Letícia Aragão Santiago
THE Δ337T MUTATION ON THE TRβ CAUSES ALTERATIONS ON GLUCOSE HEPATIC METABOLISM.
Letícia Aragão Santiago, RN, MS, PhD
Mice bearing the genomic mutation Δ337T on the TRΒ gene present the classical signs of resistance to thyroid hormone, with high serum TH and TSH. This mutant thyroid hormone receptor (TR) is unable to bind TH and is constitutively bound to a co-repressor, having also a dominant negative effect on normal function of other TRs. We have previously reported that homozygous (TRΒΔ337T) mice for this mutation showed reduced body weight (BW), length and white adipose tissue mass, despite augmented relative food intake. We have also observed lower HOMA-IR and higher tolerance to glucose on a glucose tolerance test and marked sensitivity to insulin on an insulin sensitivity test.
Conversely, hyperthyroid mice have normal tolerance to glucose but higher insulin sensitivity. TRΒΔ337T mice developed profound hypoglycemia after insulin administration. Therefore we postulated that the mutant TRΒ interferes importantly with hepatic glucose metabolism. In order to investigate whether low blood glucose after insulin administration was due to reduction in hepatic glucose production, we performed the pyruvate tolerance test (PTT). Glycemia was analyzed after overnight fasting after administration of 2g/kg BW of pyruvate. Compared to wild-type (WT) littermates, TRΒΔ337T mice displayed lower blood glucose at 30 and 60 minutes after pyruvate administration (P<0.05). In addition, hepatic glycogen content, analyzed by enzymatic assay, was 4.5-times lower in TRΒ Δ337T mice than in WT (P<0.01). However, in hyperthyroid (50μg of T3/100g BW/day/14 days) WT mice of the same genetic background, we found a reduction of higher magnitude (30-times reduction) in glycogen content. In contrast, hypothyroid mice had a higher hepatic glucose content (p<0.0001). This suggests that TRΒΔ337T mice exhibited an attenuated hyperthyroid-like phenotype regarding hepatic glycogen deposit, which was also found previously for TH-induced brown adipose tissue hypertrophy. These effects may correlate to the dominant negative effect of the mutant TRΒ.
Collectively the data suggest that TRΒΔ337T mice have deficit on hepatic glucose production, by reduced gluconeogenesis and lower glycogen deposit. To determine gluconeogenic genes expression, we performed mRNA expression analysis by real-time PCR of the rate-limiting enzymes phosphoenolpyruvate carboxylase kinase (PEPKC) and glucose 6-phosphatase (G6Pase), together with PGC1α, a key transcriptional factor that positively regulates these enzymes and has been shown to be essential to gluconeogenesis. PGC1α mRNA was reduced in TRΒΔ337T mice (2.9-times, P<0.001), accompanied by a tendency for reduced expression of enzymes that are responsible for the first and last catalytic reactions in the gluconeogenic pathway. In our preliminary data, we found a decrease, yet not significant, of mRNA for PEPCK (P=0.0589) and G6Pase (P=0.1010). The reduction in PGC1α expression is consistent with the up-regulation of its gene expression by TH via TRΒ, and therefore in this aspect the TRΒΔ337T mice behave as the hypothyroid phenotype.
In conclusion, our data show that mice carrying the Δ337T dominant negative mutation on the TRΒ exhibit impaired glucose homeostasis related to a reduction in hepatic glycogen content and reduced expression of PGC1α mRNA and possible reduction on PEPCK and G6Pase expression as well, leading to decreased gluconeogenesis and hepatic glucose production.