Albert G. Burger

DNA Binding Properties of Peroxisome Proliferator-activated Receptor Subtypes on Various Natural Peroxisome Proliferator Response Elements IMPORTANCE OF THE 5′-FLANKING REGION

The three subtypes of the peroxisome proliferator-activated receptors (PPARα, β/δ, and γ) form heterodimers with the 9-cis-retinoic acid receptor (RXR) and bind to a common consensus response element, which consists of a direct repeat of two hexanucleotides spaced by one nucleotide (DR1). As a first step toward understanding the molecular mechanisms determining PPAR subtype specificity, we evaluated by electrophoretic mobility shift assays the binding properties of the three PPAR subtypes, in association with either RXRα or RXRγ, on 16 natural PPAR response elements (PPREs). The main results are as follows. (i) PPARγ in combination with either RXRα or RXRγ binds more strongly than PPARα or PPARβ to all natural PPREs tested. (ii) The binding of PPAR to strong elements is reinforced if the heterodimerization partner is RXRγ. In contrast, weak elements favor RXRα as heterodimerization partner. (iii) The ordering of the 16 natural PPREs from strong to weak elements does not depend on the core DR1 sequence, which has a relatively uniform degree of conservation, but correlates with the number of identities of the 5′-flanking nucleotides with respect to a consensus element. This 5′-flanking sequence is essential for PPARα binding and thus contributes to subtype specificity. As a demonstration of this, the PPARγ-specific element ARE6 PPRE is able to bind PPARα only if its 5′-flanking region is exchanged with that of the more promiscuous HMG PPRE.

Evidence that hyperglycaemia per se does not inhibit hepatic glucose production in man

SummaryThe effect of hyperglycaemia on hepatic glucose production (Ra) was investigated in nine healthy men using sequential clamp protocols during somatostatin infusion and euglycaemia (0–150 min), at plasma glucose levels of 165 mg · dl−1 (9.2 mM, 150–270 min) and during insulin infusion (1.0 mU · kg−1 · min−1, 270–360 min) in study 1 or during hypo-insulinaemia and plasma glucose levels of 220 mg · dl−1 (12.2 mM; 270–390 min) in study 2. Somatostatin decreased Raand glucose disposal rate (Rd) but increased plasma free fatty acids (FFA) and lipid oxidation during euglycaemia. Increasing plasma glucose to 165 mg · dl−1 (9.2 mM) and hypo-insulinaemia increased Rd, but no suppressive effects on Ra, plasma FFA and lipid oxidation were observed. By contrast hyperinsulinaemia (study 1), as well as a further increase in plasma glucose (study 2), both decreased Ra. However, more pronounced hyperglycaemia increased insulin secretion despite somatostatin resulting in a fall in plasma FFA and lipid oxidation. Our data questions the accepted dogma that hyperglycaemia inhibits Raindependently of insulin action.