California Pacific Currents 2002
The Socialization of Cells, Their Microenvironment, and the Roots of Cancer
Cells are universes within themselves, with a myriad of molecular players orchestrating a vast array of complex activities. Cancer cells retain this incredible potential, even while becoming masters of disguise, adaptation, and subterfuge. The research scientist is a “guest” in this strange world, peering in and attempting to resolve some of the ways in which cancer cells hijack the normal cell machinery and use it for their aberrant activities. John L. Muschler, PhD, has been looking — in some unexpected places — for clues as to how cancer cells go wayward and become increasingly menacing.
Dr. Muschler’s research universe is breast cancer and, more specifically, the extracellular matrix (ECM). The ECM occupies the space outside and between cells and organs. All cell types rely upon the ECM. This scaffolding can be seen as a lattice, similar in appearance to a finely crocheted lace. As a network, it provides mechanical support for cells and organs, as well as channels along which cells divide and migrate. It also provides the lines of communication through which roving protein messengers influence how cells respond to one another and facilitate cooperation within the larger context of the particular organ and the organism as a whole.
Extracellular Origins of Cancer
Genetic mutation, through which individual “renegade” cells arise, has been regarded by some researchers as the sole cause of cancer. This theory, however, does not explain how a defect in a single protein allows these cells to adopt a host of strategies to survive and proliferate in situations where healthy cells cannot. Metastasis is a prime example: healthy cells do not meander around the body and lodge in new organs. Under normal conditions, breast cells do not colonize bone and lung, two common sites of metastasis in breast cancer.
The primary goal of Muschler’s work is to understand how tumor cells respond to aberrant signals from the ECM. To do this, he compares the interactions of tumor cells versus normal cells with the ECM. Inappropriate responsiveness to the matrix, which serves as a barrier that normally limits the expansion of cells, is believed to be an important step in the progression and metastases of cancer. Muschler sees his job as defining the communication lines between the cell and particular components of the ECM under healthy versus cancerous conditions.
Examining mammary epithelial cells, Muschler is able to dissect functions of particular receptors on the cell membrane that communicate with and adhere to the ECM. Changes in their receptors can lead to a cellular disorganization that, if sustained, may contribute to the development of cancer. In essence, tumors can arise through a disruption of the structure that maintains cell residence in the format intended by nature.
Integrins Are Important Links between the Environment and the Cell
With the discovery of integrins, a family of molecules that tie the cell surface to the matrix outside the cell, part of the mystery was solved. Integrins are receptors embedded in the cell membrane that attach weakly to “messengers” in the ECM. An arrangement of numerous, weak bonds anchors the cell into the matrix in a way that allows it to explore its environment without detaching from it or becoming too firmly glued to it. This affiliation is essential for the cell to divide and migrate, while still observing the confines of the organ.
“In the extracellular matrix [research] world,” explains Dr. Muschler, “integrins were believed to do the majority of cell-matrix communication. I arrived at the bench with the same bias; the integrins had been discovered recently and people were putting great stock in them. Numerous members of this family were discovered and they appeared on most cell types. It was still poorly understood, however, how cells adhered to the extracellular matrix and how these adhesive interactions regulated cell function.”
Another Piece of the Puzzle: Dystroglycan
While a slew of researchers headed down the integrin track, Muschler had a hunch. “I realized that there was something missing, because you could effectively remove integrins by the addition of antibodies that limit their activity, but still observe at least partial ability of the cells to interact with the ECM. Therefore, integrins were not the only factors.”
A potential link for Muschler came through advances in a very different area of research. In muscle cells, a protein called dystrophin cross-links muscle fibers to one another and to proteins in the ECM, a cushioning system necessary for muscle contraction. A deficiency of dystrophin, caused by a mutation in the gene that codes for it, is the underlying defect in Duchenne muscular dystrophy. The cumulative stress of muscle contraction without proper support by dystrophin progressively damages the muscle. The outcome is that most individuals with Duchenne muscular dystrophy lose the ability to walk before leaving adolescence; other motor functions are also lost subsequent to that.
Dystrophin was found to work through a membrane-bound receptor. This receptor was identified as dystroglycan, a membrane-spanning protein that connects the interior and exterior cell environments. The leap for Muschler was the realization that dystroglycan might be a critical communication vehicle in mammary cells as well. “Dystroglycan had been overlooked in studies of mammary epithelial cell function,” explains Dr. Muschler, whose work went on to demonstrate that dystroglycan was central to epithelial cell structure and growth. “My studies showed that dystroglycan facilitates formation of normal cell architecture and helps dictate when and how cells proliferate.”
Might there be a corresponding role in cancer? “At that time, I had not yet made the connection between dystroglycan and cancer. The critical insight,” elaborates Dr. Muschler, “came when I noticed that dystroglycan was altered in the majority of breast cancer cell models.” Abundant and functional in normal cells, analysis of breast cancer cells showed that these cells were characterized by a loss of dystroglycan and its corresponding role in maintaining cell and tissue order. “Through this line of research,” he adds, “I was able to discern a role for dystroglycan in breast cancer.”
With a link established and momentum building, Muschler was faced with the logistical hurdles of forging ahead in a new field. “Dystroglycan research is unlike research in the integrin field, where antibodies and chemicals can be ordered from catalogues.” The research demanded new tools to delve into dystroglycan function. These new tools were developed through collaborations with researchers such as Dr. Kevin Campbell of the University of Iowa. “Over time, dystroglycan-specific reagents and a mouse experimental model were in place to explore the role of dystroglycan.”
Dystroglycan and Its Role in Cancer Growth
His tenacity ultimately paid off: Muschler was able to show that the loss of dystroglycan observed in many breast cancers correlated with more aggressive, or advanced, cancer cell behavior. Thus, dystroglycan seems to get lost or “misplaced” by tumor cells as they progress, an omission that contributes to the tumor’s ability to metastasize.
The reciprocal also proved true: restoration of dystroglycan function reduced the tumor-like potential of the cell. By introducing dystroglycan back into the system, Muschler found that it was possible to thwart the cancer’s ability to progress to a more aggressive kind of mass. This discovery lends insight into the mechanisms that tumors exploit to grow. It also affords a potential tool for the management of breast cancer. “This information not only helps us understand the root causes of tumor cell behavior but also creates new strategies aimed at taming cancer cells. If we can find ways of restoring this protein, we can potentially slow cancer cell growth and reduce aggressiveness in some tumors.” Work, he says, that is still a ways off.
As for acceptance of his findings, “Many still believe the integrins do most of the work,” he admits, “but I am finding otherwise. There remains a lot of exciting work to be done.” Apparently the granting agencies agree. Muschler receives support through the Department of Defense Breast Cancer Research Program (U.S. Army) IDEA Grant. Muschler is also a current recipient of the New Investigator Award from the California Breast Cancer Research Program, which is funded in part by the state of California tobacco tax.
Is this everybody’s cup of tea? For John Muschler, PhD, “What could be more interesting than to understand how cells assemble themselves and cooperate to form a living organism?” Whether it be further investigation into the role of the ECM, or dystroglycan, or other such molecules, Muschler’s work holds promise for a clear understanding of how normal cells function and communicate, as well as gives us clues as to how these functions are disturbed in cells that become tumorous. Knowing how or where the signal comes from that creates a cancer cell may someday lead to reinstating the proper signals and even finding a cure for diseases such as breast cancer.