Pancreatic cancer associated with obesity and diabetes: an alternative approach for its targeting

Besides storing excess energy as TG, AT secretes several factors regulating energy metabolism in various organs. These adipokines including adiponectin, leptin, resistin, and ghrelin play an important role in glucose and lipid metabolism. Among them, adiponectin and leptin are the most important and are therefore the focus here in discussing obesity-associated PC.

Adiponectin is also referred to as AdipoQ, which acts on several tissues to control energy homeostasis and insulin sensitivity [45, 46]. It regulates carbohydrate as well as lipid metabolism through the adenosine monophosphate-activated protein kinase (AMPK) pathway. The expression of circulatory AdipoQ is decreased in obesity and diabetes. However, the role of circulating AdipoQ in PC remains debatable regarding its impact on pancreatic tumor progression. Adiponectin serve as negative regulator that mediate its function by acting on its two receptors i.e. AdipoR1 and AdipoR2. Mechanistically, AdipoQ increases insulin synthesis and secretion by preventing apoptosis of pancreatic β-cells through activation of ERK and AKT pathways [47] (Fig. 2). Huang et al. demonstrated that subcutaneous implant of mouse pancreatic cell lines (H7 and Panc02) in AdipoQ knockout (APNKO) mice has reduced tumor weight and size as well as increased apoptosis by up-regulating cleaved caspase-3 as compared to wild type (WT) littermates. In addition, knockdown of AdipoR1, the major receptor of AdipoQ in these mouse cell lines (H7 and Panc02) followed by subcutaneous injection reduced tumor weight, size, and expression of Ki-67 (proliferation marker). Further, AdipoQ was observed to decreases apoptosis and increases PC cell proliferation and migration by activating the AMPK-Sirt1-PGC1α pathway [48] (Fig. 2). Similarly, in a case-control study, Dalamaga et al. studied the blood levels of AdipoQ in PC and control cases both before and after controlling for age, gender, BMI, smoking status, alcohol consumption, history of diabetes, and family history of PC. Higher AdipoQ levels were associated with PC. At tissue level, utilizing 16 tumor tissues, the authors observed positive or strong positive expression of AdipoR1 in 87.5% of cases while positive or strong positive expression of AdipoR2 was observed in >97% cases. Based on this, the investigators suggested to investigate the role of AdipoQ as a marker for early detection of PC. Further, Kadri et al. observed no correlation between adiponectin levels and PC [49]. Similarly, Pezzilli et al. did not observe any significant correlation among adiponectin levels and PC at serum level [50]. However, retrospective and prospective studies indicate that early detection of low circulatory AdipoQ levels may or may not be associated with the development of PC, because single nucleotide polymorphisms of the AdipoQ gene are common [51‐54] and the presence of these SNPs in AdipoQ, but not its receptors, are associated with altered serum adiponectin levels [55].

Inhibitory role of AdipoQ in halting tumor progression has also been observed [49]. In this regard some clinical studies suggest that circulating AdipoQ inhibit tumor cell proliferation by decreasing AKT and beta catenin levels across multiple malignancies (breast, colon and prostate) [56, 57]. In the case of PC, the molecular mechanism by which up-regulated AdipoQ levels inhibit cancer progression is still unclear; possibilities include 1) increasing insulin sensitivity via phosphorylation of insulin receptors, which down-regulates insulin/IGF-1 signaling, 2) down-regulating the expression of inflammatory cytokines that inhibit NF-κB activation, 3) directly activating the AMPK pathway to activate the p53 tumor suppressor gene, and 4) promoting cancer cell apoptosis via peroxisome proliferator-activated receptor gamma (PPARγ) activation and inhibiting angiogenesis [58, 59]. One study fed genetically engineered PC mice (Kras^G12D/Pdx-1-Cre) with a calorie-restricted diet and observed delays in formation of pancreatic intraepithelial neoplasms (PanIN) [60, 61]. Delayed progression of PanIN to PDAC was accompanied by increased AdipoQ and Sirt1 levels as well as decreased mTOR and IGF-1 expression [61]. In another study, Kato et al. incubated recombinant AdipoQ with the Pan02 murine cell line and noted decreased cell proliferation and increased apoptosis at 5 and 10 μg/ml, respectively. Further, orthotopic implantation of Pan02 cell line showed a significant increase in tumor volume by higher vascularization (more microvessel density) and decreased apoptosis in AdipoQ knockout mice as compared to WT animal [58, 62]. Overall, the findings from this study suggested AdipoQ to be a tumor suppressive role in PC by directly inhibiting proliferation and inducing apoptosis [62]. Interestingly, a recent study by Messaggio and co-workers showed that the decreased expression of AdipoQ receptors in pancreatic tumor tissues as compared to adjacent normal tissue. To elucidate the role of AdipoQ, its agonist AdipoRon was applied to both mouse and human cell lines and was found to inhibit PC tumor growth and proliferation by down-regulating leptin-induced STAT3 signaling. These results suggest AdipoRon could be a potential therapeutic agent for PC [63].

Leptin was the first adipokine identified in AT in 1993; it controls food intake and energy expenditure via a feedback mechanism in the brain [64]. After secretion from AT, leptin enters into circulation and reaches a level depending upon the AT size [65]. Under normal physiological conditions, leptin decreases appetite and increases fatty acid oxidation through its receptor (OBR or LEPR). However, in obesity and diabetes, elevated circulatory levels of leptin do not drive the same appetite feedback responses [66]. Like AdipoQ, leptin has a role in PC pathogenesis. In PC tumor cells, leptin binds to both full-length receptor (OBR1) as well as the short form (OBRs) to mediate downstream signaling [67]. Leptin receptor (OBR) and hypoxia inducible factor-1 (HIF-1) are predominantly co-expressed in PC cell lines and tissues during hypoxic conditions. HIF-1 binds to the hypoxia-responsive element (HRE) in the OBR promoter, regulating OBR transcription. Co-expression of OBR and HIF-1 in PC tissues was associated with poor prognosis, decreased overall survival and increased metastasis to distant organs in PC patients (Fig. 2). Silencing of HIF-1 inhibited leptin receptor expression in PC cells, suggesting that a positive feedback loop between HIF-1 and leptin/OBR mediates PC progression [67]. In another in vitro study, recombinant human leptin promoted PC cell migration and invasion but had no effect on proliferation [68]. The migration of PC cells occurred via the janus kinase 2 and signal transducer and activator of transcription 3 (JAK2/STAT3) pathway, which targets its downstream effector matrix metalloproteinase 13 (MMP13). The in vivo impact of leptin-over expressing PC cells was tested by orthotopic implantation into athymic nude mice, which led to greater tumor growth and lymph node metastasis. Over expression of leptin in PC cells and mouse tumors resulted in up-regulation of MMP13 levels, suggesting that leptin/MMP13 signaling is important for metastasis. In addition, MMP13 levels correlated with OBR expression in lymph node metastatic human PC tissues. The authors concluded that PC cell migration, invasion and metastasis occur via the JAK2/STAT3/MMP13 pathway [68] (Fig. 2).

A high fat/caloric diet leads to obesity, insulin resistance and increased leptin levels, all of which contribute to pancreatic adiposity. The accumulation of lipid molecules into the pancreas leads to activation and deposition of inflammatory cytokines (e.g., interleukin-6), which potentiate PC cell growth, migration and invasion [69]. Leptin activates Notch signaling and its receptors, leading to activation of its downstream molecules (survivin and Hey2) required for PC proliferation (Fig. 2). Notch signaling also up-regulates stem cell markers (CD44, CD24 and ESA) in PC cells. Inhibition of leptin (by IONP-LPrA2) after subcutaneous implantation of PC cells delayed tumor onset and decreased tumor size as well as cancer stem cell markers [70]. In another study by the same group reported that BxPC-3 and MiaPaCa-2 PC cells were treated in the presence of 5-FU, leptin, notch inhibitor (DAPT) and leptin inhibitor (IONP-LPrA2). They observed that decreased 5-FU cytotoxicity (by decreasing pro-apoptotic markers), increased cell proliferation and anti-apoptotic factors was due to leptin treatment. Moreover, IONP-LPrA2 reduced PC tumorspheres (treated with 5-FU) via notch signaling and suggesting that leptin might be involved in reducing the cytotoxic effect of chemotherapeutic drug and facilitating chemoresistance [71]. The leptin-notch signaling axis targeting has been projected as potential mediator for benefitting PC patients with obesity. Overall, the effect of AdipoQ and leptin in the progression of PC is still under investigation in obese people and further studies are warranted before targeting these adipokines in PC therapy.

The gut microbiome (hidden organ) comprises at least 10¹⁴ microorganisms belonging mostly to the phyla Firmicutes and Bacteroidetes, which play an important role in obesity and other metabolic disorders [72]. Recent evidences suggest that diet, environmental factors and microbial components can contribute to the development of cancer in liver and pancreas through a gut-liver/pancreas axis [73]. As shown in Fig. 3, a high-fat diet can alter the gut microbiome and trigger an inflammatory cascade. Gram-negative bacteria secrete lipopolysaccharide (LPS), which induces low-grade inflammation through its binding to toll-like receptors (TLRs) and CD14 co-receptors present on monocytes, macrophages and neutrophils [74, 75]. Furthermore, altered gut microbiota may lead to decreased intestinal tight junction proteins (ZO-1 and occludin), which allows LPS entry into circulation [76]. Binding of LPS to its up-regulated receptors (CD14 or TLRs) on immune cells induces PC cell proliferation [77, 78]. Additionally, these immune cells also play a role in cancer cell invasion, angiogenesis and metastasis [79‐81] by recruiting myeloid differentiation primary response gene 88 (MyD88) or TIR-domain-containing adapter-inducing interferon-β (TRIF) adaptor molecules. Activation of these molecules leads to inflammation by up-regulating p44/42 mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK) and NF-κB pathways (Fig. 3). Therefore, an altered gut microbiota may promote cancer by driving inflammatory responses [82]. In support of this, germ-free (absent microflora) mice are less prone to carcinogenesis probably due to a decrease in tumor-associated inflammation [83, 84]. Similar results were observed when WT mice were treated with broad-spectrum antibiotics to inhibit the microbiota [85]. As final evidence, antigenic peptide secreted from Helicobacter pylori (which causes gastric ulcers) has been associated with PC pathogenesis [86]. H. pylori components translocate into the pancreas from the gut and activate NF-κB, thereby increasing the expression of pro-inflammatory cytokines involved in PC initiation and progression [87]. A recent study by Sethi et al. demonstrated that the gut microbiome modulation may have impact on tumor growth in a mouse model. Initially, the authors orally administered a cocktail of broad-spectrum antibiotics to C57BL/6J mice for 15 days. Then at 15 days, a pancreatic cell line derived from Kras^G12D/+; Trp53^R172H/+; Pdx1^cre (KPC) mice was injected subcutaneously or intrasplenically (to induce liver metastasis). Results of this study showed that absence of gut microbiota led to a significant decrease in subcutaneous tumors, and decreased degree of liver metastasis. Besides, an absence of gut microbiota shows a significant increase in anti-tumor mature T cells [Th1 (IFN gamma⁺CD4⁺CD3⁺) and Tc1 cells (IFN gamma⁺CD8⁺CD3⁺)] in the tumor microenvironment with an unknown mechanism. Finally, the relative abundance of Bacteroidetes and Firmicutes phyla decreased in fecal samples upon antibiotic administration in KPC mice. The authors concluded that modulation of gut microbiota on tumor progression could be a novel immunotherapeutic strategy [88].

In general, pancreatic tumors depend on carbohydrate metabolism for their survival, growth and resistance to chemotherapy. Dietary carbohydrates are usually fully metabolized in the small intestine, with the exception of resistant starch. Gut microbiota further process the starch in the large intestine through fermentation, and as a result, short-chain fatty acids (acetate, butyrate and propionate) are released. Resistant starch by avoiding degradation in small intestine imparts several health benefits via decreasing circulatory glucose levels, body weight, and inflammation without causing any side effects [89]. Interestingly, media engineered to mimic resistant starch (low glucose concentration) decreased PC cell proliferation compared with control media. The decrease in cell proliferation is due to down-regulation of ERK and mTOR signaling (Fig. 3). Similarly, mice bearing sub-cutaneous PC tumors fed a resistant starch diet showed lower tumor weight than controls on a normal diet. Additionally, resistant starch also inhibits the growth of inflammation-causing organisms including Bacteroides acidifaciens, Ruminococcus gnavus, Clostridium cocleatum and Escherichia coli in mice by modulating gut microbiota [90].

Early metastasis (primarily at lymph nodes and liver) and chemoresistance are responsible for PC aggressiveness. However, treatment with gemcitabine, first line therapy for metastatic PC, results in altered gut microbiota, which affects PC growth. Administration of gemcitabine in nude mice bearing subcutaneous PC cell line tumors leads to increased growth of Proteobacteria and Akkermansia muciniphila, which potentiate inflammation and/or mucin degradation. The imbalance of gut microbiome due to gemcitabine treatment also disrupts the intestinal integrity; this, in turn, favors the entry of microorganisms or their components into the circulation to reach distant organs. In the pancreas, the microbe-associated molecular patterns (such as LPS and endotoxins) on the microbial surfaces bind to TLRs, activating inflammation through NF-kB signaling. In addition, gemcitabine-treated mice have greater LPS-induced inflammation and lower levels of inosine (a naturally occurring metabolite of adenosine), which has anti-inflammatory and immunosuppressive effects [91]. Furthermore, fecal microbiota obtained from KPC mice was recolonized into antibiotic treated WT mice which shows higher bacterial population access into the pancreas. Ablation of gut microbiota in Ptfla^Cre; LSL-Kras^G12D (KC) mice by oral antibiotics were recolonized with feces derived from WT or KPC mice and pancreatic tumor growth acceleration was observed in KPC derived feces only. Similarly, recolonization of feces (from KPC animal bearing pancreatic tumors) in germ free (GF)-KC mice shows increased pancreatic tumor growth as compared to GF-WT mice. This tumor acceleration might be associated with a decrease in activated T-cell infiltration in GF condition. They hypothesized that antibiotic treatment results in an increased intratumoral CD8:CD4 T-cell ratio which activates immunogenicity in PC. Future studies are warranted to identify microbial signatures that influence growth of PC tumors [92]. Taken together, a better understanding of the role of gut microbiota in PC tumor progression could open up new avenues in PC therapy development.

In obesity, pro-inflammatory cytokines are released from AT macrophages and infiltrate into AT; however the exact mechanism for these events is not known. In obese rats and humans, elevated inflammatory cytokine TNF-α activates other cytokines, in particular, IL-6, promoting angiogenesis and metastasis [93‐95]. Therefore, the possible common mechanism by which obesity induces inflammation in several cancers (pancreatic, lymphoma and glioblastoma) might be through TNF-α-induced NF-κB signaling [96‐98]. In addition, TNF-α secreted from cancer cells triggers cancer associated fibroblasts to stimulate macrophage infiltration [99, 100]. This infiltration occurs in several cancers through TNF-α-induced IL-6 to up-regulate STAT3 signaling [101]. Mice with PC tumors and diet-induced or genetic obesity expressed significantly higher STAT3 in the PC tumors. The up-regulation of STAT3 can drive PC progression through the activation of anti-apoptotic and proliferative proteins (Bcl-X_L, Mcl-1, Survivin, c-Myc and cyclin D1) as well as matrix metalloproteinases [102‐104]. Currently, studies are focused on the role of AT-derived inflammatory cytokines in modulating signaling pathways that can indirectly influence the progression of PC.

Despite the harsh hypoxic environment, PC survives in part due to expression of HIF1-α, which prevents apoptosis and increases the synthesis of glycolytic enzymes and transporter proteins [105]. According to the Warburg effect, the cancer cell depends on glycolysis to produce energy instead of aerobic respiration [106‐108]. The most important rate-limiting glycolytic enzymes are pyruvate kinase (PKM2), which catalyzes the conversion of phosphoenol pyruvate to pyruvate, and lactate dehydrogenase (LDHA), which then catalyzes conversion of pyruvate to lactate. The glycolytic pathway releases high-energy phosphates in the form of nicotinamide adenine dinucleotide, which enters the mitochondria for energy synthesis. LDHA is overexpressed throughout carcinogenesis, while PKM2 expression increases during the transition of cystic lesions to cancer. A possible explanation is that cystic lesions require high levels of LDHA, which induces PKM2 splicing in a later stage of tumor proliferation [109]. Furthermore, activation of EGFR initiates translocation of PKM2 to the nucleus where it binds to β-catenin, resulting in up-regulation of cyclin D1, Stat3, Oct4 and HIF, which induce cell proliferation [110, 111]. Therefore, both glycolytic enzymes (PKM2 and LDHA) are possible targets for PC treatment in preclinical studies.

In addition to adipokines, pre-adipocytes as well as mature AT secrete cytokines and growth factors that have a role in tumor growth. In pancreatic tumor progression, cross-talk between PSC and PC is mediated through several growth factors including platelet-derived growth factor, transforming growth factor, vascular endothelial growth factor and hepatocyte growth factor (HGF) [112, 113]. HGF has received much attention due to its mitogenic signal and its angiogenic effects on AT [114, 115]. In the case of obesity, HGF is released from the AT and the resulting circulatory levels contribute to pancreatic cell proliferation [116]. Exogenous supplementation of HGF induces proliferation in a murine pancreatic cell line (Pan02) through its receptor c-MET, whereas in the absence of c-MET, HGF had no direct effects in a murine pancreatic cell line and indirectly inhibited apoptotic cell death [117]. HGF inhibition by means of neutralizing antibody (AMG102) inhibited tumor growth and metastasis as compared to gemcitabine treatment [118]. Over expression of c-Met renders PC cells resistant to gemcitabine and radiation [44, 119] through an unknown mechanism. As one possibility, Cui and co-workers demonstrated that the Forkhead box M1 (FOXM1) transcription factor regulates c-MET expression via ERK, AKT and STAT3 pathways, creating a positive feedback loop that promotes tumor growth. Further, inhibition of c-MET, FOXM1, ERK, AKT and STAT3 signaling pathways with their respective inhibitors abolished the c-MET positive loop [120]. Therefore, the HGF/c-MET feedback loop regulates tumor proliferation, invasion and migration [121] and may be a novel target for growth factor-induced tumor growth.

In obesity, TG accumulates in the pancreas along with other organs and results in inflammation, higher expression of cytokines and remodeling of extracellular matrix (ECM). Hyaluronic acid or hyaluronan (HA) is a glycosaminoglycan and ubiquitous component of ECM which increases interstitial fluid pressure (IFP) and also reduces entry of chemotherapeutic drugs in PC tumors [122]. In tumor progression, the cross-talk between cancer cells and ECM is very important. Normally, HA synthesized by hyaluronan synthase (HAS) and secreted into the ECM under controlled conditions. However, increased expression of HA was observed in insulin-resistant mice aorta [123] and in the pancreas of diabetic mice [124]. In addition, expression of HA in the ECM is associated with diet-induced insulin resistance and was reversed upon treatment with the drug pegylated recombinant human hyaluronidase (PEGPH20), which improves insulin sensitivity in muscle tissue [125].

The PC stroma cells and the ECM express abundant HA to maintain a supportive tumor microenvironment [126]. Binding of HA to its receptors [cluster of differentiation-44 (CD44) or receptor for HA-mediated motility (RHAMM)], activates Ras and PI3K signaling, leading to increased cell proliferation, migration, and metastasis. Further, the activated PI3K pathway in cancer cells also increases drug resistance via activation of a multi-drug receptor [127‐129]. The HA receptor CD44/RHAMM mediates cell-cell/matrix interactions and up-regulation of HA (around 12-fold increase) is observed in PC [130‐133]. PC cells increase expression of HA via epigenetic regulation (decreased DNA methylation) and concomitant up-regulation of its enzyme HAS [134]. HA exists in low and high molecular weight forms. In vitro treatment with low molecular weight HA (25-75 kDa) increased PC cell motility compared to treatment with high molecular weight HA (400-600 kDa) [135, 136]. In conclusion, inhibition of HA synthesis may be a possible therapeutic strategy against PC and obesity-associated PC. Recently, PEGPH20 has gained interest to target HA for improving intratumoral microenvironment in PC. The different concentrations of HA along with mouse PC cells were implanted in immunodeficient mice that showed high IFP which reduce delivery of chemotherapeutic drugs. So targeting HA, a single high dose of PEGPH20 had a significant reduction on IFP in KPC mice. Further, a combination of PEGPH20 and gemcitabine showed decrease in cell proliferation and increased apoptosis in KPC mice [137]. In a randomized phase II clinical study, metastatic PC patients (231 were selected from a total of 279 patients) were treated with nab-paclitaxel/gemcitabine (AG) or PEGPH20 + nab-paclitaxel/gemcitabine (PAG). Patients (n=84) who had HA-high tumors showed improvement in the progression-free survival, overall survival and reduction in the thromboembolic (TE) incidence by PAG alone. Furthermore, PAG treatment was accompanied by more muscle spasm, neutropenia, myalgia and TE as compared to AG. Overall, srudy finding suggested that tumor HA could be a promising therapeutic target for PC patients with high HA [138].

Adrenomedullin (AM) is expressed by F-cells of the pancreas and plays a role in PC along with its receptor (adrenomedullin receptor ADMR). In 1993, AM was initially isolated from an adrenal medulla tumor called a pheochromocytoma. It is also expressed in AT and acts on pancreatic β-cells to inhibit insulin secretion; however, its effects on β-cells are poorly understood [152]. The circulatory levels of AM are very low under normal conditions; however, its levels are elevated in PC to cause insulin resistance [153]. Pancreatic beta, endothelial and stellate cells express ADMR. Its autocrine function in modulating tumor growth and progression has been evaluated in certain PC cell lines, i.e. Panc-1, BxPC3, and MPanc96 as well as human PSC and endothelial cells [154]. In a study, treatment with AM antagonist reduced PC tumor growth which indicating that AM plays a role in promoting PC progression. Furthermore, silencing of ADMR inhibited tumor growth and metastasis in liver and lung tissues of xenograft mice [155]. In PCDM, plasma levels of AM were significantly higher compared to diabetic patients alone, and its expression is higher in tumor and hypoxia conditions [152, 156].

AM is transported in the pancreas by extracellular vesicles, which contain proteins, lipids, and nucleic acids and are secreted by all cell types into circulation [157]. These vesicles play an important role in the transportation of biological components to other cells and tissues [158]. Extracellular vesicles form exosomes (30-100 nm) by inward or reverse budding of vesicular bodies called microvesicles (100-1000 nm) or by outward blebbing of membrane and apoptotic bodies (500-2000 nm) [159, 160]. Exosomes derived from PC cells have the capacity to promote metastasis in a tissue such as liver by residing in a pre-metastatic niche. The niche contains macrophage migration inhibitory factor engulfed by Kupffer cells, which induces secretion of fibronectin in the liver. The secreted fibronectin inhibits infiltration of macrophages and neutrophils derived from the bone marrow and promotes tumor growth [161]. PC exosomes control the specific site of organ metastasis by producing integrins, molecules that mediate cell adhesion. For example, Kupffer, lung fibroblast and epithelial cells recognize integrins such as αvβ5, α6β1 and α6β4, respectively, and subsequently recruit PC cells to these organs [162]. PC exosomes can also transfer AM to pancreatic β-cells (via caveolin-dependent endocytosis and micropinocytosis), which causes insulin resistance through ADMR-AM interactions. Furthermore, the presence of exosomal AM results in β-cell damage by increasing the production of reactive oxygen/nitrogen species and by increasing endoplasmic reticular stress markers (Bip and Chop) [163]. PC cell exosomes containing AM enter AT by the same mechanism that occurs in pancreatic β-cells. Internalization of AM results in lipolysis by activation of hormone-sensitive lipase through p38 and MAPK/ERK pathways; the resulting effect is growth and differentiation of the cancer cells [164]. Another similar peptide to AM is AM-2, which was identified in rats in 2004. AM2 has a similar function as AM in promoting angiogenesis, tumor development, progression and metastasis through MAPK signaling [165]. However, no studies have examined the role of AM-2 in PCDM.

Another important molecule is vanin 1 (VNN1, pantetheinase) present on the surface of epithelial and myeloid cells and highly expressed in the gut and liver tissue [166, 167]. VNN1 is mainly responsible for the breakdown of pantetheine to pantothenic acid (vitamin B₅) and cysteamine [168]. It is actively involved in inflammation, migration, stress, and glucose and lipid metabolism. Alteration of glucose and lipid metabolic pathways in the liver leads to development of insulin resistance and eventual T2DM. Mice exhibiting diet-induced obesity and Zucker diabetic fatty rats (model for T2DM) have more VNN1 activity in plasma as well as higher expression in the liver compared to normal controls [169]. Based on gene expression profiling, PCDM patients express higher VNN1 and MMP9 levels in peripheral blood as compared to patients with T2DM alone [170]. VNN1 reduces inflammation in PCDM by altering the levels of cysteamine and glutathione. VNN1 along with cysteamine protect the pancreatic β-cells from the oxidative stress generated during streptozotocin-induced diabetes in animals [171]. Enhanced γ-glutamylcysteine synthetase activity observed in vanin-1^-/- deficient mice with low levels of cysteamine resulted in an accumulation of endogenous glutathione (GSH) levels [172, 173]. By contrast, over expression of VNN1 decreased expression of GSH and PPARγ, resulting in increased oxidative stress in PCDM [174], through an unknown mechanism. These findings suggest that decreased GSH and PPARγ might contribute to islet dysfunction in PCDM and that vanin-1 and MMP9 could serve as novel pharmacological targets to treat early asymptomatic PCDM patients.