The induction and activation of STAT1 by all-trans-retinoic acid are mediated by RARβ signaling pathways in breast cancer cells

Retinoic acid receptor-β (RARβ) and signal transducer and activator of transcription 1 (STAT1) are important mediators of the antiproliferative and apoptotic actions of retinoids and cytokines/growth factors, respectively. Expression of both RARβ and STAT1 is lost in most breast cancer cell lines hut it can be induced by retinoids in estrogen receptor-positive cells. We investigated a possible functional connection between these two mediators and present evidence supporting RARβ as a tumor suppressor. First, by using different receptor-selective retinoids, we demonstrated that RARβ induction in MCF-7 cells by all-trans-retinoic acid (atRA) was associated with the activation of STAT1 gene transcription. The direct involvement of RARβ in atRA-induced STAT1 gene activation was further demonstrated by showing that transfection with an anti-sense RARβ construct blocked atRA-induced STAT1 expression in MCF-7 cells whereas introduction of a sense-RARβ construct resulted in STAT1 induction by atRA in MDA-MB 231 cells. In addition, we showed that STAT1 was phosphorylated/activated under atRA treatment of MCF-7 cells; this process required the involvement of RARβ and protein synthesis. STAT1 phosphorylation/activation was accompanied by increased tyrosine kinase activity that was not due to the activation of JAK1, JAK2 or Tyk 2, suggesting the possible involvement of an unidentified tyrosine kinase.