Arsenic interferes with the ability of human fat cells to regulate their blood sugar, according to new research. The effect is evident at exposure levels below what is necessary for overt toxicity. This result may help explain how the heavy metal contributes to type II diabetes, a chronic, life-changing disease.

In this new paper, Paul et al. show how exposure to two forms of arsenic can alter the cell signaling pathway that enables insulin to regulate the uptake of glucose into the cell. The authors have identified the signaling pathway affected by arsenic and deciphered how seemingly small changes impair the cell’s glucose regulating system. The miscommunication that results because of those changes keeps the cell from importing the energy rich sugar it needs as fuel, leaving too much glucose in the blood.

Context
What did they do?
What did they find?
What does it mean?
Resources

Context

The number of Americans with diabetes has doubled in the last 25 years according to the U.S. Centers for Disease Control and Prevention. Causes for this staggering rise are not known but risk factors for the more prevalent type II diabetes include diet, age, obesity, lifestyle and ethnicity. Lab research and human epidemiological studies are gradually revealing that environmental contaminants, such as persistent organic pollutants and bisphenol A may also play a role in this skyrocketing disease.

Epidemiologists have reported associations between arsenic exposure and diabetes in the past, but the results have been inconsistent and inconclusive (Longnecker and Daniels 2001, Navas-Cien et al. 2005).

Arsenic is a naturally occurring toxic metal and a potent human carcinogen. Much research has been conducted showing a correlation between arsenic exposure and several forms of cancer, including bladder, lung and skin (Lasky et al. 2007). In contrast, far less is known about the nonmalignant effects of arsenic, for example its association with heart disease (Mumford et al. 2007). Now research is emerging demonstrating that at low, nonlethal concentrations, arsenic can also disrupt normal endocrine system function (for example, Kaltreider et al. 2001, Davey et al. 2007).

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Type II Diabetes

A complex collection of hormonal feedback loops control how much glucose is circulating in the blood. Insulin is a key member of the team of signaling molecules in this process: When glucose is detected by the pancreas, it stimulates the pancrease to secrete insulin; higher glucose means more insulin. The insulin then causes muscle and fat cells to absorb glucose, bringing glucose levels back down.

People with Type II diabetes don't respond normally to insulin. At first they become 'insulin resistant.' A given amount of insulin doesn't stimulate as much glucose uptake as it used to. The pancreas responds to this by producing more insulin, enough to stimulate the fat and muscle cells to take up more glucose. Over time, their resistance to insulin increases until finally their pancreas can't produce enough insulin to bring the glucose levels down to the target range. The uncontrollable rise in blood sugar then causes Type II diabetes.

In contrast, type I diabetics do not make their own insulin and so must take insulin to regulate blood glucose.

What did they do?

In this study, Paul et al. exposed cells from a widely used line of human fat cells (called 3T3-L1) to nontoxic concentrations of two forms of arsenic, arsenite (AsIII) and methylarsonous acid (MAsIII). Arsenic can be metabolized, or changed in the body, into a number of forms, including AsIII and MAsIII. AsIII and MAsIII were previously shown by these researchers to interfere with the communication pathway that links insulin with glucose uptake into the fat cells (see box to right).

The authors began by finding the concentration of AsIII (50 µM) and MAsIII (2 µM) and the exposure period (4 hours) at which insulin dependent glucose uptake by fat cells was inhibited but not by harming the cell. These arsenic concentrations and exposure period were used in all subsequent experiments.

To understand how the heavy metal alters this crucial cellular messaging system, they measured the impact of arsenic exposure on glucose uptake by the cells, the number of glucose transporters (molecules that help glucose get through the cell membrane) and the activity of signaling pathway proteins.

What did they find?

Paul et al. knew from previous research the role of the insulin receptor, glucose transporter and one other cellular player, PKB/Akt. The activation of PKB/Akt by the addition of phosphate groups by the enzymes PDK-1 and PDK-2 induces GLUT4 translocation to the cell membrane and increases import of glucose. AsIII and MAsIII are both known to suppress the activity of PKB/Akt. This universally important cell protein is an essential part of the insulin signaling pathway that governs glucose uptake by the cell.

But how do the two forms of arsenic interfere with its function? To find out, Paul et al. carefully worked through the signaling pathway, finding out where along the way, between the initial arrival of insulin and the final importation of glucose, they could detect changes in protein activity caused by exposure to arsenic.

First, they showed that treating fat cells with these nontoxic concentrations of the arsenics did not impact the concentrations of the phospholipid PIP3 within the cells. Insulin binding leads to an increase in this phospholipid, which in turn leads to activation of two proteins, PDK-1 and PDK-2 (more detail). The fact that PIP3 levels were not affected means that the impact of arsenic takes place in this signaling chain after PIP3.

Insulin signaling

A signaling pathway is similar to a dramatic play, comprised of players (receptors, enzymes and transporter proteins within the cell) and the script (the order in which the cellular players interact to cause some biological effect).

The insulin signaling pathway is complex. Diagrammed below, it consists of a group of proteins that sense insulin in the blood and induce the cells to import glucose from the blood into the cell. These proteins, insulin receptors and glucose transporters in the cell membrane as well as other proteins inside the cell, transfer the message that insulin is present in the blood and so glucose should be taken up by the cell.

Click for an expanded view; Adapted from Paul et al.

In the insulin signaling pathway, diagrammed above, the players consist of insulin itself, the insulin receptor, a series of enzymes and the glucose transporter proteins (GLUT4). In the short version of the play, insulin in the blood binds with its receptor on the surface of the fat cell and through the interaction of a series of enzymes, a phospholipid and phosphate, causes the glucose transporter to move to the cell membrane and transport glucose from the blood into the cell. This increases glucose in the cell and decreases it in the blood.

They used an experiment with radioactively labeled glucose (graph to left) to measure glucose uptake in control and arsenic-treated cells. This experiment involved 3 types of cells: Normal cells (labeled normal in graph to left), a cell type with a mutation that prevents continuous production of activated PBK/AKt (labeled PBK/Akt blocked), and a cell type that always produces PBK/Akt (labeled PBK/Akt continuous).

In the control without insulin, the 'PBK/Akt continuous' cells took in glucose, demonstrating that PBK/Akt activation was sufficient for glucose transport. Normal cells didn't import glucose.

In experiments with insulin, cells without arsenic transported glucose effectively. In the normal fat cell line and PBK/Akt blocked, arsenic reduces the cells ability to import glucose. In PBK/Akt continuous, arsenic treatment has no effect on glucose uptake (black bars). Taken together, these comparisons demonstrate the critical role of activated PBK/Akt.

Because the experiment was conducted at only one dose level--determined to be sufficient to impair glucose regulation without causing overt toxicity--the results do not provide insights into how low a dose is enough to interfere with insulin activity. More work will be necessary to establish a dose-response relationship. The results are certainly relevant to occupational exposure to arsenic and to places in the world where water sources are heavily contaminated by the metal. Previous research with arsenic, however, has shown it is capable of altering glucocorticoid action at levels well beneath those used in this experiment.

Given the increase in numbers of people with type II diabetes and the damaging effects of the disease on the physical and emotional health of patients, this research brings to light the need to reduce the environmental exposure to arsenic.

Resources:

Davey, JC, JE Bodwell, JA Gosse and JW Hamilton. 2007. Arsenic as an endocrine disruptor: Effects of arsenic on estrogen receptor-mediated gene expression in vivo and in cell culture. Toxicological Sciences. Published online Feb. 5, 2007.

Kaltreider, RC, AM. Davis, JP Lariviere, and JW Hamilton 2001. Arsenic Alters the Function of the Glucocorticoid Receptor as a Transcription Factor. Environmental Health Perspectives 109:245-251.

Lasky, T, S Wenyu, A Kadry and MK Hoffman. 2004. Mean total arsenic concentrations in chicken 1989 – 2000 and estimated exposures for consumers of chicken. Environmental Health Perspectives 112:18–21.

Longnecker, MP and JL Daniels. 2001. Environmental Contaminants as Etiologic Factors for Diabetes. Environmental Health Perspectives 109(suppl 6):871-876.

Mumford, J, K Wu, Y Xia, R Kwok, Z Yang, J Foster and WE Sanders, Jr. 2007. Chronic arsenic exposure and cardiac repolarization abnormalities with QT interval prolongation in a population-based study. Environmental Health Perspectives. Published online Feb. 14, 2007.

Nachman, KE, JP Graham, LB Price and EK Silbergeld. 2005. Arsenic: A roadblock to potential animal waste management solutions. Environmental Health Perspectives 113:1123–1124.