The Inflammasome and Pyroptosis

The tumor terrain is composed of diverse subsets of cancer cells, multiple immune cells and inflammatory mediators. It is a complex ecologic system that cancer uses to support its progression, creating environmental signals that drive a persistent robust chronic inflammation and changes the immune system from our potent defender into one benefiting the tumor.

This sequence of events is stimulated when cellular damage and /or stress is detected by the innate immune system, primarily macrophages, neutrophils and dendritic cells. Generally, this activates an acute inflammatory response that is designed to contain and resolve the inflammatory damage and allow healing. One prominent mechanism that assists in quieting normal inflammation is apoptosis, a non-inflammatory form of cell death that removes the damaged cells, reducing exposures that promote an ongoing inflammatory response. 

But cancer is capable of prolonging, amplifying, and maintaining this inflammatory response resulting in persistent signals of damage or danger which can activate the inflammasome pathway within immune cells that acts as sensors of danger or injury in the terrain. This in turn can drive a highly inflammatory form of programmed cell death called Pyroptosis. 

Table of Contents:

  1. Pyroptosis Is a Duel Edged Sword

  2. The Inflammasome

  3. The Paradox

  4. Current Research

  5. Integrative Perspectives

Pyroptosis Is a Duel Edged Sword

It can be beneficial and cause cancer cells to rupture and release tumor antigens that allow the immune system to recognize and attack the tumor.

But if it is persistently designed to remain chronic it can result in detrimental responses. 

  • It continues producing signals from damage associated molecular patterns (DAMPS) which cause ongoing sustained cytokine activation that continues amplifying inflammatory responses.

  • The recruiting of immunosuppressive cells, TAMs and MDCs (see below)

  • Increased vascular creation and porosity to allow tumor spread

  • Damages tissues to create a more suitable cancer microenvironment of low oxygen, more cellular stress and oxidative metabolic stress. 

  • Alters normal resolving anti-inflammatory molecules creating misinterpretation and failure to resolve inflammation.

  • Changes healing macrophages to become suppressive immune cells

  • Diminution of T cell strength, resulting from the exhaustion of their responses from continuous activity and preventing them from being able to mount a response to remove cancer cells.

  • The creation of a supportive metabolic environment which impairs effective immune responses.

  •  Creates metabolic by-products created in the tumor microenvironment create low oxygen, high lactic acid and low pH which damage mitochondria which creates reactive oxidation stimulating more inflammation.

  • Effects Potassium efflux, causing the movement of potassium from its normal high intracellular levels when cell damage occurs. This then creates ATP loss from the cells which activates inflammasome signaling.

(Chronic inflammatory states uncommonly utilize the inflammasome, and then only episodically and for short durations of time. In many situations the immune system regulates its expression to prevent an excessive response.) 

So the tumor microenvironment can also be directly manipulated, by cancer, producing signals that resemble danger signals that activate the following 

  • It commandeers control of normal wound healing causing persistent cell death, oxidative stress and inflammatory signals preventing local down regulation of inflammation

  • Recruiting of suppressive immune cells

  • Promoting new blood vessel formation to allow cancer cell spread and obtain the nutrients needed to grow

  • Supports epithelial-mesenchymal transition. Normally epithelial cells adhere and contact other cells, and are oriented in a specific direction and are anchored to a basement membrane. In embryologic development and wound healing the body enlists the epithelial-mesenchymal transition, EMT, program to allow epithelial cells to loosen their bonds, change position, become mobile and travel to other areas. There they are able to transition into needed types of cells in that area. In cancer, the altered microenvironment allows this EMT to enable its spread and metastasis.

Cancer cells continue stimulation and activation of this amplified inflammatory response and immune system manipulation resulting in activation of inflammasome causing pyroptosis and becoming a self-sustaining, augmented and persistent inflammatory response, which maintains a microenvironment beneficial for continued cancer survival.

The Inflammasome

DAMAGE SENSORS 

Immune cells have a group of sensors, called nucleotide -binding domain-like receptors (NLR’s) which are complex proteins that are intracellular, especially macrophages and monocytes. NLRP3 is the most important sensor to respond to damage/ injury/cellular distress. 

ADAPTOR

  • The adapter stage is then needed in the assembly the inflammasome. When the NLRP3 sensor becomes activated by the above triggers it changes its shape to interconnect with multiple apoptosis-associated speck -like proteins (ASC) which act to amplify the sensor connection, but also provides an additional binding site called the CARD (caspase recruitment domain) which is then able to bind and activate caspase-1.

  • CASPASE-1 ACTIVATION

This adaptor /caspase-1 complex becomes activated and cleaves inactive interleukin precursors of IL-1B and IL18 making them active proinflammatory cytokines that can be released by the monocytes and macrophages 

The two phases needed to activate the inflammasome are:

Priming is readying the environment for activation being produced by signals of both the normal innate immune system and unresolved chronic inflammation. In acute situations it is a normal preparation,temporary and adaptive. But in cancer and chronic inflammation it is long lasting and promotes continued terrain damage.

  • It increases cytokine proteins to upregulate and increase inflammatory and inflammasome components.

  •  Increases production of inactive inflammasome elements, NLRP3 protein,IL1-b,IL18

  • The tumor creates conditions of of low oxygen, supporting survival and inflammation

  • STAT3 is often active in tumors and promotes genes to suppresses immune actions, increase cell proliferation and maintains chronic inflammation

  • Augments other interconnected pathways that respond to damage 

  • Produces the molecules needed for inflammasome response

With the terrain ready, Triggers then

  • Create the actual inflammasome

  • Activate the adaptor and caspace-1

  • Release IL1-b 

So when the inflammasome is activated,rather than slow ongoing inflammation,the immune system supercharges an amplified escalated response through activation of caspase -1.

This produces pyroptosis.

Pyroptosis is a highly inflammatory response, caused by inflammasome activation which breaks up or lysis the cells exposing cellular contents and stimulating an enhanced continued inflammatory response.

The Paradox

This heightened inflammatory response does kill tumor cells, brings immune cells to the terrain and exposes tumor antigens for attack.

However, the paradox in cancer results in

  • A tumor microenvironment that is persistently inflamed, while at the same time it is immune suppressed. Initially the immune system can attack the tumor with its anti tumor armamentarium, specifically T cells, but when extended for long periods their ability becomes exhausted and weakened.

  • Chronic inflammasome stimulation recruits cytokines that also support the cancer; 

along with tumor associated macrophages(TAMs), that inhibit CD8 T cytotoxic cells and NK along and MDSC cells, immune suppressor cells.

TAMs produce growth factors for the tumor, suppress immune responses and support new blood vessel growth to allow spread.

MDSC cells are the result of the tumor cells signaling the bone marrow causing alteration of their normal immune maturation and acting to suppress natural killer (NK) and cytotoxic T cells.

In this microenvironment a cycle of persistent chronic inflammation develops causing continuous inflammasome production and pyroptosis due to persistent danger signaling. 

In oncology research, the objective for treatment innovation is

  • Targeting the immune cells, mainly the TAMs, to disrupt their signaling can be more effective than targeting the tumor

  • Directly activating the inflammasome within the tumor cells themselves to induce pyroptosis in the cancer cells. 

Current Research

NLRP3 and Pancreatic Ductal AdenoCarcinoma (PDAC)

In pancreatic ductal carcinoma, tumor cell derived proinflammatory IL-1b helps stimulate a tumor micro environment that promotes the transformation of normal cells to cancer cells and at the same time initiates an immunosuppressive terrain with proinflammatory cells; using M2 macrophages, regulatory B cells and T17 helper cells.

Upregulation of IL-1b occurs in several other cancers, head and neck, breast, lung cancers and melanoma, and it is associated with poorer prognosis by sustained activation of NF-kB,which supports cytokine production and mitogen activated kinase (MAPK) pathways which increase cancer proliferation.

The study by Drs Das, Shapiro and Bar-Sagi published in Cancer Research,2020, demonstrated that the pancreatic ductal adenocarcinoma tumor cell derived proinflammatory cytokine IL-1b is necessary for the development of a tumor microenvironment that supports tumor growth. It promotes immunosuppression; mediated by myeloid derived cells, M2, macrophages and inhibits anti-tumor immune reactions. IL-1b also recruits tumor associated macrophages(TAMs )which produce growth factors for the tumor, suppress immune responses and support new blood vessel growth to allow spread. 

Using a mouse model, the researchers showed that the loss of IL-1b signaling in the microenvironment of PDAC allowed CD8 cytotoxic T cells to be activated and enabled enhanced survival of the PDA bearing mice.

Breast Cancer

In animal and human models, Dr Guo and Fu demonstrated in the Journal Scientific Reports that mice made deficient in inflammasome components had diminished breast cancer tumor growth. Also, inflammasome activation of IL-1b promoted infiltration of bone marrow derived myeloid tumor suppressor cells and tumor associated macrophages (TAM) into the tumor microenvironment which would support cancer growth.

To counteract these inflammasome effects that favor tumor growth, their research demonstrated that blocking the IL-1 pathway inhibited effects of tumor growth and spread.

This study by Drs. J In, Fuchs, and others measured IL-1b in tissue extracts from >200 breast cancer patients. IL-1b was detected in 90% of invasive breast cancers and was much higher than in ductal carcinoma in situ (DCIS) and benign lesions. The trend also suggested having more aggressive tumor indicators such as, ER negativity, high grade tumor, p53 positivity.

Their conclusions suggested that IL-1b was found in the microenvironment of most breast cancers and higher levels were associated with tumor aggressiveness and the ability to invade.

Colitis Associated Cancer

A study using a mouse model of colitis demonstrated that the inflammatory neutrophils produce large amounts of IL-1b that is associated with development of colitis associated colon cancer. Decreasing of these neutrophils or the blocking of the IL-1b substantially reduced damage to the mucosal lining and the development of cancer. The conclusion was that in colitis associated cancer, IL-1b promotes the growth of cancer.

Other Research

In a recent focus discussion with oncologists the discussion indicated that a focus of cancer research was in modulating IL-1b. And indeed there is research being funded currently for IL-1b inflammasome blockers to target the inflammatory process in the tumor microenvironment and impact cancer outcomes.

Anakinra, a IL-1 receptor antagonist which while being used in rheumatoid arthritis ,is being studied in combination with other chemotherapy for acute myelogenous leukemia, colorectal and prostate cancer and multiple myeloma.Also early trials for breast and pancreatic cancer are being initiated.

Canakinumab, a monoclonal antibody, IL-1 blocker, is showing effects in reducing mortality in lung cancer patients by slowing tumor growth, reducing inflammation and preventing greater tumor invasiveness.

Currently there are Phase 3 trials, with these drugs, for non-small cell lung cancer combined with Immune checkpoint drugs.

Other early trials are ongoing for triple negative breast cancer and colorectal cancer

The goals of these clinical trials with IL-1b inhibiting drugs are to ascertain 

  • Can they regulate the tumor microenvironment 

  • How do they work with PD-1 immune checkpoint inhibitors

  • Do they help prevent metastasis 

Integrative Perspectives

To reduce this cycle it is most important to treat the primary underlying problems but also prevent the necessary triggers that initiate the cycle. If only one feature, either priming or triggering, is present the overall marked inflammatory response is reduced significantly.

Integrative Approaches to Block Inflammasome Triggering

Curcumin

In multiple studies, curcumin was shown to reduce IL-1b and inhibit caspace-1.

Omega -3 Fatty Acids

A study found omega-3 fatty acids inhibited NLRP-3 activation and decreased inflammatory dysfunction in a mouse model by reducing inflammasome assembly. From the Journal Immunity, June 2013.

Resveratrol

Inhibits NLRP-3 activation and protects mitochondria. Science Direct August 2020 by Drs Olcum, Tastan, Genc and others.

It was also shown in multiple other scientific studies.

Quercetin

In multiple studies quercetin reduced NLRP3 expression and caspase 1 production.

EGCG

In multiple studies,it reduced IL-1b and caspace-1.

Ginseng

Ginseng was shown to reduce IL-1b in two studies.

Sulforaphane

In multiple studies, sulforaphane reduced IL-1b and caspase -1.

It is important to recognize that the tumor microenvironment is not just triggered by the cancer but develops within the body and actually is initiated by earlier disruptions in the terrain that allows cancer development and then supports its growth.

So these supplements, when prescribed by an experienced physician, are a strategy for seeking preventive measures to provide balance in the terrain.