Spermidine promotes adipogenesis of 3T3-L1 cells by preventing interaction of ANP32 with HuR and PP2A

We have shown previously that the polyamine spermidine is indispensable for differentiation of 3T3-L1 preadipocytes. In the present study, we examined the mechanism of spermidine function by using the polyamine biosynthesis inhibitor alpha-difluoromethylornithine in combination with the metabolically stable polyamine analogues gamma-methylspermidine or (R,R)-alpha,omega-bismethylspermine. At the early phase of differentiation, spermidine-depleted 3T3-L1 cells showed decreased translation of the transcription factor C/EBP beta (CCAAT/enhancer-binding protein beta), decreased PP2A (protein phosphatase 2A) activity and increased cytoplasmic localization of the RNA-binding protein HuR (human antigen R). The amount of HuR bound to C/EBP beta mRNA was reduced, whereas the amount of bound CUGBP2, an inhibitor of C/EBP beta translation, was increased. ANP32 (acidic nuclear phosphoprotein 32) proteins, which are known PP2A inhibitors and HuR ligands, bound more PP2A and HuR in spermidine-depleted than in control cells, whereas immunodepletion of ANP32 proteins from the lysate of spermidine-depleted cells restored PP2A activity. Taken together, our data shows that spermidine promotes C/EBP beta translation in differentiating 3T3-L1 cells, and that this process is controlled by the interaction of ANP32 with HuR and PP2A.