Obesity-linked PPARγ S273 phosphorylation promotes insulin resistance through Growth Differentiation Factor 3

Abstract

Overnutrition and obesity promote adipose tissue dysfunction, often leading to systemic insulin resistance. The thiazolidinediones (TZDs) are a potent class of insulin-sensitizing drugs and ligands of PPAR that improve insulin sensitivity, but their use is limited due to significant side effects. Recently, we demonstrated a mechanism by which TZDs improve insulin sensitivity distinct from receptor agonism and adipogenesis: reversal of obesity-linked phosphorylation of PPAR at Serine 273. However, the role of this modification has not been tested genetically. Here we demonstrate that mice encoding an allele of PPAR which cannot be phosphorylated at S273 are protected from insulin resistance, without exhibiting differences in body weight or TZDassociated side effects. Indeed, hyperinsulinemic-euglycemic clamp experiments confirm improved insulin sensitivity, as evidenced by increased whole-body glucose uptake. RNA-seq experiments reveal PPAR S273 phosphorylation specifically enhances transcription of Gdf3, a BMP family member. Ectopic expression of Gdf3 is sufficient to induce insulin resistance in lean, healthy mice. We find that Gdf3 can impact metabolism by inhibition of BMP signaling. Together, these results highlight the diabetogenic role of PPAR S273 phosphorylation and focuses attention on a putative target, Gdf3.

Regulation of metabolic homeostasis by the TGF‐β superfamily receptor ALK7

“…This is in agreement with results from our group and others showing effects of GDF-3 ablation on weight gain only under situations of nutrient overload [25,109]. Hall et al [100] also showed that viral mediated overexpression of GDF-3 in adipose tissue reduced insulin sensitivity, but did not affect body weight in normal Chow diet, which is in line with previous GDF-3 overexpression studies [108]. Looking for a signaling mechanism of GDF-3 action, Hall et al tested GDF-3 activities in HEK293 (human kidney fibroblasts) and C2C12 (mouse myoblast) cell lines, but failed to observe activation of Smad2 and Smad3, effectors of the canonical signaling pathway activated by ALK4 and ALK7 receptors.…”

“…Also, given the hyperinsulinemia of mice with a global inactivation of ALK7 signaling [38], attributing the enhanced lipolysis in ALK7-deficient mice to effects of insulin on GDF-3 expression may be problematic. Finally, a more recent report by Hall et al [100] found GDF-3 expression to be downregulated by 60% in adipose tissue macrophages of knock-in mice lacking phosphorylation at serine 273 of PPARc. The mutant mice displayed improved insulin sensitivity, mimicking some of the effects of PPARc agonists, such as thiazolidinedione compounds [111], but showed no differences from wild-type mice in body weight when fed on a Chow diet.…”

“…[97,98] and may thus affect fat deposition through non-cell-autonomous effects. They are also produced in adipose tissue, where their mRNA expression is prominently increased by high-fat diet and during obesity [82,97,99,100]. SVF cells isolated from lean mice express high levels of Inhbb mRNA encoding activin B, which then decline upon adipocyte differentiation, indicating that this ligand is mainly produced by adipocyte precursors, endothelial, or myeloid cells [Lee and Ib añez, in preparation].…”

“…mRNA encoding GDF-3 has been found in both SVF and differentiated adipocytes, and in both cases, its expression is enhanced during high-fat diet [82,100]. Within the SVF, a series of studies have identified adipose tissue macrophages as the main source of GDF-3 in this fraction [82,100,101].…”

“…Signaling pathways implicated in regulation of catabolic activities by ALK7 in BAT. Numbers in brackets indicate references supporting a particular pathway or functional connection: (1) [140]; (2) [124,141]; (3)[124,142]; (4)[143][144][145]; (5)[146];(6) [37,147];(7) [41];(8) [83];(9) [71];(10) [86];(11) [28,31];(12) [25,26];(13) [82,97,99,100]. Adapted from ref [41]…”