Wnt Signaling: From Mesenchymal Cell Fate to Lipogenesis and Other Mature Adipocyte Functions

Abstract

Wnt signaling is an ancient and evolutionarily conserved pathway with fundamental roles in the development of adipose tissues. Roles of this pathway in mesenchymal stem cell fate determination and differentiation have been extensively studied. Indeed, canonical Wnt signaling is a significant endogenous inhibitor of adipogenesis and promoter of other cell fates, including osteogenesis, chondrogenesis, and myogenesis. However, emerging genetic evidence in both humans and mice suggests central roles for Wnt signaling in body fat distribution, obesity, and metabolic dysfunction. Herein, we highlight recent studies that have begun to unravel the contributions of various Wnt pathway members to critical adipocyte functions, including carbohydrate and lipid metabolism. We further explore compelling evidence of complex and coordinated interactions between adipocytes and other cell types within adipose tissues, including stromal, immune, and endothelial cells. Given the evolutionary conservation and ubiquitous cellular distribution of this pathway, uncovering the contributions of Wnt signaling to cell metabolism has exciting implications for therapeutic intervention in widespread pathologic states, including obesity, diabetes, and cancers.

Figure 1

Adipose tissues interact with myriad organs to maintain global homeostasis. Adipocytes are located in discrete depots and niches throughout the body and have fundamental and complex roles in storage and release of energy in response to local and global needs, thermoregulation, mechanical support, and secretion of adipokines to regulate energy balance, metabolism, and immune responses.

Figure 2

Wnts play fundamental and diverse roles in tissue development and maintenance. A: Wnts are lipid-modified secreted glycoproteins (∼30–40 kDa) characterized by a signal peptide sequence, 23 conserved cysteine residues, varying numbers of N-glycosylation sites (N-glyc), and two conserved lipid modifications: palmitic acid (C16:0) at Cys77 and palmitoleic acid (C16:1) at Ser209. Palmitoleoylation (C16:1) at Ser209 is required for the interaction between Wnts and their dedicated chaperone protein, Wntless; palmitoylation (C16:0) at Cys77 is required for functionality of secreted Wnts. Wntless is an evolutionarily conserved transmembrane protein required for intracellular trafficking and secretion of lipidated Wnts. Wntless (∼62 kDa) is predicted to have seven transmembrane domains (TMD), a signal peptide sequence, an endocytosis motif, and a hydrophobic lipocalin domain thought to be the site of interaction with Wnts. B: After Wnt proteins are synthesized in the ER, they undergo significant posttranslational modifications, including N-glycosylation and lipidation. The ER acyltransferase Porcupine catalyzes addition of palmitic (C16:0) and palmitoleic (C16:1) acid moieties at conserved Cys77 and Ser209 residues, respectively. Wntless binds to and chaperones Wnt proteins from the ER through the trans-Golgi network and to the plasma membrane in secretory vesicles. Once Wnts are secreted, Wntless is transported back via a retromer complex to the Golgi for reuse or to lysosomes for degradation.

Figure 3

β-catenin is the central player in the canonical Wnt pathway. A: β-catenin is comprised of a central 12-unit Armadillo repeat domain (ARM) flanked on either side by distinct N-terminal domain and C-terminal domain (CTD). The N-terminal region consists of conserved serine (S33, S47, S45) and threonine (Thr41) residues that are sequentially phosphorylated by CK1α and GSK3β; phosphorylation of these sites promotes binding to β-TrCP, subsequent ubiquitination, and proteasomal degradation. The ARM region forms a superhelix featuring a positively charged groove that serves as a platform for interactions with various β-catenin binding partners, including E-cadherin, Axin, APC, and TCF/LEF proteins. These partners share overlapping binding sites in the ARM groove and thus typically cannot bind simultaneously. DIX domain, Dishevelled/Axin domain; HMG box domain: high-mobility group box domain; RGS domain: regulator of G-protein signaling domain; TMD, transmembrane domain. B: In the absence of extracellular Wnts (inactive [left panel]), β-catenin is bound by a destruction complex comprised of Axin, APC, GSK3β, CK1α, and PP2A. CK1α and GSK3β sequentially phosphorylate (P) free cytosolic β-catenin, targeting it for ubiquitination (Ub) by β-TrCP and subsequent proteasomal degradation. When canonical Wnts bind to their frizzled receptors and LRP coreceptors (active [right panel]), they trigger an intracellular signaling cascade that leads to hypophosphorylation, stabilization, and accumulation of cytosolic β-catenin. β-catenin can then translocate to the nucleus, displace Groucho/TLE repressors, and coactivate TCF/LEF transcription factors to mediate Wnt target gene expression.

Figure 4

Canonical Wnt signaling regulates MSC fate determination and differentiation. A: Activation of canonical Wnt/β-catenin signaling in MSCs suppresses adipogenesis and promotes osteoblastogenesis. Wnt signaling suppresses adipocyte differentiation by inhibiting expression of PPARγ and C/EBPα, the central regulators of adipogenesis. B: The complexities of Wnt signaling are evident during adipogenesis: although canonical Wnt signaling inhibits adipocyte differentiation, many Wnts, frizzled receptors, and central regulatory components of the pathway are constitutively expressed or induced in mature mouse adipocytes derived from MSCs. Thus, the canonical Wnt signaling machinery is also highly operative in terminally differentiated adipocytes.

Figure 5

Wnt signaling is associated with metabolic disease in humans. Emerging genetic evidence in humans links specific members of the Wnt signaling pathway to body fat distribution, obesity, cardiovascular disease, and type 2 diabetes.