THYROID-HORMONE RECEPTOR-ALPHA INHIBITS RETINOIC ACID-RESPONSIVE GENE-EXPRESSION AND MODULATES RETINOIC ACID-STIMULATED NEURAL DIFFERENTIATION IN MOUSE EMBRYONIC STEM-CELLS

Thyroid hormone (T-3) and retinoic acid (RA) are essential for normal vertebrate development and are known to coregulate several genes. Earl y development is predominantly retinoic acid sensitive, yet thyroid ho rmone receptor-alpha (T(3)R alpha) is expressed along with retinoic ac id receptors (RAR)-alpha, -beta, and -gamma. To determine the role of unliganded T(3)R alpha in early development and on RA-stimulated neura l development, we used homologous recombination techniques to inactiva te both T(3)R alpha gene alleles in mouse embryonic stem (ES) cells. L oss of both T(3)R alpha alleles resulted in an increase in basal and R A-induced expression of the endogenous RA-responsive genes, RAR beta a nd alkaline phosphatase, which demonstrates that T(3)R alpha has an in hibitory effect on the RA response. A similar magnitude of T(3)R inhib ition of the RA response was seen in transient transfection assays of RA response elements in both ES and JEG cells. Cotransfection experime nts were used to demonstrate that inhibition of the RA response could be mediated by T(3)R alpha 1. The addition of T(3)R alpha 1, but not t he T(3)R alpha variant c-erbA alpha 2, to T(3)R alpha-null ES cells re stored the inhibitory effect on RA-induced gene expression. RA-stimula ted neural differentiation was seen in the wild-type, but not in T(3)R alpha-null ES, cells, consistent with reports of abnormal neural deve lopment as a consequence of premature RA stimulation. Our results demo nstrate that the early expression of unliganded T(3)R alpha functions to modulate the RA response and RA-stimulated neural differentiation.