Fibroblast heterogeneity: a new phenotype associated with photoaging/solar elastosis

We here present a novel subtype of fibroblasts isolated from sun-exposed skin of elderly donors and characterized by its prominent and stable myo-chondrocyte-type differentiation. As we depict a similar chondrocyte-type expression pattern at sites of solar elastosis in human skin, we propose that this fibroblasts phenotype develops in the course of massive and perseverative UV-dependent damage as part of the process terminating in the most severe appearance of photoaging - solar elastosis. Along with the abnormal expression profile, these “old” fibroblasts generate a chondrocyte-type matrix with a layer of myofibroblast-differentiation in close vicinity to the epidermis, when propagated as skin equivalents (SEs) in 3D organotypic cultures. Importantly, these fibroblasts are strongly reduced in their ability to support epidermal growth and differentiation, allowing for rapid/premature epidermal atrophy. Having identified TGFß as an important player in this scenario, we show that the addition of TGFß to normal young fibroblast induces while inhibition of TGFß to the old fibroblast abolishes the myo-chondrogenic expression profile. Accordingly, inhibiting TGFß in the in vivo-like SEs, not only abrogates the aberrant phenotype but allows these fibroblasts to regain their normal mutual interaction with the epidermal keratinocytes and to support epidermal regeneration and longevity. Further, we provide evidence for a role of chronic solar UV radiation in initiating chondrogenic gene expression in young fibroblasts. Concomitantly, we show however, that the same UV treatment abolishes myofibroblast differentiation in the SEs with the “old” chondrogenic fibroblasts, and that this selective inhibition is already sufficient to restore the fibroblasts’ ability for successfully supporting epidermal growth and differentiation. From this it is tempting to suggest, that besides all damaging potential, “mild” solar UV radiation may exert also beneficial effects, e.g. in promoting activation of photoaged skin. To compare in vitro and in vivo aging of human dermal fibroblasts we cultivated cells in vitro from young donors (21/22 years of age) over 22 passages and compared gene expression to fibroblasts from an old donor (66 years of age). To compare in vitro and in vivo aging of human dermal fibroblasts we compared the gene expression from a scaffold-based OTC model (scaOTC), which allows epidermal regeneration for several months (Boehnke et al., 2007; Stark et al., 2004, 2006). sca-OTCs were cultivated from 3 different fibroblast sources (one from a young (22 years) and 2 from old donors (66 years and 74 years), all co-cultured with the same normal human epidermal keratinocytes. scaOTCs were cultivated for three and 9 weeks and gene expression compared based on the in vivo age of the constitutent fibroblasts. While all fibroblasts were able to promote the establishment of a well-stratified and differentiated epidermis yet epidermal differentiation demonstrated a somewhat impaired epidermal differentiation in 3-week-old scaOTCs , as compared to that seen on young fibroblasts. In scaOTCs with old fibroblasts, the epithelia became more disorganized and in part atrophic. After 9 weeks, this phenotypic difference was even more aggrevated, leading to a largely atrophic epithelium with hardly any vital keratinocytes, while the young fibroblasts continued to support regeneration of a still perfectly organized epidermis.