Bacteria And Pancreas Cancer

"All disease begins in the gut"--Hippocrates

The microbiome is the collection of 10-100 trillion organisms that both live in and on the human body, primarily in the gastrointestinal tract. Predominantly bacteria, these microorganisms live out their lives for the most part unnoticed in dynamic symbiosis with us. However their role is not to be underestimated and likely they define us as humans.

The microbiota are extremely diverse. The Meta-HIT consortium reported a gene catalog of 3.3 million non-redundant genes in the human gut microbiome alone, as compared to the ～22,000 genes present in the entire human genome. The microbiome is established within 20 minutes of birth and an infant's GI tract microbiome begins to resemble that of an adult as early as 1 year of life.

Dysbiosis, or alterations in the core microbiome, has been shown to play an outsize role in human disease, ranging from dysfunctions in metabolism, autoimmunity, cardiovascular disease, and cancer. In fact, infectious agents are thought to underlie the development of 10–20% of the global cancer cases each year, and this percentage is even higher in digestive tract-related malignancies such as liver and gastric cancers, being as high as 80%. In these malignancies, the gut microbiota is in direct contact with high-risk organs. As it turns out, pancreas cancer (PDAC) is one of them.

How Do Gut Bacteria Play A Role In Cancer Development?

Gut bacteria likely play a major role in carcinogenesis by their ability to promote inflammatory responses, change the tumor immune microenvironment, alter tumor metabolism, and modulate tumor sensitivity to drugs. Particular to pancreas cancer, the microbiota modulates the tumor microenvironment (TME) locally through interactions with immune responses, hence, influencing cancer initiation, development, and treatment, and the effects range from harmful to beneficial.

The Microbiome And The Pancreas

Historically, the pancreas was thought to be a sterile organ, but recent studies have found the existence of bacteria populations in both normal pancreatic tissue and PDAC tumor samples. Nejman et al. examined the bacterial composition of 1010 tumor samples across 7 different tumor types and 516 normal samples. They found that pancreatic cancer had one of the highest proportions of tumor positive for bacterial DNA, and bacteria belonging to the Proteobacteria phylum were most abundant. Geller et al. conducted real-time quantitative polymerase chain reaction (qPCR) to detect bacterial 16S ribosomal DNA in 113 human PDAC samples and 20 normal pancreas samples. They were able to detect bacterial DNA in 15% of the normal pancreas samples and 76% of the PDAC samples.

In addition to higher levels of bacteria in PDAC samples, several studies have shown a difference in the gut microbiota between PDAC patients and healthy individuals, with a composition of gut microbiota that was also unique in PDAC patients. PDAC contained significantly higher Bacteroidetes and lower Firmicutes and Proteobacteria when compared to healthy controls. What this means is still unknown.

What has also been found is that oral microbiomes differ between PDAC patients and healthy controls too. For example, Farrell et al. analyzed oral microbiota and reported that the levels of Neisseria elongata and Streptococcus mitis were significantly reduced in patients with PDAC relative to healthy controls. Fan et al. reported that Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans were associated with a higher risk of PDAC. Michaud et al. (2013) reported that high levels of antibodies against the oral bacteria Porphyromonas gingivalis were also associated with a twofold increased risk of PDAC. Again, what this means clinically is unknown.

What Might This Mean?

Gnansekaran et al. evaluated the direct effects of P. gingivalis on PDAC development and proliferation using cell lines and a xenograft model. They found that P. gingivalis infection enhanced proliferation of PDAC cells, and the enhanced tumor cell proliferation correlates with P. gingivalis intracellular survival. Hypoxia increased P. gingivalis intracellular survival. Consistent with the in vitro results, the authors found that P. gingivalis infection also led to enhanced tumor growth in vivo. A prior study in an oral squamous cell model indicated that of the effects of P. gingivalis on cancer cell growth was mediated through a Toll-like receptor 2 (TLR2)-dependent mechanism.

Dysbiosis might also lead to the development of pre-cancerous states of the pancreas, specifically chronic pancreatitis (CP) and IPMN.

Recent analyses compared the gut microbiome in patients with CP and healthy controls (HC). Ciocan et al. analyzed intestinal microbiota profiles in severe alcoholic hepatitis (sAH) or alcoholic chronic pancreatitis (ACP) compared to alcoholic healthy controls (A-HC). The authors reported that patients with ACP have lower bacterial diversity compared to that of A-HC. A more active intestinal microbiome was observed in patients with ACP (e.g., Klebsiella, Enterococcus, and Sphingomonas). A lower abundance of Faecalibacterium was observed in ACP compared to both A-HC and sAH.

Gaiser et al. analyzed the microbiota in 105 patients with pancreatic cyst fluid including IPMN, IPMN with cancer, SCN, and MCN. The authors reported that significantly higher bacterial DNA copies were found in the cyst fluid of IPMN and cancer compared with non-IPMN (SCN and MCN). At the phylum level, IPMN LGD was found to be dominated by Proteobacteria, while IPMN HGD and cancer were generally diverse and dominated by Firmicutes or Proteobacteria. Despite the large individual variation, the authors found that IPMN HGD was enriched in Fusobacteria, Granulicatella, and Serratia, suggesting the potential role of these bacteria in the progression of pancreatic precursors to PDAC.

But How?

Although specific tumor-related bacterial profiles have now been identified, the "how" has remained somewhat elusive. Only recently has accumulating evidence suggested that the microbiome modulates innate and adaptive immune programs and contributes to the formation of an immunosuppressive tumor microenvironment (TME). This is very important for how PDAC evades the immune system response.

The microbiota in the TME can activate the immune system and recruit immune cells. Immune cells are induced by microbiota to differentiate into different subtypes of immune cells, which secrete the appropriate factors that play pro- or anti-neoplastic roles in tumorigenesis and progression. For example, immune cells with antitumor effects, M1 macrophages (secreting IL-1β/IL-6/TNF), CD8+ T cells, and Th1 (differentiated from CD4+ T cells and secreting IFNγ) may be reduced by the presence of microbiota. In contrast, immune cells with pro-tumor effects, M2 macrophages (secreting TGF/IL-10/CCL18), B cells, MDSC, and CD4+ differentiated into Th2 (secreting TGF/IL-6), Th17 (secreting IL-17), and Treg (secreting IL-10) are increased.

Can Any Of This Be Used Clinically Right Now?

The distinct microbiota found in PDAC patients offers novel opportunities to develop diagnostic/screening biomarkers and the tests may be easily performed in the clinic since salivary and fecal samples will be less invasive and easier to obtain than tissue biopsy from metastatic tumors. Farrell et al. first explored the possibility of using salivary microbial profile as a diagnostic biomarker by using the Human Oral Microbe Identification Microarray (HOMIM) to compare the salivary microbiota between 10 PDAC patients and 10 matched controls to identify bacterial candidates. This was then validated in a cohort of 28 PDAC samples, 28 matched controls, and 27 chronic pancreatitis samples. The authors observed a significant difference in the salivary microbiota between PDAC patients and controls. Specifically, levels of Neisseria elongata and Streptococcus mitis were significantly lower in patients with pancreatic cancer, and the combination of these two microbial biomarkers showed a sensitivity of 96.4% and specificity of 82.1% in differentiating PDAC patients from healthy subjects.

Prognostic Markers

A very interesting study by Kharofa et al. compared the bacterial profiles of patients with PDAC who were either characterized as long-term survivors (6 year survival or more) or not (median survival of 1.8 years) after pancreas surgery. They found that stool from patients cured from PDAC had more relative abundance of Faecalibacterium prausnitzii and Akkermansia muciniphila.

Riquelme et al. examined the intratumoral microbiome composition of 68 resected PDAC tumors; 36 of these patients were considered long-term survivors (> 5 years) and 32 were considered short-term survivors (<5 years). They found that the intratumoral microbiome diversity was significantly higher in the long-term survivors compared with the short-term survivors. A specific microbiome signature (Pseudoxanthomonas, Streptomyces, Saccharopolyspora, Bacillus clausii) was highly predictive of long-term survival.

Microbiome As A Predictive Marker For Treatment Response

Gemcitabine is one of the main chemotherapy drugs used to treat PDAC, but its benefit can be limited. One of the reasons may be that the microbiota damages the antitumor properties of gemcitabine. Geller et?al. proved that most of the microorganisms associated with pancreatic tumors are Gammaproteobacteria, including Enterobacter and Pseudomonas, which can produce cytidine deaminase (CDD) and promote the metabolism of gemcitabine into its inactive form, 2’,2’-difluorodeoxyuridine, leading to the degradation and resistance of gemcitabine. They found that the combination of gemcitabine and antibiotics is more effective than gemcitabine alone.

A retrospective study by Weniger et al. of PDAC patients receiving adjuvant gemcitabine found better PFS in PDAC patients without Klebsiella pneumoniae (which belongs to the class Gammaproteobacteria) in their bile culture than those with K. pneumoniae. Antibiotic treatment with quinolones was associated with improved overall survival in patients who were positive for K. pneumoniae.

Fecal Transplants To Help Cure Cancer?

Riquelme et al. performed human fecal microbiota transplants from PDAC patients, PDAC survivors, and healthy controls to tumor-bearing mice to evaluate the role of the gut microbiome in shaping the tumor microbiome, the immune system, and PDAC progression. The authors showed that gut or tumor microbiomes from PDAC survivors induced an antitumor response and enhanced the infiltration of CD8+ T cell in tumor-bearing mice.

Two trials are currently recruiting patients with gastrointestinal cancers utilizing fecal transplants to augment the benefit of immune checkpoint inhibitors (here and here).

Conclusion

In less than a decade great strides have been made in linking the microbiome to PDAC, as both a biomarker and a potential treatment pathway. The modulation of microbiota has the potential to augment drug efficacy and reduce toxicity, and future studies should integrate microbiome-based biomarkers as well as evaluate the role of fecal transplantation, probiotics, dietary changes, and antibiotics in altering treatment response and patient outcomes. Clearly this field is only in its infancy but holds tremendous promise in the fight against PDAC.

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