Conversion of adult pancreatic alpha-cells to beta-cells after extreme beta-cell loss.

Pancreatic insulin-producing beta-cells have a long lifespan, such that in healthy conditions they replicate little during a lifetime. Nevertheless, they show increased self-duplication after increased metabolic demand or after injury (that is, beta-cell loss). It is not known whether adult mammals can differentiate (regenerate) new beta-cells after extreme, total beta-cell loss, as in diabetes. This would indicate differentiation from precursors or another heterologous (non-beta-cell) source. Here we show beta-cell regeneration in a transgenic model of diphtheria-toxin-induced acute selective near-total beta-cell ablation. If given insulin, the mice survived and showed beta-cell mass augmentation with time. Lineage-tracing to label the glucagon-producing alpha-cells before beta-cell ablation tracked large fractions of regenerated beta-cells as deriving from alpha-cells, revealing a previously disregarded degree of pancreatic cell plasticity. Such inter-endocrine spontaneous adult cell conversion could be harnessed towards methods of producing beta-cells for diabetes therapies, either in differentiation settings in vitro or in induced regeneration.

The zinc finger-containing transcription factor Gata-4 is expressed in the developing endocrine pancreas and activates glucagon gene expression.

Gene inactivation studies have shown that members of the Gata family of transcription factors are critical for endoderm development throughout evolution. We show here that Gata-4 and/or Gata-6 are not only expressed in the adult exocrine pancreas but also in glucagonoma and insulinoma cell lines, whereas Gata-5 is restricted to the exocrine pancreas. During pancreas development, Gata-4 is expressed already at embryonic d 10.5 and colocalizes with early glucagon+ cells at embryonic d 12.5. Gata-4 was able to transactivate the glucagon gene both in heterologous BHK-21 (nonislet Syrian baby hamster kidney) and in glucagon-producing InR1G9 cells. Using gel-mobility shift assays, we identified a complex formed with nuclear extracts from InR1G9 cells on the G5 control element (-140 to -169) of the glucagon gene promoter as Gata-4. Mutation of the GATA binding site on G5 abrogated the transcriptional activation mediated by Gata-4 and reduced basal glucagon gene promoter activity in glucagon-producing cells by 55%. Furthermore, Gata-4 acted more than additively with Forkhead box A (hepatic nuclear factor-3) to trans-activate the glucagon gene promoter. We conclude that, besides its role in endoderm differentiation, Gata-4 might be implicated in the regulation of glucagon gene expression in the fetal pancreas and that Gata activity itself may be modulated by interactions with different cofactors.

Targeted deletion of the PRL receptor: effects on islet development, insulin production, and glucose tolerance.

PRL and placental lactogen (PL) stimulate beta-cell proliferation and insulin gene transcription in isolated islets and rat insulinoma cells, but the roles of the lactogenic hormones in islet development and insulin production in vivo remain unclear. To clarify the roles of the lactogens in pancreatic development and function, we measured islet density (number of islets/cm(2)) and mean islet size, beta-cell mass, pancreatic insulin mRNA levels, islet insulin content, and the insulin secretory response to glucose in an experimental model of lactogen resistance: the PRL receptor (PRLR)-deficient mouse. We then measured plasma glucose concentrations after ip injections of glucose or insulin. Compared with wild-type littermates, PRLR-deficient mice had 26-42% reductions (P < 0.01) in islet density and beta-cell mass. The reductions in islet density and beta-cell mass were noted as early as 3 wk of age and persisted through 8 months of age and were observed in both male and female mice. Pancreatic islets of PRLR-deficient mice were smaller than those of wild-type mice at weaning but not in adulthood. Pancreatic insulin mRNA levels were 20-30% lower (P < 0.05) in adult PRLR-deficient mice than in wild-type mice, and the insulin content of isolated islets was reduced by 16-25%. The insulin secretory response to ip glucose was blunted in PRLR-deficient males in vivo (P < 0.05) and in isolated islets of PRLR-deficient females and males in vitro (P < 0.01). Fasting blood glucose concentrations in PRLR-deficient mice were normal, but glucose levels after an ip glucose load were 10-20% higher (P < 0.02) than those in wild-type mice. On the other hand, the glucose response to ip insulin was normal. Our observations establish a physiologic role for lactogens in islet development and function.

Experimental models of beta-cell regeneration.

The control of glucose metabolism by pancreatic endocrine cells throughout life relies on a tight regulation of the mass of insulin-producing beta-cells. How this homoeostasis is achieved is not well understood. Over the last few years, experimental rodent models with altered beta-cell mass, and, more recently, new transgenic approaches designed to tackle this problem, have provided abundant information. Processes such as beta-cell proliferation and apoptosis, or even beta-cell differentiation from poorly characterized progenitor cells, whether immature or differentiated, appear to be implicated. A complex picture is thus emerging in which the nature of the pancreatic lesion appears to determine the kind of regenerative response. The environment formed by acinar and ductal cells, and also by vascular and neuronal structures, which surround islets and penetrate into their beta-cell core, might play crucial roles so far unsuspected, which should be explored in the near future.

Pax-6 and c-Maf functionally interact with the alpha-cell-specific DNA element G1 in vivo to promote glucagon gene expression.

Specific expression of the glucagon gene in the rat pancreas requires the presence of the G1 element localized at -100/-49 base pairs on the promoter. Although it is known that multiple transcription factors such as Pax-6, Cdx-2/3, c-Maf, Maf-B, and Brain-4 can activate the glucagon gene promoter through G1, their relative importance in vivo is unknown. We first studied the expression of Maf-B, c-Maf, and Cdx-2/3 in the developing and adult mouse pancreas. Although Maf-B was detectable in a progressively increasing number of alpha-cells throughout development and in adulthood, c-Maf and Cdx-2/3 were expressed at low and very low levels, respectively. However, c-Maf but not Cdx-2/3 was detectable in adult islets by Western blot analyses. We then demonstrated the in vivo interactions of Pax-6, Cdx-2/3, Maf-B, and c-Maf but not Brain-4 with the glucagon gene promoter in glucagon-producing cells. Although Pax-6, Cdx-2/3, Maf-B, and c-Maf were all able to bind G1 by themselves, we showed that Pax-6 could interact with Maf-B, c-Maf, and Cdx-2/3 and activate transcription of the glucagon gene promoter. Overexpression of dominant negative forms of Cdx-2/3 and Mafs in alpha-cell lines indicated that Cdx-2/3 and the Maf proteins interact on an overlapping site within G1 and that this binding site is critical in the activation of the glucagon gene promoter. Finally, we show that specific inhibition of Pax-6 and c-Maf but not Cdx-2/3 or Maf-B led to decreases in endogenous glucagon gene expression and that c-Maf binds the glucagon gene promoter in mouse islets. We conclude that Pax-6 and c-Maf interact with G1 to activate basal expression of the glucagon gene.