Development of High Fat Diet-Induced Hyperinsulinemia in Mice Is Enhanced by Co-treatment With a TLR7 Agonist

Abstract

Metabolic syndrome (MetS) is common in Systemic Lupus Erythematosus (SLE) patients and is associated with increased cardio-renal risk. Toll-like receptor 7 (TLR7) stimulation promotes the development of SLE through mechanisms including activating type I Interferon (IFN) and autoreactive B cells. The current study tested whether combined TLR7 agonist treatment and exposure to a high fat, high sucrose “Western diet” intervention affects the early-stage development of SLE or MetS features. Female C57BL/6 mice were untreated or treated with the TLR7 agonist imiquimod (IMQ) and fed a high-fat diet (HFD; fat 42% kcal, sucrose 34% kcal) or control diet (fat 12.6% kcal, sucrose 34% kcal) for 6 weeks. Supporting early-stage induction of autoimmunity, spleen weights were significantly increased and anti-nuclear antibody (ANA) positivity was detected in IMQ-treated mice. Increased body weight, gonadal fat pad mass, and plasma leptin levels were observed between HFD and control animals for both IMQ and untreated mice. However, the increase in these parameters with HFD was slightly but significantly diminished in IMQ-treated mice. Both the HFD and IMQ treatments significantly increased fasting blood glucose levels. Notably, IMQ treatment affected fasting insulin concentrations in a diet-dependent manner, with hyperinsulinemia observed in IMQ-HFD treated mice. Together, this indicates that the IMQ model of SLE is associated with metabolic alterations, impaired glycemic control, and hyperinsulinemia under HFD conditions. This model may be helpful in further investigating the relationship between MetS and SLE, and supports a role of TLR7 signaling in promoting or accelerating the development of dysglycemia and hyperinsulinemia.

TLR7 activation by imiquimod worsens glycemic control in female FVB/N mice consuming a high‐fat diet

“…Not only are TLRs known for their role in innate immunity, but a role in the progression of obesity‐related inflammation and metabolic complications has also emerged. The current study, as well as recent work by our group (Kakalij et al., 2022 ) and others (Hanna Kazazian et al., 2019 ; Revelo et al., 2016 ) suggest that enhanced TLR7 activation may exacerbate the development of MetS features, including impaired glucose homeostasis and insulin resistance. We report that pharmacological TLR7 activation promoted impairment of glucose tolerance in female FVB/N mice exposed to a HFD for 12 weeks.…”

“…C57BL/6 mice are regarded as susceptible to developing MetS when placed on a hypercaloric diet (Leonardi et al, 2020). We did not detect a significant independent effect of IMQ on FVB/N fasting blood glucose in the current study at either 6 or 12 weeks, whereas there was a significant increase in C57BL/6J mice at 6 weeks (Kakalij et al, 2022), perhaps reflecting strain differences in the response to HFD. Revelo et al reported that 8 days of IMQ treatment impaired glucose tolerance but did not raise fasting blood glucose in male C57BL/6 mice maintained on a normal control diet (Revelo et al, 2016).…”

“…Although HFD animals showed an increased left ventricle weight-to-tibia length ratio, we did not detect any significant effect of IMQ treatment to exacerbate left ventricular hypertrophy, nor was there any significant effect of either treatment on left ventricular ANP or BNP expression. Similarly, no IMQ-induced exacerbation of HFDinduced left ventricular hypertrophy was observed in our previous study; indeed, HFD-related ventricular hypertrophy appeared to be blunted by IMQ in C57BL/6 mice, for reasons that are not currently clear (Kakalij et al, 2022). The liver seemed to be a more sensitive target in this study, with increased liver-to-tibia ratio of IMQ-treated HFD-fed animals and upregulated TLR7 mRNA expression.…”

“…In using an oral glucose challenge in the present study, it is possible that IMQ‐ and HFD‐induced alterations in incretins as well as insulin. Although we lacked sufficient plasma for measurements of insulin or incretin hormones in the present study, we have previously reported increased fasting insulin levels following 6 weeks of IMQ plus HFD‐treatment of female C57BL/6J mice compared to HFD alone (Kakalij et al., 2022 ). C57BL/6 mice are regarded as susceptible to developing MetS when placed on a hypercaloric diet (Leonardi et al., 2020 ).…”

“…Several experimental models of lupus exist, with a more recently described model being topical treatment with imidazoquinoline derivatives such as imiquimod (IMQ), which are recognized by TLR7 (Hemmi et al, 2002;Yokogawa et al, 2014). Although the original study by Yokogawa et al (2014) did not report any metabolic parameters in IMQ-treated mice that were maintained on regular rodent chow, we recently reported that combining IMQ and a high-fat diet (HFD) for 6 weeks increased fasting blood glucose and insulin in female C57BL/6J mice (Kakalij et al, 2022). However, the autoimmune phenotype of these mice was mild in our hands, and unpublished data from our lab did not reveal substantial target organ damage in the C57BL/6 background, thereby limiting the model's usefulness.…”