The Role of the BMK1-MEKK3 Pathway in Breast Cancer

Institution: Scripps Research Institute
Investigator(s): Ta-Hsiang Chao, Ph.D. -
Award Cycle: 2000 (Cycle VI) Grant #: 6FB-0027 Award: $64,152
Award Type: Postdoctoral Fellowship
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease

Initial Award Abstract (2000)
Breast cancer and other cancers of epithelial origin (e.g., lung, bladder) often show increased growth potential through a cell surface epidermal growth factor (EGF) receptor. A complex group of cellular proteins are responsible for activity of EGF. These include the mitogen-activated protein (MAP) kinases. Our interest is in a specific type of MAP, called big MAP kinase, BMK1. This BMK1 growth controlling protein appears to have unique structural elements, and may provide a key understanding of how growth control operates in breast cancer. We have demonstrated that EGF and heregulin (the activator of the Her-2/neu) can work through BMK1. More importantly, my preliminary studies have demonstrated that the activity of BMK1 is essentially required for heregulin-induced cell proliferation and cell cycle progression in breast cancer cell lines. This strongly suggests the BMK1 activity is involved in breast cancer cell proliferation.

By using a yeast system we were able to identify a key regulator of BMK1 pathway. These new discoveries allow us to define a novel signaling pathway, perhaps unique to breast cancer. As a logical extension of my preliminary studies, I will focus on further studies of how the BMK1 activity is regulated and its role in breast cancer development. My objectives are to: 1) study the structure/function of a protein, called MEKK3, for the activation of BMK1, 2) isolate regulatory molecule(s) of MEKK3 using an approach in yeast cells as developed from my previous research.

Growth control in normal and cancer cells has complex unique and common elements. This project could provide a better understanding of how this BMK1 pathway is essential to uncontrolled growth in breast cancer cells. This would put us in a position to both design breast cancer-specific inhibitors and to develop rapid ‘read-out’ assays to determine the effectiveness of inhibitors at other points in these complex pathways.

Final Report (2001)
Note: The PI resigned the grant in the second year to accept a position in industry.

The erbB2 (Her2/neu) gene encodes a transmembrane glycoprotein, which is a member of the epidermal growth factor receptor (EGFR) family and is among those most frequently altered in human tumorigenesis. Moreover, extensive investigation has demonstrated that overexpressionof erbB2 plays a critical role in the development and progression of human breast cancer. ErbB receptors promote tumor progression by sending oncogenic signals into cells through its cytoplasmic signaling domain. In this regard, I have discovered a signaling effector (MEKK3) that mediates the oncogenic message generated from erbB2 to a mitogen-activated protein kinase whose activity is required for the proliferation of breast cancer cells.

To assess the structural basis for the regulatory role of MEKK3 in cancer cell proliferation, my first aim was to genetically engineered a number of MEKK3 mutants containing modifications in regions which are critical for its activity. These mutants have been successfully expressed in breast cancer cells, and their effects on ErbB dependent cell activation awaits future study. A second aim that was to investigate the mechanism how ErbBs transmit the signal to MEKK3. My approach in this aim was to isolate the upstream regulator(s) for MEKK3. For this reason, I generated MEKK3 as "bait" and used it in a library screening based on the yeast-two hybrid system. We obtained several cellular proteins that physically interacted with MEKK3. One of the MEKK3-binding clones was named Trio. I cloned and generated several mutants forms of Trio. They were sequenced and expression and introduced into HEK293 mammalian cell for study. My preliminary data indicated that the kinase domain within Trio is essential for its physical interaction with MEKK3.

The relevance of Trio and other proteins isolated from the yeast two-hybrid screening in MEKK3 activation pathway by ErbB ligand stimulation will require further study. Since MEKK3 is regulating an ErbB-dependent mitogenic pathway, whose activity is required for the development of certain breast cancers, my studies might provide new thinking into mechanisms by which the oncogenic properties of erbBs are regulated.