Introduction
This article reveals the unexpected effect of bone morphogenic protein-binding endothelial regulator (BMPER) in enhancing insulin sensitivity and explores the possible mechanism of its action. The article concludes that BMPER may potentially be used for the treatment of diabetes in the future.
Molecular mechanisms of the hormone insulin
To understand the following article, it must first be understood how insulin acts as a chemical messenger to decrease blood glucose levels. Shortly after eating, food is broken down in the digestive system into their fundamental building blocks. Sugars in food are broken down into monosaccharides. These simple sugars are diffused into blood capillaries in the small intestines and enter blood circulation, thus causing a rise in blood glucose level, a condition known as hyperglycaemia. Hyperglycaemia may cause various complications that are potentially life threatening. This is where insulin plays a crucial role in regulating blood glucose level.
When blood glucose level rises, a type of cells in the pancreas called beta cells sense these changes and respond by secreting insulin. insulin moves along the blood and binds to insulin receptors (IR) found on the cell membrane of target cells. Generally, the binding of insulin to IR causes autophosphorylation of the IR and subsequently the phosphorylation of proteins inside the cell known as insulin receptor substrates (IRS). (Phosphorylation refers to the attachment of phosphate groups onto molecules which may change their conformation and thus their interactions with other molecules.) The phosphorylated IRS will then activate a signal transduction cascade that ultimately leads to desired effects of target cells, which will be explored in the following paragraphs.
Since the molecular mechanisms of insulin, as for any signalling molecule, are extremely complicated, we will focus on the pathway that leads to increased glucose uptake of cells from the bloodstream. The binding of insulin to its receptor activates IRS1 which activates the enzyme PI3K, PI3K converts PIP2, which is phospholipid from the cell membrane, into PIP3. PIP3 then phosphorylates another enzyme PKB which facilitates the insertion of a glucose transporter GLUT4 into the cell membrane. Finally, GLUT4 increases the permeability of the cell to glucose, thus leading to glucose uptake into cells from the bloodstream and the subsequent decrease of blood glucose levels.1 Some of these molecules will be used to validate the activation of the IR signalling pathway.
In addition, through various intracellular mechanisms, insulin not only promotes fat and glycogen synthesis from glucose, but also inhibits the breakdown of fats of body tissues or glycogen in the liver into glucose. This can ensure that glucose taken up in cells is stored properly and not quickly released into the bloodstream again.
About Type 2 Diabetes Mellitus
Diabetes Mellitus (DM) is a disorder characterised by chronic hyperglycaemia, common symptoms include thirst, excessive urination, increased appetite, weight loss, lethargy, poor wound healing and infections.2 Type 2 Diabetes Mellitus (T2DM) is the most common type of DM, accounting for approximately 90-95% of DM cases.3 It refers to a specific type of diabetes caused by the inability of cells to respond to insulin, or insulin resistance.4 Patients with T2DM may have normal or even very high insulin levels in blood, however, insulin fails to regulate their blood glucose level. The pathogenesis of T2DM is not completely understood but it is generally believed that obesity, sedentary lifestyle, family history of diabetes, various
health conditions and certain medications may potentially contribute to insulin resistance. 5
BMPER and endothelial cells
Bone morphogenic protein-binding endothelial regulator (BMPER) is a type of protein produced in endothelial cells (EC) which make up the walls of blood vessels. BMPER serves mainly as an inhibitor of bone morphogenetic protein (BMP) function, another protein which regulates the maintenance of endothelial structure as well as vascular inflammatory responses.6 Although the effect of BMPER in insulin resistance was never studied before, endothelial dysfunction and inflammation have been observed during the development of T2DM, suggesting a possible role of ECs in glucose homeostasis.
The loss of BMPER leads to glucose dysregulation
To study the effects of loss of BMPER, two types of mice were first generated using genetic engineering methods such as CRISPR and Cre-Lox recombination (not to be discussed in this article). The BMPER gene which codes for the protein BMPER was made inactive in a group of mice named BMPER inducible knockout (iKO) mice. Another type of mice which retained the functional BMPER gene was called wild-type (WT) mice, it served as a control.
After confirming the identities of WT and iKO mice by measuring their BMPER levels in blood plasma as well as in a variety of tissues (iKO mice have lower BMPER levels), a series of experiments were performed on them. Firstly, insulin levels in plasma were much higher in iKO mice than in WT mice, a condition known as hyperinsulinemia. Hyperinsulinemia is very likely the result of insulin resistance. When the body is resistant to insulin, the secretion of insulin from the pancreas fails to control blood glucose level, the pancreas thus produces more insulin to try to compensate for the weakened effect of the hormone, leading to hyperinsulinemia.
To confirm the above hypothesis, the glucose and insulin were injected separately into the blood of both types of mice. When glucose was injected, a longer time is required for glucose to return to normal levels in iKO mice, showing glucose intolerance in iKO mice. Similarly, when insulin is injected, a smaller decrease in blood glucose level was exhibited in iKO mice, reflecting the inability of the iKO mice to respond to insulin, implying insulin resistance.
Furthermore, hyperinsulinemic-euglycemic clamp studies were performed in mice for quantitative analysis. In this study, insulin was continuously infused to maintain a certain high concentration of insulin in plasma, glucose concentration was also held at a constant normal level by continuous glucose infusion. When the steady state is achieved, the glucose infusion rate equals the rate of uptake of glucose in the body. The infusion rate was 24 arbitrary units in iKO mice as opposed to 10 in WT mice. (fig. 1) More specific parameters are also measured, the glucose production in the liver was higher in iKO in mice while glucose uptake in muscles, adipose tissues and the heart were lower in iKO mice. The above results reflect decreased sensitivity of insulin in the liver and peripheral tissues.
Fig. 1: Glucose infusion rate (GIR) of wild type (WT) and BMPER inducible knockout (iKO) mice in a hyperinsulinemic-euglycemic clamp study
Since the BMPER gene is highly expressed in endothelial cells (EC), meaning that it is often used to code for proteins, researchers hypothesized that EC is an important source for BMPER production. To test the hypothesis, scientists generated another type of mice with defective BMPER genes only in the EC, they are called endothelium-specific BMPER inducible knockout (eKO) mice. The experiments above were repeated and eKO mice show very similar results to iKO mice, suggesting that EC is an important source of BMPER in the body.
Next, ECs were tested under metabolic stress, a condition of excess nutrient uptake which simulates the unhealthy diet of diabetic patients. ECs were treated with high glucose, advanced glycation end products (AGEs), or palmitic acids, all of which are molecules synthesized under metabolic stress. All experiments resulted in the decrease in production and secretion of BMPER, suggesting a correlation between metabolic stress and decreased BMPER levels.
Deriving the pathway of action of BMPER
Although evidence shows that BMPER regulates insulin sensitivity, the underlying molecular mechanisms remain unclear. Upon investigation, researchers noticed that iKO mice showed decreased phosphorylation of IRS1 and PKB. If you can still recall the signalling pathway of insulin, IRS1 and PKB are involved in the downstream pathway of insulin receptors (IR), therefore BMPER may activate the insulin signalling pathway through IR. In fact, it was later found that IRS1 phosphorylation induced by BMPER or insulin was blocked in liver cells isolated from mice which are unable to synthesize IR, which is consistent with the hypothesis.
However, one question remains unresolved, BMPER does not directly bind to IR, which raises queries on how can BMPER modulate blood glucose through IR. To answer this question, scientists tested different proteins with BMPER in an attempt to spot proteins that interact with BMPER. Finally, they identified another protein, Niemann-Pick C1 (NPC1), as an interacting protein of BMPER. Following the identification of NPC1, a series of studies were performed in liver cells. It was found that BMPER that were introduced into the system interacted with NPC1 molecules produced by liver cells, and that they were found together mainly in vesicles inside of the cells. When the NPC1 gene was inactivated in cells, BMPER could not induce phosphorylation of IRS1 while insulin can. Therefore, NPC1 specifically mediates BMPER-promoted, but not insulin-promoted insulin signalling. Even though BMPER does not bind to IR, BMPER, IR and NPC1 are still found in the same complex in cells, the above evidence suggests that NPC1 may serve as a scaffold protein – a protein that simultaneously binds to both BMPER and IR, for the interaction of the two molecules.
In previous studies, BMPER can be endocytosed by cells. In the liver cells used in the experiment, BMPER was also endocytosed before exhibiting its effect. When chlorpromazine (CPM), an inhibitor of a certain type of endocytosis was administered, BMPER cannot be endocytosed and it the phosphorylation of IRS1 was not observed. Thus, endocytosis of BMPER into cells is a crucial step for the activation of the IR pathway by BMPER.
BMPER supplementation alleviates glucose dysregulation
Since loss of BMPER resulted in defective glucose metabolism in mice, it is normal to wonder if BMPER can cure mice with defective glucose metabolism. In an experiment, scientists produced genetically engineered mice that can synthesize more BMPER with the help of adeno-associated viruses (not to be discussed in this article), it was found that plasma insulin and glucose levels were decreased in this group of mice compared to normal mice. Then, a type of diabetic mice with severe insulin resistance was used, it initially had significantly lower BMPER levels compared to normal mice but genetic engineering successfully recovered BMPER levels in diabetic mice, insulin and glucose levels also returned to normal levels. Polyuria (abnormally large amount of urine) and glycosuria (abnormally high levels of glucose in urine) are typical symptoms of T2DM. A significant decrease in urine volume and urinary glucose was observed in the genetically engineered diabetic mice. Not surprisingly, similar results were observed when BMPER protein is directly injected into mice.
It should also be noted that the using the above methods cannot alleviate insulin resistance in mice which fail to synthesize IR because BMPER acts with the help of IR to control blood glucose level.
Conclusion
This article uncovers the role of BMPER in glucose metabolism. When loss-of-function experiments were performed, insulin resistance and the subsequent inability to regulate blood glucose was observed. When gain-of-function experiments were performed, BMPER decreased blood glucose in mice, an effect similar to that of insulin. Moreover, BMPER acts in the IR pathway, the interaction between BMPER and NPC1 and its endocytosis into cells are essential steps for it to exhibit its effects on insulin sensitivity.
The endothelium (or the wall of blood vessels) tightly controls material exchange between the blood and tissues, yet its role in the development of T2DM is not well understood. The above study shows that metabolic stress decreases the production of BMPER in ECs, which may subsequently lead to insulin resistance and the progression of T2DM.
Discussion
Currently, there are many different types of drugs prescribed to patients with type 2 diabetes mellitus, such as metformin (biguanides), sulfonylureas and glinides. They act by various different mechanisms to lower blood glucose levels. For instance, metformin increases insulin sensitivity and decreases production of glucose in the liver, while sulfonylureas and glinides stimulate the pancreas to secrete more insulin. Unfortunately, these drugs are all associated with side effects, metformin may cause nausea or diarrhoea while sulfonylureas may cause weight gain or hypoglycaemia (a condition of too low blood glucose levels that may lead to unconsciousness). Therefore, it is generally recommended to tackle T2DM with lifestyle changes instead of medication.7, 8, 9
In this study, when diabetic mice are genetically engineered to produce more BMPER or are directly injected with BMPER, their blood glucose returned to normal levels. BMPER also alleviated some common symptoms of T2DM, such as polyuria and glycosuria. The mice remained healthy and no obvious signs of other health complications were observed. It can be concluded that BMPER exhibits therapeutic potential in treating T2DM.
An important point to take note of is that these experiments are performed on mice. Although humans and mice are very similar biologically, they may still have slight physiological differences. While BMPER supplementation brought diabetes in mice into remission, it may or may not work in humans. Its side effects on humans are also unknown. Further investigation must be done to study the effects of BMPER before performing clinical trials on humans. With that said, BMPER supplementation through gene therapy or protein delivery may still be a potential approach to treat T2DM and insulin resistance in humans, possibly saving countless number of lives troubled by diabetes.
Article written based on: Mao, H., Li, L., Fan, Q., Angelini, A., Saha, P., Wu, H., . . . Pi, X. (2021, March 26). Loss of bone morphogenetic protein-binding endothelial regulator causes insulin resistance. Retrieved April 02, 2021, from https://www.nature.com/articles/s41467-021-22130-2
Other References
Taniguchi, C., Emanuelli, B., & Kahn, C. (2006, February 1). Critical nodes in signalling pathways: Insights into insulin action. Retrieved April 02, 2021, from https://www.nature.com/articles/nrm1837?proof=t
Centre for Health Protection. (2020, November 10). Centre for HEALTH protection, Department of health - diabetes mellitus. Retrieved April 02, 2021, from https://www.chp.gov.hk/en/healthtopics/content/25/59.html
MedicalNewsToday. (2019, April 1). Type 2 DIABETES statistics: Facts and trends. Retrieved April 02, 2021, from https://www.medicalnewstoday.com/articles/318472#Diabetes-and-ethnicity
Mayo Clinic Staff. (2021, January 20). Type 2 diabetes. Retrieved April 02, 2021, from https://www.mayoclinic.org/diseases-conditions/type-2-diabetes/symptoms-causes/syc-20351193
Wu, Y., Ding, Y., Tanaka, Y., & Zhang, W. (2014, September 6). Risk factors contributing to type 2 diabetes and recent advances in the treatment and prevention. Retrieved April 02, 2021, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4166864/
Moser M;Binder O;Wu Y;Aitsebaomo J;Ren R;Bode C;Bautch VL;Conlon FL;Patterson C;. (2003, August 23). BMPER, a NOVEL endothelial cell PRECURSOR-DERIVED PROTEIN, ANTAGONIZES bone morphogenetic Protein signaling and endothelial cell differentiation. Retrieved April 02, 2021, from https://pubmed.ncbi.nlm.nih.gov/12897139/
MedicalNewsToday. (2019, March 26). Can diabetes be cured? A review of therapies and lifestyle changes. Retrieved April 04, 2021, from https://www.medicalnewstoday.com/articles/317074#Managing-type-2-diabetes
Mayo Clinic Staff. (2021, January 20). Type 2 diabetes. Retrieved April 04, 2021, from https://www.mayoclinic.org/diseases-conditions/type-2-diabetes/diagnosis-treatment/drc-20351199
Drug Office, Department of Health. (2012, January). Information on diabetic medications. Retrieved April 04, 2021, from https://www.drugoffice.gov.hk/eps/do/en/consumer/news_informations/dm_01.html
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