Expression of activin A in human keratinocytes at early stages of cultivation

Langerhans cells are strongly reduced in the skin of transgenic mice overexpressing follistatin in the epidermis

Activins are members of the transforming growth factor-beta (TGF-beta) family and are important for skin morphogenesis and wound healing. TGF-beta1 is necessary for the population of the epidermis with Langerhans cells (LC). However, a role for activin in LC biology is not known. To address this question, we analyzed skin from transgenic mice overexpressing the activin antagonist follistatin in the epidermis. Using immunofluorescence, we observed a striking decrease in the number of LC in the epidermis of transgenic mice in comparison to wild-type mice. Nevertheless, these LC expressed normal levels of major histocompatibility complex (MHC)-class II and Langerin/ CD207 in situ. In explant cultures of whole ear skin the number of dendritic cells (DC), which migrated into the culture medium, was reduced. This reduction was even more pronounced in cultures of epidermal sheets. Virtually all emigrated cutaneous DC displayed typical morphology with cytoplasmic "veils", showed translocation of MHC-class II to the surface membrane, and expressed the maturation marker 2A1. Thus, cutaneous DC from transgenic mice seemed to mature normally. These results demonstrate that overexpression of follistatin in the epidermis affects LC trafficking but not maturation and suggest a novel role of the follistatin-binding partner activin in LC biology.

Activin a in the regulation of corneal neovascularization and vascular endothelial growth factor expression

Activin A, a dimeric glycoprotein that belongs to the transforming growth factor-beta superfamily, governs cellular differentiation in a wide variety of models and has been implicated in the regulation of angiogenesis. We examined the role of activin A and its downstream signaling pathway in a murine model of inflammatory corneal neovascularization induced by mechanical injury (debridement), and in vitro in corneal epithelial cells. Activin A expression increased steadily from day 2 until day 8 after mechanical debridement in vivo, paralleling vascular endothelial growth factor (VEGF) expression. Administration of recombinant activin A in mice increased the area of neovascularization, VEGF expression, and the kinase activities of p38 and p42/44 MAPKs after mechanical debridement. Systemic inhibition of activin A in vivo with a neutralizing antibody reduced the area of neovascularization, VEGF expression, and p38 and p42/44 MAPK activity, whereas administration of an isotype-matched control antibody had no effect. In vitro treatment with activin A increased VEGF secretion, as well as p38 and p42/44 MAPK activity in corneal epithelial cells, whereas concurrent administration of specific inhibitors of p38 or p42/44 MAPK abolished the stimulatory effect of activin A on VEGF production. We conclude that activin A stimulates inflammatory corneal angiogenesis by increasing VEGF levels through a p38 and p42/44 MAPK-dependent mechanism.

Follistatin production by skin fibroblasts and its regulation by dexamethasone

Activin and follistatin (FS) appear to play a role in the development of the skin and its appendages, in the inflammatory process, angiogenesis, and in wound healing. Although there is information on the expression of activin subunits and receptors in fibroblasts and keratinocytes, there are no reports on the regulation of FS expression in these cells. In the present study we analyzed the splicing variants of FS mRNAs in fibroblasts from genital and nongenital skin by RT-PCR and northern analysis, and examined the induction of FS mRNA and protein by hormones and growth factors in skin fibroblasts from human and nonhuman primates. FS mRNA was highly expressed in all fibroblast strains with similar expression regardless of donor species (human or monkey), donor age (neonate or adult), or the organ from which the fibroblast strains were established (skin or pituitary, genital or non-genital skin). Moreover, the band density corresponding to FS-288 was <5-10% of the value for FS-315 in skin fibroblasts as in all other tissues examined. Fibroblast FS mRNA and protein production were biphasically regulated by dexamethasone: low concentrations (0.01 and 0.1 nM) increased whereas higher concentrations (>1 nM) suppressed FS expression. On the other hand, androgens, activin and PACAP38 were without effect. These data establish cultured skin fibroblasts as a model to study FS gene expression in humans, and support a role for follistatin in the normal immune response and in the anti-inflammatory actions of glucocorticoids.

Activin A induces terminal differentiation of cultured human keratinocytes

Activin A, a member of the TGFbeta-superfamily, is well known to play important roles in the growth and differentiation of various target cells. We have previously demonstrated that activin A is produced at an early stage of cultivation at both the protein and the mRNA levels in cultured human keratinocytes. In this study, the effects of activin A on differentiation and proliferation of human keratinocytes were examined. Activin A (> or =1 nM) induced cornified envelope formation and the synthesis of loricrin, keratin 1, involucrin, and transglutaminase 1. In addition, transglutaminase activity and mRNA level of transglutaminase 1 were increased by activin A. [3H]Thymidine incorporation and cell number were reduced by activin A (> or =1 nM) compared with control, suggesting an inhibitory effect of activin A on cell proliferation. On the basis of these findings, it is likely that activin A contributes to differentiation and suppression of proliferation in human keratinocytes.

Identification of receptors and Smad proteins involved in activin signalling in a human epidermal keratinocyte cell line

BACKGROUND

Activin A is a multifunctional protein, which is a member of the transforming growth factor-beta (TGF-beta) superfamily. Smad proteins have recently been shown to transduce signals for the TGF-beta superfamily of proteins, and Smad2 was implicated in activin signalling in Xenopus embryos.

RESULTS

We identified the receptors and Smad proteins activated by activin A in a human epidermal keratinocyte cell line, HaCaT. The major activin receptors expressed on HaCaT cells were activin type II receptor (ActR-II) and activin type IB receptor (ActR-IB). We have also shown that in HaCaT cells, activin A induced the phosphorylation of Smad3 and, to a lesser extent, of Smad2. On the other hand, TGF-beta induced an efficient phosphorylation of both Smad2 and Smad3. Activin A preferentially induced the nuclear translocation of Smad3 in HaCaT cells, whereas TGF-beta strongly induced the nuclear translocation of Smad2, as well as other Smads. Moreover, a constitutively active form of ActR-IB efficiently stimulated the formation of a heteromeric complex between Smad3 and Smad4 in COS cells transfected with Smad cDNAs.

CONCLUSIONS

These results suggest that activin A binds to a receptor complex of ActR-II and ActR-IB, and preferentially activates Smad3 in HaCaT human keratinocytes.