The cancer stem cell hypothesis asserts that malignancies arise in tissue stem and/or progenitor cells through the dysregulation or acquisition of self-renewal [1
]. Stem cells are long lived and capable of acquiring multiple mutations over time to transform to malignancy while differentiated cells turn over rapidly [2
]. Clonal expansion of stem cell populations through dysregulated self-renewal is, thus, hypothesized to be an early step in carcinogenesis [3
]. This hypothesis is supported by recent studies demonstrating that breast tissue from women who carry germline BRCA1 mutations contains islands of cells which uniformly express the stem cell marker ALDH1. These expanded stem cell colonies display loss of heterozygosity for the normal BRCA1 allele [4
]. Experimental knockdown of BRCA1 in normal mammary cells leads to an increase in the ALDH-positive stem cell population in vitro
and in mouse models[5
If primitive breast cells are the targets for transforming events, then interventions aimed at reducing this cell population may provide novel risk reduction strategies. Current strategies, which included prophylactic mastectomy for BRCA1 or BRCA2 carriers, as well as the antiestrogens tamoxifen and raloxifene, all are associated with potential toxicities. These toxicities have limited the widespread utilization of Tamoxifen and raloxifene in cancer prevention [6
]. Furthermore, hormonal interventions selectively prevent estrogen receptor (ER) positive breast cancers, but are less effective, and may actually increase the development of ER negative breast cancers [8
]. This highlights the important need to develop non-toxic strategies to effectively prevent both ER negative and ER positive breast cancer. If the cancer stem cell hypothesis is valid, then strategies aimed at targeting stem cell self-renewal pathways represent rational approaches for cancer prevention. One such pathway is the Wnt signaling pathway which is dysregulated in breast cancer, as well as many other malignancies [11
]. Although the development of specific pharmacologic Wnt inhibitors has proved a challenge, there is evidence that curcumin, a dietary phenol found in spices, is able to downregulate the Wnt signaling pathway[14
]. Interestingly, there is substantial evidence in preclinical models that curcumin is a potent chemopreventive dietary agent [15
]. This suggests that the protective effects of curcumin might be due to Wnt inhibition of self-renewal in breast stem/progenitor cells.
Another dietary polyphenol, isolated from black and long peppers, which has been reported to reduce cancer incidence in chemical rodent models of lung cancer, is piperine[20
]. It has been suggested that piperine may also enhance the bioavailability of curcumin if these chemopreventive agents are given in combination [25
]. The cancer preventive effects of piperine, as a single agent, in breast carcinogenesis have not yet been explored.
In order to determine whether the dietary polyphenols, curcumin and piperine, are able to modulate the self-renewal of normal and malignant breast stem cells, we examined the effects of these compounds on mammosphere formation and on expression of the breast stem cell marker aldehyde dehydrogenase (ALDH). We demonstrate that curcumin and piperine are able to inhibit breast stem cell self-renewal and provide evidence that these effects are mediated through inhibition of Wnt signaling. These studies suggest that stem cell self-renewal assays may serve as biomarkers for cancer prevention studies. Such assays also provide potential mechanisms for the action of dietary polyphenols as cancer preventive agents.