Control of mRNA translation preserves endoplasmic reticulum function in beta cells and maintains glucose homeostasis.

Type 2 diabetes is a disorder of hyperglycemia resulting from failure of beta cells to produce adequate insulin to accommodate an increased metabolic demand. Here we show that regulation of mRNA translation through phosphorylation of eukaryotic initiation factor 2 (eIF2alpha) is essential to preserve the integrity of the endoplasmic reticulum (ER) and to increase insulin production to meet the demand imposed by a high-fat diet. Accumulation of unfolded proteins in the ER activates phosphorylation of eIF2alpha at Ser51 and inhibits translation. To elucidate the role of this pathway in beta-cell function we studied glucose homeostasis in Eif2s1(tm1Rjk) mutant mice, which have an alanine substitution at Ser51. Heterozygous (Eif2s1(+/tm1Rjk)) mice became obese and diabetic on a high-fat diet. Profound glucose intolerance resulted from reduced insulin secretion accompanied by abnormal distension of the ER lumen, defective trafficking of proinsulin, and a reduced number of insulin granules in beta cells. We propose that translational control couples insulin synthesis with folding capacity to maintain ER integrity and that this signal is essential to prevent diet-induced type 2 diabetes.

Acute Exercise Regulates hTERT Gene Expression and Alternative Splicing in the hTERT-BAC Transgenic Mouse Model.

INTRODUCTION: Aerobic exercise maintains telomere length through increased human telomerase reverse transcriptase (hTERT) expression and telomerase enzyme activity. The impact of acute exercise on hTERT alternative splicing (AS) is unknown. PURPOSE: This study aimed to examine hTERT AS in response to acute treadmill running. METHODS: A bacterial artificial chromosome mouse model containing the 54-kilobase hTERT gene locus inserted into its genome (hTERT-BAC) was utilized. The gastrocnemius, left ventricle, and brain were excised before (Pre), upon cessation (Post), and during recovery (1, 24, 48, and 72 h; n = 5/time point) from treadmill running (30 min at 60% maximum speed). Full-length (FL) hTERT and the "minus beta" (-ß) AS variant (skips exons 7 and 8 and does not code for active telomerase) were measured by gel-based and droplet digital reverse transcription-polymerase chain reaction methods. SF3B4 and SRSF2 protein expression were measured by Western blotting. RESULTS: Compared with Pre, FL hTERT increased at Post before decreasing during recovery in the gastrocnemius (48 and 72 h; P ≤ 0.001) and left ventricle (24 h; P = 0.004). The percentage of FL hTERT in the gastrocnemius also increased during recovery (1 and 72 h; P ≤ 0.017), whereas a decrease was observed in the left ventricle (1, 24, and 48 h; P ≤ 0.041). hTERT decreased in the brain (48 h), whereas FL hTERT percentage remained unaltered. SF3B4 protein expression decreased throughout recovery in the gastrocnemius and tended to be associated with FL hTERT (r = -0.348, P = 0.075) and -ß in opposite directions (r = 0.345, P = 0.067). CONCLUSIONS: Endurance exercise increased hTERT gene expression, and altered FL hTERT splicing in contractile tissues and may maintain telomere length necessary to improve the function and health of the organism.

Intronic Cis-Element DR8 in hTERT Is Bound by Splicing Factor SF3B4 and Regulates hTERT Splicing in Non-Small Cell Lung Cancer.

Splicing of the hTERT gene to produce the full-length (FL) transcript is necessary for telomerase enzyme activity and telomere-dependent cellular immortality in the majority of human tumors, including non-small cell lung cancer (NSCLC) cells. The molecular machinery to splice hTERT to the FL isoform remains mostly unknown. Previously, we reported that an intron 8 cis-element termed "direct repeat 8" (DR8) promotes FL hTERT splicing, telomerase, and telomere length maintenance when bound by NOVA1 and PTBP1 in NSCLC cells. However, some NSCLC cells and patient tumor samples lack NOVA1 expression. This leaves a gap in knowledge about the splicing factors and cis-elements that promote telomerase in the NOVA1-negative context. We report that DR8 regulates FL hTERT splicing in the NOVA1-negative and -positive lung cancer contexts. We identified splicing factor 3b subunit 4 (SF3B4) as an RNA trans-factor whose expression is increased in lung adenocarcinoma (LUAD) tumors compared with adjacent normal tissue and predicts poor LUAD patient survival. In contrast to normal lung epithelial cells, which continued to grow with partial reductions of SF3B4 protein, SF3B4 knockdown reduced hTERT splicing, telomerase activity, telomere length, and cell growth in lung cancer cells. SF3B4 was also demonstrated to bind the DR8 region of hTERT pre-mRNA in both NOVA1-negative and -positive NSCLC cells. These findings provide evidence that DR8 is a critical binding hub for trans-factors to regulate FL hTERT splicing in NSCLC cells. These studies help define mechanisms of gene regulation important to the generation of telomerase activity during carcinogenesis. IMPLICATIONS: Manipulation of a core spliceosome protein reduces telomerase/hTERT splicing in lung cancer cells and results in slowed cancer cell growth and cell death, revealing a potential therapeutic strategy.

FOXP3 as an X-linked tumor suppressor.

The FOXP3 gene was initially identified because its mutation caused lethal autoimmune diseases in mice and humans. Mice with heterozygous mutations of FoxP3 (mouse version of the FOXP3 gene) succumb to mammary tumors spontaneously, while those with prostate-specific deletions develop prostate intraepithelial neoplasia. Somatic mutations, deletion, and epigenetic inactivation of FOXP3 are widespread among human breast and prostate cancers. Unlike autosomal tumor suppressor genes that are usually inactivated by mutations in both alleles, X-linked FOXP3 mutations in cancer samples are usually heterozygous, with the wildtype allele selectively inactivated in cancer. This skewed X-inactivation suggests a new approach to reactivation of FOXP3 for cancer therapy.

Translation attenuation through eIF2alpha phosphorylation prevents oxidative stress and maintains the differentiated state in beta cells.

Accumulation of unfolded protein within the endoplasmic reticulum (ER) attenuates mRNA translation through PERK-mediated phosphorylation of eukaryotic initiation factor 2 on Ser51 of the alpha subunit (eIF2alpha). To elucidate the role of eIF2alpha phosphorylation, we engineered mice for conditional expression of homozygous Ser51Ala mutant eIF2alpha. The absence of eIF2alpha phosphorylation in beta cells caused a severe diabetic phenotype due to heightened and unregulated proinsulin translation; defective intracellular trafficking of ER cargo proteins; increased oxidative damage; reduced expression of stress response and beta-cell-specific genes; and apoptosis. However, glucose intolerance and beta cell death in these mice were attenuated by a diet containing antioxidant. We conclude that phosphorylation of eIF2alpha coordinately attenuates mRNA translation, prevents oxidative stress, and optimizes ER protein folding to support insulin production. The finding that increased proinsulin synthesis causes oxidative damage in beta cells may reflect events in the beta cell failure associated with insulin resistance in type 2 diabetes.