Calcineurin-independent inhibition of 3T3-L1 adipogenesis by Pasteurella multocida toxin: Suppression of Notch1, stabilization of β-catenin and pre-adipocyte factor 1

Pasteurella multocida toxin (PMT) is a potent mitogen and a specific activator of Gq-dependent signalling pathways. PMT impairs osteoblast differentiation and causes bone loss and fat reduction in vivo. We examined the effect of PMT on cell signalling pathways involved in 3T3-L1 adipocyte differentiation. We demonstrate that PMT treatment before or together with differentiation induction factors inhibits adipogenesis and prevents upregulation of important adipocyte markers -peroxisome-proliferator-activated receptor γ (PPARγ) and CAATT enhancer-binding protein α (C/EBPα). Moreover, PMT completely downregulates PPARγ and C/EBPα expression in mature adipocytes. Differentiation of pre-adipocytes into adipocytes requires the suppression of pre-adipocyte factor 1 (Pref1) and Wnt signalling, along with the degradation of β-catenin. PMT prevents downregulation of Pref1 and β-catenin under differentiation-inducing conditions. In addition, PMT treatment downregulates expression of Notch1, a protein responsible for cell fate decision and implicated in regulation of adipogenesis in 3T3-L1 cells. PMT action on adipogenesis was not reversed by cyclosporin A, an inhibitor of Gαq-PLC-calcium-dependent calcineurin activation. Our results reveal new pathways involved in PMT action on cellular physiology and differentiation. Our study further demonstrates that the effect of PMT on Pref1/PPARγ/C/EBPα expression and adipogenesis does not occur just through activation of the Gαq-calcium-calcineurin pathway, but involves Wnt/β-catenin and Notch1 signalling pathways, two signalling pathways strongly linked to cancer predisposition, neurological and immunological dysfunctions, and fat and bone development.