Dual Regulatory Role of Polyamines in Adipogenesis

Background: Polyamines are required for the process of adipogenesis. Results: Polyamine depletion causes inactivation of C/EBP that is correlated with elevation of CHOP, and independently causes inhibition of mitotic clonal expansion. Conclusion: Polyamines are required for both main hubs of adipogenesis; completion of MCE and activation of C/EBP. Significance: Our results assist clarifying the mechanism by which polyamines support adipogenesis. Adipogenesis is a complex process, accompanied by a chain of interdependent events. Disruption of key events in this cascade may interfere with the correct formation of adipose tissue. Polyamines were demonstrated necessary for adipogenesis; however, the underlying mechanism by which they act has not been established. Here, we examined the effect of polyamine depletion on the differentiation of 3T3-L1 preadipocytes. Our results demonstrate that polyamines are required early in the adipogenic process. Polyamine depletion inhibited the second division of the mitotic clonal expansion (MCE), and inhibited the expression of PPAR gamma and C/EBP alpha, the master regulators of adipogenesis. However, it did not affect the expression of their transcriptional activator, C/EBP beta. Additionally, polyamine depletion resulted in elevation of mRNA and protein levels of the stress-induced C/EBP homologous protein (CHOP), whose dominant negative function is known to inhibit C/EBP beta DNA binding activity. Conditional knockdown of CHOP in polyamine-depleted preadipocytes restored PPAR gamma and C/EBP alpha expression, but failed to recover MCE and differentiation. Thus, our results suggest that the need for MCE in the adipogenic process is independent from the requirement for PPAR gamma and C/EBP alpha expression. We conclude that de novo synthesis of polyamines during adipogenesis is required for down-regulation of CHOP to allow C/EBP beta activation, and for promoting MCE.