Cell-to-cell signaling influences the fate of prostate cancer stem cells and their potential to generate more aggressive tumors.

An increasing number of malignancies has been shown to be initiated and propelled by small subpopulations of cancer stem cells (CSC). However, whether tumor aggressiveness is driven by CSC and by what extent this property may be relevant within the tumor mass is still unsettled. To address this issue, we isolated a rare tumor cell population on the basis of its CD44(+)CD24(-) phenotype from the human androgen-independent prostate carcinoma cell line DU145 and established its CSC properties. The behavior of selected CSC was investigated with respect to the bulk DU145 cells. The injection of CSC in nude mice generated highly vascularized tumors infiltrating the adjacent tissues, showing high density of neuroendocrine cells and expressing low levels of E-cadherin and ß-catenin as well as high levels of vimentin. On the contrary, when a comparable number of unsorted DU145 cells were injected the resulting tumors were less aggressive. To investigate the different features of tumors in vivo, the influence of differentiated tumor cells on CSC was examined in vitro by growing CSC in the absence or presence of conditioned medium from DU145 cells. CSC grown in permissive conditions differentiated into cell populations with features similar to those of cells held in aggressive tumors generated from CSC injection. Differently, conditioned medium induced CSC to differentiate into a cell phenotype comparable to cells of scarcely aggressive tumors originated from bulk DU145 cell injection. These findings show for the first time that CSC are able to generate differentiated cells expressing either highly or scarcely aggressive phenotype, thus influencing prostate cancer progression. The fate of CSC was determined by signals released from tumor environment. Moreover, using microarray analysis we selected some molecules which could be involved in this cell-to-cell signaling, hypothesizing their potential value for prognostic or therapeutic applications.

Action of retinoic acid receptor on EGFR gene transactivation and breast cancer cell proliferation: Interplay with the estrogen receptor.

In the present report, we investigated the action of retinoic acid (RA) on the transactivation of the epidermal growth factor receptor (EGFR) gene promoter. In a previous study, we showed that the estrogen receptor (ER) α activated by 17ß-estradiol (E2) increased EGFR expression by enhancing the binding of the transcription factor Sp1 to the EGFR minimal promoter in HeLa cells. Here, we demonstrate that ligand-activated RA receptor (RAR) α inhibited EGFR transactivation by competing with Sp1 for binding to the same promoter fragment in the same cell model. When RARα and ERα were coexpressed, the inhibitory effect of RA on transactivation of the EGFR promoter counteracted the enhancement induced by E2-activated ERα and became more pronounced in the presence of ligand-free ERα. In the MCF7 breast cancer cell line, which endogenously expresses RARα and ERα, RA exerted anti-proliferative effects in the presence of ligand-free ERα. Moreover, interplay between the pathways mediated by the two receptors was observed, as RA counteracted E2-induced cell proliferation. Our results suggest that the interference with the activity of Sp1 on the EGFR promoter could be related to the observed RA-mediated growth suppression of breast cancer cells.

Down-regulation of epidermal growth factor receptor induced by estrogens and phytoestrogens promotes the differentiation of U2OS human osteosarcoma cells.

In previous studies on HeLa cells we demonstrated estrogen-responsiveness of the epidermal growth factor receptor (EGFR) gene, as 17beta-estradiol (E(2)) and selective estrogen receptor modulators (SERMs) genistein (G), daidzein (D), and 4-hydroxytamoxifen (4OH-T) modulated its transcription in a ligand- and estrogen receptor (ER) isoform-specific way. This study describes further investigations into the role of ERs in mediating the effects induced by E(2) and SERMs on EGFR expression, and the relationship between the actions of ERs and EGFR in U2OS osteosarcoma cells stably expressing ERalpha or ERbeta. Cell number and DNA content determination revealed that E(2), G, and D inhibited proliferation and cell cycle progression and promoted apoptosis in both cell lines. In parallel, changes in cell morphology typical of osteoblast maturation were observed via optical microscopy. Consistently, quantitative PCR and Western blot analysis showed an up-regulation of markers of osteoblast differentiation and bone repair, and a decrease in EGFR expression. The transfection of specific antisense (AS) oligonucleotides strengthened our hypothesis that EGFR reduction caused changes in the proliferation/differentiation pattern comparable to those induced by ER ligands. The link between the ER and EGFR pathways was confirmed by treatment with 4OH-T, which decreased the EGFR level and produced differentiation effects via ERalpha, but induced both EGFR expression and proliferation effects via ERbeta. In conclusion, we show that also in U2OS cells, E(2) and SERMs are able to modulate the expression of the EGFR gene and can affect events strictly controlled by its signaling pathway, such as the maturation of osteoblasts.