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 rev erse tetracycline-dependent transactivator system in INS-I cell-derived sub clones INSr alpha beta and INSr beta to achieve tightly controlled and cond itional expression of wild type Pdx1 or its dominant-negative mutant, as we ll as brain-4, INSr alpha beta cells express not only insulin but also gluc agon and brain-4, while INSr beta cells express only insulin. Overexpressio n 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. I nduction of dominant-negative Pdx1 in INSr alpha beta cells resulted in dif ferentiation of insulin-producing beta -cells into glucagon-containing alph a -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 -cel ls and to cause alpha -cell differentiation. in contrast, induction of brai n-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 enco ding insulin, islet amyloid polypeptide, Glut2, and Nkx6.1. Pdx1 defines pa ncreatic cell lineage differentiation. Loss of Pdx1 function rather than ex pression of brain4 is a prerequisite for alpha -cell differentiation.