ALCAM gene expression in tumor cells, and for the positive prognostic value of high-level ALCAM in breast cancer.
Highlights
Activated leukocyte cell adhesion molecule (ALCAM) is implicated in the prognosis of multiple cancers with low level expression associated with metastasis and early death in breast cancer
ALCAM mRNA in sixteen breast cancer cell lines derived from metastatic breast cancer tumors in the brain, lymph node and the pleural cavity, and primary breast tumors in ductal epithelium were quantified by qRT-PCR
Most cell lines derived from pleural effusions (MB-157, MDA-MB-435, HCC1428, MDA-MB-453, MCF-7, MDA-MB-231 and SK-BR-3) expressed relatively low levels of ALCAM mRNA, while cells originating from the lymph node (HCC70, HCC1008 and BT549) expressed relatively high amounts of ALCAM mRNA (Fig. 1A)
Summary
Activated leukocyte cell adhesion molecule (ALCAM) is implicated in the prognosis of multiple cancers with low level expression associated with metastasis and early death in breast cancer. Despite this significance, mechanisms that regulate ALCAM gene expression and ALCAM’s role in adhesion of pre-metastatic circulating tumor cells have not been defined. We studied ALCAM expression in 20 tumor cell lines by real-time PCR, western blot and immunochemistry. Subsequent studies showed that patients with the lowest level of ALCAM transcripts develop skeletal metastasis [24], that low ALCAM correlated with an aggressive tumor phenotype and significantly negative correlation between ALCAM expression and tumor diameter and grade [25]. More recently high-level ALCAM in breast cancer tissues has emerged as a predictor of good outcome among patients treated with tamoxifen [26] and adjuvant chemotherapy [27,28]
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