Arsenic trioxide inhibits nuclear receptor function via SEK1/JNK-mediated RXRα phosphorylation

Abstract

We have previously published that 2 proven treatments for acute promyelocytic leukemia, As 2 O 3 and retinoic acid, can be antagonistic in vitro. We now report that As 2 O 3 inhibits ligand-induced transcription of the retinoic acid receptor, as well as other nuclear receptors that heterodimerize with the retinoid X receptor α (RXRα). As 2 O 3 did not inhibit transactivation of the estrogen receptor or the glucocorticoid receptor, which do not heterodimerize with RXRα. We further show that As 2 O 3 inhibits expression of several target genes of RXRα partners. Phosphorylation of RXRα has been reported to inhibit nuclear receptor signaling, and we show by in vivo labeling and phosphoamino acid detection that As 2 O 3 phosphorylated RXRα in the N-terminal ABC region exclusively on serine residues. Consistent with our previous data implying a role for JNK in As 2 O 3 -induced apoptosis, we show that pharmacologic or genetic inhibition of JNK activation decreased As 2 O 3 -induced RXRα phosphorylation and blocked the effects of As 2 O 3 on RXRα-mediated transcription. A mutational analysis indicated that phosphorylation of a specific serine residue, S32, was primarily responsible for inhibition of RXRα-mediated transcription. These data may provide some insight into the rational development of chemotherapeutic combinations involving As 2 O 3 as well as into molecular mechanisms of arsenicinduced carcinogenesis resulting from environmental exposure.

Uses for JNK: the Many and Varied Substrates of the c-Jun N-Terminal Kinases

“…h The first four sequences are murine, and the last sequence is human (2,204). i In the protein sequence, there are 11 S-P and 2 T-P motifs.…”

“…JNK has also been implicated in the inhibition of RXR␣ transactivation when cells are exposed to stress in the form of arsenic trioxide (204). Mutational analysis has suggested the requirement for Ser32, suggesting this as the novel Ser target for JNK involved in the inhibition of nuclear receptor function (Table 1) (204).…”

Role of mitogen-activated protein kinase kinase 4 in cancer

“…There is evidence that retinoid receptors can suppress tumorigenesis and that the loss of specific receptors promotes carcinogenesis in many tissues (Altucci and Gronemeyer, 2001). Retinoid acid receptor a and retinoid X receptor a (RXRa) are phosphorylated by JNK leading to inhibition of retinoic acid (RA)-mediated transcriptional events (Adam-Stitah et al, 1999;Lee et al, 2000;Mann et al, 2005;Srinivas et al, 2005). Surprisingly, RXRa is also directly phosphorylated by MKK4 on Tyr residues leading to the suppression of RA-mediated transcription (Lee et al, 2000).…”

Activation of mammalian target of rapamycin and the p70 S6 kinase by arsenic trioxide in BCR-ABL–expressing cells

“…Generation of reactive oxygen species by As 2 O 3 potentiates induction of cell killing, and an accumulating body of evidence points towards a role for reactive oxygen species, particularly H 2 O 2 , on arsenic-induced apoptosis (9,10). Recent work has uncovered an additional mechanism by which As 2 O 3 may generate its anti-leukemic responses, involving inhibition of nuclear receptor function via c-Jun NH 2 -terminal kinase -mediated retinoid X receptor a phosphorylation (11). Thus, several cellular cascades seem to be engaged in the generation of arsenic-dependent growth inhibition and apoptosis of malignant cells.…”