Emptying the Pool: Modular Insulin Secretion From the Pancreas

Turnover Rates of the Low-Density Lipoprotein Receptor and PCSK9: Added Dimension to the Cholesterol Homeostasis Model

OBJECTIVE

We measured the turnover rates of the LDLR (low-density lipoprotein receptor) and PCSK9 (proprotein convertase subtilisin/kexin type 9) in mice by metabolic labeling with heavy water and mass spectrometry. Approach and Results: In liver of mice fed high-cholesterol diets, LDLR mRNA levels and synthesis rates were markedly lower with complete suppression of cholesterol synthesis and higher cholesterol content, consistent with the Brown-Goldstein model of tissue cholesterol homeostasis. We observed markedly lower PCSK9 mRNA levels and synthesis rates in liver and lower concentrations and synthesis rates in plasma. Hepatic LDLR half-life (t½) was prolonged, consistent with an effect of reduced PCSK9, and resulted in no reduction in hepatic LDLR content despite reduced mRNA levels and LDLR synthesis rates. These changes in PCSK9 synthesis complement and expand the well-established model of tissue cholesterol homeostasis in mouse liver, in that reduced synthesis and levels of PCSK9 counterbalance lower LDLR synthesis by promoting less LDLR catabolism, thereby maintaining uptake of LDL cholesterol into liver despite high intracellular cholesterol concentrations.

CONCLUSIONS

Lower hepatic synthesis and secretion of PCSK9, an SREBP2 (sterol response element binding protein) target gene, results in longer hepatic LDLR t½ in response to cholesterol feeding in mice in the face of high intracellular cholesterol content. PCSK9 modulation opposes the canonical lowering of LDLR mRNA and synthesis by cholesterol surplus and preserves LDLR levels. The physiological and therapeutic implications of these opposing control mechanisms over liver LDLR are of interest and may reflect subservience of hepatic cholesterol homeostasis to whole body cholesterol needs.

Imaging β-Cell Function Using a Zinc-Responsive MRI Contrast Agent May Identify First Responder Islets

An imaging method for detecting β-cell function in real-time in the rodent pancreas could provide new insights into the biological mechanisms involving loss of β-cell function during development of type 2 diabetes and for testing of new drugs designed to modulate insulin secretion. In this study, we used a zinc-responsive MRI contrast agent and an optimized 2D MRI method to show that glucose stimulated insulin and zinc secretion can be detected as functionally active "hot spots" in the tail of the rat pancreas. A comparison of functional images with histological markers show that insulin and zinc secretion does not occur uniformly among all pancreatic islets but rather that some islets respond rapidly to an increase in glucose while others remain silent. Zinc and insulin secretion was shown to be altered in streptozotocin and exenatide treated rats thereby verifying that this simple MRI technique is responsive to changes in β-cell function.

A Local Counter-Regulatory Motif Modulates the Global Phase of Hormonal Oscillations

Counter-regulatory elements maintain dynamic equilibrium ubiquitously in living systems. The most prominent example, which is critical to mammalian survival, is that of pancreatic α and β cells producing glucagon and insulin for glucose homeostasis. These cells are not found in a single gland but are dispersed in multiple micro-organs known as the islets of Langerhans. Within an islet, these two reciprocal cell types interact with each other and with an additional cell type: the δ cell. By testing all possible motifs governing the interactions of these three cell types, we found that a unique set of positive/negative intra-islet interactions between different islet cell types functions not only to reduce the superficially wasteful zero-sum action of glucagon and insulin but also to enhance/suppress the synchronization of hormone secretions between islets under high/normal glucose conditions. This anti-symmetric interaction motif confers effective controllability for network (de)synchronization.