Secretory Conservation in Insulin Producing Cells: Is There a System-Level Law of Mass Action in Biology?

Abstract

Altered secretion of insulin from pancreatic β-cells can manifest into disorders. For example, a lack of endogenously produced and/or secreted insulin results in Type 1 diabetes (and other associated subtypes). Pancreatic βcells are the endocrine secretory cells that promote insulin secretion in response to glucose stimulation. Secretion in response to extracellular triggers is an interplay among various signaling pathways, transcription factors, and molecular mechanisms. The Mouse Insulinoma 6 (MIN6) cell line serves as a model system for gaining mechanistic insights into pancreatic β-cell functions. It is obvious that higher glucose consumption and increased insulin secretion are correlated. However, it has been reported that intracellular ATP levels remain ∼ constant beyond the extracellular glucose (EG) concentration of 10 mM. Therefore, any cause−effect relationship between glucose consumption (GC) and enhanced insulin secretion (eIS) remains unclear. We also found that total cellular protein, as well as total protein content in the culture "supernatant," remains constant regardless of varying EG concentrations. This indicated that eIS may be at the cost of (a) intracellular synthesis of other proteins and (b) secretion of other secretory proteins, or both (a) and (b), somehow coupled with GC by cells. To gain insights into the above, we carried out a transcriptome study of MIN6 cells exposed to hypoglycemic (HoG = 2.8 mM EG) and hyperglycemic (HyG = 25 mM EG) conditions. Expression of transcripts was analyzed in terms of Fragments Per Kilobase of transcript per Million mapped reads and Transcripts Per Million (FPKM and TPM) as well as values obtained by normalizing w.r.t. "∑(FPKM)" and "∑(TPM)." We report that HyG extracellular conditions lead to an ∼2-fold increase in insulin secretion compared to HoG measured by the enzyme-linked immunosorbent assay (ELISA) and transcripts of secreted proteins as well as their isoforms decreased in HyG conditions compared to HoG. Our results show for the first time that eIS in HyG conditions is at the cost of reduced transcription of other secreted proteins and is coupled with higher GC. The higher GC at increased extracellular glucose also indicates a yet undiscovered role of glucose molecules enhancing insulin secretion, since ATP levels resulting from glucose metabolism have been reported to be constant above an EG concentration of 10 mM. While extrapolation of our results to clinical implications is ambitious at best, this work reports novel cellular level aspects that seem relevant in some clinical observations pertaining to Type 1 diabetes. In addition, the conservatory nature of cellular secretions in insulin-secreting cells, discovered here, may be a general feature in cell biology.

(Re-)Viewing Role of Intracellular Glucose Beyond Extracellular Regulation of Glucose-Stimulated Insulin Secretion by Pancreatic Cells

“…In this work, we analyzed differential gene expression in MIN6 cells exposed to low and high extracellular glucose concentrations (EGC), from their whole transcriptome data (55,467 distinct transcripts, of which 14,427 were identified as successfully expressed genes). 65 The study of differentially expressed genes (DEGs) can be obtained with high precision when the true number of DEGs ranges from 15 to 30%, even in the absence of biological replicates. 66 We obtained a value of 26%; thus, our study of DEGs was precise enough.…”

“…The data were submitted to the NCBI-GEO (National Center for Biotechnology Information – Gene Expression Omnibus) database repository with an assigned accession number record of GSE226652. The entire differential gene expression analyses study can be viewed through the link and reference 65 . As shown in Figure 2 (inset), the fold change of housekeeping genes is unaffected under both HoG and HyG conditions.…”

“…However, within biological systems, cells can generate energy to a certain extent, prompting questions about the conservation of cellular energy. Recent research studies provide insights into how cells conserve energy and secrete specific proteins, often at the expense of other secretory proteins . Notably, glucose molecules play a dual role: they not only contribute to energy generation through GLUT2-glucokinase-mediated phosphorylation in glycolysis but also enhance insulin secretion by binding to other proteins that facilitate glucose-mediated insulin secretion.…”

“…encountered in specific wet-experimental systems. Furthermore, intracellular energy transactions, , especially viewing the role of glucose molecules as intracellular ligands rather than as a source of metabolic energy, will also need to be carefully analyzed. The computational approaches utilized here do not consider such variables arising out of wet-experimental set ups.…”

“…Recent research studies provide insights into how cells conserve energy and secrete specific proteins, often at the expense of other secretory proteins. 65 Notably, glucose molecules play a dual role: they not only contribute to energy generation through GLUT2-glucokinase-mediated phosphorylation in glycolysis but also enhance insulin secretion by binding to other proteins that facilitate glucose-mediated insulin secretion. Transcriptomic analyses of hyperglycemic-exposed-MIN6 cells identified upregulation of 3 secretion-specific genes corresponding to proteins KIF11, ATP6V0A4, and CACNB4.…”