New Insights into Chronic Pancreatitis Treatment: Potential Mechanisms and Applications of Probiotics and Prebiotics

1. Introduction

Chronic pancreatitis (CP) is a progressive fibroinflammatory syndrome with an annual incidence of 5 to 8 and a prevalence of 42-73 cases per 100,000 adults in the United States [1,2,3]. With repetitive episodes of inflammation, the pancreas is irreversibly replaced by fibrotic tissues, resulting in chronic abdominal pain, endocrine and exocrine insufficiency, reduced quality of life, and a shorter life expectancy [4]. Current therapeutic approaches primarily focus on symptom alleviation and supportive care, rather than targeting the underlying pathophysiological mechanisms [5].

In recent years, accumulating evidence has highlighted the crucial role of the pancreas in regulating gut microbiota and the reciprocal influence of gut microbiota on pancreatic function, which indicates the presence of a bidirectional relationship referred to as the "gut-pancreas axis". Gut microbiota plays a pivotal role in this axis through its involvement in metabolism and nutrition, protection against pathogens, and immune system regulation [6]. Bidirectional alteration of the gut-pancreas axis has been observed in many pancreatic diseases, including CP (Figure 1) [7]. Regarding its role in its role in the homeostasis of the gut-pancreas axis, microbiota-based treatments, such as probiotics and prebiotics, may offer effective therapeutic options for CP.

Probiotics, defined as live microorganisms that confer health benefits to the host, and prebiotics, non-digestible food components that selectively stimulate beneficial gut bacteria, have shown potential in managing various gastrointestinal and systemic disorders [8]. In the context of CP, several mechanisms by which probiotics and prebiotics might exert therapeutic effects have been proposed.

Although probiotics or prebiotics have been proposed as potential treatments for chronic pancreatitis [6,7], their efficacy has not yet been validated in animal models or clinical trials. Additionally, the possible mechanisms of their action have not been thoroughly explored. This review aims to explore the potential of probiotics and prebiotics as therapeutic agents for CP by examining their effects on the gut-pancreas axis. We will discuss possible mechanisms that ameliorate CP, including endocrine and exocrine function improvement, inflammation reduction, and pancreatic fibrosis alleviation. The effects of probiotics and prebiotics on these targets and their feasibility as intervention methods are reviewed. We seek to provide deeper insights into the potential of probiotics and prebiotics in the treatment of CP.

2. Search Strategy

We conducted a comprehensive search in PubMed on July 2024. The search strategy involved the following main queries: (1) "(probiotics OR prebiotics OR synbiotics) AND (pancreatitis)". Literature and reference screening were conducted to select potentially relevant articles. This approach provided a general overview of the current research landscape and potential therapeutic mechanisms of probiotics and prebiotics in CP. After identifying the potential mechanisms of action, the following search queries were involved: (2) (small intestine bacterial overgrowth) AND (chronic pancreatitis); (3) (small intestine bacterial overgrowth) AND (probiotics OR prebiotics OR synbiotics); (4) (short-chain fatty acid) AND (pancreas*); (5) (short-chain fatty acid) AND (probiotics OR prebiotics OR synbiotics); (6) (GLP-1) AND (pancreas*); (7) (GLP-1) AND (probiotics OR prebiotics OR synbiotics).

3. Alleviation of Small Intestinal Bacterial Overgrowth

In a healthy small intestine, several defective mechanisms maintain a relatively sterile environment: gastric acid secretion, an intact ileocecal valve, intestinal motility, immunoglobulins in intestinal secretions, and the bacteriostatic properties of pancreatic and biliary secretions [9,10]. When these protective mechanisms are disrupted, small intestinal bacterial overgrowth syndrome (SIBO) can occur. SIBO is characterized by an excessive number of bacteria in the small bowel, leading to gastrointestinal symptoms such as bloating, abdominal distension, diarrhea, and nutrient deficiencies [11,12]. A systematic review found that SIBO is present in 38% of patients with CP [13]. Current evidence links SIBO in CP to diabetes mellitus, pancreatic exocrine insufficiency, and the severity of CP, with treatment often resulting in symptomatic improvement [13,14,15,16].

The standard treatment for SIBO involves antibiotics aimed at eradicating bacteria in the small intestine [17]. However, with a combined normalization rate of 51% for antibiotics, about half of the patients may remain symptomatic despite treatment [18]. This necessitates refined treatment strategies. Probiotics and prebiotics are believed to benefit SIBO by preventing the growth of pathogenic flora through direct competition and the production of bacteriocins [12]. Several randomized controlled trials have shown that adding probiotics to antibiotic therapy results in higher clinical remission rates [19,20,21]. In a randomized prospective pilot study of patients with SIBO and chronic abdominal distension, the group receiving a combination of probiotics (Lactobacillus casei, Lactobacillus plantarum, Streptococcus faecalis, Bifidobacterium brevis) showed significantly better clinical improvement compared to the sole metronidazole group [22]. A systematic review concluded that while probiotics are unavailable to prevent SIBO, they can effectively decontaminate SIBO and relieve abdominal pain [23]. Probiotics also aid in repairing and reconstructing intestinal mucosa. In rats treated with probiotic formulations containing coconut oil and traces of peppermint-lemon-patchouli essential oil, researchers observed mitotic figures and regression of the inflammatory response in villus epithelium and crypts previously damaged by SIBO-induced gut dysbiosis [24].

Probiotic supplementation to reduce SIBO has been attempted in various diseases, including irritable bowel syndrome [25,26,27,28,29], hypothyroidism during pregnancy [30,31,32], systemic sclerosis [33], liver diseases [34,35,36], and gastric and colorectal cancer [37]. However, there is a lack of research evidence on the application of probiotics for SIBO in chronic pancreatitis. Further investigation is needed to explore the potential benefits of probiotics in alleviating SIBO in CP.

4. Facilitation of Short-Chain Fatty Acids Production

Short-chain fatty acids (SCFAs), primarily acetate, propionate and butyrate, are produced via fermentation of dietary fibers by gut microbiota. They have significant effects on various tissues, including the pancreas. Sodium butyrate is capable of inhibiting histone deacetylases (HDACs), which are crucial in inflammation and fibrogenesis. Post-treatment with sodium butyrate significantly reduces the expression of α-smooth muscle actin, interleukin-1β, inducible nitric oxide synthase, and 3-nitrotyrosine, thereby alleviating L-arginine-induced pancreatic damage and fibrosis in rats [38]. SCFAs modulate pancreatic fibrosis by inhibiting macrophage infiltration and M2 phenotype switching [39]. SCFAs have also been confirmed to play an immunoregulatory and anti-inflammatory role. Cathelicidin-related antimicrobial peptide (CRAMP) is an immunoregulatory antimicrobial peptide and can be produced by acinar cells. It modulates the phenotypic switch of intrapancreatic macrophages and changes the production of transforming growth factor-β, thereby defending against inflammation. Research has revealed that the production of CRAMP is regulated by SCFAs produced by gut microbiota [40]. Additionally, SCFAs, especially butyrate, exhibit anti-inflammatory effects by inhibiting the activation of NF-κB and HDACs [41,42,43,44]. SCFAs also act directly on acinar cells to stimulate secretion, similar to incretins, through increasing cellular calcium concentration [45,46,47,48].

Extensive studies have investigated SCFAs' effects on insulin secretion, acting as ligands to G-protein-coupled receptors (GPCRs), specifically free fatty acid receptor-2 (FFA2, previously termed GPR43) and FFA3 (previously termed GPR41). These receptors are found in various human tissues, including gut enteroendocrine cells and pancreatic islets [49,50]. FFA2 and FFA3 receptors on enteroendocrine cells trigger GLP-1 secretion [49], which has multiple positive effects and will be discussed in the next part. Enhanced secretion of insulin after SCFA treatment has been reported in a number of studies and is thought to be associated with FFA2 and FFA3 receptors on β-cells, but contradicting evidence also exists in several researches [50,51]. Therefore, no clear consensus has been achieved on the effect of SCFAs on FFA2 and FFA3 receptors in pancreatic islets.

Patients with CP exhibit a reduced abundance of SCFA producers, such as Faecalibacterium and Fusicatenibacter [52]. There is a noticeable reduction in Faecalibacterium prausnitzii from healthy controls to CP non-diabetics to CP diabetics [53]. Depletion of SCFA-producing Gram-positive bacteria worse CP independently of TLR4, but supplementing exogenous SCFAs ameliorates the condition [39]. These studies implicated the role of SCFAs in protecting pancreatic function from damage of CP. Therefore, supplementing probiotics or prebiotics that contribute to SCFA production may offer a novel intervention for managing CP.

Both in vivo and in vitro studies confirm that probiotics can increase SCFA levels. Probiotics capable of producing SCFAs are summarized in Table 1. In an in vitro human gut model, an aqueous probiotic suspension, containing L. plantarum, L. rhamnosus, L. acidophilus and Enterococcus faecium, exerted anti-inflammatory effects through increased SCFA production, especially butyrate [54].

Prebiotics also show potential as clinical targets by promoting the growth and activity of probiotics. Prebiotics, typically complex carbohydrates such as starch, pectin, xylan, and arabinogalactan, serve as substrates for bacterial fermentation, resulting in the production of SCFAs [62]. The metabolism of different polysaccharides is associated with the production of different SCFAs. For example, pectin metabolism leads to a proportional increase in acetate concentration, while starch fermentation significantly boosts butyrate production over other SCFAs [63,64]. Overall, the microbial hydrolysis of insoluble substrates can promote the biosynthesis of high concentrations of SCFAs, with about 60% present as acetate, while butyrate and propionate each account for approximately 20% of gastrointestinal SCFAs [65]. Colonic SCFAs increase after consuming inulin or arabinoxylan oligosaccharides-enriched food in healthy humans [66,67]. Inulin supplementation elevates the abundance of butyrate-producing microbiota, including Bifidobacterium, Clostridium cluster IV, and Akkermansia muciniphila [68]. When supplemented with oligofructose or inulin as the sole energy source, cross-feeding interactions between bifidobacteria and butyrate-producing bacteria like Faecalibacterium prausnitzii are observed. These interactions may enhance the colon ecosystem and contribute to combined bifidogenic and butyrogenic effects [69,70].

In summary, the use of probiotics and prebiotics to produce SCFAs shows promise as a management for CP. This approach could help modulate inflammation, fibrosis, and pancreatic function, offering a potential therapeutic avenue worth further exploration.

5. Activation of Glucagon-like Peptide 1 Receptors in the Pancreas

Glucagon-like peptide 1 (GLP-1) is released from gut enteroendocrine cells at low levels during fasting and increases significantly within minutes of food digestion. GLP-1 is a multifaceted hormone with broad pharmacological potential, including incretin-like activity, stimulation of glucose-dependent insulin secretion, and inhibition of glucagon secretion, food intake, and gastric emptying [71,72]. These properties have led to the development of GLP-1 receptor (GLP-1R) agonists for treating T2DM, and subsequently, obesity [73]. The multifunctional role of GLP-1 in the pancreas suggests additional potential for clinical management.

The physiological importance of GLP-1R on β-cells has been well-established in animal studies. GLP-1 normalizes glucose tolerance and enhances glucose-dependent insulin secretion via GLP-1R on pancreatic β-cells [74]. The mechanisms by which GLP-1 restores glucose sensitivity in β-cells involve crosstalk between membrane ion channels, cyclic AMP (cAMP)-dependent signaling, and intracellular glucose metabolism. Additionally, GLP-1 inhibits glucagon secretion, although the expression levels of GLP-1R on α-cells are debated. Some studies report GLP-1R on a subset of α-cells [75], suggesting direct inhibition of glucagon secretion, while others show very low or undetectable levels [76,77,78]. Moreover, GLP-1 acts on GLP-1R on pancreatic δ-cells, stimulating somatostatin secretion, which inhibits glucagon secretion from α-cells via the somatostatin-2 receptor (SSTR2) [78,79].

Beyond regulating blood glucose through modulating levels of insulin and glucagon, GLP-1 inhibits β-cell apoptosis, induces β-cell proliferation, and increases β-cell mass [80]. In diabetic mouse models, GLP-1R activation alleviates ER stress in β-cells via cAMP-dependent enhancement of ATF4 translation, promoting β-cell survival [81]. Although GLP-1R agonists can increase β-cell mass in diabetic rodent models, this effect is modest and short-lived, with older rodents showing reduced response [82,83,84]. Nevertheless, these drugs are believed to help prevent further loss of β-cell mass and function, especially if treatment begins early in disease progression. In baboons subjected to partial pancreatectomy and treated with the GLP-1R agonist exenatide, immunofluorescent staining revealed ductal cells co-expressing insulin, suggesting exenatide might promote the differentiation of ductal cells into β-like cells [85].

While most GLP-1 research focuses on α and β cells in the endocrine pancreas, GLP-1 also affects the exocrine pancreas. GLP-1R is expressed in a significant proportion of pancreatic acinar cells, though at lower levels than in β cells [86,87,88]. In caerulein-induced experimental pancreatitis, GLP-1R agonists increased pancreas weight and induced anti-inflammatory protein expression while reducing proinflammatory markers [89]. Preclinical studies show that GLP-1R activation increases acinar cell mass and protein content via S6 phosphorylation, independent of DNA content or cell proliferation changes [90]. GLP-1 induces amylase secretion in pancreatic acini through stimulated cAMP production and increased protein kinase A-mediated phosphorylation [86]. Moreover, elevation of plasma enzyme levels in human subjects treated with GLP-1R agonists is dose-independent and reversible [91].

The effects of GLP-1 and its analogs on the exocrine pancreas have raised concerns about the risk of pancreatitis. Most of the evaluations were done in patients with type 2 diabetes mellitus (T2DM). A population-based cohort study found that incretin users had a 1.5-fold increased risk of any pancreatitis and a 2.0-fold increased risk of acute pancreatitis, although no increased risk was found for chronic pancreatitis [92]. A review of liraglutide clinical trials reported a higher incidence of pancreatitis, but conclusions were inconclusive due to confounding variables [93]. While the risk of GLP-1 causing pancreatitis remains uncertain, further research is needed to understand its effects in CP compared to T2DM, as the existing evidence is all based on the T2DM population.

Pancreatic stellate cells (PSCs) are activated in the CP microenvironment, contributing to pancreatic fibrosis progression. With the activation of PSC, GLP-1R on it is markedly increased. Studies suggest that GLP-1R agonist liraglutide does not induce inflammatory gene expression in activated PSCs but does induce proliferation [94]. Other studies found chronic GLP-1R agonist treatment can lead to PSC activation, causing the expression of fibrosis markers and chronic inflammation [95]. In the context of CP, further research is needed to determine whether GLP-1R agonists exacerbate pancreatic fibrosis and to understand the underlying mechanisms. It remains to be explored whether there are ways to modify GLP-1R agonists to enhance their positive effects on pancreatic endocrine and exocrine functions while minimizing their impact on PSCs.

In many animal models of other diseases, certain probiotics have been found to induce GLP-1 secretion (Table 2). In addition, using probiotics as oral vectors for recombinant GLP-1R agonists delivery has been explored to replace costly chemical synthesis and inconvenient injections. Probiotics can efficiently target the pancreas, offering high bioavailability. Lactobacillus paracasei L14 transformed with a plasmid encoding the exendin-4 gene has shown efficient secretion and facilitated transport of exendin-4, enhancing insulin secretion and maintaining β cells [96]. Engineered probiotic yeast Saccharomyces boulardii administered orally has also produced bioactive GLP-1R agonists [97]. Apart from delivering GLP-1R agonists, protease-resistant modified GLP-1 (mGLP-1) has been constructed with added arginine to ensure the structural integrity of mGLP-1 released in vivo [98]. In addition to producing bioactive GLP-1R agonists, engineered probiotics as carriers also exert their inherent function of regulating the microbiota. Engineered Clostridium butyricum significantly improved gut microbiota dysbiosis in rats via downregulating the relative abundance of Porphyromonadaceae at the family level and upregulating Lactobacillus at the genus level [99]. Similarly, engineered Escherichia coli Nissle 1917 expressing GLP-1 regulated intestinal flora and increased probiotic diversity in mice [100].

An increase in GLP-1 secretion levels has also been observed following the addition of prebiotics, including dietary resistant starch [101,102], resistant maltodextrin [103], fructooligosaccharides [103,104], chondroitin sulfate [105], and Dendrobium officinale polysaccharide (DOP) [106]. The stimulative effect of prebiotics on GLP-1 secretion may be through stimulating SCFA production [101,102].

Table 2. Summary of probiotics that can promote GLP-1 expression in various disease models.

Genus	Species	Disease models	References
Lactobacillus	L. casei CCFM419	T2DM	[107]
	L. plantarum MTCC5690		[108]
	L. fermentum MTCC5689		[108]
	Lactobacillus CGMCC No. 21661		[109]
	L. rhamnosus NCDC 17		[110]
	L. paracasei JY062	Glycolipid metabolic disorders	[111]
	L. reuteri	Glucose metabolism disorder induced by acrylamide; glucose-tolerant humans	[112,113]
	L. paracasei subsp. paracasei L. casei W8	isolated pig intestine	[114]
Lacticaseibacillus	L. paracasei L-21	STC-1 cell line	[115]
Bifidobacterium	selenium-enriched B. longum DD98	T2DM	[116]
	B. animalis subsp. lactis MN-Gup		[117]
	B. animalis subsp. lactis NJ241	Parkinson’s disease	[118]
	B. animalis subsp. lactis GCL2505	Metabolic syndrome	[119]
	B. longum subsp. longum B-53	STC-1 cell line	[115]
Akkermansia	Pasteurized A. muciniphila	T2DM	[120]
Bacteroides	B. thetaiotaomicron	alcoholic fatty liver disease	[121]
Limosilactobacillus	L. fermentum MG4295	T2DM	[122]
Clostridium	C. butyricum	chronic unpredictable mild stress; T2DM	[123,124]

Table 2. Summary of probiotics that can promote GLP-1 expression in various disease models.

Genus	Species	Disease models	References
Lactobacillus	L. casei CCFM419	T2DM	[107]
	L. plantarum MTCC5690		[108]
	L. fermentum MTCC5689		[108]
	Lactobacillus CGMCC No. 21661		[109]
	L. rhamnosus NCDC 17		[110]
	L. paracasei JY062	Glycolipid metabolic disorders	[111]
	L. reuteri	Glucose metabolism disorder induced by acrylamide; glucose-tolerant humans	[112,113]
	L. paracasei subsp. paracasei L. casei W8	isolated pig intestine	[114]
Lacticaseibacillus	L. paracasei L-21	STC-1 cell line	[115]
Bifidobacterium	selenium-enriched B. longum DD98	T2DM	[116]
	B. animalis subsp. lactis MN-Gup		[117]
	B. animalis subsp. lactis NJ241	Parkinson’s disease	[118]
	B. animalis subsp. lactis GCL2505	Metabolic syndrome	[119]
	B. longum subsp. longum B-53	STC-1 cell line	[115]
Akkermansia	Pasteurized A. muciniphila	T2DM	[120]
Bacteroides	B. thetaiotaomicron	alcoholic fatty liver disease	[121]
Limosilactobacillus	L. fermentum MG4295	T2DM	[122]
Clostridium	C. butyricum	chronic unpredictable mild stress; T2DM	[123,124]

In conclusion, the potential of engineered probiotics to express GLP-1 analogs offers a promising avenue for treating CP. Unfortunately, there is currently a lack of experimental evidence regarding the use of engineered probiotics in CP. However, considering their mechanisms of action and the positive effects observed in other disease models, their application in the treatment of CP holds great promise. This approach could improve pancreatic function and manage symptoms more effectively, although further research is needed to fully understand the implications and optimize treatment strategies.

6. Conclusion

Current treatments for CP lack innovation, underscoring the need for novel therapeutic approaches. The gut microbiota can influence pancreatic function through its metabolic activities in the gut, via the gut-pancreas axis. Probiotics and prebiotics may hold the potential for treating CP via this axis.

The three possible intervention mechanisms discussed in this review—alleviating small intestine bacterial overgrowth, facilitating SCFAs production, and activating GLP-1R in the pancreas—are largely based on theoretical extrapolations from existing research, much of which is derived from other pancreatic disease models. Although there is a scarcity of experimental evidence specifically targeting CP, these mechanisms show strong potential for its treatment, including improvement of pancreatic endocrine and exocrine functions, and maintaining cellular and structural integrity. Therefore, there is an urgent need for experimental validation in the field of chronic pancreatitis. This exploration forms the core focus of this review, highlighting the promising potential of these interventions to address the pressing need for improved chronic pancreatitis therapies.