Role of heme regulated eIF2α kinase in pancreatic beta cell function and viability

The eukaryotic translation initiation factor 2 alpha (eIF2α) is part of the initiation complex that drives the initiator amino acid methionine to the ribosome, a crucial step in protein translation. In stress conditions such as virus infection, endoplasmic reticulum (ER) stress, amino acid or heme deficiency eIF2α can be phosphorylated and thereby inhibit global protein synthesis. This adaptive mechanism prevents protein accumulation and consequent cytotoxic effects. Heme-regulated eIF2α kinase (HRI) is a member of the eIF2α kinase family that regulates protein translation in heme deficiency conditions. Although present in all tissues, HRI is predominantly expressed in erythroid cells where it remains inactive in the presence of normal heme concentrations. In response to heme deficiency, HRI is activated and phosphorylates eIF2α decreasing globin synthesis. This mechanism is important to prevent accumulation of heme-free globin chains which cause ER stress and apoptosis. RNA sequencing data from our group showed that in human islets and in primary rat beta cells HRI is the most expressed eIF2α kinase compared to the other family members. Despite its high expression levels, little is known about HRI function in beta cells. The aim of this project is to identify the role of HRI in pancreatic beta cells. This was investigated taking a loss-of-function approach. HRI knock down (KD) by RNA interference induced beta cell apoptosis in basal condition. HRI KD potentiated the apoptotic effects of palmitate or proinflammatory cytokines, two in vitro models for type 2 and type 1 diabetes, respectively. Increased cytokine-induced apoptosis was also observed in HRI-deficient primary rat beta cells. Unexpectedly, we observed a mild increase in eIF2α phosphorylation in HRI-deficient cells. The levels of mRNA or protein expression of C/EBP homologous protein (CHOP) and activating transcription factor 4 (ATF4) were not modified. HRI KD cells have decreased spliced X-box binding protein 1 (XBP1s), an important branch of the ER stress response. However, overexpression of XBP1s by adenovirus in HRI KD cells did not protect from HRI siRNA-induced apoptosis. HRI deficiency decreased phosphorylation of Akt and its downstream targets glycogen synthase kinase 3 (GSK3), forkhead box protein O1 (FOXO1) and Bcl-2-associated death promoter (BAD). Overexpression of a constitutively active form of Akt by adenovirus in HRI-deficient beta cells partially decreased HRI KD-mediated apoptosis. Interestingly, BAD silencing protected from apoptosis caused by HRI deficiency. HRI silencing in beta cells also induced JNK activation. These results suggest an important role of HRI in beta cell survival through modulation of the Akt/BAD pathway. Thus, HRI may be an interesting target to modulate beta cell fate in diabetic conditions.

Efficacy and Safety of Ticagrelor for Long-term Secondary Prevention of Atherothrombotic Events in Relation to Renal Function: Insights from the PEGASUS TIMI-54 trial

Aims: We evaluated the relationship of renal function and ischaemic and bleeding risk as well as the efficacy and safety of the P2Y12 platelet receptor inhibitor ticagrelor in stable patients with prior myocardial infarction (MI). Methods & Results: Patients with a history of MI 1-3 years prior from the Prevention of Cardiovascular Events in Patients with Prior Heart Attack Using Ticagrelor Compared to Placebo on a Background of Aspirin (PEGASUS)-TIMI 54 were stratified based on estimated glomerular filtration rate (eGFR), with<60 ml/min/1.73m2 prespecified for analysis of the effect of ticagrelor on the primary efficacy composite of cardiovascular death, MI, or stroke (MACE) and the primary safety endpoint of TIMI major bleeding. Of 20,898 patients, those with eGFR<60 (N=4,849, 23.2%) had a greater risk of MACE at 3 years relative to those without, which remained significant after multivariable adjustment (HRadj 1.54, 95% CI 1.27–1.85, p<0.001). The relative risk reduction in MACE with ticagrelor was similar in those with eGFR<60 (ticagrelor pooled vs. placebo: HR 0.81; 95% CI 0.68–0.96) vs. ≥60 (HR 0.88; 95% CI 0.77–1.00, pinteraction=0.44). However, due to the greater absolute risk in the former group, the absolute risk reduction with ticagrelor was higher: 2.7% vs. 0.63%. Bleeding tended to occur more frequently in patients with renal dysfunction. The absolute increase in TIMI major bleeding with ticagrelor was similar in those with and without eGFR<60 (1.19% vs. 1.43%), whereas the excess of minor bleeding tended to be more pronounced (1.93% vs. 0.69%). Conclusion: In patients with a history of MI, patients with renal dysfunction are at increased risk of MACE and consequently experience a particularly robust absolute risk reduction with long-term treatment with ticagrelor.