Thyroid hormone nuclear receptors (TRs) are ligand-dependent transcription factors which regulate growth, differentiation, and development. The molecular mechanism by which TRs mediated these effects remains unclear. A prevailing hypothesis is that TRs exert their biological effects by cooperating with other transcription factors. We have recently shown that the human TR subtype beta1 (hTRbeta1) interacts with the tumor suppressor p53, which plays a critical role in cell-cycle regulation and tumorigenesis. This interaction of hTRbeta1 with p53 leads to an impairment of TR function. The present study examined whether hTRbeta1 could modulate the function of p53. Mapping of the domains of p53 responsible for the interaction with hTRbeta1 indicated that the regions involved resided in the DNA-binding domain and carboxy terminus of p53. In agreement with this finding, hTRbeta1 increased the binding of p53 to p53 DNA-binding elements. This increase in DNA binding, however, resulted in repression of p53-dependent transcription activation in transfected cells. Furthermore, hTRbeta1 led to an inhibition of the p53-mediated induction of bax and gadd45 expression. In contrast, the p53-induced expression of p21 was not affected by hTRbeta1, suggesting that the expression of p53-regulated genes is differentially modulated by hTRbeta1. Because the expressions of bax, gadd45, and p21 are directly regulated by p53, these results indicate that hTRbeta1 can modulate p53-regulated gene expression and support the hypothesis that there is cross-talk between these two regulatory pathways. The cross-talk between these two transcription factors could play an important role in the biology of normal and cancer cells.