The Inhibition of Splicing of Glucose-6-phosphate Dehydrogenase mRNA During Starvation by Heterogeneous Nuclear Ribonucleoprotein K

Splicing of nascent RNA transcripts is an essential step in gene expression and a target of nutrient regulation. Glucose-6-phosphate dehydrogenase (G6PD) is a lipogenic gene whose expression is regulated exclusively at this posttranscriptional stage. Expression of G6PD mRNA increases 15- to 17-fold during refeeding and is inhibited 80-90% by starvation and the addition of polyunsaturated fat to the diet. The large changes in the accumulation of G6PD mRNA are due to changes the rate of pre-mRNA splicing and not changes in its transcription. In this regard, dietary carbohydrate enhances intron removal, which increases the accumulation of G6PD mRNA. Starvation alters pre-mRNA splicing by decreasing the rate of intron removal, leading to intron retention and a decrease in the accumulation of mature mRNA. A regulatory element within exon 12 of the G6PD pre-mRNA that contains both an ESS and an ESE mediates these changes in splicing efficiency. SR proteins, like SRp20, along with hnRNP K, L and A2/B1 bind to this regulatory region. Starvation caused an increase in the expression of heterogeneous nuclear ribonucleoprotein (hnRNP) K protein and this increase coincided with the increase in the binding of hnRNP K to the regulatory element and a decrease in the expression of G6PD mRNA in vivo. HnRNP K bound to two C-rich motifs forming an ESS within exon 12. Overexpression of hnRNP K decreased the splicing and expression of G6PD mRNA, while siRNA-mediated depletion of hnRNP K caused an increase in the splicing and expression of G6PD mRNA. HnRNP K binding to the C- rich motifs blocked binding of serine-arginine rich, splicing factor 3 (SRSF3), a splicing enhancer. Thus hnRNP K is a nutrient regulated splicing factor responsible for the inhibition of the splicing of G6PD during starvation.