Obesity and T2D are closely linked metabolic complications, both of which are increasing at alarming rates [38
]. AMPK has a key role in the development of obesity and T2D [39
]. Activation of AMPK inhibits adipogenesis [4
]. Activation of AMPK promotes myogenesis [8
]. However, the mechanisms linking AMPK to MSC differentiation remains unclear. Wnt/β-catenin signaling pathway regulates morphogenesis during early developmental stages. Activation of Wnt signaling stabilizes β-catenin, which enters nuclei and binds to TCF/LEF transcription factors to induce Wnt target genes [40
]. β-Catenin is a primary mediator of the canonical Wnt/β-catenin signaling pathway [41
]. Down-regulation of Wnt/β-catenin signaling was linked to the promotion of adipogenesis [14
We hypothesized that AMPK inhibits adipogenesis through Wnt/β-catenin signaling pathway via phosphorylation of β-catenin. We treated C3H101/2 cells with chemicals to either activate or inhibit AMPK, as well as ectopic expression of wild-type and mutated AMPKα subunit, and AMPK inhibition reduces Ser 552 phosphorylation in β-catenin. The phosphorylation of β-catenin by AMPK was further confirmed by in vitro
kinase assays. Ser 552 is a site previously identified as a phosphorylation site by protein kinase A (PKA) [18
] and Akt [20
]. We used a specific inhibitor of PKA as well as Akt to exclude the possibility that such phosphorylation was mediated by these two kinases.
To analyze the biological functions of this phosphorylation, we analyzed the effect of this phosphorylation on the activity of β-catenin/TCF mediated transcription using a TCF/LEF reporter construct. The β-catenin/TCF mediated transcription was dramatically enhanced due to β-catenin Ser 552 phosphorylation. Therefore, AMPK induces β-catenin phosphorylation at Ser 552, which stabilizes β-catenin and enhances β-catenin/TCF mediated transcription, inhibiting adipogenesis.
The next question is whether β-catenin phosphorylation is one of the main mechanisms for AMPK to inhibit adipogenesis. Using Ser 552 mutated β-catenin (S552A), AMPK was not effective to regulate β-catenin/TCF mediated PPARγ transcription and adipogenesis, strongly suggesting that Ser 552 is a major mechanism for AMPK to regulate adipogenesis from MSC.
Our observation that AMPK mediates adipogenesis via phosphorylation of β-catenin has important physiological implications. AMPK has a central role in the regulation of energy metabolism and is regulated by numerous factors [43
]. First, obesity epidemic is becoming increasingly serious and it is known that obesity inhibits AMPK [44
]11]. In addition, AMPK is modulated by adipokines such as leptin, adiponectin and resistin, and cytokine, such as interleukin-6 [45
]. Furthermore, AMPK is activated and regulates energy metabolism during exercise [46
]. Therefore, obesity, inflammation and exercise likely affect adipogenesis through the cross-talking of AMPK with Wnt/β-catenin signaling, exerting long-term effects on tissue development and physiological functions. Because both AMPK and Wnt/β-catenin signaling are widely expressed in various tissues and conserved in animal kingdom, such cross-talking likely has broad biological effects waiting to be discovered.