Retinoid toxicity for fibroblasts and epithelial cells is separable from growth promoting activity

Hormesis: Wound healing and keratinocytes

Hormetic dose responses (i.e., a biphasic dose/concentration response characterized by a low dose stimulation and a high dose inhibition) are shown herein to be commonly reported in the dermal wound healing process, with the particular focus on cell viability, proliferation, and migration of human keratinocytes in in vitro studies. Hormetic responses are induced by a wide range of substances, including endogenous agents, numerous drug and nanoparticle preparations and especially plant derived extracts, including many well-known dietary supplements as well as physical stressor agents, such as low-level laser treatments. Detailed mechanistic studies have identified common signaling pathways and their cross-pathway communications that mediate the hormetic dose responses. These findings suggest that the concept of hormesis plays a fundamental role in wound healing, with important potential implications for agent screening and evaluation, as well as clinical strategies.

Retinoic acid induces changes in electrical properties of adult neurons in a dose- and isomer-dependent manner

The electrical activity of neurons is known to play a role in neuronal development, as well as repair of adult nervous tissue. For example, the extension of neurites and motility of growth cones can be modulated by changes in the electrical firing of neurons. The vitamin A metabolite retinoic acid also plays a critical role during nervous system development and is also known to elicit regenerative responses, namely the induction, enhancement, and directionality of neurite outgrowth. However, no studies have previously reported the ability of retinoic acid to modify the electrical activity of neurons. In this study, we determined whether retinoic acid might exert effects on the nervous system by altering the electrical properties of neurons. Using cultured adult neurons from Lymnaea stagnalis, we showed that acute application of retinoic acid can rapidly elicit changes in neuronal firing properties. Retinoic acid caused the presence of atypical firing behavior such as rhythmic bursting and altered the shape of action potentials, causing increases in half-amplitude duration and decay time. Retinoic acid also caused cell silencing, whereby neuronal activity was halted within an hour. These effects of retinoic acid were shown to be both dose and isomer dependent. We then showed that the effects of retinoic acid on cell firing (but not silencing) were significantly reduced in the presence of an retinoid X receptor pan-antagonist HX531. This study suggests that some of the effects of retinoic acid during neuronal development or regeneration might possibly occur as a result of changes in electrical activity of neurons.

Thiazolidinediones: potential as therapeutics for psoriasis and perhaps other hyperproliferative skin disease

The thiazolidinediones constitute a family of synthetic compounds that act as high-affinity ligands for persoxisome proliferator-activated receptor-gamma (PPAR-gamma), a member of the nuclear hormone receptor family. Although originally developed to facilitate glucose control in patients with Type 2 diabetes, a number of studies showed that these agents effectively inhibited epithelial cell proliferation and tissue inflammation. Many of the initial cell growth inhibition studies were conducted with malignant epithelial cells from various sites; however, in addition to malignant epithelial cells, other studies showed that rapidly proliferating epidermal keratinoctyes in culture were also sensitive to the growth-inhibiting action of these moieties. Additional studies subsequently demonstrated that some patients with plaque psoriasis responded to treatment with one or another member of the thiazolidinedione family. Due to the potential therapeutic benefit of these compounds in diseases such as psoriasis, studies have been conducted to elucidate mechanisms by which growth inhibition is achieved. Interference with a number of growth-influencing signalling pathways has been demonstrated. Of interest, some of the growth-inhibiting effects are seen under conditions in which PPAR-gamma activation may not be responsible for the activity. Based on therapeutic potential, additional ongoing studies are aimed at developing novel thiazolidinediones that may have better efficacy than the currently available agents. Other studies are aimed at identifying optimal ways to use these agents in the treatment of hyperplastic skin diseases such as psoriasis.

Heparin-binding epidermal-growth-factor-like growth factor activation of keratinocyte ErbB receptors Mediates epidermal hyperplasia, a prominent side-effect of retinoid therapy

Sun-protected human skin was maintained in organ culture and treated with all-trans retinoic acid in the presence or absence of reversible or irreversible pharmacologic antagonists of c-erbB receptor tyrosine kinase activity. In the absence of these inhibitors, all-trans retinoic acid induced epidermal hyperplasia comparable to that induced in intact skin by all-trans retinol or all-trans retinoic acid itself. There was a strong correlation between inhibition of epidermal hyperplasia in organ culture and inhibition of epidermal-growth-factor-dependent keratinocyte growth in monolayer culture. In additional studies it was shown that all-trans retinoic acid could overcome the known inhibitory effects of calcium on expression of HB-EGF-like growth factor mRNA in organ-cultured skin. Further, it was shown that an antibody to HB-EGF-like growth factor inhibited retinoid-stimulated epidermal hyperplasia in organ culture and reduced proliferation in cultured keratinocytes. In contrast, the c-erbB receptor tyrosine kinase antagonists and the neutralizing HB-EGF-like growth factor antibody were ineffective in inhibiting all-trans-retinoic-acid-dependent survival and proliferation of human dermal fibroblasts. Taken together, these data indicate (i) that retinoid-induced epidermal hyperplasia in human skin proceeds through c-erbB, and (ii) that HB-EGF-like growth factor is one of the c-erbB ligands mediating this effect.

Effects of all-trans retinoic acid on UVB-irradiated human skin substitute

Retinoids are frequently used for treatment of photodamaged skin. We wished to find out whether photodamage could be attenuated by applying all-trans retinoic acid (RA) during repetitive irradiation. For this purpose, we used human cutaneous cells and tissue: pure monolayer cultures containing either keratinocytes or fibroblasts, and human skin substitute (SS) containing both cell types. All cultures were exposed to 8 mJ/cm(2) of UVB and were immediately treated with RA (0, 1.5, or 3 microM). The irradiation and RA treatment protocol was repeated until the cells of the nonirradiated culture had reached confluence. In the irradiated SS, RA preserved the structure (epidermal stratification and differentiation) and ultrastructure (well-organized intermediate filaments and desmosomes) in a state comparable to that observed in nonirradiated SS. As well RA maintained secretion of basement membrane components (laminin and type-IV collagen). Following irradiation, cutaneous cells also displayed more proliferative capacity when SS was treated. In the irradiated monolayer cultures, RA maintained the proliferative capacity of fibroblasts and decreased their differentiation whereas the opposite effect was seen on keratinocytes. In conclusion, RA clearly helps protect human skin against photodamage induced by repeated exposure to UVB.

Heterocyclic derivatives of all-trans retinoic acid: in vitro effects on fibroblast/keratinocyte growth and differentiation, and in vivo effects on guinea-pig skin

We investigated the biological effects of five all-trans retinoic acid derivatives, bearing heterocyclic ring systems in the side chain. Growth assays performed on submerged human fibroblast and keratinocyte cultures revealed that (E)4-[2-(5-terbuthyl-thiophen-2-yl)propenyl]benzoic acid (compound 5) is the best compound among the studied derivatives for it exhibits a weaker antiproliferative activity and induces, like all-trans retinoic acid does, a significant increase in fibroblast and keratinocyte growth. The morphological and morphometrical analyses of submerged human fibroblast cultures and human epidermis reconstructed in vitro showed that the compound 5 behaves similarly to all-trans retinoic acid: it induces a decrease in all the cell parameters of submerged fibroblast cultures, and modulates the differentiation of keratinocytes in in vitro reconstructed epidermis. Compound 5 induces thickening of epidermis in vivo, one of the most remarkable pharmacological effects of retinoids on skin, but compared to all-trans retinoic acid, it induces a weaker irritation on guinea-pig skin in terms of both erythema and scaling. Compound 5 could then represent a promising candidate for the treatment of certain dermatological diseases.