Polycomb protein Ezh2 regulates pancreatic β-cell Ink4a/Arf expression and regeneration in diabetes mellitus

Abstract

Proliferation of pancreatic islet b cells is an important mechanism for self-renewal and for adaptive islet expansion. Increased expression of the Ink4a/Arf locus, which encodes the cyclin-dependent kinase inhibitor p16INK4a and tumor suppressor p19 Arf , limits b-cell regeneration in aging mice, but the basis of b-cell Ink4a/Arf regulation is poorly understood. Here we show that Enhancer of zeste homolog 2 (Ezh2), a histone methyltransferase and component of a Polycomb group (PcG) protein complex, represses Ink4a/Arf in islet b cells. Ezh2 levels decline in aging islet b cells, and this attrition coincides with reduced histone H3 trimethylation at Ink4a/ Arf, and increased levels of p16INK4a and p19Arf . Conditional deletion of b-cell Ezh2 in juvenile mice also reduced H3 trimethylation at the Ink4a/Arf locus, leading to precocious increases of p16INK4a and p19 Arf . These mutant mice had reduced b-cell proliferation and mass, hypoinsulinemia, and mild diabetes, phenotypes rescued by germline deletion of Ink4a/Arf. b-Cell destruction with streptozotocin in controls led to increased Ezh2 expression that accompanied adaptive b-cell proliferation and re-establishment of b-cell mass; in contrast, mutant mice treated similarly failed to regenerate b cells, resulting in lethal diabetes. Our discovery of Ezh2-dependent b-cell proliferation revealed unique epigenetic mechanisms underlying normal b-cell expansion and b-cell regenerative failure in diabetes pathogenesis.[Keywords: Pancreas; islet of Langerhans; histone; epigenetics; diabetes; cell cycle] Supplemental material is available at http://www.genesdev.org. Received September 18, 2008; revised version accepted March 13, 2009. Proliferation of insulin-secreting b cells in pancreatic islets is an important mechanism for establishing, maintaining, and adapting islet organ function to meet host physiological demands (for review, see Cozar-Castellano et al. 2006;Heit et al. 2006a). Harnessing our understanding of these mechanisms could accelerate development of islet replacement strategies in diseases like autoimmune (type 1) diabetes, but the molecular basis of self-renewal in organs like islets is poorly understood. Islet b cells expand in neonatal humans, mice, and other species, but this proliferation decays thereafter Meier et al. 2008), which may promote pandemic (type 2) forms of diabetes mellitus. Thus, investigation of regenerative failure in b cells may elucidate important mechanisms underlying diabetes pathogenesis. p16 INK4a and p19 Arf (hereafter Ink4a and Arf) encoded by the Cdkn2a locus are negative regulators of the cell cycle and are thought to limit proliferation in islet b cells (Krishnamurthy et al. 2006) and other tissues (Zindy et al. 1997). Ink4a inhibits specific cyclin-dependent kinases (CDK), including CDK4, a key regulator of b-cell proliferation (Rane et al. 1999), while Arf inhibits the ubiquitin ligase activity of MDM2, thereby stabilizing p53 (for review, see Matheu et al. 2008). Germline Ink4a deficiency in mice permits increased b-...

Ezh2, the histone methyltransferase of PRC2, regulates the balance between self-renewal and differentiation in the cerebral cortex

“…Loss of polycomb function in a variety of cell types leads to upregulated transcription of the negative regulators of the cell cycle encoded by the Ink4/Arf locus (Cdkn2a/Cdkn2b) (11,33,34). Cdkn2a and Cdkn2b are not normally expressed in the developing cerebral cortex (35)(36)(37), and we found the H3K27me3 modification present on the Cdkn2a and Cdkn2b promoters in the wild-type E12 cortex (Fig.…”

Maternal diet and aging alter the epigenetic control of a promoter–enhancer interaction at the Hnf4a gene in rat pancreatic islets

“…This hypothesis is supported by our functional assays in BRIN cells showing that loss of H3K27me3 leads to reactivation of the silent state. A previous study reported reduction of H3K27me3 repressive mark at the Ink4a/Arf locus in mouse β-cells, with age, that leads to increased expression (35). This was associated with a decline of the histone methyltransferase Ezh2 with age (35).…”

The regulation of pre- and post-maturational plasticity of mammalian islet cell mass

“…In particular, ageing de-represses this locus, leading to increased levels of cell cycle inhibitors (p16 cip1 and p19), decreased B-lymphoma Moloney murine leukaemia virus insertion region-1 (BMI1) banding [135] and decreased levels of enhancer of zeste homologue 2 (EZH2) [136], a histone methyltransferase and member of the polycomb group of proteins.…”