Delineating the Nuclear Receptors Involved in Retinoid Responsiveness in CTCL Cells

Introduction: Cutaneous T Cell Lymphoma (CTCL) has an annual incidence of one per 6.4 million persons in the United States and represents 3.9% of all non-Hodgkin lymphomas. Numerous treatment modalities exist, including both oral Bexarotene and topical Bexarotene 1% gel. Bexarotene is a synthetic retinoid that selectively activates retinoid X receptor (RXR) isotypes. Retinoids, natural metabolic derivatives and synthetic analogs of vitamin A, are physiologically important in establishing mucosal immunity. Retinoids induce gut homing in T lymphocytes effectively targeting immune cells away from the skin niche. Although it is appreciated that retinoids activate the retinoic acid receptor (RAR) and retinoid X receptor (RXR) family of nuclear receptors to elicit cellular changes, the molecular details of retinoic acid action remain poorly defined within the context of CTCL therapy. The specific nuclear receptor isotypes involved in CTCL responsiveness and if this responsiveness entails the expression of molecules associated with gut homing is not known. The significance of this knowledge gap is championed by the fact that the full benefit of Bexarotene in CTCL is limited due to adverse metabolic side effects, namely, hypercholesterolemia, hypothyroidism, and hyperlipidemia. The promiscuous pairing nature of RXR with other nuclear receptors such as the liver X receptor and thyroid hormone likely accounts for the detrimental effects. Delineating how retinoids influence CTCL at the molecular level would greatly increase the specificity and efficacy of retinoid-based therapies.

Results: Here we employ a battery of agonists and antagonists, including Bexarotene, to delineate the specific nuclear receptors utilized by retinoids to evoke adhesion changes within CTCL cells. When a spectrum of T cell lymphomas/leukemias are cultured with the physiologically relevant all-trans-retinoic acid (atRA) retinoid isomer, only CTCL lineages increase integrin-mediated adhesion to the gut trafficking ligand, Mucosal Adressin Cell Adhesion Molecule (MAdCAM-1). Our data further demonstrate that the individual activation of RAR or RXR nuclear receptors isotypes was sufficient to induce adhesion to MAdCAM-1 within the CTCL cells. Our data also establish that the induced immune cell adhesion was largely attributable to the specific activity of the RAR-alpha receptor isotype. However, longer exposure to other isotype-specific agonists did induce adhesion. Most importantly, we show that the adhesion to MAdCAM-1 was significantly augmented upon activation of both RAR and RXR nuclear receptors in a synergistic manner. In addition, this synergism was only observed with the RAR-alpha isotype.

Conclusions: The current study provides molecular resolution as to which nuclear receptors transduce retinoid exposure into biologically relevant CTCL cell adhesion. Our work suggests that Bexarotene therapy can be augmented by combinatorial drug therapy with a RAR-alpha agonist to induce synergistic receptor activation. Decreasing the concentration of Bexarotene required to elicit a desired response and increasing the specificity by targeting distinct receptor isotypes would plausible minimize the adverse side effects associated with retinoids. The significance of this work is underscored by the historical success of retinoid-based therapies such as Bexarotene and the potential undiscovered therapies still embodied within vitamin A.