Pdx1 level defines pancreatic gene expression pattern and cell lineage differentiation

The absence of Pdx1 and the expression of brain-4 distinguish alpha -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 INSr alpha beta and INSr beta to achieve tightly controlled and conditional expression of wild type Pdx1 or its dominant-negative mutant, as well as brain-4, INSr alpha beta cells express not only insulin but also glucagon and brain-4, while INSr beta cells express only insulin. Overexpression of Pdx1 eliminated glucagon mRNA and protein in INSr alpha beta cells and promoted the expression of beta -cell-specific genes in INSr beta cells. Induction of dominant-negative Pdx1 in INSr alpha beta cells resulted in differentiation of insulin-producing beta -cells into glucagon-containing alpha -cells without altering brain4 expression. Loss of Pdx1 function alone in INSr beta cells, which do not express endogenous brain-4 and glucagon, was also sufficient to abolish the expression of genes restricted to beta -cells and to cause alpha -cell differentiation. in contrast, induction of brain-4 in INSr beta cells initiated detectable expression of glucagon but did not affect beta -cell-specific gene expression. In conclusion, Pdx1 confers the expression of pancreatic beta -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 alpha -cell differentiation.