Treatment of hormone-refractory breast cancer: apoptosis

Following surgery, the hormone dependence of breast tumors is exploited for therapy using antagonists such as tamoxifen, although occasional hormone-resistant clones do appear. Another chemotherapeutic strategy uses microtubule inhibitors such as taxanes. Unfortunately, these agents elicit toxicities such as leukocytopenia, diarrhea, alopecia, and peripheral neuropathies and are also associated with the emergence of drug resistance. We have previously described a tubulin-binding, natural compound, noscapine, that was nontoxic and triggered apoptosis in many cancer types albeit at 10 Mmol/L or higher concentrations depending on the cell type. We now show that a synthetic analogue of noscapine, 9-bromonoscapine, is f10-fold to 15-fold more potent than noscapine in inhibiting cell proliferation and induces apoptosis following G2-M arrest in hormone-insensitive human breast cancers (MDA-MB-231). Furthermore, a clear loss of mitochondrial membrane potential, release of cytochrome c, activation of the terminal caspase-3, and the cleavage of its substrates such as poly(ADP-ribose) polymerase, suggest an intrinsic apoptotic mechanism. Taken together, these data point to a mitochondrially mediated apoptosis of hormone-insensitive breast cancer cells. Human tumor xenografts in nude mice showed significant tumor volume reduction and a surprising increase in longevity without signs of obvious toxicity. Thus, our data provide compelling evidence that 9-bromonoscapine can be useful for the therapy of hormone-refractory breast cancer. [Mol Cancer Ther 2006;5(9):2366–77] Introduction Breast cancer is one of the most frequent malignancies among women in the U.S. and is the leading cause of death worldwide between the ages of 40 and 55 years (1, 2). This disease is controlled by surgery and radiotherapy, and is commonly supported by adjuvant chemotherapies or hormonotherapies (3). It is well established that the ovarian hormones, estrogen and progesterone, are essential for the growth and maintenance of the mammary ductal tissue (4). During postlactational regression of breasts, extensive apoptosis of ductal cells is required, whereas myoepithelial cells and basal lamina persist, and are reused during the resumption of extensive cell proliferation (5, 6). Likewise, breast tumor cells are also heavily dependent on estrogen and progesterone hormones for their maintenance and growth (4). Fortunately, very effective antagonists for these hormones exist, such as tamoxifen, which is widely used for the treatment of these tumor types usually subsequent to surgical resection (7). However, occasional loss of receptors due to genetic lesions in tumor cells and overexpression of drug efflux pumps lead to resistance towards hormone-mimetic drugs as well as other chemotherapeutic agents. Although second-generation selective estrogen receptor modulators such as raloxifene and second-line treatment options such as the aromatase inhibitors (letrozole and anastrazole) are somewhat effective, they are primarily useful against tumors that have a positive hormone receptor status. The challenge, thus, lies in the emergence of hormone-refractory tumors which no longer respond to the antihormone therapy. Another effective target for the treatment of these hormone-insensitive breast tumors is the microtubule cytoskeleton (8–12). Microtubules are ubiquitous polymers assembled from the noncovalent head-to-tail association of a-tubulin and h-tubulin (13–15). The process of microtubule assembly is highly dynamic, consequently, microtubules extend and shorten continually within the cellular milieu (16–20). This dynamic property is critical for microtubules to carry out many of their cellular functions (20–23). Especially, accurate chromosome segregation during mitosis, which requires an exquisite regulation of spindle microtubule dynamics, even a minor alteration of the microtubule dynamics could halt mitotic progression (21, 24–28). This is probably the primary basis for the use of microtubule-interfering agents in cancer chemotherapy (29, 30). Small molecules that stabilize and bundle microtubules such as taxanes are currently used to treat hormone-refractory breast tumors (31). However, due to their toxicity in normal healthy cells as well as cancer cells, they display several side effects such as leukocytopenias, alopecia, diarrhea, and peripheral neuropathies (30, 32). Received 4/13/06; revised 6/26/06; accepted 7/12/06. Grant support: NIH grant to H.C. Joshi. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Requests for reprints: Ritu Aneja or Harish C. Joshi, Department of Cell Biology, Emory University School of Medicine, 615 Michael Street, Atlanta, GA 30322. Phone: 404-727-0445; Fax: 404-727-6256. E-mail: [email protected] or [email protected]. Copyright C 2006 American Association for Cancer Research. doi:10.1158/1535-7163.MCT-06-0205 2366 Mol Cancer Ther 2006;5(9). September 2006 American Association for Cancer Research Copyright © 2006 on October 9, 2011 mct.aacrjournals.org Downloaded from DOI:10.1158/1535-7163.MCT-06-0205