Effects of ageing and senescence on pancreatic ß-cell function.

Ageing is generally associated with deterioration of organ function and regenerative potential. In the case of pancreatic ß-cells, an age-related decline in proliferative potential is well documented, and was proposed to contribute to the increased prevalence of type 2 diabetes in the elderly. The effects of ageing on ß-cell function, namely glucose-stimulated insulin secretion (GSIS), have not been studied as extensively. Recent work revealed that, surprisingly, ß-cells of mature mice and humans secrete more insulin than young ß-cells in response to high glucose concentrations, potentially serving to counteract age-related peripheral insulin resistance. This functional change appears to be orchestrated by p16(Ink4A) -driven cellular senescence and downstream remodelling of chromatin structure and DNA methylation, enhancing the expression of genes controlling ß-cell function. We propose that activation of the cellular senescence program drives life-long functional maturation of ß-cells, due to ß-cell hypertrophy, enhanced glucose uptake and more efficient mitochondrial metabolism, in parallel to locking these cells in a non-replicative state. We speculate that the beneficial aspects of this process can be harnessed to enhance GSIS. Other age-related mechanisms, which are currently poorly understood, act to increase basal insulin secretion levels also in low glucose conditions. This leads to an overall reduction in the amplitude of insulin secretion between low and high glucose at old age, which may contribute to a deterioration in metabolic control.

EZH2 promotes a bi-lineage identity in basal-like breast cancer cells.

The mechanisms regulating breast cancer differentiation state are poorly understood. Of particular interest are molecular regulators controlling the highly aggressive and poorly differentiated traits of basal-like breast carcinomas. Here we show that the Polycomb factor EZH2 maintains the differentiation state of basal-like breast cancer cells, and promotes the expression of progenitor associated and basal-lineage genes. Specifically, EZH2 regulates the composition of basal-like breast cancer cell populations by promoting a 'bi-lineage' differentiation state, in which cells co-express basal- and luminal-lineage markers. We show that human basal-like breast cancers contain a subpopulation of bi-lineage cells, and that EZH2-deficient cells give rise to tumors with a decreased proportion of such cells. Bi-lineage cells express genes that are active in normal luminal progenitors, and possess increased colony-formation capacity, consistent with a primitive differentiation state. We found that GATA3, a driver of luminal differentiation, performs a function opposite to EZH2, acting to suppress bi-lineage identity and luminal-progenitor gene expression. GATA3 levels increase upon EZH2 silencing, mediating a decrease in bi-lineage cell numbers. Our findings reveal a novel role for EZH2 in controlling basal-like breast cancer differentiation state and intra-tumoral cell composition.

Modeling ductal carcinoma in situ: a HER2-Notch3 collaboration enables luminal filling.

A large fraction of ductal carcinoma in situ (DCIS), a non-invasive precursor lesion of invasive breast cancer, overexpresses the HER2/neu oncogene. The ducts of DCIS are abnormally filled with cells that evade apoptosis, but the underlying mechanisms remain incompletely understood. We overexpressed HER2 in mammary epithelial cells and observed growth factor-independent proliferation. When grown in extracellular matrix as three-dimensional spheroids, control cells developed a hollow lumen, but HER2-overexpressing cells populated the lumen by evading apoptosis. We demonstrate that HER2 overexpression in this cellular model of DCIS drives transcriptional upregulation of multiple components of the Notch survival pathway. Importantly, luminal filling required upregulation of a signaling pathway comprising Notch3, its cleaved intracellular domain and the transcriptional regulator HES1, resulting in elevated levels of c-MYC and cyclin D1. In line with HER2-Notch3 collaboration, drugs intercepting either arm reverted the DCIS-like phenotype. In addition, we report upregulation of Notch3 in hyperplastic lesions of HER2 transgenic animals, as well as an association between HER2 levels and expression levels of components of the Notch pathway in tumor specimens of breast cancer patients. Therefore, it is conceivable that the integration of the Notch and HER2 signaling pathways contributes to the pathophysiology of DCIS.

Epigenetic crosstalk.

In the November 15 issue of Nature, report that disruption of histone methylation in the fungus Neurospora crassa results in the elimination of DNA methylation. This demonstrates that chromatin structure can pattern DNA methylation and suggests that different epigenetic layers engage in complex crosstalk.

Imprinting: focusing on the center.

Recent studies have focused on the identification of imprinting centers and on the elucidation of the mechanisms by which they control imprinting. These studies begin to shed light on the means by which imprinting marks are established in the gametes and on the various molecular strategies utilized to execute differential expression of the two parental alleles.

The tmp gene, encoding a membrane protein, is a c-Myc target with a tumorigenic activity.

The c-Myc oncoprotein induces cell proliferation and transformation through its activity as a transcription factor. Uncovering the genes regulated by c-Myc is an essential step for understanding these processes. We recently isolated the tumor-associated membrane protein gene, Tmp, from a c-myc-induced mouse brain tumor. Here we show that Tmp is specifically highly expressed in mammary tumors and T-cell lymphomas which develop in c-myc transgenic mice, suggesting that Tmp expression is a general characteristic of c-Myc-induced tumors. In addition, Tmp expression is induced upon serum stimulation of fibroblasts as shown in a time course closely correlated with c-myc expression. We have isolated the Tmp promoter region and identified a putative c-Myc binding element, CACGTG, located in the first intron of the gene. We show here that constructs containing the Tmp regulatory region fused to a reporter gene are activated by c-Myc through this CACGTG element and that the c-Myc-Max protein complex can bind to this element. Moreover, an inducible form of c-Myc, the MycER fusion protein, can activate the endogenous Tmp gene. We also show that Tmp-overexpressing fibroblasts induce rapidly growing tumors when injected into nude mice, suggesting that Tmp may possess a tumorigenic activity. Thus, TMP, a member of a novel family of membrane glycoproteins with a suggested role in cellular contact, is a c-Myc target and is possibly involved in c-Myc-induced transformation.

Chromosomal mapping of Tmp (Emp1), Xmp (Emp2), and Ymp (Emp3), genes encoding membrane proteins related to Pmp22.

We have recently characterized a novel mammalian gene family, encoding membrane glycoproteins with four trans-membrane domains. This gene family includes the previously studied PMP22, which is involved in the Charcot-Marie-Tooth neuropathy, and three novel genes: TMP, XMP, and YMP (HGMW-approved symbols EMP1, EMP2 and EMP3, respectively). The Tmp (tumor-associated membrane protein) gene was isolated from a c-myc induced mouse brain tumor and is expressed in several highly proliferative cell types. We have now isolated cDNAs of the mouse Xmp and Ymp genes and determined the chromosomal localization of mouse Tmp, Xmp, and Ymp. Tmp was mapped to mouse chromosome 6, Xmp was mapped to chromosome 16, and Ymp was mapped to chromosome 7. Tmp and Ymp map to paralogous chromosomal regions, whereas Xmp maps to a chromosomal region that is putatively paralogous to a region on chromosome 11, to which Pmp22 was previously mapped. These data suggest that this family of membrane glycoproteins evolved as a result of chromosomal duplications.

Characterization of a tumor-associated gene, a member of a novel family of genes encoding membrane glycoproteins.

To isolate genes involved in tumor formation and in embryogenesis, a subtracted cDNA library was constructed from a c-myc-induced mouse brain tumor. A gene isolated in this screen, named TMP (tumor-associated membrane protein), codes for a putative glycoprotein with four transmembrane domains. The TMP gene was found to be highly expressed in brain tumor cells but not in normal brain. It is also expressed at high levels in undifferentiated embryonic stem cells, but markedly down-regulated in these cells after their differentiation into embryoid bodies. The TMP amino acid sequence bears high homology to the growth arrest specific protein PMP22/GAS-3, which is involved in several human peripheral neuropathies. The expression patterns of the TMP and PMP22 genes in NIH-3T3 fibroblasts were compared at different proliferation states. The results suggest an inverse pattern of expression for the two homologs, TMP expression being high during cell proliferation and PMP22 expression being high during growth arrest. To further characterize the TMP gene we have isolated its human homolog and examined its expression in embryonic and adult tissues. In our search for human sequences homologous to TMP and PMP22, we identified two new genes which we have named XMP and YMP. Thus, we present a novel family of membrane glycoproteins, one member of which is closely associated with proliferation and another with growth arrest.