Tamoxifen-Induced Endometrial Cell Transformation

Institution: Salk Institute for Biological Studies
Investigator(s): Zhimin Lu, M.D., Ph.D. -
Award Cycle: 2000 (Cycle VI) Grant #: 6FB-0108 Award: $85,968
Award Type: Postdoctoral Fellowship
Research Priorities
Etiology and Prevention>Etiology: the role of environment and lifestyle

Initial Award Abstract (2000)
Breast cancer is the most common form of cancer in American women, accounting for one out of three cancer diagnoses. Tamoxifen, which has anti-estrogen activity, is effective in both breast cancer treatment and prevention. Tamoxifen reduces the rate of primary breast cancer development by 45% among high risk women. However, it has been reported that long-term treatment with tamoxifen increases the occurrence of uterine cancer.

Besides having anti-estrogen activity, tamoxifen is also an inhibitor for the intracellular enzyme Protein Kinase C. Protein Kinase C is a family of enzymes composed of 11 members including PKC d. The evidence has shown that PKC d may suppress the development and growth of tumors; as such, the inhibition of PKC d promotes tumor formation. Therefore, there is a possibility that the ability of tamoxifen for inhibition of PKC d is the cause for uterine cell proliferation and cancer formation.

To address this question, we will first investigate whether tamoxifen, through the inhibition of PKC d, has the tumor promoting effect on rat or mouse cells. Next, the effect of PKC d inhibition caused by tamoxifen on carcinogenesis will be further examined in human uterine cells. Finally, the differences in gene expression after tamoxifen treatment will be investigated in order to investigate which cancer related genes are turned on and /or which tumor suppressor genes are turned off upon treatment.

Through these studies, we hope we can understand the mechanism of tamoxifenís severe side effects and find approaches for developing more effective breast cancer therapies and prevention.

Final Report (2002)
Tamoxifen has been used as a therapeutic agent for breast cancer treatment and prevention, but the benefit comes at a price of increased risk of endometrial cancer that arises from the inner membrane of the uterus. Besides its anti-estrogen effects for cancer treatment, other tamoxifen effects are elicited via estrogen receptor-independent routes. Tamoxifen also inhibits activity of a group of enzymes called protein kinase C (PKC). PKC has more than 11 family members including PKC . PKC ? has been found to have the potential function to suppress tumor growth. Then, the question is whether tamoxifen-induced endometrial cancer is due to the inhibition of PKC .

We have found that tamoxifen, like other PKC chemical inhibitors, promotes rapid cell growth in a cell line in which a gene called c-Src is highly expressed (a rat fibroblast cell line, 3Y1/c-Src). The c-Src gene is a normal cellular counterpart of viral Src gene, which can cause tumor formation. Mutations in c-Src genes can lead to cancer formation, whereas abnormally high amounts ("expression") of c-Src partially transforms cells. The effects of treatment with tamoxifen appear to resemble those of rottlerin, a PKC ?specific chemical inhibitor that turns on enzymes (called ERK1/2 MAP kinases) important in the regulation of cell growth. Therefore, tamoxifen may promote the growth of the partially transformed cells by activation of ERK1/2, which is possibly through inhibition of PKC. Moreover, Tamoxifen treatment, which mimics the effect of rottlerin, greatly increases the activity of another enzyme (phospholipase D or PLD). PLD is commonly elevated in response to stimuli that can cause cell transformation.

To test whether tamoxifen has any effect on cell transformation, the soft agar assay has been used to test the transforming ability of cells given that only transformed or cancer cells can grow in it. The results showed that tamoxifen, acting like rottlerin significantly increased the transforming ability of 3Y1/c-Src cells but not their parental non-transformed cells. Therefore, tamoxifen promotes partially transformed cells to a much more transformed stage through the inhibition PKC . This may be an important mechanism for tamoxifen-induced human uterus cancer.

These studies provide important insights into the mechanism of tamoxifen-induced severe side effects and may provide approaches for developing more effective breast cancer therapies and prevention.

The PHD domain of MEKK1 acts as an E3 ubiquitin ligase and mediates ubiquitination and degradation of ERK1/2
Periodical:Molecular Cell
Index Medicus: Mol Cell
Authors: Zhimin Lu
Yr: 2002 Vol: 9 Nbr: Abs: Pg:945-956