Scott M. Thacher

Therapeutic Applications for Ligands of Retinoid Receptors

Synthetic retinoids, ligands for the RAR and RXR members of the steroid/thyroid superfamily of nuclear hormone receptors, are used for the treatment of psoriasis, acne, photoaging and cancer. Retinoid mechanisms of action for these conditions largely involve effects on epithelial differentiation and modulation of inflammation with some impact on the immune system. Retinoid medicinal chemistry in recent years has identified ligands highly specific for one of the three RAR subtypes (RAR-alpha) and for the RXR family of receptors, as well as antagonists for the RARs, RARalpha and the RXRs. Structure-activity relationships among the novel retinoid classes are reviewed along with potential therapeutic activities and side effects. RAR-alpha specific retinoids inhibit cancer cell growth but lack other retinoid toxicities, including skin irritation now ascribed to RAR-gama. RXR-specific retinoids lower blood glucose in animal models of type 2 diabetes albeit with a potential for mild hypothyroidism. Function-selective retinoids, especially a class of RAR antagonists called inverse agonists, have unexpected gene regulatory activity. Given the diverse properties and tissue distributions of the retinoid receptors, synthesis of additional classes of receptor-specific and function-selective ligands has the potential to produce novel therapeutic applications.

Determination of retinoid activity by an enzyme-linked immunosorbent assay

In normal human keratinocytes, retinoic acid suppresses the expression of the plasma membrane associated enzyme transglutaminase (TGm) at the pretranslational level. This finding led us to develop an enzyme-linked immunosorbent assay (ELISA) for the evaluation of the biological activity of retinoids, i.e., natural and synthetic derivatives of vitamin A. In this assay, keratinocytes are cultured in a 96-well cluster in the presence of different retinoid concentrations. The expression of TGm is then quantified, without any extraction or purification step, using a TGm-specific monoclonal antibody and a peroxidase-conjugated secondary antibody. The dose-response curves obtained show this ELISA to be a sensitive and reproducible assay to determine the potency of retinoids.

Localization of the 230-kilodalton bullous pemphigoid antigen in cultured keratinocytes: formation of a prehemidesmosome.

The hemidesmosome is the major attachment structure of the epidermal basal cell visible ultrastructurally in skin. The importance of its components to cultured cell attachment to substratum is not understood, however. A component of the hemidesmosome, the 230-kDa bullous pemphigoid antigen (p230), has been shown to be present in an insoluble or particulate fraction of cultured cells. In order to more fully characterize its potential importance for cell-matrix adhesion in cultured keratinocytes, specific antibodies were raised to the C-terminal region of p230 expressed as a bacterial fusion protein. Such antibodies recognize the hemidesmosome of epidermis, binding on the cytoplasmic region of its plaque. In addition, keratinocytes cultured in a 0.15 mM Ca(2+)-defined medium contain a detergent-resistant pool of p230 which appears to lie in the same focal plane as the culture substrate and has a patchy or irregular distribution by indirect immunofluorescence. Treatment of cultured cells at 4 degrees C with trypsin or pronase sufficient to release keratinocytes from the culture dish does not affect the electrophoretic migration of p230 on SDS-gels, suggesting that p230 is not exposed to the extracellular space. In cells cultured in 0.15 mM Ca2+, 230-kDa BP antigen is localized to discrete clusters resting near the basal plasma membrane of the cell by immunogold staining following brief detergent treatment and fixation. These clusters are approximately 0.1 micron in diameter, which is similar in size to the in vivo hemidesmosome. Fully formed electron dense hemidesmosomal plaques are not observed under the same culture conditions, however. It appears that these clusters are early precursors of the hemidesmosome.

Retinoic acid (RA) receptor transcriptional activation correlates with inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced ornithine decarboxylase (ODC) activity by retinoids: a potential role for trans-RA-induced ZBP-89 in ODC inhibition.

Evaluation of retinoic acid receptor (RAR) subtype–selective α and γ agonists and antagonists and a retinoid X receptor (RXR) class–selective agonist for efficacy at inhibiting both induction of ornithine decarboxylase (ODC) by the tumor promoter 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) in mouse epidermis and rat tracheal epithelial cells and the appearance of papillomas in mouse epidermis treated in the 2‐stage tumor initiation–promotion model indicated that (i) RXR class–selective transcriptional agonists, such as MM11246, were not involved in ODC inhibition; (ii) RAR‐selective agonists that induce gene transcription from RA‐responsive elements (RAREs) were active at low concentrations; (iii) RAR‐selective antagonists that bind RARs and inhibit AP‐1 activation on the collagenase promoter but do not activate RAREs to induce gene transcription were less effective inhibitors; and (iv) RARγ‐selective retinoid agonists were more effective inhibitors of TPA‐induced ODC activity than RARα‐selective agonists. These results suggest that RARE activation has a more important role in inhibition of ODC activity than RXR activation or AP‐1 inhibition and that RARγ‐selective agonists would be the most useful inhibitors of epithelial cell proliferation induced by tumor promoters. The natural retinoid all‐trans‐RA induced expression of transcription factor ZBP‐89, which represses activation of the GC box in the ODC promoter by the transcription factor Sp1. Int. J. Cancer 91:8–21, 2001.

A new class of RAR subtype selective retinoids: correlation of pharmacological effects with receptor activity.

The synthesis and biological activity of a series of structurally related retinoids with different RAR subtype selectivities are described. These retinoids bind to all three RAR subtypes but in functional transactivation assays, they show RARbeta or RARbeta,gamma selectivity with weak RARalpha activity. The subtype selectivity of these retinoids was found to correlate with their efficacy (ODC inhibition) and toxicity (topical irritation and teratogenicity) profiles. The degree of RARgamma transactivation activity correlates with their topical toxicity and teratogenicity as measured by the inhibition of chondrogenesis. Of the RARbeta selective retinoids reported here, retinoid 12 is the most promising, as it is completely devoid of two common retinoid related toxicities, namely topical irritation and teratogenesis.

Synthesis and biological activity of 1,2,3,4-tetrahydroquinoline and 3,4-(1H)-dihydroquinolin-2-one analogs of retinoic acid

Abstract Retinoids are natural and synthetic analogs of the hormone retinoic acid. Retinoids are currently being investigated clinically as drugs in several areas, including dermatology and oncology. We report the synthesis and biological activity of a new series of potent, RAR-specific retinoids substituted with a 1,2,3,4-tetrahydroquinoline or a 3,4-(1H)-dihydroquinolin-2-one group.