AC 55649 is a potent and selective agonist of retinoic acid receptor β2 (RARβ2) isotype with a pEC50 value of 6.9 ± 0.4 [1].

RARβ2 is a tumor suppressor gene. In several human neoplasms, it is frequently hypermethylated [2].

In MCF7 cells, AC 55649 significantly inhibited cellular growth, appeared to inhibit DNA synthesis as seen in experiments of [3H] thymidine incorporation. The extent of the inhibition was comparable to the one resulted from the presence of all-trans-retinoic acid [1]. In vitro, RARβ2 induced the neurite outgrowth in adult mouse spinal cord [3]. Before neurite outgrowth, cells were stimulated with either AC 55649 or retinoic acid for 1 week. Treatment with retinoic acid largely increased the neurite outgrowth of cells, compared with the control. This result was revealed by immunocytochemistry. AC 55649 also induced the neurite outgrowth of NTERA-2 cells. This indicated that AC 55649 mediated neuronal differentiation in a manner similar to that found with retinoic acid. Using another differentiation marker, Neurofilament L, similar results were also evident [1].

Retinoids influence both differentiation and morphogenetic events during development of the vertebrate limb. These effects are mediated by nuclear retinoid receptors. In the posterior limb bud, transcripts of RARβ2 but not RARβ1 are enriched to three-fold. Transcripts of RARβ1 are initially present throughout the limb bud ectoderm and mesenchyme, then become restricted within the loose connective tissue and perichondrial regions of the limb [4]. In Xenopus, treated with AC 55649 at 100 nM, approximately 65% of larvae survived and practically all started to form a pair of bilaterally symmetric limb buds. Treatments with AC 55649 provoked the death of larvae at the metamorphosis onset at around stage 62. Stage 62 is a stage showing peak levels of T3 and T4 thyroid hormones. Meanwhile all larvae without treatment were metamorphosed [5].

References:
[1].  Piu F, Gauthier NK, Olsson R, et al. Identification of novel subtype selective RAR agonists. Biochemical pharmacology, 2005, 71(1): 156-162.
[2].  Jerónimo C, Henrique R, Hoque MO, et al. Quantitative RARβ2 Hypermethylation A Promising Prostate Cancer Marker. Clinical Cancer Research, 2004, 10(12): 4010-4014.
[3].  Maden M, Hind M. Retinoic acid, a regeneration-inducing molecule. Developmental dynamics, 2003, 226(2): 237-244.
[4].  Cuervo R, Chimal-Monroy J. Chemical activation of RARβ induces post-embryonically bilateral limb duplication during Xenopus limb regeneration. Scientific reports, 2013, 3.
[5].  Smith SM, Kirstein IJ, Wang ZS, et al. Differential expression of retinoic acid receptor-β isoforms during chick limb ontogeny. Developmental Dynamics, 1995, 202(1): 54-66.