Potential roles of PP2A-Rac1 signaling axis in pancreatic β-cell dysfunction under metabolic stress: Progress and promise

Abstract

Recent estimates by the International Diabetes Federation suggest that the incidence of diabetes soared to an all-time high of 463 million in 2019, and the federation predicts that by 2045 the number of individuals afflicted with this disease will increase to 700 million. Therefore, efforts to understand the pathophysiology of diabetes are critical for moving toward the development of novel therapeutic strategies for this disease. Several contributors (oxidative stress, endoplasmic reticulum stress and others) have been proposed for the onset of metabolic dysfunction and demise of the islet β-cell leading to the pathogenesis of diabetes. Existing experimental evidence revealed sustained activation of PP2A and Rac1 in pancreatic β-cells exposed to metabolic stress (diabetogenic) conditions. Evidence in a variety of cell types implicates modulatory roles for specific signaling proteins (α4, SET, nm23-H1, Pak1) in the functional regulation of PP2A and Rac1. In this Commentary, I overviewed potential cross-talk between PP2A and Rac1 signaling modules in the onset of metabolic dysregulation of the islet β-cell leading to impaired glucose-stimulated insulin secretion (GSIS), loss of β-cell mass and the onset of diabetes. Potential knowledge gaps and future directions in this fertile area of islet biology are also highlighted. It is hoped that this Commentary will provide a basis for future studies toward a better understanding of roles of PP2A-Rac1 signaling module in pancreatic β-cell dysfunction, and identification of therapeutic targets for the treatment of islet β-cell dysfunction in diabetes.

Keywords: Diabetes; Metabolic stress; Pancreatic β-cell; Protein phosphatase 2A; Rac1; SET; α4.

Figure 1:. Potential mechanisms that underlie functional (in)activation of Rac1 in pancreatic β-cells

Published evidence suggests that Rac1 is regulated by a variety of proteins/factors, including GEFs, GDIs. In addition, using proteomics approaches, we have identified novel interaction partners for Rac1 in pancreatic β-cells under basal and hyperglycemic conditions. Interestingly, association of some of these proteins/factors with Rac1 was significantly increased under high glucose-treatment conditions. Furthermore, Rac1 has been shown to undergo a variety of post-translational modifications, including prenylation, palmitoylation, phosphorylation, ubiquitination and SUMOylation. As discussed in the text, prenylation is the most studied for Rac1 in the islet β-cell. Potential regulation of its function by other post-translational modification steps need to be assessed in the context of its role in the pathogenesis of islet function under metabolic stress. Lastly, Rac1, which is primarily cytosolic and translocates to the plasma membrane fraction, following cellular activation for optimal regulation of its effectors. We have recently demonstrated that Rac1 associates with the nuclear fraction (in an active, but unprenylated configuration) under conditions of metabolic stress. We proposed that it might regulate apoptotic factors in the nuclear fraction to elicit regulatory effects leading to cell demise (see text for additional details).

Figure 2:. Multifactorial regulation of PP2A-like enzymes in the islet β-cell

We proposed earlier that PP2A plays a central role in the islet cell survival and demise. As depicted in the figure, functional regulation of PP2A is under the control of many modulators. Note that most, but not all, of these modulators are known to regulate PP2A function via their effects (stimulation or inhibition based on conditions) on the CML of PP2Ac. Chronic exposure to glucose has been shown to increase the CML of PP2Ac leading to its activation. CER has been shown to activate PP2A without affecting the CML of PP2Ac. Effects of inositol phosphates and sulfonylureas on CML remains to be studied. Metabolites of glucose have been shown to inhibit CML of PP2Ac leading to its inactivation. Relevant to this article are effects of two regulatory proteins, namely SET and α4 of PP2A function. Overall, the functional regulation of PP2A is complex due to its subunit composition, subcellular distributions as well as properties of its regulatory proteins/factors (see text for additional details).

Figure 3:. Potential contributors to accelerated signaling of PP2A-Rac1 module in metabolic stress induced dysfunction of the islet β-cell

The proposed model identifies at least two major contributors, namely α4 and SET, that mediate functional activation of PP2A and Rac1 signaling in pancreatic β-cells subjected to metabolic stress conditions. α4 contributes to PP2A activation via multiple pathways including prevention of PP2Ac degradation, CML of PP2Ac and holoenzyme assembly. Mechanisms underlying its ability to activate Rac1 still remain to be elucidated. SET is inhibitory to PP2A function. Therefore, it is likely that metabolic stress promotes its dissociation from PP2Ac from the SET-PP2Ac complex by preventing its requisite post-translational modification steps, including phosphorylation. SET could contribute to Rac1 activation via its nuclear-cytoplasmic shuttling, and/or yet unidentified mechanism(s). These need to be assessed in the islet β-cell. Lastly, in addition to SET and α4, stress-induced dysfunction might require the interplay between other proteins/factors including nm-23 and Pak1. Again, a methodical investigation of these individual signaling steps might be necessary to assign roles for these molecules in the pathogenesis of β-cell dysfunction in diabetes (see text for additional details).

Figure 4:. A working model depicting potential signaling steps involved in PP2A-Rac1 signaling modules leading to metabolic dysfunction of the islet β-cell under metabolic stress.

Metabolic stress induces the CML of PP2Ac, recruitment of regulatory B subunits, holoenzyme assembly and activation of PP2A leading to dephosphorylation and inactivation of key survival proteins. In addition, metabolic stress conditions also promote activation of Rac1 leading to stimulation of NADPH oxidases, and associated increase in the intracellular oxidative stress. This, in turn, promotes activation of stress kinases (p38 MAPK and JNK1/2) leading to mitochondrial dysregulation culminating in loss of mitochondrial membrane permeability pore transition and release of cytochrome C into the cytosolic compartment and associated increase in caspase-3 activity. Earlier studies from our laboratory have documented significant alterations in nuclear compartment in β-cells following exposure to metabolic stress, including caspase-3 mediated degradation of nuclear lamin B and loss of nuclear integrity. Furthermore, we have recently reported that sustained activation of Rac1 under metabolic stress conditions leads to inappropriate movement of Rac1 to the nuclear fraction in clonal β-cells, normal rodent islets and human islets to result in activation of apoptotic factors, including p53. It is proposed that α4 and SET play significant roles in promoting PP2A/Rac1 signaling modules leading to islet β-cell dysfunction under these conditions. The author realizes that this model might be construed as simplified (and relatively linear) since published evidence implicates several competing mechanisms that might underlie β-cell dysfunction including inflammation, lipotoxicity, ER stress and accumulation of amyloid toxic oligomers. However, at least at the outset, it might be worthwhile to test this model to precisely define the molecular/cellular mechanisms involved in Rac1-PP2A signaling axis in islet β-cell dysfunction under metabolic stress. Successful outcome from these studies might pave way toward identification of key targets involved in islet β-cell dysfunction under diabetogenic conditions (see text for additional details).