Dact1 was first identified as an evolutionarily conserved, Dvl-interacting protein (19
); however, its mechanism of action and function remained ambiguous (19
) and was reported to be cell/tissue and context dependent (23
). Before this study, the presence and role of Dact1 in adipogenesis/adipose tissue was unexplored. Here, we demonstrate that Dact1
is primarily expressed in both human and murine preadipocytes and in SVFs of adipose tissue. Furthermore, Dact1
is downregulated during adipogenic conversion of primary cultures and cell lines. With this profile, Dact1
could be considered a surrogate preadipocyte marker, however, we demonstrate that its biological action contrasts with other previously described “vanishing preadipocyte” genes (e.g., Gata2/3
, and Wnt10b
). Through both gain- and loss-of-function approaches, we have shown that expression of Dact1
confers proadipogenic potential and that its presence in the preadipocyte is required for efficient adipocyte differentiation.
Our observations of the effects of Dact1 on adipogenesis and on signaling in preadipocytes are most consistent with its role as an antagonist of Wnt/β-catenin signaling in this tissue. This has only previously been demonstrated in Xenopus
embryos and mammalian embryonic fibroblasts (19
). Resolution of the discrepancies in the apparent signaling function of Dact proteins may lie in our observations showing that manipulation of Dact1 in preadipocytes elicits a differential and specific effect on the levels of other proteins configuring a Wnt/β-catenin signaling network. This suggests that changes in Dact1 protein concentrations have downstream effects on a whole network of signaling proteins, which may contribute exquisite modulation to the strength of Wnt/β-catenin signaling tone. It is noteworthy that some of the effects of Dact1 (i.e., altering Dvl levels) are strictly cell-autonomous, whereas others (i.e., altering Wnt and sFRP levels) can also be expected to affect signaling in neighboring cells. The latter is likely to be highly relevant in vivo in an otherwise cellularly heterogeneous adipose tissue, where gene expression and differentiation state is likely to vary significantly between neighboring cells. Dact1 may play a significant role in determining not only the sensitivity of the intracellular Wnt/β-catenin pathway but also the strength of the extracellular autocrine/paracrine Wnt signals to neighboring cells. We speculate that this type of modulation, when regulated by nutrient availability, may be the basis of a local self-regulating mechanism to titrate the differentiation of an appropriate number of preadipocytes to specific storage demands.
These studies also present several new components of the Wnt/β-catenin network not previously described in adipose tissue and/or studied during adipogenesis. These include four proteins expressed primarily in preadipocytes (Dact1 Dvl2, Dvl3, and Wnt3a) and three expressed primarily by mature adipocytes (Dvl1, sFRP1, and sFRP5). Importantly, we also demonstrate that these molecules are coordinately regulated in vivo in response to nutritional and dietary challenges, in genetic forms of obesity and to pharmacological agents typically producing adipose tissue expansion and improving insulin sensitivity.
Given their intrinsic peculiarities, it is not unexpected that our studies have revealed some differences between the expression profiles obtained from in vivo and in vitro models of adipogenic regulation. One important factor that is worth considering is that the in vitro systems used here represent a homogenous population of preadipocytes that are simultaneously exposed to and synchronously respond to defined adipogenic stimuli and/or genetic manipulation, thus allowing construction of detailed temporal profiles. In contrast, whole adipose tissues as used for in vivo studies represent a heterogeneous population of cell types each with distinct gene expression profiles and sensitivities to nutritional and pathological cues and context-dependent feedback loops. These coupled with local paracrine signals are likely to affect the dynamic balance between mature adipocytes and preadipocytes. Furthermore, it is evident that adipose tissue expansion in vivo does not involve the simultaneous recruitment of all preadipocytes into the adipogenic program. Nonetheless, it is clear from our findings that key determinants of Wnt/β-catenin signaling tone are regulated in response to refeeding and that the sustained induction of Dact1 in preadipocytes could be one mechanism facilitating, on one hand, the preselection of preadipocytes for subsequent differentiation and being the source of paracrine signals that prevents the differentiation of more distal preadipocytes, resulting in an appropriate titrated adipose tissue expansion in response to nutritional status.
That being said, our in vitro observation that changes in Dact1 levels lead to reciprocal changes in Wnt10b in preadipocyte cell lines are recapitulated in adipose tissue obtained from three in vivo models that reflect actively expanding adipose tissue (short-term high-fat diet, young ob/ob, and TZD-treated wild-type mice). Conversely, our data also show that under conditions of insulin resistance when adipogenesis and fat deposition are compromised and fat accumulation plateaus, this network becomes uncoupled, but Dact1 appears to be appropriately regulated in the opposite direction. This suggests that Dact1 may act as a gate-keeper, facilitating the accurate adaptation of adipose tissue expansion to storage requirements based on nutritional and metabolic status.
In summary, we present evidence of a functional network formed by Dact1, sFRP, and Wnt ligands that facilitates cross talk in adipose tissue between preadipocytes and mature adipocytes, thereby ensuring appropriate titration of adipose tissue expansion in response to nutrient availability. We speculate that dysregulation of this network may be the pathogenic basis leading to an altered balance between the processes of adipocyte growth versus preadipocyte recruitment—ultimately leading to a spectrum of adipose tissue cellularity ranging from hypertrophy to hyperplasia. Similarly, modulation of this network by targeting Dact1 may be of therapeutic value to improve the metabolic efficiency of the adipose tissue and prevent obesity-associated metabolic complications.