Targeting Macrophage Migration Inhibitory Factor in Acute Pancreatitis and Pancreatic Cancer

Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine implicated in the pathogenesis of inflammation and cancer. It is produced by various cells and circulating MIF has been identified as a biomarker for a range of diseases. Extracellular MIF mainly binds to the cluster of differentiation 74 (CD74)/CD44 to activate downstream signaling pathways. These in turn activate immune responses, enhance inflammation and can promote cancer cell proliferation and invasion. Extracellular MIF also binds to the C-X-C chemokine receptors cooperating with or without CD74 to activate chemokine response. Intracellular MIF is involved in Toll-like receptor and inflammasome-mediated inflammatory response. Pharmacological inhibition of MIF has been shown to hold great promise in treating inflammatory diseases and cancer, including small molecule MIF inhibitors targeting the tautomerase active site of MIF and antibodies that neutralize MIF. In the current review, we discuss the role of MIF signaling pathways in inflammation and cancer and summarize the recent advances of the role of MIF in experimental and clinical exocrine pancreatic diseases. We expect to provide insights into clinical translation of MIF antagonism as a strategy for treating acute pancreatitis and pancreatic cancer.

Acute pancreatitis (AP) is one of the most common gastroenterological diseases with an increasing global incidence and is complicated by considerable comorbidity, mortality, and financial burden (Peery et al., 2019;Petrov and Yadav, 2019). In the course of the disease, injured pancreatic acinar cells secrete inflammatory mediators such as interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), MIF, chemokines and their ligands that mediate recruitment and infiltration of neutrophils and monocytes at the injury site (Lugea et al., 2017), further aggravating local injury and systemic inflammation (Linkermann et al., 2014). As a result, anti-inflammatory treatment strategies have been tested in AP (Gukovskaya et al., 2017). Despite an enormous amount of preclinical research (Habtezion et al., 2019;Lee and Papachristou, 2019;Saluja et al., 2019) and clinical trials (Moggia et al., 2017), no effective targeted pharmacological treatment for AP has been discovered. Therefore, the current treatment of AP is limited to supportive care as well as the management of local and systemic complications (Vege et al., 2018).
AP, chronic pancreatitis, and pancreatic cancer are the common diseases of the exocrine pancreas (Petrov and Yadav, 2019). About 10% of AP patients will develop chronic pancreatitis (Sankaran et al., 2015) and its global incidence is 10 cases per 100,000 general population per year (Xiao et al., 2016). The estimates of incidence and mortality for pancreatic cancer are 8.14 cases and 6.92 deaths per 100,000 persons annually, respectively (Xiao et al., 2016). In China, pancreatic ductal adenocarcinoma (PDAC) is expected to be the second leading cause of cancer-related death by 2030 (Siegel et al., 2019).
Emerging evidence suggests that inflammatory cytokines including MIF, TNF-α, interferon gamma, and transforming growth factor beta are increased in the setting of cancer (Lippitz, 2013). An elevated cytokine concentration profile is associated with reduced survival in pancreatic cancer patients (Babic et al., 2018). There remains no effective pharmacological treatment for PDAC and the prognosis is still extremely poor (Lippitz, 2013;Ryan et al., 2014). There appears to be a compelling role for MIF in pancreatic diseases. Circulating MIF levels are significantly higher in obese or type 2 diabetic populations compared to healthy controls (Morrison and Kleemann, 2015), are significantly elevated in experimental and human AP and correlated with disease severity (Sakai et al., 2003), and are also highly up-regulated in exosomes (Costa-Silva et al., 2015) and in PDAC tissue (Funamizu et al., 2013;Lippitz, 2013;Tan et al., 2014). Small molecular MIF inhibitors, anti-MIF antibodies, and genetic ablation of MIF have all been tested and show protective effects in experimental AP (Sakai et al., 2003;Matsuda et al., 2006;Guo et al., 2018;Li et al., 2019;Zhu et al., 2020) and PDAC (Winner et al., 2007;Denz et al., 2010;Funamizu et al., 2013;Tan et al., 2014;Costa-Silva et al., 2015;Guo et al., 2016;Yang et al., 2016;Wang et al., 2018;Suresh et al., 2019) models.
The aim of this review is to describe what is known about the structure and function of MIF with a particular focus on signaling pathways involved in inflammation and cancer. The role of MIF in AP and PDAC and the potential for MIF targeted treatment strategies are also emphasised.

LITERATURE SEARCH
A systematic literature search was conducted in Ovid Medline (PubMed), Scopus, Science Citation Index expanded, and Google Scholar to find related articles. The key words were "acute pancreatitis," "pancreatitis," "chronic pancreatitis," or "pancreatic cancer" in combination with "macrophage migration inhibitory factor" or "D-Dopachrome tautomerase." All studies investigating MIF in experimental and clinical exocrine pancreatic diseases were collated. Reference lists of relevant reviews and other non-primary data sources regarding this context captured by the search strategy were also manually screened. Only publications in English were included. In total, 15 studies investigating MIF in AP and 14 studies in PDAC were summarized in this review. We did not identify original study investigating MIF and chronic pancreatitis.

STRUCTURE AND FUNCTION OF MIF
MIF is a highly conserved protein of 12.5 kDa, with evolutionarily ancient homologues in plants, protozoans, nematodes, and invertebrates (Sparkes et al., 2017). The MIF protein is a 115amino acid polypeptide that folds to form two antiparallel α-helices that pack against a 4-stranded β-sheet (Trivedi-Parmar and Jorgensen, 2018). On the basis of X-ray crystallography data, the biologically active form of MIF is a homotrimer (Suzuki et al., 1996). MIF is different from other cytokines because its structure contains three evolutionarily stable catalytic sites that are associated with tautomerase and oxidoreductase activities (Rosengren et al., 1996;Sun et al., 1996;Suzuki et al., 1996).
MIF is secreted by the anterior pituitary and immune cells  and is ubiquitously stored and expressed in a variety of cells including epithelial, endothelial, mesenchymal, dendritic, and other cell types (Jankauskas et al., 2019). Constitutive release of MIF from cells results in its high concentration in the extracellular space (Lee et al., 2016;Lang et al., 2018). However, the process by which MIF is released is not fully understood. Unlike other cytokines, MIF exists as a preformed type in multiple cell sub-populations throughout the body Bacher et al., 1997), and is particularly distributed and expressed in cells of the nervous (Nishibori et al., 1996) and endocrine systems (Calandra and Roger, 2003) that have direct contact with the natural environment, (e.g. lung, skin, and gastrointestinal). MIF production is largely facilitated in response to an array of stimuli including hypoxia, hydrogen peroxide, lipopolysaccharide (LPS), TNF-α, thrombin, and angiotensin II (Jankauskas et al., 2019). Waeber et al. (Waeber et al., 1997) reported that MIF was highly expressed in several insulin-secreting cell lines, colocalized with insulin-containing secretory granules, and was secreted in response to glucose stimulation in a time-and concentration-dependent manner. Immunoneutralization of MIF by anti-MIF IgG reduced the first and second phase of the glucose-induced insulin secretion response by 39 and 31%, respectively. Whether pancreatic acinar cells produce and release MIF remains to be determined.

PATHOPHYSIOLOGIC ROLE OF MIF
MIF circulates normally at levels from 2 ng/ml to 6 ng/ml, following a circadian rhythm that correlates with plasma cortisol under physiological conditions (Petrovsky et al., 2003). This is of particular clinical relevance as low concentration of glucocorticoids induce release of MIF into circulation and in turn, circulating MIF overrides glucocorticoid-mediated inhibition of cytokine secretion, and has been shown to fully abolish the protective effect of glucocorticoids in a lethal model of endotoxininduced inflammation (Calandra et al., 1995;Bloom et al., 2016a). In the case of acute inflammatory diseases, MIF has been demonstrated to be implicated in the pathogenesis of glomerulonephritis, acute lung injury, sepsis, and AP, and its elevation is closely associated with disease severity or progression (Harris et al., 2019). In addition to its cytokine activity, mammalian MIF also harbors diverse catalytic functions. In this regard, tautomerase activity is the most widely studied function, exhibiting the ability to catalyze tautomerization of phenylpyruvate, p-hydroxyphenylpyruvate, and D-dopachrome (Rosengren et al., 1996;Rosengren et al., 1997). Moreover, MIF interferes in cell cycle regulation by negatively interacting with c-Jun activation binding protein-1 (JAB1, also referred to the fifth component of the constitutive photomorphogenic-9 signalosome, CSN5)-dependent pathways, resulting in degradation of cyclindependent kinase inhibitor p27 Kip1 and cell cycle progression (Kleemann et al., 2000).

D-DOPACHROME TAUTOMERASE
D-dopachrome tautomerase (D-DT, also referred to MIF-2) comprises 117 amino acids with a molecular weight of 13 kDa (Zhang et al., 1995;Nishihira et al., 1998). It has a highly homologous tertiary structure and similar biological properties to MIF (Sugimoto et al., 1999;Merk et al., 2011). Whereas plasma D-DT and MIF circulate in similar concentrations under basal or pathological conditions, LPS-treated macrophages release 20-fold more MIF than D-DT, indicating D-DT derived from nonmacrophage sources prominently contribute to plasma D-DT expressions in vivo (Merk et al., 2011). The differences and coincidences between D-DT and MIF have been reviewed by Illescas et al. (Illescas et al., 2020). Regardless of the similarities, D-DT seems to play different or even opposed role from MIF under some circumstances. For example, D-DT lacks both the CXXC redox motif and pseudo (E)LR motifs present on MIF, while the former one is important in sensing redox signals and the latter one is essential for its chemokine function (Esumi et al., 1998;Bernhagen et al., 2007;Merk et al., 2011). In adipose tissues, D-DT and MIF are differentially expressed and have distinct roles in adipogenesis.
While D-DT is negatively correlated with obesity and reverses glucose intolerance, MIF is positively correlated with obesity and insulin resistance (Kim et al., 2015). D-DT also binds to JAB1 and the interaction affinity between JAB1 and D-DT is comparable to that observed between JAB1 and MIF (Merk et al., 2011).

MIF SIGNALLING PATHWAYS IN INFLAMMATION AND CANCER
The MIF-related key signaling pathways in inflammation and cancer are delineated in Figure 1.
MIF can individually bind to CD74 or CXCR2/4 independent of whether the other receptor type is co-expressed (Bernhagen et al., 2007;Schwartz et al., 2009). Extracellular MIF triggers mononuclear (monocytes, neutrophils, and T cells) cell chemotaxis via MIF/CXCR2 and MIF/CXCR4. Monocyte arrest elicited by MIF depends on CD74 and the complex formed by MIF/CXCR2 or MIF/CXCR4 in the context of inflammation and atherosclerosis (Bernhagen et al., 2007). This effect can be suppressed by either a MIF genetic deficiency or antibodies to MIF, CXCR2 or CD74 (Bernhagen et al., 2007). Moreover, MIF can directly interact with CXCR7 (Alampour-Rajabi et al., 2015), resulting in MIF/CXCR7mediated functional responses that include promotion of human CXCR7 internalization, activation of MIF-mediated ERK1/2 zeta-chain associated protein kinase-70 signaling pathway as well as B cell chemotaxis. It is not completely elucidated whether CD44 is also involved in the receptor complexes of CD74 with the CXCRs. Therefore, tissue responsiveness to MIF depends on its specific expression of MIF receptors and co-receptors, (i.e. CD44 and CXCRs).

Intracellular MIF-Mediated Signaling Pathways
In addition to its extracellular activities, it is reported that endocytic MIF can transcriptionally and post-transcriptionally override the immunosuppressive effects of glucocorticoids (Calandra and Roger, 2003). Other studies (Roger et al., 2001;Galvão et al., 2016;Lang et al., 2018;Shin. et al., 2019) published in the past few years collectively support a regulatory effect of endogenous MIF on Toll-like receptor 4 (TLR4) and the NLR family pyrin domain containing 3 (NLRP3) inflammasome signaling pathways. These are thought to play a critical role in AP for developing further pancreatic injury and systemic inflammation (Sharif et al., 2009;Sendler et al., 2020). In response to the stimulation by LPS and Gram-negative bacteria (canonical TLR4 activators), the MIFdeficient macrophages were found to be hyposensitive (reduced production of pro-inflammatory cytokines, e.g., TNF-α and FIGURE 1 | MIF-mediated signal transduction and regulation in inflammation and cancer. Intracellularly, MIF overrides glucocorticoids induced suppression of NF-ĸB synthesis and nuclear translocation, leading to increased cytokine production, (e.g. IL-6 and TNF-α). MIF also facilitates NLRP3 inflammasome assembly through interacting with ASC, vimentin and NLRP3, resulting in pyroptosis and elevated production of IL-1β and IL-18. MIF upregulates the expression of TLR4 by facilitating the transcription of NF-ĸB, which allows rapid recognition of LPS by TLR4, promoting the production of cytokines (including MIF). Intracellular MIF also binds to JAB1, resulting in tumor cell cycle progression. After MIF is released, extracellular MIF binds to the transmembrane receptor complex CD74 and CD44, resulting in the subsequent phosphorylation of ERK/MAPK and PI3K/AKT. ERK1/2 activates transcription elements AP1/ETS, which leads to expression of pro-inflammatory cytokines. On the other side, the activation of ERK/COX2 inhibits p53-dependent apoptosis, promoting tumor cell proliferation. PI3K/AKT activation phosphorylated Bcl-2 family member protein BAD and Foxo transcription factor Foxo3a, enhancing cancer cell survival and invasion. During hypoxia, the binding of MIF to CD74 contributes to HIF1α activation and stabilization, which then upregulates the expression of angiogenic growth factors including VEGF, LOX, and CTGF that consequently promote angiogenesis. Meanwhile, CD74/CD44 receptor complex releases CD74 intracellular domain (CD74-ICD), it translocates into the nucleus and increases NF-ĸB activation, leading to increased caspase-1 activation and NLRP3 production. Extracellular MIF also binds to G-protein-coupled chemokine receptors CXCR2, CXCR4, and CXCR7 individually or dependent on CD74 to form MIF/CXCR2 and MIF/CXCR4 complex, activating leucocyte chemotaxis. Abbreviations: NF-ĸB, nuclear factor kappa B; IL, interleukin; TNF, tumor necrosis factor; NLRP3, NLR Family Pyrin Domain Containing three; TLR4, toll-like receptor four; LPS, lipopolysaccharide; CD74, Cluster of Differentiation 74; ERK, extracellular signal-regulated kinase; MAPK, mitogen-activated protein kinase; PI3K, phosphoinositide 3-kinase; AP1, activator protein 1; COX2, cyclooxygenase-2; BAD, Bcl-2 agonist of cell death; FoxO3a, forkhead box O3a; HIF1α, hypoxia inducible factor 1 alpha; VEGF, vascular endothelial growth factor; LOX, lysyl oxidase; CTGF, connective tissue growth factor; CXCR, C-X-C chemokine receptor.

MIF AND EXOCRINE PANCREATIC DISEASES MIF and Experimental AP
Results from studies investigating MIF and experimental AP are summarized in Table 1. Investigation of MIF in AP began in 2003, when Sakai and colleagues demonstrated a functional role for MIF in experimental models of AP induced by taurocholic acid (TCA-AP), caerulein (CER-AP), and choline-deficient, ethionine-supplemented diet (CDE-AP) (Sakai et al., 2003). In isolated peritoneal macrophages from ascitic fluid of TCA-AP rats, co-incubation with anti-MIF antibodies significantly inhibited IL-8 production (Sakai et al., 2003). MIF levels of serum, ascitic fluid, and lung tissue, but not pancreas or liver, were significantly increased in the TCA-AP in rats. Similarly, MIF levels in the lung were also significantly increased in the CDE-AP in mice. As for CER-AP in rats, the MIF levels were only markedly elevated in pancreatic ascites and thus peritoneal macrophages were considered to be the cellular sources of ascitic MIF. In 2006, Matsuda et al. (Matsuda et al., 2006) reported that plasma and lung MIF levels were increased by 7-fold and 4.7-fold, respectively, in a severe AP model induced by three injections of caerulein and a low dose of LPS in mice (CER/LPS-AP). The MIF levels in pancreatic tissue and serum were increased in L-arginine-induced AP (ARG-AP) in mice (Ohkawara et al., 2017) and their expression was up-regulated in the intrahepatic bile duct cells in a sodium taurocholateinduced AP (STC-AP) in rats (Wang et al., 2019).

MIF and Human AP
Clinical studies of MIF and AP are displayed in Table 2. Clinical studies (Sakai et al., 2003;Makhija et al., 2007;Rahman et al., 2007;Dambrauskas et al., 2010;Deng et al., 2017) that measured circulating MIF levels collectively and consistently demonstrated that the admission circulating MIF levels were proportionally associated with severity of AP patients. Sakai et al. (Sakai et al., 2003) determined serum MIF levels of 28 patients with mild AP and 18 with severe AP within 72 h disease onset in parallel with 12 healthy individuals, revealing that severe AP were associated with markedly higher serum MIF levels compared to mild AP and healthy controls (both p < 0.01). But there was no statistical difference between mild AP and controls. Similar results were observed in the study conducted by Rahman et al. (Rahman et al., 2007), although the serum samples were collected earlier at 24 h after AP onset. In addition, serum MIF levels were significantly raised in patients who developed pancreatic necrosis or multiple organ failure, indicating MIF could act as a sensitive biomarker to predict local and systemic complications for AP. Another study (Dambrauskas et al., 2010) comprising 108 AP patients (60 mild, 48 severe) also found that serum MIF levels within 72 h disease onset could be used as discriminator of severe and necrotizing AP. A study performed by our group  identified MIF as an early predictor for discriminating AP patients who had persistent organ failure (n 20) from those without (n 50) and healthy controls (n 10) with an area under the receiver operating characteristic curve (AUC) of 0.90 (95% confidence interval [CI], 0.81-0.96). A most recent study (Shen et al., 2021) demonstrated that MIF at a cut-off value of 2.3 ng/ml has the best discriminative power (AUC, 0.950; 95% CI, 0.914-0.987) for predicting severe AP which was higher than Acute Physiology and Chronic Health Evaluation II (AUC, 0.899), Bedside Index for Severity in Acute Pancreatitis (AUC, 0.886), and serum IL-6 (AUC, 0.826). Therefore, MIF could be used as a potential early severity predictor in patients with AP and this needs further validation.
Previous studies described the 'G' to 'C' single nucleotide polymorphism (at −173 position) of MIF in patients with systemic-onset juvenile idiopathic arthritis (Donn et al., 2001) and sarcoidosis (Amoli et al., 2002), and a CATT repeat microsatellite (at -794 position) to be associated with lower disease severity in rheumatoid arthritis . Accordingly, Makhija et al. (Makhija et al., 2007) compared the MIF gene polymorphism of a United Kingdom cohort of 164 AP patients with 197 healthy controls. It is shown that the distribution of MIF-173 genotype was significantly different between the two groups (p 0.046), whilst there was no difference regarding the distribution of MIF-794 microsatellite genotypes and alleles. However, these findings warrant confirmation from a larger population.
Intriguingly, MIF has been linked to exosomes in PDAC. Costa-Silva et al. (Costa-Silva et al., 2015) demonstrated that MIF levels in plasma exosomes isolated from a mouse pancreatic cancer model were markedly increased compared to noncancerous controls, even at an early stage of pancreatic intraepithelial neoplasia. The process of MIF-expressing PAN02 exosome "education", (whereby naive, wild-type mice were injected retro-orbitally every other day for three weeks with 5 μg of PAN02-derived exosomes), induced liver pre-metastatic niche formation in treated mice which was also inhibited by MIF knockdown.
Yang et al. (Yang et al., 2016) demonstrated the role of MIF-miR-301b-NR3C2 axis in the pathogenesis of PDAC. Over-expression of MIF induced a marked increase of miR-301b and reduction of NR3C2 levels, resulting in profound proliferation, migration, and invasion of pancreatic cancer cell lines (PANC-1 and Capan-2). While inhibition of miR-301b abolished MIF-induced suppression of NR3C2 in vitro, Mif −/− mice were associated with reduced metastasis and improved survival in a pancreatic cancer mouse model (LSL-Kras G12D , LSL-Trp53 R172H/+ , Pdx-1-Cre) in vivo.

MIF and Human Pancreatic Cancer
Clinical studies of MIF and pancreatic cancer are presented in Table 5. Collectively, these clinical studies have determined the circulating MIF levels (Winner et al., 2007;Fredriksson et al., 2008;Chen et al., 2010;Kondo et al., 2013;Tan et al., 2014;Schinagl et al., 2016) and MIF expression in dissected pancreatic tissue (Cui et al., 2009;Denz et al., 2010;Funamizu et al., 2013;Tan et al., 2014;Guo et al., 2016;Schinagl et al., 2016;Yang et al., 2016;Wang et al., 2018) of PDAC patients and healthy controls. All the studies reported that MIF levels in serum/plasma or pancreatic tissue were significantly higher in PDAC patients than healthy controls or paired non-cancerous tissue with preferable diagnostic utility, except for one study (Fredriksson et al., 2008) in which there was 30% increase in PDAC without significant difference compared with healthy controls (p 0.15). Higher MIF expression in tumor tissue was associated with worse survival of PDAC patients (Funamizu et al., 2013;Wang et al., 2018).
In addition, Denz et al. (Denz et al., 2010) reported that MIF mRNA expression in pancreatic tissue was higher in PDAC than chronic pancreatitis, both were higher than normal controls. Chen et al. (Chen et al., 2010) compared the serum MIF in PDAC patients and controls including AP, chronic pancreatitis and healthy donors, showing that MIF had an area under the curve of receiver operating characteristic of 0.78 in discriminating pancreatic cancer from controls. Tan et al. (Tan et al., 2014) focused on diabetes mellitus-associated pancreatic cancer (DM-PC) and found that MIF expression in pancreatic tissues of DM-PC was markedly higher when comparing to chronic pancreatitis or pancreatic cancer without DM. Similarly, serum MIF levels were also higher in new-onset than long term DM-PC, pancreatic cancer without DM, or new-onset T2DM patients (all p < 0.001).

MIF TARGETED TREATMENT STRATEGIES
Different MIF antagonism strategies are depicted in Figure 2.

Small Molecular Inhibitors
Pharmacological inhibition of MIF with small molecule inhibitors have shown promise in the suppression of inflammation in various animals models such as severe sepsis (Calandra et al., 2000), rheumatoid arthritis (Mikulowska et al., 1997), allergic airway inflammation (Amano et al., 2007), colitis (Ohkawara et al., 2002), glomerulonephritis (Brown et al., 2002), and chronic obstructive pulmonary disease (Russell et al., 2016). Development of MIF inhibitors has been comprehensively and elegantly reviewed elsewhere (Xu et al., 2013;Kok et al., 2018;Trivedi-Parmar and Jorgensen, 2018). Small inhibitory agents of MIF have also been widely demonstrated in experimental cancer models, including lung cancer (Mawhinney et al., 2015), bladder cancer (Choudhary et al., 2013), adenoid cystic carcinoma (Liu et al., 2013), melanoma, and colon cancer (Ioanou et al., 2014). Briefly, small-molecule inhibitors of MIF primarily focus on rational structure-based design that target the MIF tautomerase activity and MIF-CD74 binding (Brown et al., 2009;Xu et al., 2013;Dickerhof et al., 2014;Spencer et al., 2015;Trivedi-Parmar and Jorgensen, 2018). The drought of a reliable, consensus-based in vitro assays for MIF biological activity has been a significant challenge to the development of small molecule MIF inhibitors. Further work should focus on developing a robust high-throughput clinically relevant MIF bioassay that can be applied for second-pass screening, glucocorticoid override, cellular proliferation, and cytokine release detection to expedite the discovery of efficient small molecular MIF inhibitors (Bloom et al., 2016b). In AP and PDAC, only ISO-1 and 4-IPP have been investigated in experimental animal models 1) mean plasma MIF levels in PDAC patients was 7,240 pg/ml; 2) plasma PTX3 levels were positively correlated with levels of MIF (r 0.38, p 0.001) Kondo et al. (2013) Serum. PC (n 17), AP (n 26), CP (n 26), healthy controls (n 16) 1) compared with healthy controls, serum MIF levels in PC patients and pancreatitis patients were 10.6-fold and 9.2-fold higher, respectively; 2) the AUC of serum MIF in discriminating PC patients from healthy controls or all controls (including AP, CP, and healthy controls) were 1.00 or 0.78, respectively Pancreatic tissue. PDAC patients (n 11), healthy controls (n 7) 1) MIF protein expression in PC nests was 2.7-fold higher than normal pancreatic ducts; 2) the AUC of pancreatic MIF levels in discriminating PC from normal pancreas was >0.7 with p < 0.001 Cui et al. (2009) Pancreatic tissue. PDAC patients (n 11), CP (n 9), Paired tissue. PDAC patients (n 11), healthy controls (unknown) MIF mRNA expression in pancreatic tissues was higher in PDAC than CP, stromal tissue of PDAC, and normal ductal area 1) miRNA profiling identified 53 differentially expressed miRNAs in MIF-high vs. MIF-low tumors, a higher expression of miR-301b, miR-15b, miR-10b, miR-93, and miR-590-5p in MIF-high tumors were also associated with poor survival in PDAC cases; 2) MIF expression in the tumor was positively correlated with miR-301b and negatively correlated with NR3C2 expression Yang et al. (2016) Exosomes isolated from plasma. PDAC patients: With liver metastasis (n 18), with no evidence of disease five years post-diagnosis (n 10), with progression of disease post-diagnosis (n 12), healthy controls (n 15) 1) MIF levels in exosomes isolated from PDAC with progression of disease post-diagnosis were significantly higher than PDAC with no evidence of disease 5 years post diagnosis and healthy controls; 2) MIF levels were lower in PDAC patients with liver metastasis that those with progression of disease, without significance

Anti-MIF Antibodies
In AP, two studies (Sakai et al., 2003;Matsuda et al., 2006) have demonstrated the efficacy of anti-MIF antibody in CER/LPS and CDE diet AP models, respectively, revealing a promising therapeutic target. A newly established series of nanobodies (NbE5 and NbE10) have shown to attenuate lethality in in vivo septic shock model (Sparkes et al., 2018), is of great interest to be applied in other conditions of inflammatory end-organ damage, such as severe AP. A number of MIF monoclonal antibodies developed by researchers at Baxter have revealed significant anti-MIF activity in human PC3 prostate cancer cell lines and in vivo xenograft model (Hussain et al., 2013). In vitro, BaxG03, BaxB01, and BaxM159 reduced cell growth and viability by inhibiting ERK1/2 and AKT pathways. The antibodies also inhibited MIF-promoted migration and invasion. In vivo, treatment with anti-MIF antibodies reduced tumor growth in a dose-dependent manner. Recently, a phase 1 clinical trial using imalumab (Bax69) in solid tumors and metastatic colorectal adenocarcinoma demonstrated that imalumab has a maximum tolerated dose of 37.5 mg/kg every 2 weeks and a biologically active dose of 10 mg/kg weekly (Mahalingam et al., 2020). Further investigation is warranted to define the role of anti-MIF antibody as a treatment strategy for pancreatic cancer. Of note, the development of antibodies may be mitigated by their short half-life, high costs associated with production, and potential immunogenicity.

Targeting MIF Receptors
As MIF relies largely on CD74 to regulate the downstream cellular events, treatment targeting CD74 holds great potential to inhibit MIF signaling. A humanized anti-CD74 monoclonal antibody, milatuzumab has shown to significantly prolong the survival duration of multiple myeloma xenograft mice models (Stein et al., 2004;Stein et al., 2009). It has reached phase 1 clinical trial in multiple myeloma (Kaufman et al., 2013) and systemic lupus erythematosus (Wallace et al., 2016), indicating no severe adverse effects. Apart from CD74, inhibitors against CXCL2 (Reparixin) and CXCL4 (Plerixafor) are also of interest for further investigation (Steinberg and Silva, 2010;Goldstein et al., 2020). However, as MIF/CD74 pathway also plays an important role in wound repair by activating pro-survival and proliferative pathways that protects the host during injury (Farr et al., 2020), complete inhibition of CD74 could cause some unpredictable side effects which need precaution.

MIF-Related Gene Therapy
First of all, MIF knockout animals are long lived with no characteristic health issues, giving the opportunity for the initiation of MIF-related gene therapy (Harper et al., 2010). MIF related gene therapy such as Mif gene knockout/ knockdown, DDT gene knockdown, or CD74 gene knockdown has exhibited great potential in pre-clinical studies of AP (Matsuda et al., 2006;Zhu et al., 2020) and PDAC (Winner et al., 2007;Denz et al., 2010;Funamizu et al., 2013;Tan et al., 2014;Costa-Silva et al., 2015;Guo et al., 2016;Yang et al., 2016;Wang et al., 2018;Suresh et al., 2019). Furthermore, it was demonstrated that a xenograft model of head and neck squamous cell carcinoma with MIF knockdown was more sensitive to cisplatin and 5-fluorouracil treatment than control (Kindt et al., 2013). On the other hand, overexpressing MIF in pancreatic cancer cells reduced the sensitivity to gemcitabine (Funamizu et al., 2013). Taken together, it supports that MIF downregulation may potentiate the effect of chemotherapy agents in cancer. There remain ongoing opportunities to develop additional MIF suppression therapies for clinical evaluation.

CONCLUSION
In summary, we have comprehensively reviewed the role of MIF in AP and PDAC. It is apparent from the review that investigations of MIF in AP are at a relatively early stage. Up to now, in vivo AP studies have measured circulating MIF levels, confirmed MIF expression in target organs, and commenced applying MIF inhibitory drugs for efficacy testing. Whether pancreatic acinar cells express MIF and how MIF contributes to the early acinar cell events in AP, (i.e. calcium overload, mitochondrial dysfunction, oxidative stress, endothelial reticulum stress, and trypsinogen activation) remains unclear. It is notable that anti-MIF antibodies and the MIF inhibitor ISO-1, have shown encouraging potential for improving pancreatic damage and associated organ injury in AP animal models. Clinical studies have identified circulating MIF as a potential biomarker for early prediction of AP severity which needs further validation. Future research is warranted to detail the underlying molecular mechanisms, (i.e. TLR4/NLRP3) of MIF in pancreatic acinar cells and AP. In chronic pancreatitis, the role of MIF remains elusive. In pancreatic cancer, MIF enhances the proliferation and invasion of tumor cells, resulting in increased tumor growth and metastasis in vivo. Early studies of MIF knockdown or use of specific inhibitors support MIF as a potential target for PDAC. Future research is required to bring forward a range of promising treatment approaches to clinical evaluation.