Pdx1 Level Defines Pancreatic Gene Expression Pattern and Cell Lineage Differentiation

Abstract

The absence of Pdx1 and the expression of brain-4 distinguish α-cells from other pancreatic endocrine cell lineages. To define the transcription factor responsible for pancreatic cell differentiation, we employed the reverse tetracycline-dependent transactivator system in INS-I cell-derived subclones INSαβ and INSrβ to achieve tightly controlled and conditional expression of wild type Pdx1 or its dominant-negative mutant, as well as brain-4. INSrαβ cells express not only insulin but also glucagon and brain-4, while INSrβ cells express only insulin. Overexpression of Pdx1 eliminated glucagon mRNA and protein in INSrαβ cells and promoted the expression of β-cell-specific genes in INSrβ cells. Induction of dominant-negative Pdx1 in INSrβ cells resulted in differentiation of insulin-producing β-cells into glucagon-containing α-cells without altering brain4 expression. Loss of Pdx1 function alone in INSrβ cells, which do not express endogenous brain-4 and glucagon, was also sufficient to abolish the expression of genes restricted to β-cells and to cause α-cell differentiation. In contrast, induction of brain-4 in INSrβ cells initiated detectable expression of glucagon but did not affect β-cell-specific gene expression. In conclusion, Pdx1 confers the expression of pancreatic β-cell-specific genes, such as genes encoding insulin, islet amyloid polypeptide, Glut2, and Nkx6.1. Pdx1 defines pancreatic cell lineage differentiation. Loss of Pdx1 function rather than expression of brain4 is a prerequisite for α-cell differentiation.