Hormonal Regulation of TGF Beta During Mammary Development

Institution: Lawrence Berkeley National Laboratory
Investigator(s): Mary Helen Barcellos-Hoff, Ph.D. -
Award Cycle: 1998 (Cycle IV) Grant #: 4PB-0136 Award: $758,817
Award Type: Request for Applications
Research Priorities
Biology of the Breast Cell>Biology of the Normal Breast: the starting point

Initial Award Abstract (1998)
The development of both the normal breast and breast cancers are regulated by ovarian hormones, progesterone and estrogen. However, it has long been postulated that these hormones have their effects by inducing certain growth factors that regulate cell growth rates. We propose that transforming growth factor-b (TGF-b), which has been found in different studies to both promote and inhibit human breast cancer, is differentially regulated by estrogen and progesterone, and in turn that it mediates breast development.

It is already well established that TGF-b can profoundly inhibit epithelial cell proliferation and morphogenesis. However, it is not known when this action occurs in the normal breast or how it is regulated. The activity of TGF-b is restrained by its production in a latent complex. TGF-b needs to be released from this complex in an extracellular process called activation in order to influence cells. Studies in our laboratory showed that certain antibodies recognize TGF-b only upon its release from the latent complex, i.e. following activation. We believe that activation is a primary regulator of TGF-b action and requires further study in mammary glands in order to understand the complex role of TGF-b in breast biology and cancer.

We have used these antibodies to obtain preliminary data that indicate that the level of activation is variable in mouse mammary epithelial cells (the cells that line the milk ducts and give rise to most mammary cancers) and is associated primarily with periods of mammary gland growth. Our preliminary data show that the endbud, a multicellular structure necessary for ductal elongation, contains a population of cells with a lot of active TGF-b, in addition to proliferating cells and cells undergoing programmed cell death. Likewise during pregnancy, mammary cells are highly proliferative but TGF-b is present in a certain subset. Since TGF-b profoundly inhibits mammary epithelial cell proliferation in culture, activation during growth periods appears paradoxical. However, TGF-b can also induce cell death, or apoptosis, and apoptosis often accompanies periods of proliferation, perhaps in order to insure that growth stops at the appropriate moment, or in order to weed out damaged cells. In the proposed studies we will define the role of TGF-b during these physiological states by determining whether the TGF-b positive cells are undergoing proliferation or apoptosis. We will also determine whether TGF-b positive cells also express the receptors for these hormones.

Our hypothesis is that during the two major periods of mammary growth, i.e. puberty and pregnancy, estrogen and progesterone differentially regulate TGF-b activity, which in turn both positively regulates apoptosis and negatively regulates proliferation. In combination with our TGF-b antibodies, we will use two new transgenic mouse models in which TGF-b or progesterone receptor have been compromised in order to define the contribution of TGF-b to hormonal regulation of mammary growth and development.

Final Report (2002)
Mammary gland development and neoplasia are regulated by ovarian hormones, progesterone and estrogen. However it has long been postulated that the action of these hormones is through the induction of certain proteins, known as growth factors, that regulate proliferation. We proposed that transforming growth factor- 1 (TGF-), a growth factor that is implicated as both a tumor suppressor and a tumor promoter in human breast cancer, is differentially regulated by estrogen and progesterone, and in turn, regulates cell proliferation during mammary development. It is well-established that TGF- can profoundly inhibit mammary epithelial cell proliferation and shape changes (morphogenesis). However it is not known when this action occurs in normal breast and how it is regulated. The activity of TGF- is restrained because it is produced as a latent complex. In order to influence cell proliferation, TGF- has to be released from this complex in an extracellular process called activation. We believe that activation is a primary regulator of TGF- action and requires further study in mammary gland in order to understand the complex role of TGF- in mammary biology and neoplasia.

We began with preliminary data that TGF-1 activity was restricted to certain mammary epithelial cells during periods of mammary proliferation such as puberty, pregnancy and estrus. During this grant, we showed that the vast majority (90%) of cells in this TGF-1 positive population do not proliferate. Moreover, this cell population was found to include all the cells that express estrogen receptor alpha (ER) and most of the cells that express progesterone receptor. To test the consequences of TGF-1 activity, we studied mice that have less than 10% of normal levels of TGF-1. We found significantly increased proliferation in the mammary epithelium of TGF-1-compromised mice at puberty, estrus and pregnancy, and also when estrogen and progesterone was administered after ovary removal. When epithelium from TGF-1-compromised mice was grafted into the mammary fat pads of normal mice, the graft grew faster than similarly grafts from normal mice, indicating that TGF-1 synthesized in the mammary epithelium is controlling proliferation. We then studied whether the ER cell population was affected by TGF-1 depletion. Compared to normal mice, significantly more ER expressing cells proliferate at estrus and in mice administered estrogen and progesterone after ovary removal. In addition the number of ER expressing cells was increased in TGF-1-compromised mice at these stages. In summary, estrogen and progesterone regulate TGF-1 activation (and therefore activity), which in turn regulates proliferation. TGF-1 activity co-localizes in cells expressing ER and prevents these cells from proliferating in response to estrogen and progesterone.

We now hypothesize that TGF- regulates the ER subpopulation that maintains proliferative potential. Since ER regulation is a key component of mammary homeostasis, we predict that disruption of TGF- action is involved in the genesis of ER positive breast cancer. This hypothesis has bearing on the therapeutic uses of anti-estrogens and potential for developing new therapies. Further, it provides new insight into the basic biology regulating tissue homeostasis in the mammary gland.

Transforming Growth Factor B and Breast Cancer: Mammary gland development.
Periodical:Breast Cancer Research
Index Medicus: Breast Cancer Res
Authors: Barcellos-Hoff MH, Ewan KBR
Yr: 2000 Vol: 2 Nbr: 2 Abs: Pg:92-99