Growth Factor Receptor Activation in Breast Cancer

Institution: Scripps Research Institute
Investigator(s): Ichiro Maruyama, Ph.D. -
Award Cycle: 1997 (Cycle III) Grant #: 3IB-0108 Award: $267,291
Award Type: IDEA
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease



Initial Award Abstract (1997)
Cell-surface receptors are proteins that bind factors such as hormones or growth factors, and transmit the external signal inside the cell, ultimately resulting in processes such as cell growth and differentiation. These receptors reside on the cell membrane and typically consist of three parts: a sensing segment that sticks outside the cell; a middle segment that runs through the cell membrane; and a segment that sticks inside the cell and transmits the signal inside the cell. Epidermal Growth Factor Receptor (EGFR) is such a cell surface receptor. It is present in a variety of different cell types, and is implicated in breast cancer. When EGFR is turned on (activated), it can enhance the mobility of cancer cells and increase their capacity to metastasize. Therefore, knowledge of a molecular mechanism for the receptor activation is a crucial step in the understanding of human breast cancer.

It believed that one way a receptor can be activated is by a hormone binding to two molecules of the receptor and bringing them together. However, an accumulating number of observations suggest that linking of two receptor molecules is not sufficient for the receptor activation. For example, the artificial linking of two receptor molecules by antibodies does not always activate the receptors. To clarify the controversial observations, we analyzed the structure of the membrane-spanning region of the aspartate receptor, a cell surface receptor for the amino acid aspartate. Based on the results, we have proposed a model for the activation mechanism in which the membrane-spanning segment rotates or twists in the plane of the cell membrane to transmit the factor-binding signal into the inside of the cell. We propose to determine whether this model also explains how other receptors, including EGFR, are activated. We will artificially mutate the membrane-spanning region of EGFR to find the structure of the segment critical for the signaling. We will also analyze the movement of the membrane-spanning segment during the signaling. Mutant receptors that can be activated without bound factors may also be isolated in this project.

Knowledge of a molecular mechanism for activation of EGFR should be a crucial step, not only in an understanding of the pathogenesis of human breast cancer, but also towards development of anti-cancer drugs. Using computer modeling, small chemicals or peptides that inhibit activation of EGFR can be designed based on knowledge of the molecular mechanism.


Final Report (2000)
Epidermal Growth Factor Receptor (EGFR) is a cell surface receptor that transmits external signals inside the cell. It is present in a variety of cell types, and is implicated in beast cancer. When EGFR is turned on (activated), it can enhance the mobility of cancer cells and increase their capacity to metastasize. Therefore, knowledge of a molecular mechanism for turning on the receptor is a crucial step in understanding human breast cancer and developing anti breast cancer drugs.

It is believed that EGFR can be activated when a protein called EGF binds to two molecules of the receptor and brings them together. However, an increasing number of observations suggest that linking two receptor molecules is not sufficient for receptor activation. We have hypothesized that EGFR can be turned on by EGF when it restricts the movement of two EGFR molecules relative to each other in its twin (dimer) structure. EGF may act as a wedge between the two molecules of EGFR to restrict the receptor's rotation or twist. This restricted motion of the receptor may enhance activation of the receptor molecules through the addition ofphosphate residues to the receptor molecules (phosphorylation).

We have analyzed the structure function relationship of EGFR by constructing mutant EGFR molecules. To monitor the movement of the receptor molecules and effect of EGF on the receptor movement, we put an amino acid residue, cysteine, in the middle of the receptor molecule. If the cysteine residue on a receptor is close to a cysteine on the other receptor, a bridge between the two receptors is formed (cross linked dimer) and the receptor movement is restricted. We put cysteine in nine distinct places on the receptor molecule and observed the formation of cross linked dimers. All the recombinant receptors formed a dimer structure in the absence of bound EGF. The efficiency of the dimer formation was highly dependent upon the location of cysteine inserted into EGFR. The nine sites of cysteine insertion could be divided into two classes; a group (5 sites) more efficiently formed a dimer structure than the other (4 sites) in the presence or absence of EGF. More than 80% of such a cysteine mutant receptor formed a dimer structure in the absence of bound EGF. EGF marginally enhanced the formation of a dimer structure of all the mutant receptors.

These results indicate that the receptor efficiently form a dimer structure without bound EGF. This is consistent with the observations in which linking of two receptor molecules is not sufficient for EGFR activation. Rather, relative orientation of two molecules in its dimer structure seems to be critical for its activation. Further studies on the active and inactive dimer structures of EGFR may provide us with clues to understand breast cancer growth and metastasis.