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.
Highlights
Retinoic acid (RA), the bioactive derivative of dietary Vitamin A and beta-carotene, is a powerful signaling molecule controlling cell proliferation [1]
We show that in RA-sensitive cells with a functional RARa signaling RA leads to growth inhibition consequent to the concerted upregulation of neutral sphingomyelinase, one of the enzymes leading to the synthesis of the antiproliferative/propaptotic ceramide (CER), and downregulation of sphingosine kinase 1 (SK1), the enzyme leading to the synthesis of the prosurvival sphingosine-1-phosphate (S1P)
In this study we provide mechanistic evidence that RA can act as a growth inhibitor or a growth promoter according to the functional status of RARa
Summary
Retinoic acid (RA), the bioactive derivative of dietary Vitamin A and beta-carotene, is a powerful signaling molecule controlling cell proliferation [1]. RA exerts an inhibitory action on cell proliferation via specialized transcription factors, the nuclear RA receptors, RARs [2]. Retinoic 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. In the course of one of our studies we observed that disruption of RA-receptor alpha, RARa, abrogates the RA-mediated growth-inhibitory effects and unmasks the growth-promoting face of RA In the absence of a functional RARa, 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
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