A classic metabolic concept posits that insulin promotes energy storage and adipose expansion, while catecholamines stimulate release of adipose energy stores by hydrolysis of triglycerides through β-adrenergic receptor (βARs) and protein kinase A (PKA) signaling. Here, we have shown that a key hub in the insulin signaling pathway, activation of p70 ribosomal S6 kinase (S6K1) through mTORC1, is also triggered by PKA activation in both mouse and human adipocytes. Mice with mTORC1 impairment, either through adipocyte-specific deletion of Raptor or pharmacologic rapamycin treatment, were refractory to the well-known βAR-dependent increase of uncoupling protein UCP1 expression and expansion of beige/brite adipocytes (so-called browning) in white adipose tissue (WAT). Mechanistically, PKA directly phosphorylated mTOR and RAPTOR on unique serine residues, an effect that was independent of insulin/AKT signaling. Abrogation of the PKA site within RAPTOR disrupted βAR/mTORC1 activation of S6K1 without affecting mTORC1 activation by insulin. Conversely, a phosphomimetic RAPTOR augmented S6K1 activity. Together, these studies reveal a signaling pathway from βARs and PKA through mTORC1 that is required for adipose browning by catecholamines and provides potential therapeutic strategies to enhance energy expenditure and combat metabolic disease.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to and degrades the low-density lipoprotein receptor (LDLR), contributing to hypercholesterolemia. Adipose tissue plays a role in lipoprotein metabolism, but there are almost no data about PCSK9 and LDLR regulation in human adipocytes. We studied PCSK9 and LDLR regulation by insulin, atrial natriuretic peptide (ANP, a potent lipolytic agonist that antagonizes insulin), and LDL in visceral adipose tissue (VAT) and in human cultured adipocytes. PCSK9 was expressed in VAT and its expression was positively correlated with body mass index (BMI). Both intracellular mature and secreted PCSK9 were abundant in cultured human adipocytes. Insulin induced PCSK9, LDLR, and sterol-regulatory element-binding protein-1c (SREBP-1c) and -2 expression (SREBP-2). ANP reduced insulin-induced PCSK9, especially in the context of a medium simulating hyperglycemia. Human LDL induced both mature and secreted PCSK9 and reduced LDLR. ANP indirectly blocked the LDLR degradation, reducing the positive effect of LDL on PCSK9. In conclusion, PCSK9 is expressed in human adipocytes. When the expression of PCSK9 is induced, LDLR is reduced through the PCSK9-mediated degradation. On the contrary, when the induction of PCSK9 by insulin and LDL is partially blocked by ANP, the LDLR degradation is reduced. This suggests that NPs could be able to control LDLR levels, preventing PCSK9 overexpression.

Cardiac natriuretic peptides (NP) are involved in cardio-renal regulation but also in lipolysis and lipid metabolism. The signaling receptor NPRA and the clearance receptor NPRC mediate the response to NP that is largely dependent on the ratio between the two receptors. Lipolysis increases when NPRC is selectively downregulated by starving or very-low calorie diet. Insulin is an anti-lipolytic hormone and also induces sodium-retention, suggesting a possible functional link with NP. Therefore we examined the insulin-mediated regulation of NP receptors in differentiated human adipocytes and tested the association between NP receptors expression in visceral adipose tissue (VAT) and insulin, HOMA index and metabolic profiles in patients undergoing renal surgery. Differentiated human adipocytes from VAT and SGBS cell line were treated with insulin in the presence of glucose. Further experiments in low-glucose media were performed to simulate in vivo starving conditions and to study glucose-insulin mediated genes involved in lipogenesis. Lipolysis was assessed by glycerol release assay. Fasting blood samples and VAT samples were taken from patients in the day of renal surgery. We observed a potent insulin-mediated and glucose-dependent up-regulation of NPRC, through the PI3K pathway, associated with lower lipolysis in differentiated adipocytes. No effect was observed on NPRA. Low-glucose medium hampered the insulin effect, likely through the modulation of glucose flux genes. An expression ratio in favor of NPRC in adipose tissue was associated with higher fasting insulinemia, higher HOMA index and higher total, LDL, and non-HDL cholesterol. Insulin and glucose-dependent NPRC induction in adipocytes, especially in the contest of insulin resistance and increased insulin levels, might be a key factor linking hyperinsulinemia, metabolic syndrome and higher blood pressure.