Michael Klaus

Retinoic Acid-induced Expression of Apolipoprotein D and Concomitant Growth Arrest in Human Breast Cancer Cells Are Mediated through a Retinoic Acid Receptor RARα-dependent Signaling Pathway

Apolipoprotein D (apoD) is a human plasma protein, belonging to the lipocalin superfamily, that is produced by a specific subtype of highly differentiated breast carcinomas and that is strongly up-regulated by retinoic acid (RA) in breast cancer cells. In this work, we have examined the molecular mechanisms mediating the induction of apoD gene expression by retinoids in T-47D human breast cancer cells. Northern blot analysis revealed that Ro40-6055, a synthetic retinoid that selectively binds and activates the retinoic acid receptor RARα, induced the accumulation of apoD mRNA in breast cancer cells in a time- and dose-dependent manner. The time course analysis demonstrated that apoD mRNA was induced 14-fold over control cells after 48 h of incubation with 10−8 M Ro40-6055. As little as 10−11 M of this retinoid induced apoD mRNA 5-fold over the control, whereas incubation with 10−7 M Ro40-6055 induced maximally 15-fold over control cells. RARα-selective antagonists counteracted the inductive effects of all-trans-RA, 9-cis-RA, and Ro40-6055 on the expression of apoD, when present at the same concentration as the retinoid agonists. By contrast, RARβ-, RARγ-, and RXR-selective retinoids did not affect apoD gene expression. The retinoid agonist Ro40-6055 had an antiproliferative effect on T-47D cells, with maximal growth inhibition of approximately 60% obtained after 7 days of incubation with 10−7 M. This antiproliferative effect could be counteracted by a 100-fold excess of the antagonist Ro41-5253. Treatment of the cells with retinoids that do not bind the nuclear retinoic acid receptors did not affect apoD expression, despite the fact that they did have a strong antiproliferative effect on T-47D cells. On the basis of these results, a role for RARα on apoD gene expression induction by retinoids in breast cancer cells is proposed.

Teratogenicity of arotinoids (retinoids) in vivo and in vitro

The effect of structural modifications on the arotinoid molecule, a new class of retinoids, on their teratogenicity in mice was studied. Animals were treated on days 8 and 9 of gestation, the most susceptible stages to retinoid-induced malformations in rodents. The teratogenic potency of the 13 arotinoids tested varied over a dose range of more than five orders of magnitude. Next, we tested whether the quantitative differences in the teratogenicity of these arotinoids correlates with their activity in high density (micromass) cultures of rat embryonic limb bud and midbrain cells. There was a good quantitative correlation between the in vivo teratogenicity and the in vitro activity in limb bud cells but no correlation was found in midbrain cells. Thus, the limb bud cell culture system may be useful for a preliminary testing to select non-teratogenic retinoids. For the risk assessment in humans, however, the in vitro results should be verified in animals studies.

STRUCTURE CHARACTERISTICS OF NATURAL AND SYNTHETIC RETINOIDS

Publisher Summary The term retinoids comprises the natural occurring vitamin A derivatives such as retinol, retinal, and retinoic acid as well as the large number of synthetic analogs prepared since the late 1960s. The goal of this intensive effort at synthesis in academia as well as in the pharmaceutical industry was to improve the ratio of toxic effects (e.g., hypervitaminosis A) to therapeutic activity compared to retinoic acid. In addition, compounds with better selectivity toward the various therapeutic indications (dermatology, oncology, rheumatology, immunology) have been sought. Formally, the molecule of retinoic acid consists of three main sections. All three parts have been modified extensively. The chapter tabulates a list of important and often-used retinoids. The structural formula, molecular weight, systematic nomenclature, and trivial, as well as generic name are provided for each compound.