The Phosphatase PHLPP2 Plays a Key Role in the Regulation of Pancreatic Beta-Cell Survival

Abstract

Currently available antidiabetic treatments fail to halt, and may even exacerbate, pancreatic β-cell exhaustion, a key feature of type 2 diabetes pathogenesis; thus, strategies to prevent, or reverse, β-cell failure should be actively sought. The serine threonine kinase Akt has a key role in the regulation of β-cell homeostasis; among Akt modulators, a central role is played by pleckstrin homology domain leucine-rich repeat protein phosphatase (PHLPP) family. Here, taking advantage of an in vitro model of chronic exposure to high glucose, we demonstrated that PHLPPs, particularly the second family member called PHLPP2, are implicated in the ability of pancreatic β cells to deal with glucose toxicity. We observed that INS-1 rat pancreatic β cell line maintained for 12–15 passages at high (30 mM) glucose concentrations (INS-1 HG) showed increased expression of PHLPP2 and PHLPP1 both at mRNA and protein level as compared to INS-1 maintained for the same number of passages in the presence of normal glucose levels (INS-1 NG). These changes were paralleled by decreased phosphorylation of Akt and by increased expression of apoptotic and autophagic markers. To investigate if PHLPPs had a casual role in the alteration of INS-1 homeostasis observed upon chronic exposure to high glucose concentrations, we took advantage of shRNA technology to specifically knock-down PHLPPs. We obtained proof-of-concept evidence that modulating PHLPPs expression may help to restore a healthy β cell mass, as the reduced expression of PHLPP2/1 was accompanied by a recovered balance between pro- and antiapoptotic factor levels. In conclusion, our data provide initial support for future studies aimed to identify pharmacological PHLPPs modulator to treat beta-cell survival impairment. They also contribute to shed some light on β-cell dysfunction, a complex and unsatisfactorily characterized phenomenon that has a central causative role in the pathogenesis of type 2 diabetes.

Inhibition of PHLPP1/2 phosphatases rescues pancreatic β-cells in diabetes

“…Thus, PHLPP, AKT, and MST1 form an auto-inhibitory triangle that regulates β-cell apoptosis in a tightly controlled manner. Our data are fully in line with a recently published report that shows upregulation of PHLPP1/2 in INS-1 cells in response to elevated glucose and their link to AKT ( Hribal et al, 2020 ).…”

“…Thus, PHLPP, AKT, and MST1 form an auto-inhibitory triangle that regulates b-cell apoptosis in a tightly controlled manner. Our data are fully in line with a recently published report that shows upregulation of PHLPP1/2 in INS-1 cells in response to elevated glucose and their link to AKT (Hribal et al, 2020). mTORC1 signaling is an instrumental pathway in nutrient sensing and the integration of metabolic, energetic, and hormonal stimuli to control cellular metabolism, survival, and anabolic growth (Gonzalez and Hall, 2017;Mossmann et al, 2018;Saxton and Sabatini, 2017).…”

Pleckstrin homology (PH) domain and Leucine Rich Repeat Phosphatase 1 (Phlpp1) Suppresses Parathyroid Hormone Receptor 1 (Pth1r) Expression and Signaling During Bone Growth

“…Further enhancement of cell proliferation as a result of Phlpp1 inhibition has potential implications beyond chondrocytes. Phlpp1 deletion promotes cell proliferation in a model of intervertebral disc degeneration, (19 ) and regulation of cell apoptosis by Phlpp1 is implicated in ischemic brain and spinal cord injury, ( 20,22 ) colitis, ( 21 ) cardiac dysfunction, ( 66 ) pancreatic beta‐cell survival, ( 67 ) and tumor activity. ( 8 ) As such, enhancement of signals associated with Phlpp1 inhibition could have a profound impact on various physiological and disease processes.…”

Phosphatase PHLPP2 regulates the cellular response to metabolic stress through AMPK

“…We previously identified PHLPP2 in a glucose-sensitive multi-protein particle in Jurkat T-ALL cells following mass-spectrometric analysis (unpublished). PHLPP2 functions as a sensor of glucose availability in primary tissue, such as pancreatic beta cells [ 39 ], and our study suggests it plays a similar role in some cancer cells. Other phosphatases that are activated by energy depletion and low fuel would be predicted to target AMPK, given its central role in energy homeostasis, and PP2A, PP2C, and PPM1E have been shown to inhibit its activity in cardiac and skeletal muscle, liver, and lung [ 40 – 44 ].…”