Recent evidence supports a role for EphB receptor tyrosine kinases as tumour suppressors in colorectal and prostate cancer. However, it is unclear how these receptors inhibit cancer cell tumorigenicity - an activity that is highly unusual for a family of receptor tyrosine kinases. Here, we report that the EphB4 receptor can behave as a tumour suppressor in a mouse xenograft model of breast cancer when stimulated by its ligand, ephrin-B2. In breast cancer cells, EphB4 activates an antioncogenic pathway involving Abl family tyrosine kinases and the Crk adaptor protein. This Abl-Crk pathway inhibits breast cancer cell viability and proliferation in addition to motility and invasion, and also downregulates the pro-invasive matrix metalloprotease, MMP-2. Consistent with these effects, EphB4 and the Abl-Crk pathway are constitutively active in non-transformed mammary epithelial cells. These findings identify a novel Eph receptor signalling pathway with tumour-suppressor activity and predict that therapeutic intervention to activate EphB4 signalling will inhibit tumour progression.

The Ets family of transcription factors in mouse or humans is comprised of at least 26 unique family members that contain an evolutionarily conserved DNA binding domain called the Ets domain. The Ets family includes both transcriptional activators and repressors. These factors have been implicated as critical mediators of a wide range of cellular processes, including development, differentiation, growth, and transformation (reviewed in Ref.

The oncogene v‐myb and its cellular progenitor c‐myb encode nuclear, DNA binding phosphoproteins that control the expression of certain target genes in immature hematopoietic cells. Here, we report the isolation of a myb‐related chicken gene, chicken B‐myb. We show that expression of B‐myb, unlike that of c‐myb, is not restricted to hematopoietic cells, suggesting that B‐myb functions in a broader spectrum of cell types than c‐myb. We have identified the authentic chicken B‐myb protein as a nuclear protein of approximately 110 kDa. We show that the B‐myb protein specifically recognizes v‐myb binding sites in vitro and that binding is mediated by an N‐terminally located DNA binding domain. Although B‐myb protein recognizes myb binding sites, B‐myb fails to transactivate several myb‐responsive gene constructs as well as the endogenous myb‐responsive gene mim‐1. Instead, we find that B‐myb represses v‐myb‐ and c‐myb‐mediated activation of the mim‐1 gene, most likely by competing with other myb proteins for binding sites. Our results raise the possibility that B‐myb is an inhibitory member of the myb family.

Ets transcription factors are important downstream targets of oncogenic Ras. The transcriptional activity of several Ets family members is regulated by Ras, and interfering with Ets-dependent transcription by expression of just the Ets2 DNA binding domain can inhibit or reverse Ras-mediated cellular transformation. To better understand the role of Ets proteins in Ras transformation, we have now analyzed the effects of stably expressing a variety of Ets2 constructs in Ras-transformed NIH3T3 (DT) cells. Expression of only the Ets2 transactivation domains, which also inhibits Ras or Neu/ErbB-2-mediated activation of Ets-dependent transcription, strongly inhibited anchorage-independent growth, but did not revert the transformed DT cell morphology. Unexpectedly, high expression of full-length Ets2, a transcriptional activator, broadly reversed the transformed properties of DT cells, including anchorage-independent growth, transformed morphology, and tumorigenicity, but did not impair attached cell growth. Increasing full-length Ets2 transcriptional activity by fusing it to the VP16 transactivation domain enhanced its ability to reverse DT cell transformation. Mutational analysis revealed that the mitogen-activated protein kinase phosphorylation site required for Ras-mediated activation, Ets2(T72), was not essential for Ets2 reversion activity. The distinct reversion activities of the highly expressed Ets2 transactivation domains or full-length Ets2, along with the specific reversion activity by Ets2 constructs that either inhibit or activate Ets-dependent transcription, suggests multiple roles for Ets factors in cellular transformation. These results indicate that several distinct approaches for modulating Ets activity may be useful for intervention in human cancers.