Past Studies, Cellular Mechanisms, and Implications of Low Molecular Protein Tyrosine Phosphatase as a Novel Therapeutic Target for Inhibition in the Treatment of Type 2 Diabetes Mellitus: A Review of Literature

Review Article / By /

Avneesh Bhangu1, Abdullah Naji2, Aaron Monga2

Author Affiliations
1 Faculty of Health Sciences, School of Medicine, Queen’s University, Kingston, Ontario
2 Western University of Health Sciences, College of Osteopathic Medicine of the Pacific, Pomona, California
PNWMSRJ. Published online Aug 4, 2020.

Abstract

Type 2 diabetes mellitus has grown to become one of the most important global health challenges, resulting in increased efforts to treat patients with lifestyle changes and pharmacotherapy. Protein tyrosine phosphatases are a promising therapeutic target to treat type 2 diabetes mellitus, considering their role in dephosphorylating insulin receptors. Protein tyrosine phosphatase 1B is a class 1 protein tyrosine phosphatase which has been the main target for inhibition in recent decades; however, inhibitors designed thus far have yet to achieve optimal specificity and bioavailability. Thus, novel therapeutic targets—such as low molecular protein tyrosine phosphatase—are being considered in the treatment for type 2 diabetes mellitus. Low molecular protein tyrosine phosphatase is suggested as a negative regulator of insulin receptor signaling which is functionally distinct from other protein tyrosine phosphatases. It has been shown that low molecular protein tyrosine phosphatase knockout-mice with high-fat diet-induced diabetes had significantly improved glucose tolerance compared to wild-type mice. Furthermore, an orally bioavailable small-molecule inhibitor of low molecular protein tyrosine phosphatase was able to effectively reverse obesity-induced diabetes. These findings suggest low molecular protein tyrosine phosphatase inhibitors could be a tangible therapeutic in improving insulin sensitivity and glycemic control. Despite encouraging results, further research should be performed to study potential side-effects of low molecular protein tyrosine phosphatase inhibition. With continued optimization, low molecular protein tyrosine phosphatase could prove to be a viable oral drug for type 2 diabetic patients.

Introduction

Type 2 diabetes mellitus (T2DM) has become one of the most important global health challenges, as it has been predicted that approximately 591.9 million people will be living with this disease by the year 2035.1 Despite the effectiveness of suggested lifestyle changes and pharmacotherapy, the rising incidence of T2DM cases and associating co-morbidities—particularly in the adolescent population—necessitate a search for novel treatment options.2

Recently, there has been growing interest in targeting protein tyrosine phosphatases (PTP) in the treatment of T2DM. Under normal physiological conditions, the binding of insulin to its corresponding insulin receptor (IR) on the plasma membrane stimulates autophosphorylation, which signals a cascade to promote biological effects of insulin such as glucose uptake. This process is negatively regulated by PTPs, which dephosphorylate substrate IR tyrosine residues to attenuate insulin signaling pathways.3 PTP expression levels are amplified in insulin-resistant obese patients and thus serve as a target for inhibition to increase insulin sensitivity.4

Many studies have focused on targeting class 1 protein tyrosine phosphatase 1B (PTP1B) for inhibition in the treatment of diabetes, to varying degrees of success.5 Low molecular weight protein tyrosine phosphatases (LMPTPs) are class 2 PTPs involved in multiple signaling pathways, yet their role in insulin resistance and potential as a therapeutic target remain understudied. The objective of this review is to assess the viability of LMPTP inhibitors in treating T2DM; this will be achieved by evaluating outcomes of existing PTP inhibitors in T2DM treatment, reviewing mechanisms of LMPTP activity in the IR pathway, and finally considering the implications of LMPTP as a novel therapeutic target.

PTP1B inhibitors have yet to achieve optimal specificity and oral bioavailability

The action of PTPs is important in both the transmission and attenuation of insulin signaling via dephosphorylating IR. For this reason, PTPs have been considered a target for inhibition in the treatment of T2DM. PTP1B has been of specific interest in recent decades because its gene expression is increased in subjects with T2DM, which largely contributes to insulin resistance.6 However, it has been demonstrated that mice with genetically knocked out PTP1B exhibit increased insulin sensitivity and resistance to weight gain on a high-fat diet.7

Early inhibitors were based on vanadium compounds, which are competitive inhibitors of PTP1B. It has been postulated that vanadium anions bind cysteine side chains in active sites of PTP1B, acting as antagonists to prevent binding of substrate IR phosphate groups.8 However, the catalytic site of PTP1B is well conserved amongst all PTPs9; thus, competitive inhibitors display limited specificity to PTP1B and could potentially act on several other phosphatases and kinases.

Because of this limitation, PTP1B has been experimented on using a variety of chemical entities exhibiting distinct mechanisms of inhibition. A study conducted by Erbe et al. demonstrated ertiprotafib inhibits PTP1B via non-classical kinetics and improves glycemic control via multiple mechanisms.10 This molecule was subsequently used in a phase 2 clinical trial for the treatment of T2DM; however, the trial was terminated due to insufficient efficacy, unwanted side-effects, and toxicity concerns. Inhibitors of allosteric sites on PTP1B were also explored as a therapeutic target because of greater specificity, fewer side effects, and lower toxicity.11 Trodusquemine and Claramine are highly selective allosteric inhibitors of PTP1B which have demonstrated positive results in T2DM treatment and have subsequently entered clinical trials.12, 13

PTP1B is recognized as an established therapeutic target for inhibition as treatment for T2DM. Unfortunately, it is evident that new PTP1B inhibitors are constantly being designed and renewed because optimal specificity and oral bioavailability have yet to be achieved. Problems with specificity could arise from the highly-conserved nature of the catalytic site in PTPs, whereas issues with bioavailability could stem from difficulty in inhibitors crossing cellular membranes. Although similar limitations may apply to inhibitors designed against LMPTP, it is possible that minute differences in structural and functional properties of the regulatory protein could allow for a successful inhibitor design. Therefore, novel therapeutic targets—such as LMPTP—should be considered.

LMPTP is a negative regulator of IR signaling that is functionally distinct from other PTPs

In considering LMPTP as a therapeutic target, it is important to first understand the mechanistic pathways initiated once insulin binds IR. Downstream effectors of IR include IRS-1 and IRS-2, which when phosphorylated, further activate phosphatidylinositol 3-kinase (PI3-K) and Akt. Amongst many pathways, the IR-IRS-1/2-PI3-K-Akt signaling cascade is responsible for two important cellular activities: increasing the utilization of glucose in glycogenesis, and increasing the transportation of GLUT4 from the cytoplasm to the plasma membrane for glucose uptake in adipocytes, hepatocytes, and skeletal muscle.14 Therefore, an increase in cellular insulin sensitivity could lead to increased glucose uptake and usage by cells, thus improving hyperglycemia in the context of T2DM.

Multiple lines of evidence suggest LMPTP as an in vivo negative regulator of insulin signaling pathways, through modulating IR phosphorylation and activation. The physiological role of LMPTP in insulin signaling was first established by Chiarugi et al.; in this study, a dominant negative form of LMPTP was overexpressed in NIH3T3-IR cells to demonstrate that LMPTP directly interacts with IR upon insulin stimulation.15 Furthermore, in a study conducted by Pandey et al., researchers suppressed expression levels of LMPTPs in vivo via LMPTP-specific antisense oligonucleotides.16 Knockdown of LMPTP expression levels resulted in increased tyrosine phosphorylation of IR, and a subsequent increase in glucose tolerance.

LMPTP has also been demonstrated to exhibit distinct phosphocysteine hydrolysis mechanisms from PTP1B in the active site, despite the conserved nature amongst PTPs.17 Therefore, alternate mechanisms of inhibition could be designed to allow for greater molecular specificity against LMPTP. Considering LMPTP is suggested as a negative regulator of IR that is functionally distinct from other PTPs, this provides a rationale for its inhibition in the possible treatment of T2DM. Furthermore, the preservation of IR function from decreased LMPTP expression levels suggests LMPTP inhibitors may not only be useful in treating type 2 diabetic patients, but also in preventing T2DM in pre-diabetic patients who are developing insulin resistance.

Implications of LMPTP as a novel therapeutic target

Inhibiting LMPTPs may be an attractive option to treat T2DM, given their critical relationship in regulating IR and novelty as a therapeutic target. In a study conducted by Stanford et al., the role of LMPTP in the context of insulin resistance was characterized, and potential as a therapeutic target for T2DM was evaluated for the first time in vivo.17 It was demonstrated that LMPTP knockout (KO) mice with high-fat diet-induced diabetes had significantly improved glucose tolerance in comparison to wild-type (WT) mice. Further investigation illustrated that LMPTP activity promotes obesity-associated diabetes specifically in the liver. The researchers also developed a small-molecule inhibitor with a novel uncompetitive binding mechanism specific to LMPTP. This orally bioavailable inhibitor was effectively able to reverse obesity-induced diabetes, characterized by an increase in liver phosphorylation which led to improved insulin sensitivity and glucose tolerance.17

Because LMPTP is involved in the regulation of a diverse number of pathways, successful LMPTP inhibition could have applications in other prominent diseases such as cancer and cardiomyopathy. A study conducted by Chiarugi et al. showed LMPTP is a positive regulator of tumor onset and growth of NIH3T3 fibroblasts, through dephosphorylating ephrin receptor.15 Therefore, inhibiting LMPTP activity could offset metastasis through maintaining phosphorylation of ephrin receptor. LMPTP expression has also been shown to be increased in end-stage heart failure, thus playing a critical role in cardiac function. In a study conducted by Wade et al., LMPTP KO mice were resistant to pressure overload hypertrophy and heart failure via increased IR phosphorylation, as well as increased protein kinase A and ephrin receptor expression.18 Considering T2DM is heavily associated with vascular disease, it is possible a successful LMPTP inhibitor could not only improve glycemic control in T2DM patients but also alleviate co-morbid microvascular complications such as diabetes-induced cardiomyopathy.

Evidently, LMPTP inhibitors have wide applicability; however, it is important to consider that LMPTP acts on many phosphotyrosine-containing cellular proteins throughout the body. For this reason, LMPTP inhibitors could impair normal physiological processes occurring in different tissues, manifesting in unwanted side-effects. Therefore, in designing LMPTP inhibitors for therapeutic treatment, a focus must be placed on localizing inhibitor function to impacted tissues. Despite this, the positive findings from using a novel uncompetitive LMPTP inhibitor in the Stanford et al. study are encouraging for the future of T2DM treatment.17

Conclusion

Protein tyrosine phosphatases are considered attractive therapeutic targets in the treatment of T2DM. PTP1B has been the main target for inhibition in recent decades due to a demonstrated increase in PTP1B expression levels and associated decrease in insulin sensitivity in T2DM patients. Therefore, PTP1B inhibitors could potentially increase insulin sensitivity and glucose tolerance. However, inhibitors designed thus far have yet to successfully achieve optimal specificity and bioavailability. Thus, novel therapeutic targets—such as LMPTP—are being considered in the treatment for T2DM.

Previous studies have shown LMPTP to attenuate insulin pathways such as the IR-IRS-1/2-PI3-K-Akt signaling cascade (responsible for glycogen synthesis and glucose uptake) through dephosphorylating IR. In turn, knockdown of LMPTP has been shown to increase IR phosphorylation, and improve glycemic control. Furthermore, because of distinct hydrolysis mechanisms in the active site of LMPTP from other PTPs, greater drug specificity can be achieved. Therefore, these findings provide a rationale for targeting LMPTP for inhibition.

Stanford et al. were the first researchers to design an orally bioavailable uncompetitive inhibitor against LMPTP, for the treatment of T2DM.17 Study findings suggest obesity-induced diabetes was effectively reversed by LMPTP inhibition in hepatocytes, which resulted in increased IR phosphorylation, thus improving insulin sensitivity and glycemic control. Moreover, previous studies have demonstrated LMPTP inhibitors could have further application in the treatment of cancerous growth and diabetes-induced cardiomyopathy.

Despite encouraging results generated by the Stanford et al. study, additional factors must be considered before implementation of LMPTP inhibitors into clinical trials.17 For example, IR expression has been shown to be decreased in insulin-resistant tissues involved in energy homeostasis, creating the possibility that LMPTP inhibitors may not sufficiently increase insulin sensitivity in diabetic patients.19 Further investigation is thus warranted in using LMPTP inhibitors in conjunction with insulin-sensitizing drugs such as Rosiglitazone, known to partially restore IR expression levels. Dosage is another parameter that must be optimized in LMPTP inhibitors. Too low a dose may not have an effect, whereas too large a dose could be detrimental. With the incidence of T2DM steadily increasing despite the effectiveness of current lifestyle and pharmacological interventions, LMPTP inhibitors could have immense implications in the healthcare field. As is covered in this review, LMPTP has demonstrated great potential as a novel therapeutic target; with continued optimization, LMPTP inhibitors could prove to be a viable oral drug for type 2 diabetic patients.

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Article information:

Published Online: August 4, 2020

Corresponding Author: Aaron Monga, OMSIV. Email: aaron.monga@westernu.edu
Conflict of Interest Declaration: The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.
Funding Source/Disclosure: The authors have not received any funding for this work from any organization.