Retinoic acid provokes a regeneration-like proliferative response in murine epidermis. A bivariate DNA/bromodeoxyuridine flow cytometric study

Retinoic acid signaling identifies a distinct precursor population in the developing and adult forebrain

We asked whether retinoic acid (RA), an established transcriptional regulator in regenerating and developing tissues, acts directly on distinct cell classes in the mature or embryonic forebrain. We identified a subset of slowly dividing precursors in the adult subventricular zone (SVZ) that is transcriptionally activated by RA. Most of these cells express glial fibrillary acidic protein, a smaller subset expresses the epidermal growth factor receptor, a few are terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling positive, and they can be mitotically labeled by sustained rather than acute bromodeoxyuridine exposure. RA activation in similar cells in SVZ-derived neurospheres depends on retinoid synthesis from the premetabolite retinol. The apparent influence of RA on precursors in vitro is consistent with key properties of RA activation in the SVZ; in neurospheres, altered retinoid signaling elicits neither cell death nor an acute increase in cell proliferation. There is apparent continuity of RA signaling in the forebrain throughout life. RA-activated, proliferative precursors with radial glial characteristics are found in the dorsal lateral ganglionic eminence and ventrolateral palliumembryonic rudiments of the SVZ. Thus, endogenous RA signaling distinguishes subsets of neural precursors with glial characteristics in a consistent region of the adult and developing forebrain.

Once and again: Retinoic acid signaling in the developing and regenerating olfactory pathway

Retinoic acid (RA), a member of the steroid/thyroid superfamily of signaling molecules, is an essential regulator of morphogenesis, differentiation, and regeneration in the mammalian olfactory pathway. RA-mediated teratogenesis dramatically alters olfactory pathway development, presumably by disrupting retinoid-mediated inductive signaling that influences initial olfactory epithelium (OE) and bulb (OB) morphogenesis. Subsequently, RA modulates the genesis, growth, or stability of subsets of OE cells and OB interneurons. RA receptors, cofactors, and synthetic enzymes are expressed in the OE, OB, and anterior subventricular zone (SVZ), the site of neural precursors that generate new OB interneurons throughout adulthood. Their expression apparently accommodates RA signaling in OE cells, OB interneurons, and slowly dividing SVZ neural precursors. Deficiency of vitamin A, the dietary metabolic RA precursor, leads to cytological changes in the OE, as well as olfactory sensory deficits. Vitamin A therapy in animals with olfactory system damage can accelerate functional recovery. RA-related pathology as well as its potential therapeutic activity may reflect endogenous retinoid regulation of neuronal differentiation, stability, or regeneration in the olfactory pathway from embryogenesis through adulthood. These influences may be in register with retinoid effects on immune responses, metabolism, and modulation of food intake.

Cellular actions of etretinate in psoriasis: enhanced epidermal differentiation and reduced cell-mediated inflammation are unexpected outcomes

Retinoids are potent cell growth and differentiation modulators, but cellular effects of therapeutic retinoids in psoriasis are unknown. We studied the effects of etretinate on pathological activation of keratinocytes and lymphocytes in patients treated systemically with this agent for 8 weeks. Ten patients with extensive psoriasis vulgaris were treated with etretinate at 0.75 mg/kg for 8 weeks. Skin biopsies obtained before and at 8 weeks of treatment were studied using immunohistochemical markers for keratinocyte proliferation or differentiation and for the presence of T-lymphocyte subsets or associated inflammatory proteins. During 8 weeks of treatment, the clinical severity decreased by a mean of 64% (p < 0.001). Compared to a similar group of patients treated with bath PUVA, psoriatic plaque erythema resolved more slowly and less completely (p < 0.05), but improvements in plaque thickness and scale were not significantly different between etretinate and PUVA treatments. Etretinate produced a 44% decrease in epidermal thickness (p < 0.001) and a 62% reduction in keratinocyte proliferation (p < 0.001) after 8 weeks of treatment. Unexpectedly, keratinocyte differentiation was enhanced following etretinate treatment as indicated by increased filaggrin production, increased number and size of keratohyaline granules, greater abundance of keratin filaments, and increased secretion of intercellular lipids from Odland bodies. The stratum corneum in resolving psoriatic lesions was unusually thin, probably caused by retinoid-induced shedding of corneocytes. "Regenerative" epidermal growth was maintained during etretinate treatment, as marked by continued expression of keratin 16 and alpha 3-integrin by suprabasal keratinocytes. Surprisingly the inflammation-associated proteins HLA-DR and ICAM-1 were no longer produced by epidermal keratinocytes following etretinate treatment, and CD3+, CD8+, and CD25+ T-lymphocyte subsets were reduced by 50-65% in lesional tissue (p < 0.01). Etretinate shows unexpected anti-inflammatory and pro-differentiation actions in psoriasis. Etretinate appears to function as a disease suppressive agent which improves hyperplasia, keratinocyte differentiation and tissue inflammation mediated by cellular immune elements.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Topical tacalcitol (1,24-(R)-dihydroxyvitamin D3) induces a transient increase in thymidine incorporation and calmodulin content in pig epidermis following tape stripping in vivo

Tape stripping induces transient increase in keratinocyte proliferation in vivo. The effects of tacalcitol (1,24-(R)-dihydroxyvitamin D3) ointment on the cell kinetics of pig epidermis after the tape stripping were investigated. The tacalcitol ointment (2 micrograms/g) was applied once to the back of pigs immediately after the tape stripping. The pig epidermal cell kinetics were analyzed at various times following the treatment. Tape stripping transiently increased thymidine incorporation of keratinocytes; the maximal effect was observed at 24 h. Tape stripping-induced increase in thymidine incorporation was markedly augmented by tacalcitol treatment. At 24 h following the tape stripping DNA-flow cytometry revealed an accelerated transition from G0/1 to S phase of cell cycle in tacalcitol treated epidermis. There was no significant difference, however, in mitotic counts and G2/M phase fractions between tape stripping-treated and tape stripping plus tacalcitol ointment-treated epidermis. We also measured calmodulin content of pig epidermis following the treatments. Although tape stripping slightly increased calmodulin content of pig epidermis, this was statistically not significant. Tape stripping plus tacalcitol ointment treatment resulted in a significant increase in calmodulin content at 24 h following the treatment. There was no significant difference in calmodulin content between tape stripping treated- and tape stripping plus tacalcitol-treated epidermis.

Differential effects of topical retinoic acid application on keratin K1 and filaggrin expression in mouse epidermis

Retinoic acid (RA) modulates epidermal homeostasis and affects differentiation-associated proteins such as keratin K1 and filaggrin. Because results from in vitro and in vivo studies have been conflicting with respect to RA effects on keratinization, we examined the terminal differentiation of epidermal cell cohorts after RA stimulation in vivo. Pulse-labelling with 5-bromo-2-deoxyuridine (BrdU) was performed by intraperitoneal injection of mice immediately or at 16 h after a single topical application of 100 nmol RA. The cell cohort labelled at the time of RA application consisted of previously unperturbed cells exposed to RA after initiation of S-phase, whereas the cohort labelled 16 h after RA application consisted of cells stimulated into the S-phase by RA. These two cohorts of partially synchronized cells were followed for up to 72 h after BrdU labelling. Such labelling combined with keratin K1 or filaggrin expression was scored by paired immunofluorescence staining of skin sections. The onset of keratin K1 expression was unchanged in both series after RA treatment, while filaggrin appeared earlier than in controls. The differential effect of RA on the maturation markers was related to the proliferative activity, the increased cell turnover, and the shortened epidermal transit time. The onset of keratin expression appeared to be regulated before the postmitotic period, whereas filaggrin expression appeared to be regulated during the late phase of the maturation process, thus being influenced by the actual epidermal kinetics and structural alterations. These results suggested that the effect of RA on epidermal differentiation is secondary to its effect on proliferation, as determined by the altered cellular age distribution following regenerative proliferation.

1,25-Dihydroxyvitamin D3 and the vitamin D analogue KH1060 induce hyperproliferation in normal mouse epidermis. A BrdUrd/DNA flow cytometric study

1,25-dihydroxyvitamin D3 (calcitriol) affects differentiation and proliferation of epidermal keratinocytes in vitro and in vivo. We have studied the topical effects of calcitriol (0.08-2.0 micrograms/ml) and of a new vitamin D analogue, the epi-20-analogue KH1060 (0.4-2.0 micrograms/ml) on epidermal proliferation in normal hairless mice. Epidermis was examined at intervals from 4 h to 8 days after a single-dose application. The mitotic rate was assessed by the stathmokinetic method and hyperplasia was scored in histological sections. Cell cycle parameters were measured by bivariate bromodeoxyuridine (BrdUrd)/DNA flow cytometry on isolated epidermal basal cells after pulse-labelling with BrdUrd. Both calcitriol and KH1060 induced a dose- and time-dependent increase in the mitotic rate and in hyperplasia, the latter drug being the most effective. Calcitriol and KH1060 induced changes in the cell cycle traverse compatible with the regenerative reaction seen after other hyperplasiogens, but with an additionally increased accumulation of cells in the G2 phase. This is similar to that seen after topical application of retinoic acid to mouse skin. Our results are thus in contrast to the anti-proliferative effects of calcitriol observed in vitro and following treatment of the hyperproliferative disease psoriasis with calcitriol as well as other vitamin D analogues.