The transcription factor Krox-20 controls Schwann cell myelination. Schwann cells in Krox-20 null mice fail to myelinate, and unlike myelinating Schwann cells, continue to proliferate and are susceptible to death. We find that enforced Krox-20 expression in Schwann cells cell-autonomously inactivates the proliferative response of Schwann cells to the major axonal mitogen β–neuregulin-1 and the death response to TGFβ or serum deprivation. Even in 3T3 fibroblasts, Krox-20 not only blocks proliferation and death but also activates the myelin genes periaxin and protein zero, showing properties in common with master regulatory genes in other cell types. Significantly, a major function of Krox-20 is to suppress the c-Jun NH2-terminal protein kinase (JNK)–c-Jun pathway, activation of which is required for both proliferation and death. Thus, Krox-20 can coordinately control suppression of mitogenic and death responses. Krox-20 also up-regulates the scaffold protein JNK-interacting protein 1 (JIP-1). We propose this as a possible component of the mechanism by which Krox-20 regulates JNK activity during Schwann cell development.

Eph receptor-ephrin signals are important for controlling repulsive and attractive cell movements during tissue patterning in embryonic development. However, the dynamic cellular responses to these signals at cell-cell contact sites are poorly understood. To examine these events we have used cell microinjection to express EphB4 and ephrinB2 in adjacent Swiss 3T3 fibroblasts and have studied the interaction of the injected cells using time-lapse microscopy. We show that Eph receptors are locally activated wherever neighbouring cells make contact. This triggers dynamic, Rac-regulated membrane ruffles at the Eph-ephrin contact sites. Subsequently, the receptor and ligand cells retract from one another, concomitantly with the endocytosis of the activated Eph receptors and their bound, full-length ephrinB ligands. Both the internalization of the receptor-ligand complexes and the subsequent cell retraction events are dependent on actin polymerization, which in turn is dependent on Rac signalling within the receptor-expressing cells. Similar events occur in primary human endothelial cells. Our findings suggest a novel mechanism for cell repulsion, in which the contact between Eph-expressing and ephrin-expressing cells is destabilized by the localized phagocytosis of the ligand-expressing cell plasma membrane by the receptor-expressing cell.

Purpose: This study investigated the functional and clinical significance of integrin avb6 upregulation in myoepithelial cells of ductal carcinoma in situ (DCIS).Experimental Design: Archival samples of DCIS and DCIS with associated invasion (n ¼ 532) were analyzed for expression of avb6 by immunohistochemistry and ability to predict recurrence and progression assessedinanindependent,uniquecohortofDCIScaseswithlong-termfollow-up.Primarymyoepithelialcells and myoepithelial cell lines, with and without avb6 expression, were used to measure the effect of avb6 on growth and invasion of tumor cell lines in vitro and in a xenograft mouse model. Involvement of TGFb signaling was established using mink lung epithelial cell (MLEC) assay and antibody inhibition, and expression and activation of matrix metalloproteinase (MMP)-9 established by Real Time-PCR and zymography.Results: Expression of avb6 is significantly associated with progression to invasive cancer (P < 0.006) and with recurrence over a median follow-up of 114 months in a series of matched DCIS cases treated with local excision. We show that expression of avb6 drives myoepithelial cells to promote tumor cell invasion in vitro and enhances mammary tumor growth in vivo. The tumor-promoting effect of avb6-positive myoepithelial cells is dependent on TGFb-driven upregulation of MMP9 and can be abrogated by inhibiting this pathway.Conclusion: These findings indicate that altered myoepithelial cells in DCIS predict disease progression and recurrence and show that upregulation of avb6 on myoepithelial cells generates a tumor promoter function through TGFb upregulation of MMP-9. These data suggest that expression of avb6 may be used to stratify patients with DCIS. Clin Cancer Res; 20(2); 344-57. Ó2013 AACR.