Endocrine Abstracts

Increased mammary metabolism is critical for initiating lactation during postpartum days 1-4. We utilised clinical and cellular approaches to investigate whether thyroid hormones, which promote lactation in rodents, are involved in initiating human lactation and regulating mammary metabolism. We recruited n=30 pregnant women following informed consent and measured serum thyroid hormones (free T4 and TSH) at 36 weeks’ gestation and on postpartum day 4. Free T4 increased from 10.4±0.2pmol/l at 36 weeks’ gestation to 11.7±0.2pmol/l on postpartum day 4 (P<0.01) whilst no change was detected in TSH. We hypothesised that thyroid hormones increase mammary metabolism to promote milk synthesis and assessed this in human mammary epithelial cells (HMECs). Reverse transcription-quantitative PCR (RT-qPCR) of thyroid hormone receptor alpha and beta (THRA and THRB) genes showed that THRB has ~7-fold greater expression than THRA (P<0.0001, n=4). THRB phosphorylates Akt, which is required for initiating lactation. Consistent with this, HMECs stimulated with 10nM T3, the most potent thyroid hormone, for 15min showed an ~2-fold increase in Akt phosphorylation (P<0.0001, n=4). Akt regulates oxidative phosphorylation, which we assessed by measuring oxygen consumption rate (OCR) and ATP synthesis. HMECs treated with 10nM T3 for ≤8hrs showed a 2-fold increase in OCR (P<0.05, n=4) without any change in cellular ATP. These findings suggested that T3 uncoupled mitochondrial respiration from ATP synthesis. Consistent with this, RT-qPCR of HMECs stimulated with 10nM T3 showed increased expression of genes encoding uncoupling proteins 2 and 3 (≥2-fold, P<0.01, n=4), which also function to divert substrates away from mitochondrial catabolism; and >20-fold increased expression of PPARGC1A (P<0.0001, n=4), which promotes mitochondrial biogenesis. In summary, these findings demonstrate that serum free T4 is increased at lactation onset. Moreover, our cellular studies indicate that thyroid hormones promote mammary mitochondrial biogenesis and may divert substrates used for ATP generation towards milk synthesis.