Endocrine Abstracts

The onset of lactation during post-partum days 1-4 is hormonally-regulated and critical for successful breastfeeding. Insulin represents a key lactogenic hormone as evidenced by women with type 1 diabetes who have delayed lactation onset. However, the role of insulin in lactation and its influence on mammary cells are unclear. We utilised clinical and cellular approaches to investigate this, and recruited n=12 women following informed consent and measured serum insulin in pregnancy (36 weeks’ gestation) and during post-partum days 1-4. Serum insulin progressively decreased from 36 weeks’ gestation to day 4 post-partum (260.6±59.1 pmol/l vs 109.8±48.8 pmol/l, P<0.05), reflecting increased maternal insulin sensitivity at lactation onset. We hypothesised that insulin promotes lactation by influencing mammary metabolism and used human mammary epithelial cells (HMECs) to evaluate this. Reverse transcription-quantitative PCR (RT-qPCR) demonstrated that HMECs express the insulin receptor. Moreover, stimulation of HMECs with 10nM insulin caused a >2-fold increase (P<0.0001, n=4) in phosphorylation of Akt, a signalling protein required for initiating lactation. Akt influences oxidative phosphorylation and glycolysis, and we assessed these processes by measuring oxygen consumption rate (OCR) and extracellular acidification rate (ECAR), respectively. HMECs treated with 10nM insulin for ≤18hrs showed increased OCR (22±0.9 vs. 17±0.4 pmol O₂/min/10 ⁶ cells for control HMECs, P<0.05, n=4) and ECAR (0.23±0.003 vs. 0.18±0.002mpH/min/10⁶ cells for control HMECs, P<0.001, n=4), consistent with increased oxidative phosphorylation and glycolysis. HMECs treated with 10nM insulin for 8hrs significantly upregulated expression of glycolytic enzymes, namely hexokinase 2 (>4-fold increase, P<0.0001, n=4) and pyruvate kinase M1/2 (1.4-fold increase, P<0.05, n=4). However, RT-qPCR analysis of HMECs showed that insulin did not increase expression of genes mediating oxidative phosphorylation, suggesting an Akt-mediated post-transcriptional mechanism. In summary, increased insulin sensitivity together with insulin-stimulated oxidative phosphorylation and glycolysis may support mammary function and milk component synthesis at the onset of lactation.