SUMMARY Lineage mapping has identified both proliferative and quiescent intestinal stem cells, but the molecular circuitry controlling stem cell quiescence is incompletely understood. By lineage mapping, we show Lrig1, a pan-ErbB inhibitor, marks predominately non-cycling, long-lived stem cells located at the crypt base that, upon injury, proliferate and divide to replenish damaged crypts. Transcriptome profiling of Lrig1+ colonic stem cells differs markedly from highly proliferative, Lgr5+ colonic stem cells; genes up-regulated in the Lrig1+ population include those involved in cell cycle repression and response to oxidative damage. Loss of Apc in Lrig1+ cells leads to intestinal adenomas and genetic ablation of Lrig1 results in heightened ErbB1-3 expression and duodenal adenomas. These results shed light on the relationship between proliferative and quiescent intestinal stem cells, and support a model in which intestinal stem cell quiescence is maintained by calibrated ErbB signaling with loss of a negative regulator predisposing to neoplasia.

Cancer-associated fibroblasts (CAFs) in the tumor stroma play a key role in tumor progression. Erdogan et al. show that CAF-mediated alignment of the fibronectin matrix is a key factor promoting directional cancer cell migration.

Epithelial metaplasia occurs when one predominant cell type in a tissue is replaced by another, and is frequently associated with an increased risk of subsequent neoplasia. In both mouse and human pancreas, acinar-to-ductal metaplasia has been implicated in the generation of cancer precursors. We show that pancreatic epithelial explants undergo spontaneous acinar-to-ductal metaplasia in response to EGFR signaling, and that this change in epithelial character is associated with the appearance of nestin-positive transitional cells. Lineage tracing involving Cre/lox-mediated genetic cell labeling reveals that acinar-to-ductal metaplasia represents a true transdifferentiation event, mediated by initial dedifferentiation of mature exocrine cells to generate a population of nestin-positive precursors, similar to those observed during early pancreatic development. These results demonstrate that a latent precursor potential resides within mature exocrine cells, and that this potential is regulated by EGF receptor signaling. In addition, these observations provide a novel example of rigorously documented transdifferentiation within mature mammalian epithelium, and suggest that plasticity of mature cell types may play a role in the generation of neoplastic precursors.

The C1/RIPE3b1 (؊118/؊107 bp) binding factor regulates pancreatic-␤-cell-specific and glucose-regulated transcription of the insulin gene. In the present study, the C1/RIPE3b1 activator from mouse ␤TC-3 cell nuclear extracts was purified by DNA affinity chromatography and two-dimensional gel electrophoresis. C1/RIPE3b1 binding activity was found in the roughly 46-kDa fraction at pH 7.0 and pH 4.5, and each contained N-and C-terminal peptides to mouse MafA as determined by peptide mass mapping and tandem spectrometry. MafA was detected in the C1/RIPE3b1 binding complex by using MafA peptide-specific antisera. In addition, MafA was shown to bind within the enhancer region (؊340/؊91 bp) of the endogenous insulin gene in ␤TC-3 cells in the chromatin immunoprecipitation assay. These results strongly suggested that MafA was the ␤-cell-enriched component of the RIPE3b1 activator. However, reverse transcription-PCR analysis demonstrated that mouse islets express not only MafA but also other members of the large Maf family, specifically c-Maf and MafB. Furthermore, immunohistochemical studies revealed that at least MafA and MafB were present within the nuclei of islet ␤ cells and not within pancreas acinar cells. Because MafA, MafB, and c-Maf were each capable of specifically binding to and activating insulin C1 element-mediated expression, our results suggest that all of these factors play a role in islet ␤-cell function.Insulin is an essential regulator of metabolism. This hormone, which is synthesized by the ␤ cells of the islets of Langerhans, increases the storage of glucose, fatty acids, and amino acids through its actions in liver, adipose tissue, and muscle. Experiments performed in vivo with transgenic animals have established that the cis-acting elements controlling ␤-cell-selective expression are located within the insulin enhancer region, which is found between nucleotides Ϫ340 and Ϫ91 relative to the transcription start site. Several key control elements within the enhancer have been identified, including C2 (Ϫ317/ Ϫ311 bp), A3 (Ϫ201/Ϫ196 bp), C1 (Ϫ118/Ϫ107 bp), and E1 (Ϫ100/Ϫ91 bp) (37,60,67). Mutations that decrease the binding affinity of the A3, C1, and E1 activators also reduce glucose-regulated transcription (37,60,67).The activator of insulin C2-element stimulated transcription is Pax6 (61). Proteins in the Pax family all contain a paired box bipartite DNA-binding domain, although Pax6 also has a homeodomain. The Pdx-1 homeodomain protein (formerly known as IPF-1, STF-1, and IDX-1) is the regulator of A3 elementactivated expression (46,48,49,50), whereas the E1 activator is a heterodimer composed of proteins in the basic helix-loophelix family that are enriched in islets (i.e., BETA2 [42]) and generally distributed (i.e., HEB [51] and E2A [2,10,17,65]). In the adult pancreas, Pax6 (61) and BETA2 (42) are found in all islet cell types, whereas Pdx-1 appears to be found only in ␤ cells, a subset of islet ␦ cells (48,49), and exocrine acinar cells (71,80). These transcription factors are necessary for ma...

Insulin gene expression is regulated by several islet-enriched transcription factors. However, MafA is the only ␤ cell-specific activator. Here, we show that MafA selectively induces endogenous insulin transcription in non-␤ cells. MafA was also first detected in the insulin-producing cells formed during the second and predominant phase of ␤ cell differentiation, and absent in the few insulin-positive cells found in Nkx6.1 ؊/؊ pancreata, which lack the majority of second-phase ␤ cells. These results demonstrate that MafA is a potent insulin activator that is likely to function downstream of Nkx6.1 during islet insulin-producing cell development.