Pancreatic Tumorigenesis: Oncogenic KRAS and the Vulnerability of the Pancreas to Obesity

A high-fat diet induces changes in mesenteric adipose tissue accelerating early-stage pancreatic carcinogenesis in mice

BACKGROUND

Increased adiposity is a significant risk factor for pancreatic cancer development. Multiple preclinical studies have documented that high-fat, high caloric diets, rich in omega-6 fatty acids (FA) accelerate pancreatic cancer development. However, the effect of a high-fat, low sucrose diet (HFD), on pancreatic carcinogenesis remains unclear. We evaluated the impact of a HFD on early-stage pancreatic carcinogenesis in the clinically relevant KrasLSL-G12D/+; Ptf1aCre/+ (KC) genetically engineered mouse model, and characterized the role of the mesenteric adipose tissue (MAT).

METHODS

Cohorts of male and female KC mice were randomly assigned to a control diet (CD) or a HFD, matched for FA composition (9:1 of omega-6 FA: omega-3 FA), and fed their diets for eight weeks.

RESULTS

After eight weeks on a HFD, KC mice had significantly higher body weight, fat mass, and serum leptin compared to CD-fed KC mice. Furthermore, a HFD accelerated pancreatic acinar-to-ductal metaplasia (ADM) and proliferation, associated with increased activation of ERK and STAT3, and macrophage infiltration in the pancreas, compared to CD-fed KC mice. Metabolomics analysis of the MAT revealed sex differences between diet groups. In females, a HFD altered metabolites related to FA (α-linolenic acid and linoleic acid) and amino acid metabolism (alanine, aspartate, glutamate). In males, a HFD significantly affected pathways related to alanine, aspartate, glutamate, linoleic acid, and the citric acid cycle.

CONCLUSIONS

A HFD accelerates early pancreatic ADM through multifaceted mechanisms, including effects at the tumor and surrounding MAT. The sex-dependent changes in MAT metabolites could explain some of the sex differences in HFD-induced pancreatic ADM.

Early elevations of RAS protein level and activity are critical for the development of PDAC in the context of inflammation

The KRASG12D mutation was believed to be locked in a GTP-bound form, rendering it fully active. However, recent studies have indicated that the presence of mutant KRAS alone is insufficient; it requires additional activation through inflammatory stimuli to effectively drive the development of pancreatic ductal adenocarcinoma (PDAC). It remains unclear to what extent RAS activation occurs during the development of PDAC in the context of inflammation. Here, in a mouse model with the concurrent expression of KrasG12D/+ and inflammation mediator IKK2 in pancreatic acinar cells, we showed that, compared to KRASG12D alone, the cooperative interaction between KRASG12D and IKK2 rapidly elevated both the protein level and activity of KRASG12D and NRAS in a short term. This high level was sustained throughout the rest phase of PDAC development. These results suggest that inflammation not only rapidly augments the activity but also the protein abundance, leading to an enhanced total amount of GTP-bound RAS (KRASG12D and NRAS) in the early stage. Notably, while KRASG12D could be further activated by IKK2, not all KRASG12D proteins were in the GTP-bound state. Overall, our findings suggest that although KRASG12D is not fully active in the context of inflammation, concurrent increases in both the protein level and activity of KRASG12D as well as NRAS at the early stage by inflammation contribute to the rise in total GTP-bound RAS.

Obesity phenotype induced by high-fat diet promotes diethylnitrosamine (DEN)-induced precancerous lesions by M1 macrophage polarization in mice liver

Liver precancerous lesions are the key to improving the efficacy of cancer treatment because of the extremely poor prognosis of HCC patients in moderate and late stages. Obesity-related HCC progression is closely related to the inflammatory microenvironment, in which macrophages are one of the major constituents. In the present study, we ask whether obesity promotes diethylnitrosamine (DEN)-induced precancerous lesions by M1 macrophage polarization. First, an association between obesity and liver precancerous lesions was determined by histopathological observations, immunochemistry and immunoblotting. The characteristics of early precancerous lesions (trabecular thickening) appeared earlier eight weeks in obese mice than in normal diet mice after DEN induction. The glutathione S-transferase placental-1 (Gstp 1) and alpha-fetoprotein (AFP) expression in obese mice after DEN induction was higher than that in the same period after DEN injection in normal diet mice. Furthermore, there was a significant increase in the total macrophage number (F4/80+) of DEN and M1 macrophage number (CD86+F4/80+) in obese mice compared with that in normal diet mice. Besides, the expressions of four pro-inflammatory factors in DEN-induced obese mice were significantly higher compared with that in normal diet mice. Additionally, angiogenesis was revealed by immunostaining assay to be associated with the inflammatory response. All the results demonstrate that obesity promotes DEN-induced precancerous lesions by inducing M1 macrophage polarization and angiogenesis.

The Role of Adipokines in Pancreatic Cancer

In modern society, inappropriate diets and other lifestyle habits have made obesity an increasingly prominent health problem. Pancreatic cancer (PC), a kind of highly aggressive malignant tumor, is known as a silent assassin and is the seventh leading cause of cancer death worldwide, pushing modern medicine beyond help. Adipokines are coming into notice because of the role of the intermediate regulatory junctions between obesity and malignancy. This review summarizes the current evidence for the relationship between highly concerning adipokines and the pathogenesis of PC. Not only are classical adipokines such as leptin and adiponectin included, but they also cover the recognized chemerin and osteopontin. Through a summary of the biological functions of these adipokines as well as their receptors, it was discovered that in addition to their basic function of stimulating the biological activity of tumors, more studies confirm that adipokines intervene in the progression of PC from the viewpoint of tumor metabolism, immune escape, and reprogramming of the tumor microenvironment (TME). Besides endocrine function, the impact of white adipose tissue (WAT)-induced chronic inflammation on PC is briefly discussed. Furthermore, the potential implication of the acknowledged endocrine behavior of brown adipose tissue (BAT) in relation to carcinogenesis is also explored. No matter the broad spectrum of obesity and the poor prognosis of PC, supplemental research is needed to unravel the detailed network of adipokines associated with PC. Exploiting profound therapeutic strategies that target adipokines and their receptors may go some way to improving the current worrying prognosis of PC patients.

Differential Effects of Dietary Macronutrients on the Development of Oncogenic KRAS-Mediated Pancreatic Ductal Adenocarcinoma

KRAS mutations are prevalent in patients with pancreatic ductal adenocarcinoma (PDAC) and are critical to fostering tumor growth in part by aberrantly rewiring glucose, amino acid, and lipid metabolism. Obesity is a modifiable risk factor for pancreatic cancer. Corroborating this epidemiological observation, mice harboring mutant KRAS are highly vulnerable to obesogenic high-fat diet (HFD) challenges leading to the development of PDAC with high penetrance. However, the contributions of other macronutrient diets, such as diets rich in carbohydrates that are regarded as a more direct source to fuel glycolysis for cancer cell survival and proliferation than HFD, to pancreatic tumorigenesis remain unclear. In this study, we compared the differential effects of a high-carbohydrate diet (HCD), an HFD, and a high-protein diet (HPD) in PDAC development using a mouse model expressing an endogenous level of mutant KRAS^G12D specifically in pancreatic acinar cells. Our study showed that although with a lower tumorigenic capacity than chronic HFD, chronic HCD promoted acinar-to-ductal metaplasia (ADM) and pancreatic intraepithelial neoplasia (PanIN) lesions with increased inflammation, fibrosis, and cell proliferation compared to the normal diet (ND) in Kras^G12D/+ mice. By contrast, chronic HPD showed no significant adverse effects compared to the ND. Furthermore, ablation of pancreatic acinar cell cyclooxygenase 2 (Cox-2) in Kras^G12D/+ mice abrogated the adverse effects induced by HCD, suggesting that diet-induced pancreatic inflammation is critical for promoting oncogenic KRAS-mediated neoplasia. These results indicate that diets rich in different macronutrients have differential effects on pancreatic tumorigenesis in which the ensuing inflammation exacerbates the process. Management of macronutrient intake aimed at thwarting inflammation is thus an important preventive strategy for patients harboring oncogenic KRAS.

The Intricate Crosstalk Between Insulin and Pancreatic Ductal Adenocarcinoma: A Review From Clinical to Molecular

Increased insulin level (or "hyperinsulinemia") is a common phenomenon in pancreatic ductal adenocarcinoma (PDA) patients and signals poor clinical outcomes. Insulin is safe in low PDA risk population, while insulin significantly promotes PDA risk in high PDA risk population. The correlation between insulin and PDA is a reciprocal self-reinforcing relationship. On the one hand, pancreatic cancer cells synthesize multiple molecules to cause elevated peripheral insulin resistance, thus enhancing hyperinsulinemia. On the other hand, insulin promotes pancreatic cancer initiation and sustains PDA development by eliciting tumorigenic inflammation, regulating lipid and glucose metabolic reprogram, overcoming apoptosis through the crosstalk with IGF-1, stimulating cancer metastasis, and activating tumor microenvironment formation (inflammation, fibrosis, and angiogenesis). Currently, taking glucose sensitizing agents, including metformin, SGLT-2 inhibitor, and GLP-1 agonist, is an effective way of lowering insulin levels and controlling PDA development at the same time. In the future, new drugs targeting insulin-related signal pathways may pave a novel way for suppressing PDA initiation and progression.

Oncogenic KRAS-Induced Feedback Inflammatory Signaling in Pancreatic Cancer: An Overview and New Therapeutic Opportunities

Pancreatic ductal adenocarcinoma (PDAC) remains highly refractory to treatment. While the KRAS oncogene is present in almost all PDAC cases and accounts for many of the malignant feats of PDAC, targeting KRAS or its canonical, direct effector cascades remains unsuccessful in patients. The recalcitrant nature of PDAC is also heavily influenced by its highly fibro-inflammatory tumor microenvironment (TME), which comprises an acellular extracellular matrix and various types of non-neoplastic cells including fibroblasts, immune cells, and adipocytes, underscoring the critical need to delineate the bidirectional signaling interplay between PDAC cells and the TME in order to develop novel therapeutic strategies. The impact of tumor-cell KRAS signaling on various cell types in the TME has been well covered by several reviews. In this article, we critically reviewed evidence, including work from our group, on how the feedback inflammatory signals from the TME impact and synergize with oncogenic KRAS signaling in PDAC cells, ultimately augmenting their malignant behavior. We discussed past and ongoing clinical trials that target key inflammatory pathways in PDAC and highlight lessons to be learned from outcomes. Lastly, we provided our perspective on the future of developing therapeutic strategies for PDAC through understanding the breadth and complexity of KRAS and the inflammatory signaling network.

Obesity and Pancreatic Cancer: Insight into Mechanisms

Simple Summary

Obesity is recognized as a chronic progressive disease and risk factor for many human diseases. The high and increasing number of obese people may underlie the expected increase in pancreatic cancer cases in the United States. There are several pathways discussed that link obesity with pancreatic cancer. Adipose tissue and adipose tissue-released factors may thereby play an important role. This review discusses selected mechanisms that may accelerate pancreatic cancer development in obesity.

Abstract

The prevalence of obesity in adults and children has dramatically increased over the past decades. Obesity has been declared a chronic progressive disease and is a risk factor for a number of metabolic, inflammatory, and neoplastic diseases. There is clear epidemiologic and preclinical evidence that obesity is a risk factor for pancreatic cancer. Among various potential mechanisms linking obesity with pancreatic cancer, the adipose tissue and obesity-associated adipose tissue inflammation play a central role. The current review discusses selected topics and mechanisms that attracted recent interest and that may underlie the promoting effects of obesity in pancreatic cancer. These topics include the impact of obesity on KRAS activity, the role of visceral adipose tissue, intrapancreatic fat, adipose tissue inflammation, and adipokines on pancreatic cancer development. Current research on lipocalin-2, fibroblast growth factor 21, and Wnt5a is discussed. Furthermore, the significance of obesity-associated insulin resistance with hyperinsulinemia and obesity-induced gut dysbiosis with metabolic endotoxemia is reviewed. Given the central role that is occupied by the adipose tissue in obesity-promoted pancreatic cancer development, preventive and interceptive strategies should be aimed at attenuating obesity-associated adipose tissue inflammation and/or at targeting specific molecules that mechanistically link adipose tissue with pancreatic cancer in obese patients.