Free Fatty Acids and Cytokines Induce Pancreatic ß-Cell Apoptosis by Different Mechanisms

Free Fatty Acids and Cytokines Induce Pancreatic ß-Cell Apoptosis by Different Mechanisms

(I've scrubbed the distracting reference numbers from some excerpts of the introduction and I'm also going to try to cite only the information pertinent to T2)

Hypercaloric diets containing large amounts of fat, also called the Western diet, contribute to a major extent to the increasing prevalence of obesity and type 2 diabetes mellitus (T2DM). T2DM is characterized by peripheral insulin resistance, pancreatic ß-cell dysfunction, and decreased ß-cell mass associated with increased rates of ß-cell apoptosis. Elevated levels of circulating free fatty acids (FFAs) contribute to the pathogenesis of T2DM. High concentrations of FFAs lead to both impairment of insulin action and ß-cell dysfunction.  Moreover, FFAs have been shown to cause ß -cell death, mainly by apoptosis. 

Of note, increased adiposity is associated not only with increased FFA release but also with adipocyte secretion of a variety of cytokines and cytokine-like adipokines, such as TNF -α, IL-6, leptin, resistin, and adiponectin. TNF-α has direct cytotoxic effects on pancreatic ß -cells, especially in combination with other cytokines.

In comments on a recent post here, Ned Kock of Health Correlator blog shared that he and I agree in our rejection of the whole "tired pancreas" theory.  He disagreed with my belief that free (or non-esterified)  fatty acids (FFA, I tend to use the acronym NEFA) are responsible for ß-cell apoptosis, fingering the adipokines like TNF-α instead.  This article would seem to support both theories.  The ß-cells are perhaps killed off by more than one mechanism.  One not necessarily coming before the other as well.

The Cytokine Mechanisms  (I'll paraphrase from the intro):

Cytokines (adipokines like TNF-α) activate nuclear factor-κB, NF-κB.  What NF-κB does is that in increases the transcription (expression) of certain proteins.  In other words, NF-κB is responsible for increasing or decreasing certain peptides involved in the functioning of cells.  From the intro, this activation leads to:

Loss of differentiated ß-cell functions by down-regulation of Isl-1 and pancreatic duodenal homeobox-1 (Pdx-1) 

Isl-1 and Pdx-1 are factors involved in the differentiation of  ß-cells.   In Baby Your Pancreas I, the cycling of ß-cells was discussed, whereby new mature (differentiated) functional ß-cells are created from progenitor cells.  Thus it would appear that this mechanism is essentially a reversal of that process.  Essentially, the ß-cells cease being ß-cells. 

Up-regulation of inducible nitric oxide synthase (iNOS) and excessive NO production

Nitric oxide (NO) has beneficial actions in the body, but excesses appear to interfere with mitochondrial function.  The mitochondria being the ultimate sites of ATP production necessary for all functions of "life". 

Up-regulation of chemokines such as monocyte chemoattractant protein-1 (MCP-1)

MCP-1 recruits monocytes, it's the initiator of autoimmune attack.

Down-regulation of the Ca⁺² pump sarcoendoplasmic reticulum Ca⁺² ATPase type 2b (SERCA-2b).  Decreased SERCA-2b expression leads to endoplasmic reticulum (ER) calcium depletion and ER stress 

The endoplasmic reticulum (ER) is the compartment in the ß-cells where proteins that are synthesized are "assembled".  Insulin, like all hormones, is a protein.  A significant characteristic of proteins for various functions is that these are large molecules that fold back on themselves in many ways so that they have a unique geometry with sites for enzymatic action and such.  One need not understand the nitty gritty of how the ER functions and facilitates insulin production in the ß-cells to understand that stressing this "organelle" leads to dysfunction and will thus impair insulin production/secretion.  The intro cites several possible mechanisms for those interested.

The Free Fatty Acid (FFA/NEFA) Mechanisms:

The mechanisms involved in FFA-induced ß-cell apoptosis in T2DM remain to be clarified. We have previously shown that FFA cytotoxicity is inversely related with cytoplasmic triglyceride accumulation. This suggests that cytoplasmic accumulation of fatty acyl-CoA is directly ß-cell toxic, whereas their esterification probably functions as a protective mechanism. 

I've spent a lot of time on this blog discussing lipotoxicity -- the interest in which came from my apparently unrelated concerns over NEFA and my racing heart issues.  Indeed at times I feel like this blog is evolving into a T2 diabetes blog which must seem odd to many considering that I'm not diabetic myself, nor does it run in my family.  Still, the IR that is associated with elevated NEFA -- such that almost everyone carrying considerable excess fat have some degree of IR in various forms -- will likely have me continuing down this research road for the foreseeable future.  Recently I've been having this "hunch" that the esterification of fatty acids into triglycerides is actually protective to the cells -- at least initially.  I've got a whole backlog of information on "lipid droplets" that are actually described as "organelles"  (essentially discrete bodies within the cells performing specific functions in the cells, analogous to organs composed of specialized cells in the complete multicellular organism).  Whether we prefer this aesthetically is another question, but clearly our bodies are healthiest when the majority of lipids are stored in our fat cells.  In cases of excessive nutrient delivery to our cells (glucose + NEFA) the cells are best served by storing the fatty acids locally as triglycerides rather than the apparently more "bioactive" free fatty acid form.  It seems to me that when we exceed the normal capacity for this in our cells, this protective form becomes again toxic.  Enough editorializing for now :-)

Increased ß-cell FFA levels might lead to de novo ceramide formation and mitochondrial cytochrome C release. 

In addition to the diacyl-CoA's, ceramides have been implicated as being the actual "agents" of lipotoxicity.  I think perhaps it's time to call this "Phase I LT" or something.  It's kind of like how your car engine doesn't run as efficiently when you haven't changed the oil in 10K miles and are putting gas in the tank that's been sitting in your gas can in the garage for a few years.  The car still runs, but ...  It is also these entities that are most consistently implicated in the fatty acid mediated peripheral (skeletal muscle) insulin resistance.  Unger implicates ceramide in apoptosis, but perhaps here's the confusion from a continuum.  In the car analogy, the engine may run less and less efficiently but perhaps it's something else that ultimately causes the vehicle to fail to start at some point.  So ... the authors continue on and describe other mechanisms for fatty acids inducing apoptosis:

Other proposed mechanisms for FFA-induced ß-cell death are the activation of protein kinase C, inhibition of protein kinase B activity, and activation of calpain-10. 

Global gene expression in FFA-treated ß-cells  indicate similarities between several of the genes induced by FFAs and those observed downstream of cytokine-induced NF-κB, including ornithine decarboxylase, b-2 microglobulin, DNA-binding protein A, and MCP-1. In line with these observations, a recent study reported that palmitate induces a pronounced ( 10-fold) NF-κB activation in insulinoma cells, comparable with the activation observed with TNF -α .  

Based on these and additional observations obtained in islets cultured at high glucose concentrations, it has been proposed that prolonged exposure to excessive concentrations of nutrients results in a proinflammatory ß-cell response, contributing to ß-cell damage and death in T2DM.

A unifying hypothesis has also been suggested for the mechanisms of nutrient- and cytokine-induced ß-cell death in T1DM and T2DM, in which activation of NF-κB is a common and crucial step for both proapoptotic stimuli.

The Current Study: 

Against this background, the aim of this study was to clarify whether common mechanisms are indeed involved in FFA- and cytokine-induced ß-cell apoptosis. The questions we asked were:

1) Is FFA-induced apoptosis associated with the activation of the transcription factor NF-κB and the induction of its downstream genes iNOS and MCP-1, as is the case for cytokines

2) Does TNF -α potentiate the toxic effects?  {See why I immediately thought of Ned?  :-) }

I could go into the experiments conducted, but I think it best to leave that to those who are interested in those details.  I would point out that the experiments were in vitro ("test tube") studies on cultured rat islets from normal Wistar rats and the INS-1E cell line (insulinoma-derived that secrete insulin in response to glucose in culture).

Their conclusions were:

1. Cytokines, but not FFAs, induce expression of the NF-κB -dependent genes iNOS and MCP-1
2. Cytokines, but not FFAs, induce NF-κB  activation
3. FFAs do not potentiate TNF-α -induced NF-κB  activation
4. Both FFAs and cytokines induce ER stress in INS-1E cells, but ER stress seems to be triggered by different mechanisms. 

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These observations indicate that FFAs and cytokines lead to ß-cell death by fundamentally distinct mechanisms, namely an NF-κB -dependent mechanism that culminates in caspase-3 activation in the case of cytokines and an NF-κB-independent mechanism in the case of FFAs.

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What follows is purely just thinking out loud.  Type 1 diabetes is a massive kill-off of the beta cells and - it would appear - the progenitor cells.    Type 2 is the combination of suppression of beta cell function, the slower kill-off of the beta cells in excess of creation of new cells through differentiation of progenitors.  In many ways it seems to me that T2 can be broken down into phases that most assuredly have no defined "boundaries" and overlap.  And for 80% or so of T2's, it is obesity that sets things in motion.  So Phase I is exceeding some threshold for proper fat storage that leads fat to become "sick" -- sick fat is insulin resistant and releases inappropriate amounts of NEFA into circulation.  It is also infiltrated with macrophages and secretes a bunch of inflammatory adipokines.  Whether the NEFA or adipokines come first is one of those futile chicken-egg debates in my opinion.   They may pretty much occur simultaneously and feedback and exacerbate each other.  Still it seems that we can define a Phase II which would be early in the diagnosis of frank diabetes.  In this phase the beta cell mass is still normal or may even be greater than normal from the compensation to counteract IR.  But the cells lose their ability to mount the acute glucose stimulated insulin response.  This seems to be initiated by lipotoxicity, perhaps the "gunking of the works" by the diacyl-CoA and ceramide moieties.  Once hyperglycemia develops, it makes the lipids more toxic and we have glucolipotoxicity.  This is something I also have a backlog of papers on -- essentially the lipids are toxic but not catastrophically so until they exert a potent enough effect to cause hyperglycemia.  In the presence of hyperglycemia it's like fatty acids on crack (or roids).   If this situation persists -- and here is where there definitely seems to be a "not much you can do about it" thing -- some are more predisposed to this due to some genetic composition -- things progress to a Phase III where beta cell mass is lost and eventually the person requires insulin therapy.  My hunch is that the Phase III is probably driven far more by the adipokine apoptosis than the fatty acid path.  But the fatty acid path must be traveled to get there to begin with.  Again ... a hunch ... and further qualified by stressing that the lines between phases would be very, very blurry.