RARα/RXR synergism potentiates retinoid responsiveness in cutaneous T-cell lymphoma cell lines

Abstract

Retinoids, natural and synthetic derivatives of vitamin A, induce cellular changes by activating nuclear retinoic acid receptors (RAR) and retinoid X receptors (RXR). Although the ability of retinoids to govern gene expression is exploited clinically for cancer therapeutics, the full benefit of retinoid-based strategies is unrealized due to detrimental side effects. Delineating the receptors that prompt cellular outcomes is critical to advancing retinoid-based approaches. Here, we identify the receptors that evoke multiple responses in cutaneous T-cell lymphoma (CTCL). The data demonstrate that RARα drives integrin β7-dependent adhesion and CCR9-mediated chemotaxis in CTCL cells. Of note, concomitant activation of RARα and RXR nuclear receptors yielded synergistic increases in adhesion and migration at concentrations where single agents were ineffective. As the established paradigm of retinoid action in CTCL is apoptosis and growth arrest, the role of RARα/RXR in these events was studied. As with adhesion and migration, RARα/RXR synergism prompted apoptosis and dampened CTCL cell proliferation. Strikingly, RARα/RXR synergism induced responses from CTCL cell lines previously reported to be unresponsive to retinoids. These data provide a novel framework that may further refine a proven CTCL therapy.

Keywords: chemotaxis; integrin; lymphoma; retinoid; vitamin A.

Figure 1

RARα activation prompts β7 expression and function in CTCL. (a) MJ or HuT78 cells were treated with DMSO or 2×EC₅₀ of well-established RAR isotype agonists for 24, 48 or 72 hrs. Surface β7 integrin expression was determined through flow cytometry and displayed as mean fluorescence intensity (MFI). (b) MJ or HuT78 cells were cultured in the presence of DMSO, 100 nM ATRA, 2×EC₅₀ RARα, β, or γ agonists for the indicated time. Static cell adhesion assays were assessed on 0.75 μg/ml of the β7 specific ligand MAdCAM-1. (c) MJ cells were cultured with the indicated RAR isotype antagonists at 500 nM for 24 hrs. Cells were further subcultured for an additional 24 hrs in the presence of 200 nM ATRA. Static cell adhesion assays were conducted as described before. Ordinate represents data that have been normalized to adhesion levels obtained with ATRA in the absence of antagonist. (d) Adhesion assays were repeated as previously described in (B) with the non-CTCL cell line, Jurkat. (e) Whole cell lysates (30 μg/lane) of CTCL or non-CTCL (Jurkat) cell lines were examined for the presence and relative abundance of the various RAR receptor isotypes.

Figure 2

RARα/RXR coactivation induces CTCL cell adhesion in a synergistic manner. (a) MJ or HuT78 cells were cultured in the presence of RAR isotype-specific agonists (1×EC₅₀) alone or in combination with 5 nM Bexarotene for 24 hrs. Cells were then assessed for adhesion to 0.75 μg/ml of MAdCAM-1. (b) CTCL cell line SeAx or MyLa cells were cultured with DMSO, 2×EC₅₀ RARα agonist, 100 nM Bexarotene, or a combination of agonists for 72 hrs. The extent of adhesion was determined on the integrin β7-ligand MAdCAM-1. (c) HuT78 cells were cultured in the presence of an RAR isotype-specific agonist (1×EC₅₀) alone or in combination with 100 nM of the pan-RXR activator SR11237 for 48 hrs. Cell adhesion was then determined as previously described. (d) Real-time quantitative PCR was conducted with primers that amplify regions encoding the human integrin α4 or β7 subunits. Templates were derived from MJ cells cultured with vehicle or RARα/RXR agonists. The ordinate reflects normalized data obtained by dividing values for RARα/RXR agonists with those from vehicle treated samples.

Figure 3

CCR9-mediated chemotaxis to CCL25 is enhanced by RARα/RXR activity. (a) CTCL cell lines MJ and HuT78 or the non-CTCL T cell lines CCRFCEM and Jurkat were treated with 1 μM ATRA or 1 μM Bexarotene for 72 hrs. Percentage of CCR9⁺ cells was detected through flow cytometry. (b) MJ and HuT78 cells were cultured with DMSO, 1 μM ATRA or 10 μM Bexarotene for 72 hrs. Directed migration towards 100 ng/ml of CCL25 was determined and displayed on the ordinate in arbitrary units (AU). (c) HuT78 or HuT102 cells were incubated with the vehicle DMSO, 1×EC₅₀ RARα agonist, 5 nM Bexarotene or combination of the two agonists. As described in (b), the relative extent of chemotaxis towards 100 ng/ml of CCL25 was measured.

Figure 4

RARα/RXR cooperatively induce apoptosis and inhibit CTCL cell proliferation. (a) HuT78, MJ cells, MyLa cells or (b) the non-CTCL cell line Jurkat were treated with DMSO, 1×EC₅₀ RARα agonist, 5 nM Bexarotene or combination thereof for 48 hrs. Data shown are the percentage of Annexin V positive cells. (c) Total cell lysates (40 μg/lane) from CTCL or non-CTCL cell lines were assessed for the indicated apoptotic protein marker. Lysates were derived from untreated cells or cells treated with RARα/RXR agonists for 72 hrs. (d) HuT78 or SeAx cells were incubated with DMSO or retinoids as described above for 48 hrs or 72 hrs, respectively. BrdU cell proliferation assays were conducted and the relative extent of proliferation is shown as arbitrary units (AU).