The Importance of Growth Inhibitory Signals in Normal Breast

Institution: University of California, Los Angeles
Investigator(s): Cindy Wilson, Ph.D. -
Award Cycle: 2002 (Cycle VIII) Grant #: 8WB-0159 Award: $150,000
Award Type: STEP Award
Research Priorities
Biology of the Breast Cell>Biology of the Normal Breast: the starting point



Initial Award Abstract (2002)
The diagnosis of breast cancer is a common and terrifying occurrence that elicits justified fear not only of the disease itself but also of the often grueling and toxic treatment regiments. Recently, however, there have been positive developments in the area of breast cancer research that have been translated into a promising new therapy for some patients. This research centers on a particular protein, HER-2, that is vastly overabundant in about 30% of breast cancers. The newly FDA approved therapy that targets HER-2 and the cancerous cells that harbor it on their surface, is called Herceptin. It is the first successful breast cancer treatment that targets a distinct genetic alteration as opposed to traditional approaches aimed essentially at all rapidly dividing cells, including normal healthy ones As such, the development of Herceptin has led the way into a new era of targeted cancer therapies. It has proven the principle that if researchers can identify specific molecular alterations that are unique to cancer cells, effective therapies can be developed.

Despite the success of Herceptin and intense investigation by our lab and others, much remains to be learned about the precise mechanism by which overabundant HER-2 leads to the development/progression of breast cancer. A more complete understanding of the events triggered by HER-2 will be required to understand why some patient's tumors do not respond to Herceptin and for the development of improved, "second generation" therapies directed at HER-2. The primary goal of this study is to the test the hypothesis that naturally occurring, growth inhibitory signals present in the normal breast tissue constitute the critical, first-line, defense mechanism against breast cancer and that HER-2 is tumor promoting because it antagonizes these inhibitors.

In order to the study the interaction between growth inhibitory signals and HER-2, we will use a cell culture assay where normal breast cells form tiny ductal structures in 3-dimensional gels made from a complex mixture of proteins that normally surrounds these cells in the human breast. Using this system, we can experimentally manipulate the concentration and activity of particular growth inhibitors to determine if the are critical for maintaining the size, shape and normal architecture of these mini breast duct structures. Once we establish the importance of a particular factor, we will test the ability of HER-2 to counter its function. We can test the influence of HER-2 in this system because we have engineered breast cells that can easily be manipulated (by the addition of an antibiotic, tetracycline) to turn on and off the HER-2 protein.

This study combines the knowledge obtained from cutting-edge DNA chip technologies with a culture assay that models normal breast structures. This potential powerful combination allows us to form a hypothesis based on a large number of gene changes associated with HER-2 overabundance and then test its functional importance in the context of breast structures. A second innovative aspect of this work is its focus on the interaction between HER-2 and growth inhibitory pathways. HER-2 is growth factor receptor and as such has mostly been studied in terms of its ability to cause cells to rapidly grow and divide. However, the idea that a major way that HER-2 accomplishes this is by antagonizing naturally occurring growth inhibitors has not been vigorously tested.


Final Report (2004)
This central topic of this research is HER-2, which is a protein that is vastly overabundant in about 30% of breast cancers. This protein has been successfully targeted by an FDA approved therapy called Herceptin that binds and blocks the function of HER-2 and slows the growth of the cancerous cells that harbor it on their surface. It is one of the first successful breast cancer treatment that targets a distinct genetic alteration as opposed to traditional approaches aimed essentially at all rapidly dividing cells, including normal healthy ones. As such, the development of Herceptin has led the way into a new era of targeted cancer therapies. Still much remains to be learned about the precise mechanism by which overabundant HER-2 leads to the development/progression of breast cancer. A more complete understanding of the events triggered by HER-2 will be required to understand why some patients do not respond to Herceptin and for the development of improved, │second generation▓ therapies directed at HER-2. The primary goal of this study is to the test the hypothesis that naturally occurring, growth inhibitory signals present in the normal breast tissue constitute the critical, first-line, defense mechanism against breast cancer and that HER-2 is tumor promoting because it antagonizes these inhibitors. HER-2 is growth factor receptor and as such has mostly been studied in terms of its ability to cause cells to rapidly grow and divide. However, the idea that a major way that HER-2 accomplishes this is by antagonizing naturally occurring growth inhibitors has not been vigorously tested.

We have been investigating the interaction between growth inhibitory proteins and HER-2 in a variety of different breast cells including nonmalignant mammary cells and cells isolated from breast cancers. We have been studying these interactions in simple culture systems where the cells grow in plastic dishes and in a more complex assay where normal breast cells form tiny ductal structures (mammary acini) in 3-dimensional gels made from a mixture of proteins that normally surrounds these cells in the human breast. Using this system, we have determined that the soluble protein called transforming growth factor beta (TGF-▀) has extremely potent growth inhibitory effects on mammary acini. This inhibitory effect is much more extreme in this assay compared to the effects observed when cells are simply grown in a monolayer in plastic dishes. We have also shown that overexpression of HER-2 can make certain cells much less sensitive to the addition of TGF-▀. Recently, however we were surprised to find that HER-2 overexpression in other breast cancer cells with highly motile and invasive properties actually exacerbates a tumor promoting response to the addition of TGF-▀.

We are currently investigating the precise mechanism by which HER-2 overexpression makes certain cancer cells less responsive to growth inhibition by TGF-▀. We believe that this property of HER-2 may be an important factor in its ability to promote tumor initiation. Furthermore, the observation that Herceptin can, at least in some situations, re-sensitize cells to TGF-▀ may help to explain its anti-tumor affects. There may therefore be therapeutic implications derived from these observations. We may be able to improve anti-HER2 therapies by testing and optimizing their ability to restore TGF-▀ sensitivity and may even be able to combine anti-HER2 drugs with those that stimulate TGF-▀. In addition, it is becoming increasing clear that the growth and invasiveness of some highly aggressive tumor cells can actually be promoted by the normally growth inhibitory protein TGF-▀. Our recent results that the HER-2 pathway can cooperate with the tumor promoting ability of TGF-▀ highlights further that the cellular context can greatly influence the biological outcome of a cancer cells exposure to a growth factor. Breast cancer is a very heterogeneous disease and we are just now beginning to recognize that this may be in part due to the type of cell from which the cancer originated.


Symposium Abstract (2003)
The diagnosis of breast cancer is a common and terrifying occurrence that elicits justified fear not only of the disease itself but also of the often grueling and toxic treatment regiments. Recently, however, there have been positive developments that have been translated into a promising new class of ôtargetedö therapies. One area of research centers on a particular protein, HER-2, that is vastly overabundant in about 30% of breast cancers. The FDA approved therapy that targets HER-2 and the cancerous cells that harbor it on their surface, is called Herceptin.

A large body of literature exists that explores the relationship between growth promoting molecules and the pathogenesis of breast cancer. Significantly less, however, is known about the growth inhibitory signals that are necessary to maintain the structure and function of the normal breast. We think it is likely that a breakdown of these inhibitory signals combines with pro-growth signals to contribute critically to the development of cancer. The primary goal of this study is to test the hypothesis that naturally occurring growth inhibitory signals present in normal breast tissue constitute the critical, first-line, defense mechanism against breast cancer and that the HER-2 protein is tumor promoting because it antagonizes these inhibitors. In order to study this interaction we use a cell culture assay where normal breast cells form tiny ductal structures in a 3 dimensional gels made from complex mixture of proteins that normally surround these cells in the human breast. Using this system, we can experimentally manipulate the concentration and function of particular growth inhibitors to determine if they are critical for maintaining the size, shape and normal architecture of these mini breast ducts. Once we establish the importance of a particular factor, we test the ability of HER-2 to counteract its function in breast cells engineered to turn on and off the HER-2 protein using the antibiotic tetracycline.

A more complete understanding of the events triggered by HER-2 will be required to understand why many tumors that carry the HER-2 alteration do not respond to Herceptin or become resistant to its effects and for the development of improved, ôsecond generationö therapies directed at HER-2. Specifically, we need to know how HER-2 interacts with other factors influencing cell growth and metastasis. We are beginning to tackle this problem by focusing on particularögrowth inhibitory molecules that our preliminary data suggests are antagonized by HER-2.