Decrease of lactogenic hormones induce epithelial-mesenchymal transition via TGFβ1 and arachidonic acid during mammary gland involution

During mammary gland involution, the epithelial mesenchymal transition (EMT) process plays an important role in tissue remodelling and in the termination of milk production. Transforming growth factor β (TGFβ) has been known as a central inducer to EMT and contributor to the mammary gland involution. However, the whole mechanism has accomplished the EMT process in mammary gland is still unclear. Here, we show that arachidonic acid, one of the major products in milk, is new player to control the EMT together with TGFβ during mammary gland involution. Firstly, we observed decrease in CDH1 (epithelial marker gene) expression and increases in VIM and TWIST1 (mesenchymal marker genes), TGFB1, and PLCG2 (arachidonic acid synthesis gene) at involution. In epithelial cells culture experiments, depletion of lactogenic hormones to mimic the involution induced TGFβ1 and PLCG2 expressions. Treatment with arachidonic acid in epithelial cells increased VIM and TWIST1 expressions without decrease of CDH1 expression, while TGFβ1 decreased CDH1 and increased VIM and TWIST1; more importantly, TGFβ1 induced the expression of PLCG2, but arachidonic acid did not induce the expression of TGFB1. Finally, arachidonic acid accelerated the TGFβ1 increasing VIM and TWIST1 expressions, meanwhile arachidonic acid synthase inhibitor partially blocked the TGFβ1 increasing VIM and TWIST1 expressions. In conclusion, TGFβ1 stimulates arachidonic acid synthesis and the arachidonic acid has a function to postulate the EMT process together with TGFβ1 during mammary gland involution.