Truth be told, before stumbling upon this abstract (full disclosure, I did not have access to the full article), I had no idea what ZAG was or did; had never heard of it. It turns out, ZAG is a protein discovered about 50 years ago that is involved with lipolysis – fat metabolism. Individuals who are leaner have higher ZAG levels compared to those who carry more weight. ZAG is also involved in cell proliferation and differentiation, cell adhesion, immunoregulation and melanin synthesis (with a potential role in vitiligo). It is ZAG’s role in immune regulation that I find most interesting, but before we get to that, let’s run through a little basic ZAG biochemistry.
What the Heck is ZAG?
The ZAG protein is ubiquitously produced throughout the body but most especially in fat, in the liver, in the brain, prostate, epithelial cells and buccal (cheek) cells and, as a result, ZAG is present in serum, cerebral spinal fluid, seminal fluid, saliva, breast milk, kidney tissues and urine, the lungs and various cancers including breast, prostate and bladder. ZAG activity is controlled by hormones, the immune system and, interestingly enough, polyunsaturated fats and essential fatty acids.
- ZAG is upregulated by glucocorticoids – cortisol (the stress and inflammation hormone), androgens, thyroid hormones (triiodothyronine – T3, in the liver only), and progestins (synthetic progesterone hormones like medroxyprogesterone acetate and likely oral contraceptives, though this is speculative on my part). It is unclear whether ZAG is affected by estradiol or synthetic estrogens. In contrast, endogenous progesterone downregulates ZAG. (Remember, progesterone competes with cortisol at the glucocorticoid receptors, higher progesterone, lower cortisol activation).
- Interferon y (IFN-y) and IL-6 increase ZAG while tumor necrosis factor alpha (TNFα) downregulates ZAG.
- ZAG also controls its own expression, using a feedforward mechanism. In other words, ZAG can make more ZAG.
- Essential fatty acids bind with ZAG and effectively downregulate its production. Along with TNF-α, essential fatty acids and progesterone are the only molecules identified to date, that downregulate ZAG. Remember this information, it will be key later on.
ZAG as a Cancer Marker? Well, Maybe.
ZAG is elevated in many cancers but the relationship between cancer and ZAG is not straightforward. With cancer, ZAG shows a non-linear pattern of expression that is time and tumor-type dependent. In other words, elevated ZAG can be both protective and detrimental depending upon the stage of the disease process. In late stage disease, elevated ZAG is associated with a condition called cancer cachexia, extreme fat and muscle loss initiated by hypermetabolism and anorexia. In the early stages of disease, ZAG expression is elevated as well and that may indicate appropriate immune activation against the cancer, but only with certain types of tumors. Highly differentiated tumors produce more ZAG compared to non-differentiated or metastasized where ZAG is lower and sometimes non-existent.
The complexity and non-linearity of the relationship between ZAG and cancer tells us that ZAG is more of a general marker of inflammation and immune activation than one that can be linked to a specific disease. This is one of the reasons I suspect elevated ZAG may not be the most specific biomarker for endometriosis, but it may point to other mechanisms potentially more diagnostic.
ZAG as an Immunomodulator
ZAG concentrations and activity are elevated during specific types of acute and chronic immune reactions – those that involve what are considered lipidic compounds and require fat metabolism for energy productions, making ZAG an immune modulator of sorts. ZAG appears to be active in disease states where aberrant cell growth and cell adhesions are necessary components and where energy sources are needed to fuel the pathological cell growth and the immune response to that cell growth; diseases like endometriosis and cancer. In contrast, ZAG is less active when there are problems with energy metabolism brought on by high calorie malnutrition, such as in obesity and hypertension or cirrhosis. How might that be possible? What functions could ZAG be responsible for that involve energy usage?
What Does ZAG Do?
This is where ZAG becomes very interesting. ZAG has two functions that I can surmise, as an immunomodulator and mediator of mitochondrial ROS. Firstly, ZAG is a protective immunomodulator, somewhat similar to the major histocompatibility complex molecules (MHC -1), CD1 antigen presenting proteins. ZAG presents specific types of lipidic compounds to T cells for destruction. It also appears to vary its function based upon the ligand that it is bound to, making its role in immunomodulation difficult to discern fully. Nevertheless, this is a huge clue. ZAG binds to toxicants and presents them to immune cells for destruction. This tells us that the diseases where ZAG is elevated involve fat soluble toxicants of some sort. Speculatively, one might place many synthetic environmental and pharmacological hormones into this category.
Could cancer and endometriosis be diseases of specific exposures? It’s an easy jump, but perhaps not a complete one. So what if ZAG binds to and presents lipidic compounds to immune cells for destruction? That doesn’t explain why we get the aberrant cell growth and cell adhesion that form the basis for tumors and endometrial implants. It also doesn’t explain how fat metabolism is involved. For that we need to look at ZAG’s second function and its role in mitochondrial regulation. (Yes, we’re back to the mighty mitochondria, the energy producing factories that power every cell in the body, and one of my favorite topics).
ZAG and Mitochondrial Function
ZAG tempers mitochondrial ROS production via an association with a set of mitochondrial uncoupling proteins (UCPs). Here it gets a bit complicated again. Recall, a common mechanism by which many toxicants, environmental and pharmaceutical, damage mitochondria is via increased reactive oxygen species or ROS production. (ROS are the cell damaging free radicals for which the array of anti-oxidant products are marketed to protect against). Whenever the mitochondria produce energy for cellular activity, ROS is a necessary byproduct, but too much ROS can be deadly and not enough ROS is also problematic. Thus, ROS production is kept in tight regulation by a number of interleaving mechanisms. ZAG appears to be one of those mechanisms but only during specific circumstances and, as of right now, only via its association with UCPs.
Mitochondrial Uncoupling Proteins – Mito What?
There are three types of mitochondrial uncoupling proteins: UCP1, UCP2, UCP3. UCP1 is involved in thermogenesis and lives mainly in brown adipose tissue or brown fat and hence, its association with weight loss. UCP2 is more ubiquitous and highly expressed in the lymphoid system, macrophages, and pancreatic islets. UCP3 is mainly expressed in skeletal muscles. Activation of UCP1 increases mitochondrial respiration fast oxygen consumption dissipation of energy as heat. In contrast, UCP2 and UCP3 reduce mitochondrial ROS production by effectively slowing down the dissipation of energy via regulating insulin secretion and fatty acid metabolism, respectively, but also via ZAG-UCP binding. ZAG increases the expression of UCP2 and UCP3, that when activated, slow down ROS production (via limiting Ca2+ uptake) and protect mitochondrial and cell functioning. Here’s the kicker, guess what activates UCP2 and UCP3? Fatty acids; the same molecules that de-activate ZAG. Guess what likely competes with fatty acids for ZAG binding (and possibly UCP binding)? That’s right, synthetic hormones – think environmental or pharmacological endocrine disruptors.
A Hypothesis Emerges: ZAG in Endometriosis
So here, in theory, we have a complex system that both activates and deactivates an immune response based upon the availability of fatty acids – nutrients – relative to lipidic toxicants. The degree to which ZAG becomes overactive depends upon upon the balance between that which activates it, lipidic toxicants, and that which deactivates it, essential fatty acids. ZAG is active when it is protecting the body from toxicants, either via direct presentation of antigens or via UCP to uncouple the production of ROS and reduce mitochondrial damage. UCPs can be activated without ZAG and reduce ROS damage but only in the presence of essential fatty acids. And so, the essential fatty acids are key to shuttling or balancing a ZAG mediated immune response to lipidic toxicants while maintaining mitochondrial integrity. A pretty cool little system.
There’s even more to this story, including potential shifts in mitochondrial oxidation pathways relative to toxicant versus fatty acid availability, the role of thyroid hormones in mediating these shifts, and interestingly enough, a role for the hypocretin/orexin system as the brain sensors in the whole process; all to be covered in subsequent posts. And of course, we have yet to address the problem with the aberrant cell proliferation and adhesion notable in endometriosis or cancer (teaser – I think this is related to mitochondrial mechanisms as well). For now though, consider what ZAG tells us about cancer and endometriosis. Elevated ZAG is a marker of disease processes that involve problems with fat metabolism; speculatively, disease processes that involve an imbalance between certain types of fat soluble toxins and fatty acids. That’s a cool discovery; one that might just tell us how to fight these diseases.
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This article was first published on October 29, 2014.