Changes to insulin sensitivity in glucose clearance systems and redox following dietary supplementation with a novel cysteine-rich protein: A pilot randomized controlled trial in humans with type-2 diabetes

We recently developed a novel keratin-derived protein (KDP) rich in cysteine, glycine, and arginine, with the potential to alter tissue redox status and insulin sensitivity. The KDP was tested in 35 human adults with type-2 diabetes mellitus (T2DM) in a 14-wk randomised controlled pilot trial comprising three 2×20 g supplemental protein/day arms: KDP-whey (KDPWHE), whey (WHEY), non-protein isocaloric control (CON), with standardised exercise. Outcomes were measured morning fasted and following insulin-stimulation (80 mU/m2/min hyperinsulinaemic-isoglycaemic clamp). With KDPWHE supplementation there was good and very-good evidence for moderate-sized increases in insulin-stimulated glucose clearance rate (GCR; 26%; 90% confidence limits, CL 2%, 49%) and skeletal-muscle microvascular blood flow (46%; 16%, 83%), respectively, and good evidence for increased insulin-stimulated sarcoplasmic GLUT4 translocation (18%; 0%, 39%) vs CON. In contrast, WHEY did not effect GCR (-2%; -25%, 21%) and attenuated HbA1c lowering (14%; 5%, 24%) vs CON. KDPWHE effects on basal glutathione in erythrocytes and skeletal muscle were unclear, but in muscle there was very-good evidence for large increases in oxidised peroxiredoxin isoform 2 (oxiPRX2) (19%; 2.2%, 35%) and good evidence for lower GPx1 concentrations (-40%; -4.3%, -63%) vs CON; insulin stimulation, however, attenuated the basal oxiPRX2 response (4%; -16%, 24%), and increased GPx1 (39%; -5%, 101%) and SOD1 (26%; -3%, 60%) protein expression. Effects of KDPWHE on oxiPRX3 and NRF2 content, phosphorylation of capillary eNOS and insulin-signalling proteins upstream of GLUT4 translocation AktSer437 and AS160Thr642 were inconclusive, but there was good evidence for increased IRSSer312 (41%; 3%, 95%), insulin-stimulated NFκB-DNA binding (46%; 3.4%, 105%), and basal PAK-1Thr423/2Thr402 phosphorylation (143%; 66%, 257%) vs WHEY. Our findings provide good evidence to suggest that dietary supplementation with a novel edible keratin protein in humans with T2DM may increase glucose clearance and modify skeletal-muscle tissue redox and insulin sensitivity within systems involving peroxiredoxins, antioxidant expression, and glucose uptake.

Oxidation of caspase-8 by hypothiocyanous acid enables TNF-mediated necroptosis

Necroptosis is a form of regulated cell death triggered by various host and pathogen-derived molecules during infection and inflammation. The essential step leading to necroptosis is phosphorylation of the mixed lineage kinase domain-like protein (MLKL) by receptor-interacting protein kinase 3 (RIPK3). Caspase 8 cleaves RIPKs to block necroptosis, so synthetic caspase inhibitors are required to study this process in experimental models. However, it is unclear how caspase-8 activity is regulated in a physiological setting. The active site cysteine of caspases is sensitive to oxidative inactivation, so we hypothesized that oxidants generated at sites of inflammation can inhibit caspase-8 and promote necroptosis. Here, we discovered that hypothiocyanous acid (HOSCN), an oxidant generated in vivo by heme peroxidases including myeloperoxidase and lactoperoxidase, is a potent caspase-8 inhibitor. We found HOSCN was able to promote necroptosis in mouse fibroblasts treated with tumor necrosis factor (TNF). We also demonstrate purified caspase-8 was inactivated by low concentrations of HOSCN, with the predominant product being a disulfide-linked dimer between Cys360 and Cys409 of the large and small catalytic subunits. We show oxidation still occurred in the presence of reducing agents, and reduction of the dimer was slow, consistent with HOSCN being a powerful physiological caspase inhibitor. While the initial oxidation product is a dimer, further modification also occurred in cells treated with HOSCN, leading to higher molecular weight caspase-8 species. Taken together, these findings indicate major disruption of caspase-8 function, and suggest a novel mechanism for the promotion of necroptosis at sites of inflammation.

P12.03.B Ascorbate alters the hypoxic pathway in glioblastoma cellsin vitro and associates with improved patient survival

Background

Glioblastomas are highly aggressive and hypoxic tumours. This environment activates the hypoxic pathway, driving glioma progression and treatment resistance. The hypoxic pathway is regulated by the hypoxia inducible factor (HIF) hydroxylases, which require oxygen as a substrate. Under normoxic conditions, the HIF hydroxylases are active, causing degradation and inhibition of HIF transcription factors. Under hypoxia, the activity of the hydroxylases reduces and HIF accumulates, activating the hypoxic response. HIF hydroxylases also require ascorbate as a cofactor for optimal function. The brain has one of the highest ascorbate levels in the human body, yet data on ascorbate levels in gliomas is scarce. Cellular ascorbate uptake occurs through solute carrier family 23 member 2 (SLC23A2). My aim is to understand the relationship between ascorbate, SLC23A2 and the hypoxic pathway in brain cancer using both in vitro cell culture and clinical samples.

Material and Methods

Ascorbate uptake was measured in human glioblastoma cell lines (T98G, U251MG, U87MG; ATCC) using reverse phase high performance liquid chromatography (HPLC-ECD). CRISPR-Cas was designed to knock-out SLC23A2. Clinical glioblastoma samples (n=37) and follow-up data were provided by the Cancer Society Tissue Bank and University of Otago Dunedin. Ethics and informed consent were obtained (H19/163, MEC/08/02/016). Ascorbate levels, measured by HPLC-ECD, and HIF-1α and downstream targets were measured using Western blotting or ELISA. A HIF score was calculated from HIF-1α and downstream target protein levels to estimate hypoxic pathway activity.

Results

In this study we have shown that T98G and U251 cells accumulate up to 15 nmol ascorbate/106 cells when exposed to 500 µM ascorbate for up to 24 hours, compared to U87MG cells with up to 3 nmol ascorbate/106 cells. Cancer Cell Line Encyclopaedia data shows that T98G and U251MG cells express higher levels of SLC23A2 compared to U87MG cells, aligning with our results. Clinical glioblastoma tissue contained a median of 7.6 µg ascorbate/100 mg tissue. Patients survival was significantly longer with above, vs below, median tumour ascorbate levels (Gehan-Breslow-Wilcoxon p = 0.027). The HIF score was negatively correlated with tumour ascorbate levels (Pearson r -0.327, p = 0.048). Patients with higher HIF-score had significantly shorter survival time compared to those with a lower HIF score (Gehan-Breslow-Wilcoxon p = 0.005).

Conclusion

Ascorbate uptake in glioblastoma cells varies between cell lines and appears reliant on the level of SLC23A2. Higher ascorbate content in clinical glioblastoma samples was associated with reduced hypoxic pathway activity and longer patient survival. Ongoing work, using SLC23A2 CRISPR-Cas knock-out cells, is investigating the effect of disrupting ascorbate uptake on hypoxic pathway signalling in glioblastoma cells.

P12.06.A Relationship between ascorbate and DNA methylation markers in clinical glioma tumours

Background

Members of the 2-oxoglutarate-dependent dioxygenase (OGDD) enzyme family play an important role in gliomas as they regulate epigenetic modifications and response to hypoxia. The OGDDs require 2-OG and O2 as substrates, and ferrous iron and ascorbate as cofactors. Both hypoxia and aberrant DNA methylation are prognostic indicators for gliomas. The ten-eleven translocase (TET) DNA demethylases are OGDDs that convert 5-methyl cytosine (5mC) to 5-hydroxymethylcytosine (5hmC), with 5hmC levels related to better prognosis. Despite this, there is limited data on the OGDD enzymes and their substrates/cofactors in glioma tissues. Our previous study showed an association between ascorbate content and markers of the hypoxic response in glioblastoma tissue. Here we determine whether there is an association between ascorbate and DNA methylation in glioma. In addition, we assess whether methylation of the methylguanine-DNA methyltransferase (DNA repair enzyme MGMT) promoter is associated with ascorbate content.

Materials and methods

Frozen clinical glioma samples from 37 patients (n=11 WHO grade I-III, n=26 glioblastoma) were obtained from the Cancer Society Tissue Bank (Ethics approval H19/163). Isocitrate dehydrogenase 1 (IDH1) mutation status was determined by sequencing. Samples were processed on dry ice in liquid nitrogen and analysed for ascorbate (high-performance liquid chromatography), global DNA methylation (mass spectrometry) and MGMT promoter analyses (methylation specific PCR).

Results

Many grade I-III tumours were IDH1 R132H mutant (6/11), and most glioblastomas were not (2/26). Glioblastoma had significantly lower ascorbate content than grade I-III tumours (p=0.026). Glioblastoma also had lower global 5hmC levels (p=0.0013). IDH1 R132H tumours tended to have a lower ascorbate content (p=0.09). Ascorbate and 5hmC levels were directly correlated (Spearman r= 0.466, p=0.004). However, cytosine and 5mC showed no association with grade or ascorbate. MGMT promoter methylation status was not associated with global methylation or ascorbate content (p=0.97, p=0.96, respectively).

Conclusion

Our data suggests that ascorbate supports TET activity in clinical glioma. It also appears that site-specific (promoter) methylation was not affected by ascorbate availability. These findings may have clinical implications, as higher 5hmC levels are associated with improved outcome, whilst continued MGMT suppression suggests chemotherapy responsiveness. However, evidence that raising tumour ascorbate leads to increased 5hmC levels, or an associated improvement in survival, requires intervention trials.

P16.11 Vitamin C levels and the hypoxic pathway in human glioma tissues

BACKGROUND

Gliomas are the most common brain cancer and survival is poor, with 11–15 months for high-grade glioblastoma patients, despite treatment. Gliomas are hypoxic tumours, which increases with tumour grade. Under hypoxia, the transcription factor hypoxia inducible factor-1 (HIF) accumulates and upregulates expression of genes involved in tumour development and progression. HIF-1 levels and activity are controlled by HIF hydroxylases which target HIF-1α for degradation and prevent co-activation. HIF hydroxylases are part of the 2-oxoglutarate (2-OG)-dependent dioxygenase enzyme family, that require 2-OG and oxygen as substrates and ascorbate and iron as co-factors. The role of ascorbate in regulating the hypoxic pathway in cancer is of interest, with previous research showing reduced HIF pathway activity with increasing tumour ascorbate levels. Brain tissue has one of the highest ascorbate levels in the body, and is one of the last to become depleted under deficiency, indicating an important role for ascorbate in this tissue. One previous study has analysed ascorbate levels in 11 human glioblastoma patients, and showed lower ascorbate in tumour tissue compared to normal brain tissue. There have been no studies investigating the relationship between ascorbate levels and the hypoxic pathway in human glioma tissues.

MATERIAL AND METHODS

Human glioma tissues (n = 39), obtained from the Cancer Society Tissue Bank Christchurch (ethics approval H19/163), were processed for ascorbate and hypoxic pathway proteins (HIF-1α, CA-IX, BNIP3, HKII, GLUT1 and VEGF). Ascorbate levels were quantified by HPLC-ED, and proteins were measured by Western blotting and ELISA. Spearman’s correlations were used to identify relationships between ascorbate and HIF pathway proteins.

RESULTS

Of the samples, 64% were GBM. Ascorbate was significantly lower in GBM compared to low-grade gliomas (p = 0.04). VEGF was significantly higher in GBM compared to astrocytomas (p = 0.01). Increased tumour ascorbate was associated with lower VEGF and CA-IX proteins. HIF-1α and BNIP3 protein were positively associated, and VEGF was positively associated with HKII and CA-IX. VEGF inversely associated with BNIP3, and CA-IX inversely associated with HKII. The hypoxic pathway score (calculated from protein levels of members of the hypoxic pathway) was reduced in tumours with higher ascorbate but this did not reach significance (p = 0.2).

CONCLUSION

This is the first study to show that ascorbate levels were reduced in high-grade gliomas compared to low-grade. Some members of the hypoxic pathway were associated with ascorbate levels. The overall hypoxic pathway score did not significantly correlate with ascorbate and increased numbers of samples are required to confirm any associations. Other variables, such as IDH-1 mutation status of the tumours may affect the correlation and will be analysed next.

The role of oxidants and vitamin C on neutrophil apoptosis and clearance

We have investigated the role of neutrophil oxidants in the surface changes that result in recognition and uptake of neutrophils by macrophages. We have shown that H2O2 produced by stimulated neutrophils is essential for the surface expression of phosphatidylserine. This does not occur in neutrophils from mice with chronic granulomatous disease and may explain the formation of granuloma in this condition. We have also investigated the role of intracellular vitamin C on neutrophil apoptosis. Cells from vitamin C-deficient mice were found to be less likely to undergo both spontaneous and oxidant-induced apoptosis, with eventual necrosis being the most probable outcome.