Using recombinase-mediated cassette exchange to engineer MIN6 insulin-secreting cells based on a newly identified safe harbor locus

Diacylglycerol kinase ζ is a positive insulin secretion regulator in pancreatic β-cell line MIN6

Some isoforms of diacylglycerol (DAG) kinase (DGK), an enzyme converting DAG into phosphatidic acid, i.e., DGKα, γ and δ, have been reportedly involved in the regulation of pancreatic β-cell function. DGKζ has also been reported to be expressed in rat pancreatic β-cells. However, its function in pancreatic β-cells remains unknown. The present study aimed to elucidate the function of DGKζ in pancreatic β-cells. The expression of DGKζ was detected in the β-cell line MIN6B and mouse pancreatic islets and in the cytoplasmic fraction from MIN6B cells. The knockdown of DGKζ with siRNA significantly decreased glucose-induced insulin secretion in MIN6B cells. The induction of DGKζ expression in MIN6CEon1 cells with a doxycycline-inducible stable expression system significantly increased glucose-induced insulin secretion. In contrast, glucose-induced insulin secretion was not changed when a kinase-dead DGKζ mutant (G356D) was overexpressed in MIN6CEon1 cells, indicating that a mechanism dependent on its kinase activity mediates the facilitatory effect of DGKζ on glucose-induced insulin secretion. Additionally, we revealed that DGKζ overexpression exhibited no effect on cell cycle of MIN6 cells. These results suggest that DGKζ plays a facilitatory role in insulin secretion in pancreatic β-cells.

A large-scale functional analysis of genes expressed differentially in insulin secreting MIN6 sublines with high versus mildly reduced glucose-responsiveness

Molecular mechanisms of glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells are not fully understood. GSIS deteriorations are believed to underlie the pathogenesis of type 2 diabetes mellitus. By comparing transcript levels of 3 insulin secreting MIN6 cell sublines with strong glucose-responsiveness and 3 with mildly reduced responsiveness, we identified 630 differentially expressed genes. Using our recently developed system based on recombinase-mediated cassette exchange, we conducted large-scale generation of stable clones overexpressing such genes in the doxycycline-regulated manner. We found that overexpressions of 18, out of 83, genes altered GSIS. Sox11 ((sex determining region Y)-box 11) was selected to confirm its roles in regulating insulin secretion, and the gene was subjected to shRNA-mediated suppression. While Sox11 overexpression decreased GSIS, its suppression increased GSIS, confirming the role of Sox11 as a negative regulator of insulin secretion. Furthermore, metabolic experiments using radiolabelled glucose showed Sox11 to participate in regulating glucose metabolism. Our data suggested that overexpression screening is a feasible option for systemic functional testing to identify important genes in GSIS.