A longstanding unsettled question is whether pancreatic beta cells originate from exocrine duct cells. We have now used genetic labeling to fate map embryonic and adult pancreatic duct cells. We show that Hnf1beta+ cells of the trunk compartment of the early branching pancreas are precursors of acinar, duct, and endocrine lineages. Hnf1beta+ cells subsequent form the embryonic duct epithelium, which gives rise to both ductal and endocrine lineages, but not to acinar cells. By the end of gestation, the fate of Hnf1beta+ duct cells is further restrained. We provide compelling evidence that the ductal epithelium does not make a significant contribution to acinar or endocrine cells during neonatal growth, during a 6 month observation period, or during beta cell growth triggered by ligation of the pancreatic duct or by cell-specific ablation with alloxan followed by EGF/gastrin treatment. Thus, once the ductal epithelium differentiates it has a restricted plasticity, even under regenerative settings.

Heterozygous HNF1A mutations cause pancreatic-islet ␤-cell dysfunction and monogenic diabetes (MODY3). Hnf1␣ is known to regulate numerous hepatic genes, yet knowledge of its function in pancreatic islets is more limited. We now show that Hnf1a deficiency in mice leads to highly tissue-specific changes in the expression of genes involved in key functions of both islets and liver. To gain insights into the mechanisms of tissue-specific Hnf1␣ regulation, we integrated expression studies of Hnf1a-deficient mice with identification of direct Hnf1␣ targets. We demonstrate that Hnf1␣ can bind in a tissue-selective manner to genes that are expressed only in liver or islets. We also show that Hnf1␣ is essential only for the transcription of a minor fraction of its direct-target genes. Even among genes that were expressed in both liver and islets, the subset of targets showing functional dependence on Hnf1␣ was highly tissue specific. This was partly explained by the compensatory occupancy by the paralog Hnf1␤ at selected genes in Hnf1a-deficient liver. In keeping with these findings, the biological consequences of Hnf1a deficiency were markedly different in islets and liver. Notably, Hnf1a deficiency led to impaired large-T-antigen-induced growth and oncogenesis in ␤ cells yet enhanced proliferation in hepatocytes. Collectively, these findings show that Hnf1␣ governs broad, highly tissue-specific genetic programs in pancreatic islets and liver and reveal key consequences of Hnf1a deficiency relevant to the pathophysiology of monogenic diabetes.

Mutations in the genes encoding transcriptional regulators HNF1beta (TCF2), HNF1alpha (TCF1), and HNF4alpha cause autosomal dominant diabetes (also known as maturity-onset diabetes of the young). Herein, we review what we have learnt during recent years concerning the functions of these regulators in the developing and adult pancreas. Mouse studies have revealed that HNF1beta is a critical regulator of a transcriptional network that controls the specification, growth, and differentiation of the embryonic pancreas. HNF1beta mutations in humans accordingly often cause pancreas hypoplasia. By contrast, HNF1alpha and HNF4alpha have been shown to regulate the function of differentiated beta-cells. HNF1alpha and HNF4alpha mutations in patients thus cause decreased glucose-induced insulin secretion that leads to a progressive form of diabetes. HNF4alpha mutations paradoxically also cause in utero and neonatal hyperinsulinism, which later evolves to decreased glucose-induced secretion. Recent studies show that Hnf4alpha deficiency in mice causes not only abnormal insulin secretion, but also an impairment of the expansion of beta-cell mass that normally occurs during pregnancy. In line with this finding, we present data that Hnf1alpha-/- beta-cells expressing SV40 large T antigen show a severe impairment of proliferation and failure to form tumours. Collectively, these findings implicate HNF1beta as a regulator of pancreas organogenesis and differentiation, whereas HNF1alpha and HNF4alpha primarily regulate both growth and function of islet beta-cells.

Accumulation of delta-aminolevulinic acid (ALA), as it occurs in acute intermittent porphyria, is a potential endogenous source of reactive oxygen species (ROS) which can then produce oxidative damage to cell structures and macromolecules. This in vivo study investigated whether melatonin could prevent the deleterious effects of ALA. Rats were injected i.p. for 2 weeks with ALA (40 mg/kg on alternate days) and/or with melatonin (50 microg/kg or 500 microg/kg daily). Administration of pharmacological doses of melatonin reduced and/or prevented ALA-induced lipid peroxidation (LPO) in both cerebral cortex and cerebellum, providing further evidence of melatonin's action as a ROS scavenger. Administration of pharmacological concentrations of melatonin to ALA-injected rats showed the protective properties of melatonin on the activities of both porphobilinogen-deaminase and delta-aminolevulinate dehydratase (ALA-D) in the cerebral cortex; the effect on ALA-D activity was unexpectedly high (at least 6-fold), indicating that, besides acting as a scavenger of hydroxyl radicals, melatonin may exert its protection on ALA-D through other mechanisms, such as increasing mRNA levels of antioxidant enzymes or/and inducing glutathione peroxidase activity. The possibility that changes in the expression of antioxidant enzymes could affect the expression of other proteins, even those not related to the cellular ROS homeostasis, should also not be discarded. The potential use of melatonin as an antioxidant and for its reactivating properties in the treatment of acute porphyrias is considered.

delta-aminolevulinic acid (ALA) promotes the generation of reactive oxygen species (ROS). Accumulation of ALA, as occurs in acute intermittent porphyria (AIP), is a potential endogenous source of ROS, which can then exert oxidative damage to cell structures. In this work we investigated the role of pharmacological concentrations of melatonin on the deleterious effect of ALA and its effect on porphyrin biosynthesis. Rat cerebellum incubations were carried out with either ALA (1.0 mM) together with increasing concentrations of melatonin (0.1-2.0 mM) or 2.0 mM melatonin together with varying ALA concentrations (0.05-2.0 mM) for different times (1-4 hr). ALA-induced lipid peroxidation was significantly diminished by melatonin in a concentration-dependent manner. In all conditions 2.0 mM melatonin restored malondialdehyde levels to control values. In incubations without ALA, melatonin markedly reduced (36-40%) the basal levels of lipid peroxidation when compared with the corresponding controls. ALA uptake and porphyrin accumulation were increased 30% in incubations with 1.0-2.0 mM ALA for 4 hr in the presence of 2.0 mM melatonin, providing evidence for the involvement of ALA-promoted ROS in the damage of enzymes related to porphyrin biosynthesis. These results are further support for the protective role of melatonin against oxidative damage induced by ALA; this protective action of melatonin is probably due to melatonin's antioxidant and free radical scavenger properties. The development of a new therapeutic approach for AIP patients employing melatonin alone or in combination with conventional treatments should be considered.

Regulation of transcription operates by the combinatorial action of sequence-specific trans-acting factors bound to upstream regulatory regions of promoters and enhancer/silencer regions. The particular linear disposition of these cis-regulatory sequences and their topological organization in chromatin, along with the cellular repertoire of transcription factors results in the formation of gene-and cell-specific constellations of chromatin bound factors. The particular array of proteins on the chromatin target modulates the transcriptional rates by multiple interactions with chromatin remodeling complexes, co-regulators, and components of the basal transcriptional machinery (1). Transcriptional regulators often exhibit a modular structure with independent DNA-binding domains (DBDs) 1 and regulatory regions. The transactivation domains are classified according to their amino acid composition as either rich in acidic side chains, glutamine, or proline residues (2). Although the precise fashion by which transactivators regulate transcription of a specific gene depends on many different promoter features and cell-specific factors, the mechanisms of action for acidic and proline-rich domains appear to be conserved in eukaryotic organisms from yeast to man (3,4).Most transcription factors are expressed in several variants that can be grouped in large families, whose members show only subtle differences. The expression of these variants is tightly regulated in a cell-type or developmental-stage specific manner. Alternative splicing is the mechanism most widely used to generate this precisely regulated, diversity of transcription factors. However, with few exceptions, the functional significance of transcription factor variants generated by alternative splicing is poorly understood (5, 6).

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