We have shown that the steroid E2 reduced islet FA and glycerolipid synthesis in vivo via ERα in β cells and prevented β cell failure in ZDF rats, a model of T2D. We have also shown that pharmacological activation of ERα66, GPER/ERα36, and ERβ suppressed lipogenesis in cultured rodent and human β cells.
Several nonexclusive mechanisms could underline the protective effect of ER activation against β cell failure in ZDF rats. First, enhanced insulin gene expression may be involved. The ZDF rat is also a model of β cell exhaustion of insulin stores in the face of fuel surfeit and massive obesity. Unlike its nondiabetic ZF counterpart, the ZDF rat harbors a mutation in the insulin promoter, decreasing insulin transcription (28
) and potentially participating in failure of ZDF islets to meet the insulin biosynthesis demand of insulin resistance. We have previously shown that ERα potentiates insulin gene transcription and increases islet insulin stores, which probably contribute to the compensatory increase in insulin secretion from β cells in insulin resistance (5
). Consistent with this view, E2 enhances PDX-1 and proinsulin gene expression in ZDF islets and causes a rise in islet insulin content. Second, E2 could enhance β cell survival in the face of hyperglycemia and hyperlipidemia (glucolipotoxicity). With ER activation preventing β cell apoptosis in streptozotocin-treated mice (3
), it is likely that E2 preserves β cell mass of ZDF rats via a similar antiapoptotic mechanism. Third, E2 improves obesity and insulin sensitivity in ZDF rats, potentially protecting β cells in an indirect manner (2
). However, our current findings support an instrumental role for E2 in preventing β cell failure via direct autocrine regulation of islet lipid homeostasis. Unlike insulin sensitizers like metformin or thiazolidinediones that improve β cell function and prevent islet lipotoxicity by suppressing circulating FFA (30
), E2 prevented β cell failure despite an elevation in circulating lipids in ZDF rats. Moreover, E2 reduced TG content and restored GSIS in cultured ZDF islets, demonstrating the importance of lipid homeostasis to β cell function in this model and suggesting a direct protective effect for E2 against lipotoxicity in these cells. Finally, the disruption of lipid homeostasis in PER
mice fed HF diet was associated with mild β cell failure to compensate for insulin resistance and hyperglycemia, a classical sign of early T2D. Taken together, these data argue that E2 improvement of lipid homeostasis is instrumental in protecting β cell function in obesity-associated models of T2D.
E2 protection likely involves the lipogenic arm of the glycerolipid/FA cycle in ZDF islets. Islets from the nondiabetic ZF rats are characterized by balanced increased lipolysis and β-oxidation in the face of increased lipogenesis, allowing islet gluco-lipo-detoxification and favoring compensation for insulin resistance (12
). In contrast, islets from diabetic ZDF rats show an imbalance between enhanced lipid esterification processes (lipogenesis) and lipolysis/β-oxidation, leading to the accumulation of MAG, DAG, and TG. By reducing FA synthesis and esterification into glycerolipid in ZDF islets, E2 prevents the sustained accumulation of potentially toxic lipid intermediates in the lipogenic arm, thus preventing tissue glucolipotoxicity (1
Previous studies have reported that E2 suppresses lipogenesis in white adipose tissue and liver in vivo (32
). However, it is unknown whether the antilipogenic effects of E2 are mediated via activation of adipocyte or hepatocyte ERs or via the central nervous system, similar to leptin signals from the mediobasal hypothalamus that trigger sympathetic outflow to white adipose tissue (34
). Several arguments support the view that E2 directly suppresses islet lipid synthesis, at least in part, via ER activation in β cells. First, E2 and selective agonists for all ERs reduced FA synthesis and TG accumulation in cultured rodent and human β cells. Second, in vivo activation of ERα with a selective agonist also suppressed islet FA synthesis and TG accumulation in mice. Conversely, in vivo activation of ERα in nonislet tissues failed to suppress islet lipid synthesis in PER
mice, which have islet-specific null deletion of ERα, and islets accumulated more lipids when these mice were fed a HF diet, in association with β cell dysfunction and mild hyperglycemia. Thus, we conclude that activation of ERα in islets is critical to prevent islet lipid synthesis and accumulation and β cell dysfunction, independent of an effect on the central nervous system or other tissues. Additionally, ERα, ERβ, and GPER/ERα36 reduced lipid synthesis to the same extent in cultured β cells when activated by synthetic agonists, despite E2 having no effect in cultured islets individually deficient in ERα, ERβ, or GPER. This suggests that ERα and ERβ exhibit nonredundant antilipogenic functions in islets.
We observed that ERs suppressed islet TG synthesis independently of an ERE. Moreover, the extranuclear ER ligand EDC and the agonist for the recently characterized extranuclear ERα36, lacking transcriptional activity (14
), both suppressed Fasn
mRNA levels and FAS activity. This demonstrated that E2 suppresses FA synthesis via input from extranuclear ERs, rather than from ERs acting as classical ligand-activated transcription factors. Furthermore, we present in vivo evidence that the extranuclear ERs signal via STAT3 activation to suppress islet FA synthesis and lipid accumulation. Further studies are needed to identify the pathways inhibiting FAS level and activity downstream of STAT3. A proposed mechanism for ER-mediated inhibition of islet lipid synthesis and accumulation is shown in Figure H.
Our findings have therapeutic implications for T2D. Previously, 2 randomized trials have shown a 30% reduction in the incidence of T2D in postmenopausal women on estrogen replacement therapy and provided some evidence for the protective effect of this therapy on β cells (35
). In ZDF rats, E2 treatment produces a massive increase in circulating lipids, as can be observed with oral estrogen (37
), which indicates that global activation of ER could be deleterious in T2D. Although ER ligands with selective extranuclear ER actions (21
) or ER pathway–selective activity (38
) have been developed, their effect is global. The results of the current study, combined with our previous findings that ERs improve islet survival (3
) and insulin biosynthesis (5
), suggest that selective enhancement or targeting of ER action in β cells may prove a novel therapeutic avenue to prevent β cell failure in T2D.