Background: Abnormal expression of dopamine receptors (DRs) has been described in multiple tumors, but their roles in breast cancer are inconclusive or contradictory since evidence of pro- and anti-tumoral effects have been reported. Herein, we analyzed the expression of DRs in breast cancer, especially in the subpopulation of cancer stem cells (CSCs), and evaluated the functional role of the receptors by pharmacological targeting. Methods: Expression of DRD1, DRD2, DRD3, DRD4 and DRD5 was investigated in human breast tumors and cancer cell lines using public databases. Correlation between gene expression and clinical outcome was studied by Kaplan-Mayer analyses. By flow cytometry, we assessed DRD1, DRD2, and DRD4 expression in cultures of MCF-7 (luminal) and MDA-MB-231 (triple-negative) cells. Using the previously reported SORE6 reporter system we examined the differential expression of DRD1, DRD2, and DRD4 in CSCs and tumor-bulk cells. The effect of pharmacological modulation of DRs on stemness and cell migration was studied by quantification of the reporter-positive fraction and wound healing assays, respectively. Results: DRD1, DRD2 and DRD4 transcripts were expressed in breast tumors. DRD4 was overexpressed compared to normal tissue and showed prognostic value. DRD1, DRD2 and DRD4 transcripts were also found in MCF-7 and MDA-MB-231 cells, but only DRD1 and DRD4 proteins were detected. DRD4 was underexpressed in CSCs compared to tumor-bulk cells, whereas DRD1 was found only in the CSCs fraction, suggesting that those receptors may have relevance in stemness control. Subtoxic concentrations of DRD1-targeting compounds did not induced significant changes in the CSCs pool. On the other hand, DRD4 inhibition by Haloperidol slightly increased the CSCs content but also reduced cell migration. Conclusions: Pharmacological modulation of DRD1 in MCF-7 or MDA-MB-231 cells seems to be irrelevant for stemness maintenance. DRD4 reduced expression in breast CSCs or its inhibition by Haloperidol favors CSCs-pool expansion. DRD4 inhibition can also reduce cell migration, indicating that DRD4 plays different roles in stem and non-stem breast cancer cells.Keywords: Dopamine receptor D1 (DRD1); breast cancer stem cells; dopamine receptor D4 (DRD4); dopamine receptor agonist; dopamine receptor antagonist.

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