Inhibition of Estrogen Signaling

In This Section:

• Estrogen Signaling in Normal Cells
• Estrogen Signaling in Cancer Cells
• Inhibiting Estrogen Signaling
• Self Test

Estrogen Signaling in Normal Cells

Some of the earliest targeted therapies were aimed at disrupting the activity of the hormone estrogen in breast cancer cells.

In premenopausal women, estrogen is made primarily in the ovaries. However, estrogen is also made in smaller amounts by other tissues, including adipose (fat) tissue and the breast. Estrogen travels through the circulatory system to reach its target organs, which include the breast.

During normal human development, the menstrual cycle, and pregnancy and lactation, estrogen and other hormones stimulate the growth and development of epithelial cells in the breast.

Estrogen is a steroid hormone that can easily cross the plasma membrane of the cell and then enter the cell's nucleus. Once inside the nucleus, estrogen binds to estrogen receptor proteins. The estrogen-receptor complex then binds to specific sequences of DNA and regulates the expression of numerous genes involved in cell growth.

Estrogen Signaling in Cancer Cells

Many years ago, it was discovered that removal of the ovaries led to the regression of some breast tumors. It is now known that this occurred because the tumors were dependent on estrogen for growth.

Approximately 75% of breast tumors rely on estrogen. These tumors are referred to as "estrogen dependent." Tumors that do not rely on estrogen for growth are called "estrogen independent."

Estrogen dependence can usually be predicted by the presence of estrogen receptors in tumor cells. The estrogen receptor protein can be detected in breast tumor biopsy specimens using a technique known as immunohistochemical staining. Tumor cells that express estrogen receptors--called ER positive--are usually estrogen dependent. Tumors that lack estrogen receptors--called ER negative--are usually estrogen independent.

Inhibiting Estrogen Signaling

Two different types of antihormone therapies are used to treat women with ER-positive breast tumors: selective estrogen receptor modulators and aromatase inhibitors.

Selective estrogen receptor modulators, or SERMs, which include drugs such as tamoxifen, compete with estrogen for binding to the estrogen receptor. When tamoxifen is bound to the estrogen receptor, estrogen is unable to bind, and estrogen signaling is disrupted.

Aromatase inhibitors, which include drugs such as anastrozole, interfere with the body's ability to make estrogen. They do so by blocking the activity of aromatase, an enzyme needed for the final steps of estrogen production. Aromatase inhibitors do not work very efficiently in premenopausal women because the ovaries make too much aromatase. Therefore, they are used mainly in postmenopausal women.

Anti-hormone therapies such as tamoxifen and anastrozole can be used to treat most stages of breast cancer. Women with early-stage breast cancer are usually treated with surgery followed by antihormone therapy and radiation therapy, and sometimes chemotherapy.

Women with inoperable, metastatic breast cancer are often given antihormone therapy, with or without chemotherapy.

Antihormone therapies can also be used to reduce the risk of developing breast cancer. Tamoxifen and another selective estrogen receptor modulator, raloxifene, and several aromatase inhibitors have been found to reduce the risk of breast cancer among women at high risk for the disease.

Self Test

Questions

1. Aromatase inhibitors act directly on the estrogen receptor.
   1. True
   2. False

Answers

1. Correct Answer: b
   1. True - Incorrect.
      Aromatase inhibitors interfere with the activity of aromatase, the enzyme responsible for the final steps of estrogen production. They do not interact directly with the estrogen receptor.
   2. False - Correct.
      Aromatase inhibitors interfere with the activity of aromatase, the enzyme responsible for the final steps of estrogen production. They do not interact directly with the estrogen receptor.

Inhibition of HER2

In This Section:

• Inhibition of HER2
• HER2 in Cancer Cells
• Inhibiting HER2
• Self Test

Inhibition of HER2

The epidermal growth factor receptor family consists of four cell surface receptors: EGF receptor, also called HER1; HER2/neu; HER3; and HER4. Binding of specific growth factors, or ligands, to three of these receptors causes them to interact, or dimerize, either with a receptor of the same type or with another family member. HER2 is called an "orphan receptor" because it does not interact directly with any ligand. Instead, it dimerizes with ligand-bound EGF receptor, HER3, or HER4.

Receptor dimerization activates signaling pathways inside the cell. These pathways lead to cell growth, proliferation, and survival.

HER2 in Cancer Cells

The HER2 gene is amplified in 20% of breast cancers. This means that the cells of these cancers have extra copies of the HER2 gene. Breast cancers with amplified HER2 genes, referred to as HER2-positive cancers, make more HER2 protein than HER2-negative cancers.

The extra HER2 protein causes increased signal pathway activation, which contributes to the uncontrolled growth and survival of these cancers.

Breast tumors that overexpress HER2 protein are more aggressive than other breast tumors. Patients with these tumors have a poorer prognosis and decreased chance of survival compared with patients whose tumors do not overexpress HER2.

Inhibiting HER2

Researchers have developed multiple strategies for interfering with HER2 signaling. These include a small molecule called lapatinib as well as monoclonal antibodies.

Herceptin® (trastuzumab) is a monoclonal antibody that binds to HER2. This prevents the receptor from activating the pathways that promote the proliferation and survival of breast cancer cells.

Herceptin® (trastuzumab) may also interfere with cancer cell growth by activating an immune response.

Herceptin® (trastuzumab) is used to treat breast cancer only if the tumor overexpresses HER2. It has been approved by the FDA for the treatment of metastatic breast cancer in combination with or following administration of standard chemotherapy. Herceptin® (trastuzumab) has also been approved by the FDA as an adjuvant treatment for some earlier-stage breast cancers. Adjuvant therapy is treatment given after primary therapy--for example, surgery--to increase the chances of long-term survival.

Self Test

Questions

1. Possible mechanisms of action of Herceptin® (trastuzumab) include:
   1. Activation of an immune response.
   2. Reduction of circulating levels of growth factor.
   3. Both A and B.

Answers

1. Correct Answer: a
   1. Correct.
      Herceptin® (trastuzumab) is thought to activate an immune response called antibody-dependent cell-mediated cytotoxicity. Other possible mechanisms of action include preventing the dimerization of EGFR family receptors, increasing the rate of degradation of HER2, and preventing the extracellular domain of HER2 from being processed properly.
   2. Incorrect.
      Herceptin® (trastuzumab) interacts with HER2, which is a cell surface receptor, and does not influence levels of circulating growth factor. Potential mechanisms of action of trastuzumab include activating an immune response, preventing the dimerization of EGFR family receptors, increasing the rate of degradation of HER2, and preventing the extracellular domain of HER2 from being processed properly.
   3. Incorrect.
      Trastuzumab interacts with HER2, which is a cell surface receptor, and does not influence levels of circulating growth factor. Potential mechanisms of action of Herceptin® (trastuzumab) include activation of an immune response, preventing the dimerization of EGFR receptors, increasing the rate of degradation of HER2, and preventing the extracellular domain of HER2 from being processed properly.

Inhibition of Insulin-Like Growth Factor Signaling

In This Section:

• IGF Signaling in Normal Cells
• IGF Signaling in Cancer Cells
• Inhibiting IGF Signaling
• Self Test

IGF Signaling in Normal Cells

The insulin-like growth factor—or IGF—signaling pathway plays an important role in normal breast development and during pregnancy and lactation. The IGF signaling pathway can be activated by two growth factors, IGF1 and IGF2. When one of these binds to an IGF1 receptor, the receptor activates downstream signaling pathways inside the cell. These signaling pathways regulate cell growth, proliferation, and differentiation.

IGF Signaling in Cancer Cells

There is strong evidence that alteration of IGF signaling plays a role in breast cancer. Preclinical studies in cell culture and animal models suggest that dysregulation of the IGF signaling pathway promotes the transformation, survival, growth, and metastasis of breast cancer cells. Epidemiologic studies have found that premenopausal women with breast cancer have high levels of circulating IGF1. And immunohistochemical staining has revealed that the IGF1 receptor is overexpressed in nearly half of all breast cancers.

Furthermore, there is evidence that the IGF signaling pathway participates in crosstalk with other signaling pathways, such as those activated by EGF receptor, HER2, VEGF receptor, and mTOR.

Inhibiting IGF Signaling

There are several potential strategies for targeting the IGF pathway. One approach is to use hormone inhibitors to reduce circulating levels of IGF1 and IGF2.

Other agents are being developed to directly target either IGF1 or the IGF1 receptor.

Several monoclonal antibodies are being developed to target the IGF1 receptor. These antibodies may prevent IGF1 or IGF2 from binding to the receptor and, thus, prevent activation of the pathway. They also appear to reduce levels of IGF1 receptor.

Several monoclonal antibodies directed against IGF1 receptor and small-molecule inhibitors of this receptor are currently being tested in breast cancer patients in Phase I and Phase II clinical trials. Many of these trials are testing these agents in combination with standard chemotherapy drugs, antihormone therapies, or drugs that target HER2 or mTOR.

Self Test

Questions

1. The IGF1R is commonly mutated in breast cancer.
   1. True
   2. False

Answers

1. Correct Answer: b
2. 1. True - Incorrect.
      There is no evidence that IGF1R mutations contribute to breast cancer. However, breast tumors may express high levels of normal IGF1R.
   2. False - Correct.
      There is no evidence that IGF1R mutations contribute to breast cancer. However, breast tumors may express high levels of normal IGF1R, leading to increased activity of the pathway.
      
      

Inhibition of PARP

In This Section:

• PARP in Normal Cells
• PARP in Cancer Cells
• Inhibiting PARP
• Self Test

PARP in Normal Cells

When a normal cell's DNA is damaged or mutated, several cellular mechanisms can come into play to detect and repair the alterations. If the DNA is repaired successfully, the cell survives. However, if the DNA cannot be repaired, the cell will undergo a form of cellular suicide called apoptosis rather than risk passing on flawed genetic information to progeny cells.

One protein involved in repairing damaged DNA is poly(ADP ribose) polymerase 1, or PARP1. When a strand of DNA is broken, or nicked, PARP1 moves to the site of damage and becomes activated. It then recruits a team of DNA repair proteins that work together to mend the broken strand of DNA.

PARP in Cancer Cells

Many standard cancer treatments, including many chemotherapy drugs and radiation therapy, damage the DNA of rapidly dividing cancer cells. If PARP is able to help repair the damage caused by these agents, tumor cells may be more likely to survive and grow.

Inhibiting PARP

However, preclinical studies suggest that standard therapies combined with PARP inhibitors may be more effective than standard therapies alone. If cancer cells are exposed to a PARP inhibitor, the protein will be unable to respond when the cell's DNA is damaged by treatments such as chemotherapy. The presence of unrepaired DNA damage will make the cell more likely to undergo apoptosis.

Clinical trials studying PARP inhibitors in combination with standard chemotherapeutic agents in breast cancer are currently under way.

It is also possible that PARP inhibitors could be effective as single agents against tumors with inherent DNA repair defects, such as breast tumors with mutations in the DNA repair proteins BRCA1 or BRCA2. Preclinical studies have shown that breast tumor cells carrying BRCA mutations undergo an arrest of the cell cycle and apoptosis when exposed to PARP inhibitors, whereas cells with normal BRCA proteins survive and continue to grow.

Several PARP inhibitors are currently being tested in clinical trials involving women with BRCA mutation-associated breast cancer.

Self Test

Questions

1. Women whose breast tumors have mutations in BRCA1 or BRCA2 overexpress PARP.
   1. True
   2. False

Answers

1. Correct Answer: b
   1. True - Incorrect.
      There is no evidence that BRCA-mutation-associated tumors have increased expression of PARP. However, it appears that cancer cells with BRCA1 or BRCA2 mutations are more sensitive to PARP inhibitors (i.e., more likely to undergo growth arrest and apoptosis) than cells with normal BRCA1 or BRCA2. This is most likely because the combination of inhibition of PARP and loss of BRCA1 or BRCA2 function results in inactivation of two major forms of DNA repair, making it more difficult for cells to maintain the integrity of their genome and leaving them more vulnerable to apoptosis.
   2. False - Correct.
      There is no evidence that BRCA-mutation-associated tumors have increased expression of PARP. However, it appears that cancer cells with BRCA1 or BRCA2 mutations are more sensitive to PARP inhibitors (i.e., more likely to undergo growth arrest and apoptosis) than cells with normal BRCA1 or BRCA2. This is most likely because the combination of inhibition of PARP and loss of BRCA1 or BRCA2 function results in inactivation of two major forms of DNA repair, making it more difficult for cells to maintain the integrity of their genome and leaving them more vulnerable to apoptosis.
      
      

Inhibiting the PI3K/Akt/mTOR Signaling Pathway

In This Section:

• PI3K/Akt/mTOR Signaling in Normal Cells
• PI3K/Akt/mTOR Signaling in Cancer Cells
• Inhibiting PI3K/Akt/mTOR Signaling
• Self Test

 

PI3K/Akt/mTOR Signaling in Normal Cells

mTOR is a critical regulator of several normal cell processes in numerous cell types, including cells of the breast. Several other proteins—including PI3-kinase, Akt, and PTEN—play roles in mTOR signaling.

PI3-kinase is an enzyme that phosphorylates certain components of the cell membrane. Once these components become phosphorylated, they bind to a protein called Akt. Akt then becomes phosphorylated and activated. This triggers activation of several downstream signaling pathways, which increase cell survival, proliferation, and cell growth.

One important player in the growth and proliferation pathways is mTOR. When activated by Akt, mTOR promotes cell growth and proliferation by stimulating protein synthesis.

In addition to receiving signals from Akt, mTOR monitors the cell's environment for the presence of growth factors and nutrients. If the cell needs additional resources, mTOR can increase nutrient uptake and promote angiogenesis.

mTOR can also increase the activity of Akt, thus enhancing the other downstream effects of this protein.

Because mTOR and its signaling partners are so powerful, the cell has mechanisms in place to regulate them. One important watchdog is PTEN. PTEN removes the phosphate groups added to membrane phospholipids by PI3-kinase. This prevents activation of Akt and its downstream pathways.

 

PI3K/Akt/mTOR Signaling in Cancer Cells

The signaling pathway that includes mTOR is highly active in many cancer cells. This can be the result of amplification or mutation of the PI3-kinase gene; amplification or mutation of the Akt gene; or loss of function of PTEN. Increased activity of some growth factor receptors can also enhance the activity of the pathway.

Activation of the mTOR pathway is associated with poor prognosis in many cancers, including breast cancer, and is linked to resistance to many types of therapy.

 

Inhibiting PI3K/Akt/mTOR Signaling

Drugs targeting PI3-kinase, Akt, and mTOR are being tested in preclinical models and clinical trials.

One mTOR inhibitor is a small molecule called rapamycin. Rapamycin enters the cell and binds to a protein called FKBP12. This complex binds to and inhibits mTOR. Inhibition of mTOR with rapamycin has been found to slow cancer cell proliferation.

mTOR inhibitors, including rapamycin, are being tested in clinical trials for breast cancer. Many of these trials are using mTOR inhibitors in combination with standard therapies or other targeted therapies, such as a monoclonal antibody against HER2.

Researchers are also working to develop assays to identify patients in whose tumors mTOR or its signaling partners are highly activated because these patients may be more likely to benefit from treatment with combinations of therapies that include mTOR inhibitors.

 

Self Test

Questions

1. mTOR is involved in the following cellular process(es):
   1. Protein synthesis
   2. Nutrient uptake
   3. Both A and B.

Answers

1. Correct Answer: c
   1. Partially correct.
      mTOR does play a role in protein synthesis, but it can also influence a cell's uptake of nutrients.
   2. Partially correct.
      mTOR does play a role in nutrient uptake, but it also influences protein synthesis.
   3. Correct.
      mTOR is involved in both protein synthesis and nutrient uptake.