Sodium 4-phenylbutyrate downregulates HSC70 expression by facilitating mRNA degradation

Intracellular trafficking of the ΔF508 cystic fibrosis transmembrane conductance regulator (CFTR) is repaired by sodium 4-phenylbutyrate (4PBA) by an undetermined mechanism. 4PBA downregulates protein and mRNA expression of the heat shock cognate protein HSC70 (the constitutively expressed member of the 70-kDa heat shock protein family) by ∼40-50% and decreases formation of a HSC70-ΔF508 CFTR complex that may be important in the intracellular degradation of ΔF508 CFTR. We examined the potential mechanisms by which 4PBA decreases HSC70 mRNA and protein expression. In IB3-1 cells, 1 mM 4PBA did not alter the activity of the Chinese hamster ovary HSC70 promoter or of a human HSC70 promoter fragment in luciferase reporter assays nor did it alter HSC70 mRNA synthesis in nuclear runoff assays. In contrast, preincubation with 4PBA increased the rate of HSC70 mRNA degradation by ∼40%. The initial rate of ³⁵S-HSC70 protein synthesis in 4PBA-treated IB3-1 cells was reduced by ∼40%, consistent with the steady-state mRNA level, whereas its rate of degradation was unaltered by 4PBA. 4PBA also reduced the steady-state accumulation of ³⁵S-HSC70 by ∼40%. These data suggest that 4PBA decreases the expression of HSC70 mRNA and protein by inducing cellular adaptations that result in the decreased stability of HSC70 mRNA.