Joan Lewis-Wambi, Ph.D.
Ph.D.: Cancer Biology, Rutgers University, New Brunswick, New Jersey
Postdoctoral: Northwestern University Comprehensive Cancer Center, Chicago, IL
Despite the benefits of endocrine therapies such as tamoxifen and aromatase inhibitors in treating estrogen receptor alpha (ER)-positive breast cancer, many tumors eventually become resistant. Identifying the underlying cellular and molecular mechanisms responsible for endocrine resistance remains a critical and immediate need. Our laboratory is interested in identifying novel pathways of endocrine-resistance in breast cancer and using that knowledge to help develop alternative treatment options for patients with endocrine resistant and metastatic disease.
One of the projects in our lab involves studying the role of pigment epithelium-derived factor (PEDF) in the development of endocrine resistance in breast cancer. PEDF is a 50 kDa glycoprotein that belongs to the non-inhibitory serine protease inhibitor (SERPIN) superfamily but it does not inhibit proteases. PEDF was first discovered as a factor secreted by retinal pigment epithelial cells, but later found to be expressed in several tissues including brain, spinal cord, eye, plasma, bone, prostate, pancreas, heart and lung. PEDF is a potent antiangiogenic factor that is showing promise as a potential anticancer agent. It is considered a potential tumor suppressor because its expression is high in normal tissues but is loss or significantly reduced in various types of malignancies. Loss of PEDF expression in breast cancer tissue has been shown to be associated with disease progression and poor survival, however, the role of PEDF in antihormone resistance and its potential as a therapeutic target for preventing and/or reversing endocrine resistance in breast cancer is not known. Recently, my laboratory found that PEDF mRNA and protein levels were dramatically reduced in antihormone resistant breast cancer cells and that stable reexpression of PEDF in the resistant cells resensitized them to tamoxifen.
In addition, tissue microarray studies of primary and recurrence tumors from patients (N=109) who initially responded to tamoxifen and subsequently failed, revealed that PEDF protein was reduced in ~52.8% of recurrence tumors compared to primary tumors. Furthermore, we have found that recombinant PEDF is capable of inhibiting the growth of endocrine resistant breast cancer cells in athymic mice and that PEDF also inhibits the growth of ER-negative breast cancer cells. Based on these findings, we hypothesize that PEDF silencing is a novel mechanism for the development of endocrine-resistance in breast cancer and its expression influences the metastatic potential of ER-expressing tumors and their ability to respond to antihormonal therapy. We plan to use lentiviruses to stably overexpress PEDF in endocrine resistant breast cancer cells to help address the following questions: how does loss of PEDF expression confer resistance to endocrine therapy? 2) How is PEDF expression regulated in breast cancer cells and what role (if any) does the estrogen receptor play in PEDF regulation? 3) What is the mechanism of action of PEDF in endocrine resistant breast cancer cells in vitro versus in vivo?
Another area of research in our laboratory involves investigating the mechanism by which estrogen paradoxically induces apoptosis in endocrine resistant breast cancer cells. Specifically, our laboratory has developed two breast cancer cell lines called, MCF-7:5C and MCF-7:2A, which were clonally selected from parental MCF-7 human breast cancer cells following long-term (>1 year) estrogen deprivation. Unlike MCF-7 cells which require estrogen/estradiol to grow and whose growth is inhibited by tamoxifen and other antiestrogens, MCF-7:5C and MCF-7:2A cells are hormone independent and are resistant to tamoxifen and aromatase inhibitors and these cells undergo apoptosis in the presence of physiologic levels of 17-estradiol (E2) in vitro and in vivo. Investigation into the mechanisms of E2-induced apoptosis in MCF-7:5C and MCF-7:2A breast cancer cells reveals that it is a mitochondrial mediated event involving the BCL-2 family proteins and endoplasmic reticulum stress.
More recently, we have found evidence to suggest that the phospholipid scramblase 1 (PLSCR1) protein might be a novel mediator of estrogen-induced apoptosis in our endocrine resistant cells. PLSCR1 is a member of the PLSCR gene family that has been implicated in multiple cellular processes including movement of phospholipids, gene regulation, immuno-activation, and cell proliferation/apoptosis. PLSCR1 and other family members (PLSCR2, PLSCR3, PLSCR4, and PLSCR5) are highly induced by interferons (IFNs) such as INF-, IFN-, and to a lesser extent INF-and their induction is associated with apoptosis. We have found that suppression of PLSCR1 using siRNA completely blocks the ability of the resistant cells to undergo apoptosis in the presence of E2 as well as other apoptosis-inducing agents.
Furthermore, we have found that calcium mobilization is dramatically altered in these cells due to suppression of PLSCR1. Currently, we are investigating how PLSCR1 regulates E2-induced apoptosis in MCF-7:5C cells and what role (if any) the other family members (i.e. PLSCR2-PLSCR5) play in this process. We are also studying the clinical significance of PLSCR1 expression in invasive breast cancer and whether PLSCR1 protein has therapeutic benefits in other types of cancers such as triple negative and inflammatory breast cancer.