Triple-negative breast cancer (TNBC), which accounts for 10% to 24% of invasive breast cancers, is typically a high-grade tumor with a great propensity to metastasize [
1]. Different studies grouped TNBC on the basis of immunophenotype and of RNA and DNA genomic profiles, identifying subtypes with variable potentiality of aggressiveness. In all studies, the more aggressive subtypes were those associated with the expression of immunomodulatory and stem-like molecules [
2,
3]. In particular, the ability of TNBC cells to proliferate, progress, and spread is also based on a limited sub-population of cells with properties similar to stem cells, defined as “breast cancer stem cells” (BCSCs) [
4]. Several stemness markers have been described to identify BCSCs, such as CD44, CD24, CD133, EpCAM, CD166, Lgr5, CD47, ALDH1, and ABCG2 [
5,
6]. Since their expression profiles showed a large variability within breast cancer subtypes, especially for TNBCs, none of them may be singly correlated with prognosis or with specific therapies of TNBC patients [
3,
5]. It is then undoubted that the simultaneous targeting of various markers expressed on BCSCs may have advantage in the treatment of highly aggressive breast tumors. In this context, CD133 and EpCAM are highly promising target antigens since, beyond to be markers of BCSCs, they have a direct relationship with malignancy of breast tumors. In particular, CD133 expression in breast cancer significantly correlates with tumor stage, tumor size, occurrence of lymph node metastases and sensitivity to neoadjuvant chemotherapy [
7,
8]. In TNBC, CD133
+ cells with cancer stem cell characteristics associate with vasculogenic mimicry [
9]. The recent use of CD133 to detect circulating tumor cells in TNBC patients [
10] has increased attention to this marker highlighting its role in establishing prognostic and predictive value in TNBCs. Concerning EpCAM, its over-expression was observed in up to 70% of breast tumors in which it strongly correlates with a higher risk of recurrence [
11]. The use of EpCAM as a marker for detecting disseminated breast cancer cells in bone marrow strongly suggests that EpCAM
+ breast cancer cells possess an enhanced ability to metastasize [
12]. Nevertheless, the sole over-expression of CD133 or EpCAM in TNBC was correlated with poorer prognosis in about 60% of tumors [
13,
14]. This evidence, on one hand, indicates that the selective removal of CD133
+ or EpCAM
+ cells may not be sufficient to eradicate cells with the most aggressive phenotype, like cancer stem cells, and on the other hand that the simultaneous targeting of the two surface antigens may be of clinical relevance in treatment of TNBC patients. In recent years, a toxin-based system to simultaneously target CD133 and EpCAM in the same cell was developed in different carcinoma models, including inflammatory breast carcinoma, showing a potent inhibition of proliferation in vitro and the regression of HNSCC (Head and neck squamous cell carcinoma) in vivo [
15]. Despite these encouraging results, the use on human tumors is far for being advantageous, due to the high costs of toxin generation and, more importantly, to the off-target effects or to the generation of anti-toxin antibodies having adverse effects against extended treatments [
16].
In breast tumor-derived cells with different phenotypes, we demonstrated that the expression of CD133 is strongly correlated to the levels of the beta2 isoform of the phosphoinositide-dependent phospholipase C (PLC-β2) [
17,
18], ectopically expressed in the large majority of primary invasive mammary tumors of all histological subtypes [
19]. Consistently, we also found that in both MDA-MB-231 and MDA-MB-468 cells, showing a TNBC basal-B and a basal-A phenotype, respectively [
20], the over-expression of PLC-β2 induced the conversion of CD133
high to CD133
low cells [
17].
Here we explored the possible role of PLC-β2 in modulating the expression of both CD133 and EpCAM in triple-negative breast tumor cells, in order to assess if strategies aimed to up-modulate this PLC isozyme may be useful in reducing the expression of these BCSCs markers and, eventually, in reducing the number of cells with a stem-like phenotype.