Patients diagnosed with adenocarcinoma of the lung are at substantial risk of metastasis. Even when the disease is detected at an early stage and removed by surgery, relapse with widespread metastases can be rapid. Don Nguyen, Joan Massagué and colleagues have found that the Wnt pathway seems to be important for lung adenocarcinoma metastasis to the brain and bone.

The rapid onset of metastasis implies that the pathways that mediate this process are present in a subset of early lung adenocarcinomas, so the authors screened clinically annotated lung adenocarcinoma samples for gene expression signatures that represent several activated pathways. One gene signature — expression changes downstream of the transcription factor TCF4 — identified a subgroup of patients with disease relapse in multiple organs. TCF4 is a member of the Wnt-regulated Lef/Tcf transcription factors, implying that Wnt signalling might be important for metastasis. To investigate further, the authors used two metastatic human lung adenocarcinoma cell lines and injected these into the hearts of immunocompromised mice, thereby modelling the dissemination of tumour cells from the lungs to distant organs. They then isolated the cells that formed brain metastases and subjected these to a further two rounds of in vivo selection. These BrM3 cells were able to efficiently form metastases in both brain and bone, and several analyses indicated that the Wnt–Tcf pathway was active to a greater extent in the BrM3 cells than in the parental cells.

To examine the functional significance of the Wnt–Tcf pathway in lung adenocarcinoma, the authors used dominant negative mutants of TCF1 and TCF4. Expression of these mutant transcription factors in BrM3 cells reduced their metastatic capacity in vivo. The authors also investigated three Wnt target genes — lymphoid enhancer binding factor 1 (LEF1), homeobox B9 (HOXB9) and bone morphogenetic protein 4 (BMP4). LEF1 and HOXB9 were highly expressed in BrM3 cells and this expression was reduced in the presence of a dominant negative Tcf. RNA interference assays showed that LEF1 and HOXB9 were needed for brain and bone metastasis, but that BMP4 was not. Moreover, expression of either LEF1 or HOXB9 induced metastasis to brain and bone in the parental cell lines. Further experiments in vitro indicated that LEF1 expression had a positive effect on colony formation and that LEF1 and HOXB9 were required for chemotactic invasion.

The authors speculate that the Wnt pathway could be activated in lung adenocarcinoma through the expression of Wnt ligands by stromal cells, a particular property of the tumour cell of origin or an epigenetic alteration in the Wnt pathway. It will be interesting to determine which of these factors are important.

Author: Nicola McCarthy - Copyright © 2009 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission