Deregulated expression of the retinoid X receptor alpha prevents muscle differentiation in P19 embryonal carcinoma cells

Abstract

We have studied the expression of the retinoid X receptor (RXR) family of receptors during the DMSO-induced differentiation of P19 murine embryonal carcinoma cells into mesoderm and muscle. RXR-alpha protein is weakly detectable in untreated P19 cells and in a mutant line of P19 cells (D3) that are resistant to DMSO-induced differentiation but begins to increase by day 3 and continues to rise gradually thereafter, whereas RXR-gamma protein is readily detected in P19 cells and decreases over the course of differentiation. Protein expression is uncoupled from mRNA levels, because DMSO induces a rapid, aggregation-independent, transient increase in RXR-alpha mRNA that diminishes by day 3 of differentiation. Thus, the expression of RXR-alpha protein is prevented at early times during DMSO-induced differentiation. Stable P19 cell clones that constitutively express RXR-alpha protein [P19(RXR-alpha)] are resistant to DMSO-induced differentiation associated with increased levels of oligonucleosomal-length DNA fragmentation. Loss of RXR-alpha expression after multiple passages results in a reversion to a DMSO-responsive phenotype. Id1 transcripts are present in P19 cells and are transiently decreased on day 2 of DMSO differentiation but remain elevated in DMSO-treated P19(RXR-alpha) and in P19 cells treated simultaneously with retinoic acid and DMSO. The mRNA for the mesoderm inducer protein Brachyury T was also deregulated in P19(RXR-alpha) cells and D3 cells compared with that of wild-type P19 cells. Together, these results show that expression of the RXR-alpha mRNA and protein in P19 cells is tightly regulated during the mesodermal/muscle differentiation of P19 cells, and that ectopic expression of the RXR-alpha protein prevents differentiation associated with increased cell death, prolonged expression of Brachyury T, and constitutive expression of Id1.