Disruption of Retinoic Acid Receptor Alpha Reveals the Growth Promoter Face of Retinoic Acid

Abstract

BackgroundRetinoic acid (RA), the bioactive derivative of Vitamin A, by epigenetically controlling transcription through the RA-receptors (RARs), exerts a potent antiproliferative effect on human cells. However, a number of studies show that RA can also promote cell survival and growth. In the course of one of our studies we observed that disruption of RA-receptor alpha, RARα, abrogates the RA-mediated growth-inhibitory effects and unmasks the growth-promoting face of RA (Ren et al., Mol. Cell. Biol., 2005, 25:10591). The objective of this study was to investigate whether RA can differentially govern cell growth, in the presence and absence of RARα, through differential regulation of the “rheostat” comprising ceramide (CER), the sphingolipid with growth-inhibitory activity, and sphingosine-1-phosphate (S1P), the sphingolipid with prosurvival activity.Methodology/Principal FindingsWe found that functional inhibition of endogenous RARα in breast cancer cells by using either RARα specific antagonists or a dominant negative RARα mutant hampers on one hand the RA-induced upregulation of neutral sphingomyelinase (nSMase)-mediated CER synthesis, and on the other hand the RA-induced downregulation of sphingosine kinase 1, SK1, pivotal for S1P synthesis. In association with RA inability to regulate the sphingolipid rheostat, cells not only survive, but also grow more in response to RA both in vitro and in vivo. By combining genetic, pharmacological and biochemical approaches, we mechanistically demonstrated that RA-induced growth is, at least in part, due to non-RAR-mediated activation of the SK1-S1P signaling.Conclusions/SignificanceIn the presence of functional RARα, RA inhibits cell growth by concertedly, and inversely, modulating the CER and S1P synthetic pathways. In the absence of a functional RARα, RA–in a non-RAR-mediated fashion–promotes cell growth by activating the prosurvival S1P signaling. These two distinct, yet integrated processes apparently concur to the growth-promoter effects of RA.

ATRA transcriptionally induces nSMase2 through CBP/p300-mediated histone acetylation

“…In ChIP experiments, we also observed enrichment of RAR- at the nSMase2 gene (data not shown), as was seen before (22). Collectively, these data agree with previous studies in MCF7 and T47D cells (9,22,23); moreover, the role that nSMase2 plays in ATRA-induced growth arrest (5) is consistent with the well-established antiproliferative role of RAR- (23,29,40,41). Additionally, as numerous studies have found that loss of RAR- signaling leads to alterations in mammary acini formation both in vivo and in vitro (27,(42)(43)(44), this suggests that the role of nSMase2 in ATRA-induced growth mRNA levels.…”

“…Indeed, emerging evidence has begun to point to a more protracted regulation of at the transcriptional level. Increases in nSMase2 expression have been reported in response to bone morphogenetic protein-2 (BMP2) (17,18), daunorubicin (19), cigarette smoke (20), confluence (6), the hedgehog signaling mediator cyclopamine (21), and all-trans retinoic acid (ATRA) (5,22,23). Notably, in the latter two cases, increased expression of nSMase2 was required for ATRA-induced growth arrest (23) and cyclopamine-induced apoptosis, respectively (21).…”

“…However, recent emerging evidence has pointed to a more protracted regulation of nSMase2 occurring at the level of expression. Indeed, increased expression of nSMase2 has been reported in response to chemotherapies (19,21), BMP2 (17,18), confluence (6), and ATRA (5,22,23).…”

Retinoic acid-producing,ex-vivo-generated human tolerogenic dendritic cells induce the proliferation of immunosuppressive B lymphocytes

“…+ B cells was due exclusively to RA, or if additional mechanisms are involved we co-cultured 2 × 10 5 iDC in the presence of an equal number of syngeneic CD19 + B cells enriched from thawed, viable PBMC in the presence or absence of 1 mM of ER-50891, a pan-RAR selective antagonist [48] for 72 h. In Fig. 4d we demonstrate that the frequency of CD19…”

Neutral Sphingomyelinase-2 Mediates Growth Arrest by Retinoic Acid through Modulation of Ribosomal S6 Kinase

“…Moreover, ATRA increased de novo Cer synthesis in human neuroblastoma cell lines (14). More recently, prolonged ATRA stimulation (72 h) of estrogen receptor-positive T47D cells increased nSMase2 expression and Cer and decreased expression of sphingosine kinase 1, responses that were suggested to be important for a reduction in cell proliferation (15). However, despite these studies, the wider effects of ATRA on the whole sphingolipid network are unknown.…”