Prolyl-peptidyl isomerase, Pin1, phosphorylation is compromised in association with the expression of the HFE polymorphic allele, H63D

There is substantial interest in HFE gene variants as putative risk factors in neurodegenerative diseases such as Alzheimer disease (AD). Previous studies in cell models have shown the H63D HFE variant to result in increased cellular iron, oxidative stress, glutamate dyshomeostasis, and an increase in tau phosphorylation; all processes thought to contribute to AD pathology. Pin1 is a prolyl-peptidyl cis/trans isomerase that can regulate the dephosphorylation of the amyloid and tau proteins. Hyperphosphorylation of these later proteins is implicated in the pathogenesis of AD and Pin1 levels are reportedly decreased in AD brains. Because of the relationship between Pin1 loss of function by oxidative stress and the increase in oxidative stress in cells with the H63D polymorphism it was logical to interrogate a relationship between Pin1 and HFE status. To test our hypothesis that H63D HFE would be associated with less Pin1 activity, we utilized stably transfected human neuroblastoma SH-SY5Y cell lines expressing the different HFE polymorphisms. Under resting conditions, total Pin1 levels were unchanged between the wild type and H63D HFE cells, yet there was a significant increase in phosphorylation of Pin1 at its serine 16 residue suggesting a loss of Pin1 activity in H63D variant cells. To evaluate whether cellular iron status could influence Pin1, we treated the WT HFE cells with exogenous iron and found that Pin1 phosphorylation increased with increasing levels of iron. Iron exposure to H63D variant cells did not impact Pin1 phosphorylation beyond that already seen suggesting a ceiling effect. Because HFE H63D cells have been shown to have more oxidative stress, the cells were treated with the antioxidant Trolox which resulted in a decrease in Pin1 phosphorylation in H63D cells with no change in WT HFE cells. In a mouse model carrying the mouse equivalent of the H63D allele, there was an increase in the phosphorylation status of Pin1 providing in vivo evidence for our findings in the cell culture model. Thus, we have shown another cellular mechanism that HFE polymorphisms influence; further supporting their role as neurodegenerative disease modifiers.