The Phosphorylation of PDX-1 by Protein Kinase CK2 Is Crucial for Its Stability

Inhibition of proteasome activity facilitates definitive endodermal specification of pluripotent stem cells by influencing YAP signalling

AIMS

The knowledge of the molecular players that regulate the generation of endoderm cells is imperative to obtain homogenous population of pancreatic β-cells from stem cells. The Ubiquitin proteasome system (UPS) has been envisaged as a crucial intracellular protein degradation system, but its role in the generation of β-cells remains elusive. Hence, it would be appropriate to unravel the potential role of UPS in endoderm specification and utilize the understanding to generate β-cells from pluripotent stem cells.

MATERIALS AND METHODS

The pluripotent stem cells (mESCs, miPSCs and hIPSCs) were subjected to differentiation towards pancreatic β-cells and assessed the proteasomal activity during endodermal differentiation. Pharmacologic agents MG132 and IU-1 were employed to inhibit and activate proteasomal activity respectively at the definitive endoderm stage to investigate its impact on the generation of β-cells. The expression of stage-specific genes were analyzed at transcript and protein levels. We also explored the role of unfolded protein response and UPS-regulated signalling pathways in endodermal differentiation.

KEY FINDINGS

We observed decreased proteasomal activity specifically during endoderm, but not during the generation of other lineages. Extraneous proteasomal inhibition enhanced the expression of endodermal genes while increasing the proteasomal activity hindered definitive endodermal differentiation. Proteasomal inhibition at the definitive endodermal stage culminated in an enriched generation of insulin-positive cells. Elevated endodermal gene expression was consistent in mESCs and hIPSCs upon proteasomal inhibition. Mechanistic insight revealed the proteasome-inhibited enhanced endodermal differentiation to be via modulating the YAP pathway.

SIGNIFICANCE

Our study unravels the specific involvement of UPS in endoderm cell generation from pluripotent stem cells and paves the way for obtaining potential definitive endodermal cells for plausible cellular therapy in the future.

CK2 activity is crucial for proper glucagon expression

AIMS/HYPOTHESIS

Protein kinase CK2 acts as a negative regulator of insulin expression in pancreatic beta cells. This action is mainly mediated by phosphorylation of the transcription factor pancreatic and duodenal homeobox protein 1 (PDX1). In pancreatic alpha cells, PDX1 acts in a reciprocal fashion on glucagon (GCG) expression. Therefore, we hypothesised that CK2 might positively regulate GCG expression in pancreatic alpha cells.

METHODS

We suppressed CK2 kinase activity in αTC1 cells by two pharmacological inhibitors and by the CRISPR/Cas9 technique. Subsequently, we analysed GCG expression and secretion by real-time quantitative RT-PCR, western blot, luciferase assay, ELISA and DNA pull-down assays. We additionally studied paracrine effects on GCG secretion in pseudoislets, isolated murine islets and human islets. In vivo, we examined the effect of CK2 inhibition on blood glucose levels by systemic and alpha cell-specific CK2 inhibition.

RESULTS

We found that CK2 downregulation reduces GCG secretion in the murine alpha cell line αTC1 (e.g. from 1094±124 ng/l to 459±110 ng/l) by the use of the CK2-inhibitor SGC-CK2-1. This was due to a marked decrease in Gcg gene expression through alteration of the binding of paired box protein 6 (PAX6) and transcription factor MafB to the Gcg promoter. The analysis of the underlying mechanisms revealed that both transcription factors are displaced by PDX1. Ex vivo experiments in isolated murine islets and pseudoislets further demonstrated that CK2-mediated reduction in GCG secretion was only slightly affected by the higher insulin secretion after CK2 inhibition. The kidney capsule transplantation model showed the significance of CK2 for GCG expression and secretion in vivo. Finally, CK2 downregulation also reduced the GCG secretion in islets isolated from humans.

CONCLUSIONS/INTERPRETATION

These novel findings not only indicate an important function of protein kinase CK2 for proper GCG expression but also demonstrate that CK2 may be a promising target for the development of novel glucose-lowering drugs.

Degradation meets development: Implications in β-cell development and diabetes

Pancreatic development is orchestrated by timely synthesis and degradation of stage-specific transcription factors (TFs). The transition from one stage to another stage is dependent on the precise expression of the developmentally relevant TFs. Persistent expression of particular TF would impede the exit from the progenitor stage to the matured cell type. Intracellular protein degradation-mediated protein turnover contributes to a major extent to the turnover of these TFs and thereby dictates the development of different tissues. Since even subtle changes in the crucial cellular pathways would dramatically impact pancreatic β-cell performance, it is generally acknowledged that the biological activity of these pathways is tightly regulated by protein synthesis and degradation process. Intracellular protein degradation is executed majorly by the ubiquitin proteasome system (UPS) and Lysosomal degradation pathway. As more than 90% of the TFs are targeted to proteasomal degradation, this review aims to examine the crucial role of UPS in normal pancreatic β-cell development and how dysfunction of these pathways manifests in metabolic syndromes such as diabetes. Such understanding would facilitate designing a faithful approach to obtain a therapeutic quality of β-cells from stem cells.

PCIF1, the only methyltransferase of N6,2-O-dimethyladenosine

N6-methyladenosine(m6A), is the most abundant post-transcriptional modification of mRNA in biology. When the first nucleotide after the m7G cap is adenosine, it is methylated at the N6 position to form N6,2-O-dimethyladenosine (m6Am). m6Am is a reversible modification located at the first transcribed nucleotide, which is present in about 30% of cellular mRNAs, thus m6Am can have a significant impact on gene expression in the transcriptome. Phosphorylated CTD interaction factor 1(PCIF1), the unique and specific methyltransferase of m6Am, has been shown to affect mRNA stability, transcription, and translation. Several studies have shown that PCIF1 is clearly associated with tumor, viral, and endocrine diseases. Moreover, PCIF1 may be related to the tumor microenvironment, immune cell typing, and programmed cell death protein 1(PD-1) drug resistance. Here, we summarize the mechanism of PCIF1 involvement in mRNA modifications, and outline m6Am modifications and diseases in which PCIF1 is involved. We also summarized the role of PCIF1 in immune and immune checkpoint blockade(ICB) treatment, and predicted the possibility of PCIF1 as a biomarker and therapeutic target.

Phosphorylation of Thymidylate Synthase and Dihydrofolate Reductase in Cancer Cells and the Effect of CK2α Silencing

Our previous research suggests an important regulatory role of CK2-mediated phosphorylation of enzymes involved in the thymidylate biosynthesis cycle, i.e., thymidylate synthase (TS), dihydrofolate reductase (DHFR), and serine hydroxymethyltransferase (SHMT). The aim of this study was to show whether silencing of the CK2α gene affects TS and DHFR expression in A-549 cells. Additionally, we attempted to identify the endogenous kinases that phosphorylate TS and DHFR in CCRF-CEM and A-549 cells. We used immunodetection, immunofluorescence/confocal analyses, reverse transcription-quantitative polymerase chain reaction (RT-qPCR), in-gel kinase assay, and mass spectrometry analysis. Our results demonstrate that silencing of the CK2α gene in lung adenocarcinoma cells significantly increases both TS and DHFR expression and affects their cellular distribution. Additionally, we show for the first time that both TS and DHFR are very likely phosphorylated by endogenous CK2 in two types of cancer cells, i.e., acute lymphoblastic leukaemia and lung adenocarcinoma. Moreover, our studies indicate that DHFR is phosphorylated intracellularly by CK2 to a greater extent in leukaemia cells than in lung adenocarcinoma cells. Interestingly, in-gel kinase assay results indicate that the CK2α' isoform was more active than the CK2α subunit. Our results confirm the previous studies concerning the physiological relevance of CK2-mediated phosphorylation of TS and DHFR.

Biophysical insights into glucose-dependent transcriptional regulation by PDX1

The pancreatic and duodenal homeobox 1 (PDX1) is a central regulator of glucose-dependent transcription of insulin in pancreatic β cells. PDX1 transcription factor activity is integral to the development and sustained health of the pancreas; accordingly, deciphering the complex network of cellular cues that lead to PDX1 activation or inactivation is an important step toward understanding the etiopathologies of pancreatic diseases and the development of novel therapeutics. Despite nearly 3 decades of research into PDX1 control of Insulin expression, the molecular mechanisms that dictate the function of PDX1 in response to glucose are still elusive. The transcriptional activation functions of PDX1 are regulated, in part, by its two intrinsically disordered regions, which pose a barrier to its structural and biophysical characterization. Indeed, many studies of PDX1 interactions, clinical mutations, and posttranslational modifications lack molecular level detail. Emerging methods for the quantitative study of intrinsically disordered regions and refined models for transactivation now enable us to validate and interrogate the biochemical and biophysical features of PDX1 that dictate its function. The goal of this review is to summarize existing PDX1 studies and, further, to generate a comprehensive resource for future studies of transcriptional control via PDX1.

SGC-CK2-1 Is an Efficient Inducer of Insulin Production and Secretion in Pancreatic β-Cells

The pyrazolopyrimidine based compound SGC-CK2-1 is a potent and highly specific CK2 inhibitor and a new tool to study the biological functions of protein kinase CK2 irrespective from off-target effects. We used this compound in comparison with the well-established CK2 inhibitor CX-4945 to analyze the importance of CK2 for insulin production and secretion from pancreatic β-cells. Both inhibitors affected the proliferation and viability of MIN6 cells only marginally and downregulated the endogenous CK2 activity to a similar level. Furthermore, both inhibitors increased the message for insulin and boosted the secretion of insulin from storage vesicles. Thus, regarding the high specificity of SGC-CK2-1, we can clearly attribute the observed effects to biological functions of protein kinase CK2.

CK2 Regulation: Perspectives in 2021

The protein kinase CK2 (CK2) family encompasses a small number of acidophilic serine/threonine kinases that phosphorylate substrates involved in numerous biological processes including apoptosis, cell proliferation, and the DNA damage response. CK2 has also been implicated in many human malignancies and other disorders including Alzheimer′s and Parkinson’s diseases, and COVID-19. Interestingly, no single mechanism describes how CK2 is regulated, including activation by external proteins or domains, phosphorylation, or dimerization. Furthermore, the kinase has an elongated activation loop that locks the kinase into an active conformation, leading CK2 to be labelled a constitutively active kinase. This presents an interesting paradox that remains unanswered: how can a constitutively active kinase regulate biological processes that require careful control? Here, we highlight a selection of studies where CK2 activity is regulated at the substrate level, and discuss them based on the regulatory mechanism. Overall, this review describes numerous biological processes where CK2 activity is regulated, highlighting how a constitutively active kinase can still control numerous cellular activities. It is also evident that more research is required to fully elucidate the mechanisms that regulate CK2 and what causes aberrant CK2 signaling in disease.

Protein Kinase CK2 Controls CaV2.1-Dependent Calcium Currents and Insulin Release in Pancreatic β-Cells

The regulation of insulin biosynthesis and secretion in pancreatic β-cells is essential for glucose homeostasis in humans. Previous findings point to the highly conserved, ubiquitously expressed serine/threonine kinase CK2 as having a negative regulatory impact on this regulation. In the cell culture model of rat pancreatic β-cells INS-1, insulin secretion is enhanced after CK2 inhibition. This enhancement is preceded by a rise in the cytosolic Ca²⁺ concentration. Here, we identified the serine residues S₂₃₆₂ and S₂₃₆₄ of the voltage-dependent calcium channel Ca_V2.1 as targets of CK2 phosphorylation. Furthermore, co-immunoprecipitation experiments revealed that Ca_V2.1 binds to CK2 in vitro and in vivo. Ca_V2.1 knockdown experiments showed that the increase in the intracellular Ca²⁺ concentration, followed by an enhanced insulin secretion upon CK2 inhibition, is due to a Ca²⁺ influx through Ca_V2.1 channels. In summary, our results point to a modulating role of CK2 in the Ca_V2.1-mediated exocytosis of insulin.

The stability of CREB3/Luman is regulated by protein kinase CK2 phosphorylation

CREB3 (Luman) is a family member of ER resident transcription factors, which are cleaved upon the induction of ER stress. Their N-terminal fragments shuttle into the nucleus where they regulate the transcription of target genes. Here, we found that human CREB3 is phosphorylated within its transcription activation domain on serine 46 by protein kinase CK2. Further analyses revealed that the phosphorylation of this site does neither affect the cleavage by S1P/S2P proteases, nor the nuclear localisation nor the transcriptional activity of CREB3. However, phosphorylation at serine 46 reduced the stability of CREB3.

Protein Kinase CK2-A Putative Target for the Therapy of Diabetes Mellitus?

Since diabetes is a global epidemic, the development of novel therapeutic strategies for the treatment of this disease is of major clinical interest. Diabetes is differentiated in two types: type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM). T1DM arises from an autoimmune destruction of insulin-producing β-cells whereas T2DM is characterized by an insulin resistance, an impaired insulin reaction of the target cells, and/or dysregulated insulin secretion. In the past, a growing number of studies have reported on the important role of the protein kinase CK2 in the regulation of the survival and endocrine function of pancreatic β-cells. In fact, inhibition of CK2 is capable of reducing cytokine-induced loss of β-cells and increases insulin expression as well as secretion by various pathways that are regulated by reversible phosphorylation of proteins. Moreover, CK2 inhibition modulates pathways that are involved in the development of diabetes and prevents signal transduction, leading to late complications such as diabetic retinopathy. Hence, targeting CK2 may represent a novel therapeutic strategy for the treatment of diabetes.

Functional interplay between the transcription factors USF1 and PDX-1 and protein kinase CK2 in pancreatic β-cells

Glucose homeostasis is regulated by insulin, which is produced in the β-cells of the pancreas. The synthesis of insulin is controlled by several transcription factors including PDX-1, USF1 and USF2. Both, PDX-1 and USF1 were identified as substrates for protein kinase CK2. Here, we have analysed the interplay of PDX-1, USF1 and CK2 in the regulation of PDX-1 gene transcription. We found that the PDX-1 promoter is dose-dependently transactivated by PDX-1 and transrepressed by USF1. With increasing glucose concentrations the transrepression of the PDX-1 promoter by USF1 is successively abrogated. PDX-1 binding to its own promoter was not influenced by glucose, whereas USF1 binding to the PDX-1 promoter was reduced. The same effect was observed after inhibition of the protein kinase activity by three different inhibitors or by using a phospho-mutant of USF1. Moreover, phosphorylation of USF1 by CK2 seems to strengthen the interaction between USF1 and PDX-1. Thus, CK2 is a negative regulator of the USF1-dependent PDX-1 transcription. Moreover, upon inhibition of CK2 in primary islets, insulin expression as well as insulin secretion were enhanced without affecting the viability of the cells. Therefore, inhibition of CK2 activity may be a promising approach to stimulate insulin production in pancreatic β-cells.