Just as avoiding apoptosis is a crucial component of transformation, the ability to avoid anoikis — apoptosis resulting from lack of adhesion — seems to be required for metastasis. Researchers have only begun to understand this process at the molecular level, but a functional screen designed by Daniel Peeper and colleagues has identified the neurotrophic receptor TRKB as an important factor in both anoikis inhibition and metastasis.

To identify suppressors of anoikis, Peeper's group screened for the ability of rat intestinal epithelial cells transduced with a retroviral cDNA expression library to survive transfer from an adhesive culture to a non-adhesive one. This move resulted in the death of most cells, but surviving clones were considered to be resistant to anoikis. One of these clones was found to carry a cDNA encoding TRKB — a tyrosine kinase receptor that has well-known roles in preventing apoptosis in the developing mammalian nervous system.

The authors showed that overexpression of TRKB in the epithelial cells allowed them to proliferate as large spheroid aggregates in suspension. TRKB is the receptor for brain-derived neurotrophic factor (BDNF), and co-expression of TRKB and BDNF promoted further proliferation. The effects of TRKB seemed to be mediated through inhibition of apoptosis, as TRKB suppressed upregulation of the pro-apoptotic executioner caspase 3 when cells were removed from a substrate. Peeper and colleagues also showed that TRKB expression activated AKT and phosphatidylinositol 3-kinase — signalling molecules that have previously been shown to suppress anoikis.

Resistance to anoikis is an important aspect of metastasis, as tumour cells must acquire the ability to survive while floating in the lymph and blood circulation. TRKB and BDNF are overexpressed in certain human tumour types, including neuroblastoma, and are associated with aggressive behaviour and poor prognosis. TRKB has not yet, however, been clearly established as an oncogene. The authors found that when TRKB was overexpressed in non-malignant epithelial cells and then transplanted into nude mice, these cells rapidly formed tumours that metastasized throughout the body. The effect was even more marked with cells that overexpressed both TRKB and BDNF.

The authors propose that TRKB activation is an important event in transformation and the initiation of metastasis. Further experiments are required to determine the exact mechanism by which it allows cells to survive in the absence of substrate. TRK-family inactivating drugs are already under development, and similar screens could be used to identify other metastasis-associated genes.