The Role of Phosphogluconate Dehydrogenase in Nrf2-mediated Proliferation andMigration of Human Hepatoma HepG2 Cells

Abstract

Nuclear factor-erythroid-2-related factor 2 (Nrf2) is a transcription factor that binds to the antioxidant response element (ARE) present in the promoter regions of its target genes, encoding mostly antioxidant enzymes and other cytoprotective proteins involved in stress response. Recent studies report that in contrast to its tumor suppressive functions in normal cells, Nrf2 facilitates tumor progression in some cancer cells. However, the molecular mechanism underlying the oncogenic properties of Nrf2 is not yet well understood. Previous studies have shown that Nrf2 also regulates the expression of some of the pentose phosphate pathway (PPP)-related enzymes. Among these, phosphogluconate dehydrogenase (PGD) is of particular interest since its key byproducts, ribulose-5-phosphate (Ru-5-P) and the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) are utilized as a precursor for nucleotide synthesis and as a reducing agent for cellular antioxidant defense, respectively. The present study was intended to explore the potential role of PGD, as a target protein of Nrf2, in the proliferation and migration of human hepatoma, HepG2 cells. Notably, Nrf2 regulates the transcription of PGD through direct binding to the ARE in its promoter region. Knockdown of Nrf2 or PGD significantly inhibited HepG2 cell proliferation and migration. Conversely, Nrf2 overexpression in HepG2 cells led to increased cell proliferation and migration, which was suppressed by silencing of PGD. While Nrf2 regulates PGD expression, knockdown of the gene encoding this enzyme downregulated the expression of Nrf2 and its target antioxidant enzymes, including heme oxgenase-1 and glutamate-cysteine ligase catalytic subunit at both transcriptional and translational levels. In particular, PGD knockdown upregulated Kelch-like ECH-associated protein 1 (Keap1) protein expression. Treatment of HepG2 cells with Ru-5-P gave rise to a decrease in Keap1 protein expression without influencing its mRNA transcript level. This was accompanied by upregulation of both Nrf2 mRNA and HO-1 protein. Collectively, the current study shows that Nrf2 promotes hepatoma cell growth and migration through activation of PGD transcription and that the PGD product, Ru-5-P, induces Keap1 degradation to activate Nrf2 signaling. Thus, there seems to exist a positive-feedback loop between Nrf2 and PGD which is exploited by hepatoma cells for their survival.