Altered thioredoxin subcellular localization and redox status in MCF-7 cells following 1,25-dihydroxyvitamin D3 treatment.

1,25-Dihydroxyvitamin D(3) (1,25D) induces apoptosis in MCF-7 cells via the intrinsic pathway involving bax translocation to mitochondria, cytochrome c release and reactive oxygen species (ROS) generation. Vitamin D up-regulated protein 1 (VDUP1), an apoptotic regulatory gene induced by 1,25D in HL-60 cells, is a negative regulator of thioredoxin (Trx1), a redox protein which neutralizes ROS and protects against oxidative stress induced apoptosis. Due to the involvement of oxidative stress in 1,25D mediated apoptosis, we analyzed whether VDUP1 or Trx1 are altered by 1,25D in MCF-7 cells. In contrast to HL-60 cells, VDUP1 mRNA was not up-regulated by 1,25D in MCF-7 cells, indicating that transcriptional up-regulation of this gene is not required for 1,25D mediated apoptosis. 1,25D did not affect the expression or activity of Trx1 in MCF-7 cells, however, Trx1 activity was higher in MCF-7 cells selected for resistance to 1,25D mediated apoptosis. In untreated MCF-7 cells, Trx1 was present only in the cytosol, and the majority was in the oxidized state. In 1,25D treated MCF-7 cells, Trx1 was present in both cytosol and nucleus, and the nuclear Trx1 pool was in the reduced state. Nuclear localization of Trx1 in 1,25D treated MCF-7 cells was confirmed by immunofluorescent microscopy. Although redox status is known to alter the ability of Trx1 to bind apoptosis signal regulating kinase 1 (ASK1), no changes in ASK1 transcript or protein levels were observed in 1,25D treated MCF-7 cells. Collectively, these studies indicate that although VDUP1 and ASK1 are not altered by 1,25D, changes in redox status and sub-cellular distribution of Trx1 occurs during 1,25D mediated apoptosis of MCF-7 cells.