The role of hnRNP A3 on age-related increased expression of FIX in the Liver

HnRNP A3 is a protein that binds the age-related increase element (AIE) of blood coagulation factor IX (FIX) and that plays critical roles in age-related gene expression, likely through an epigenetic mechanism as yet unidentified. In a previous study, we found that Ser359 phosphorylated hnRNP A3 proteins do not bind to the AIE of FIX although both unphosphorylated and Ser359 phosphorylated hnRNP proteins exist in the liver. In the present study, to explore the relationship between hnRNP A3 and FIX, we examined the age-related expression pattern of 14 single spots of hnRNP A3 detected by 2DE and subsequent MALDI-TOF/TOF/MS analyses in mouse liver. We found that the level of all four Ser359 phosphorylated hnRNP A3 proteins increased with age (from 1-21 months), while the 10 unphosphorylated hnRNP A3 proteins showed various expression patterns with age. We then examined the functional role of hnRNP A3 in FIX expression using siRNA knockdown technology targeting the hnRNP A3 gene in aged mice (12-17 months old). Inhibition of hnRNP A3 expression induced an increase in the circulating FIX level in aged mice. These results suggested that hnRNP A3 inhibits age-related FIX protein expression and that age-dependent modification of hnRNP A3, including its phosphorylation at Ser359, might be involved in the age-dependent increase in FIX expression in vivo.