BackgroundSkin atrophy is a common manifestation of aging and is frequently accompanied by ulceration and delayed wound healing. With an increasingly aging patient population, management of skin atrophy is becoming a major challenge in the clinic, particularly in light of the fact that there are no effective therapeutic options at present.Methods and FindingsAtrophic skin displays a decreased hyaluronate (HA) content and expression of the major cell-surface hyaluronate receptor, CD44. In an effort to develop a therapeutic strategy for skin atrophy, we addressed the effect of topical administration of defined-size HA fragments (HAF) on skin trophicity. Treatment of primary keratinocyte cultures with intermediate-size HAF (HAFi; 50,000–400,000 Da) but not with small-size HAF (HAFs; <50,000 Da) or large-size HAF (HAFl; >400,000 Da) induced wild-type (wt) but not CD44-deficient (CD44−/−) keratinocyte proliferation. Topical application of HAFi caused marked epidermal hyperplasia in wt but not in CD44−/− mice, and significant skin thickening in patients with age- or corticosteroid-related skin atrophy. The effect of HAFi on keratinocyte proliferation was abrogated by antibodies against heparin-binding epidermal growth factor (HB-EGF) and its receptor, erbB1, which form a complex with a particular isoform of CD44 (CD44v3), and by tissue inhibitor of metalloproteinase-3 (TIMP-3).ConclusionsOur observations provide a novel CD44-dependent mechanism for HA oligosaccharide-induced keratinocyte proliferation and suggest that topical HAFi application may provide an attractive therapeutic option in human skin atrophy.

Epidermal fatty acid-binding protein (E-FABP) was isolated from human skin and purified to homogeneity. Its molecular mass was estimated to be 15 kDa and the pI of non-denaturing protein was 5.6. Scatchard-plot analysis revealed one class of binding site for oleic acid with a Kd of 0.46 microM. Structure-binding relation experiments revealed a high affinity of E-FABP for stearic acid which decreased on reduction of the number of carbon atoms or introduction of double bonds into the fatty acid chain. Squalene, cholesterol and retinoic acid isomers showed no affinity, suggesting that E-FABP displays high specificity for fatty acids. E-FABP is a scarce cytosolic protein (0.065% of total protein). Only trace amounts could be detected in normal human skin but up to 42.5 +/- 3.4 pmol/mg of protein was found in a non-malignant defect of keratinocyte differentiation (psoriatic lesions). E-FABP levels were low in cultured human keratinocytes grown under proliferation-stimulating conditions but increased about 2-fold on induction of differentiation by Ca2+. Immunohistochemical localization showed cytosolic staining in differentiated cells of normal and psoriatic skin, suggesting a link between E-FABP and keratinocyte differentiation. The presence of E-FABP in tissues other than skin (heart, intestine and adipose tissue) excludes its specific role in fatty acid metabolism in epithelial cells or its involvement in skin lipid-barrier function.

We show that unsaturated fatty acids (FAs) bind reversibly and with high affinity to a heterocomplex of 34 kDa (FA-p34) formed by the non-covalent association of two calcium-binding proteins of the S100 family: MRP8 (S100A8) and MRP14 (S100A9). In abnormally differentiated keratinocytes (psoriasis) and in human polymorphonuclear leukocytes, FA-p34 is highly expressed (31.35 ؎ 1.6 and 349.8 ؎ 17.9 pmol of [ 3 H]oleic acid/mg protein, respectively), pointing toward a role for this heteromer in mediating effects of unsaturated fatty acids in a calcium-dependent way during cell differentiation and/or inflammation.

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BackgroundCD44 is a polymorphic proteoglycan and functions as the principal cell-surface receptor for hyaluronate (HA). Heparin-binding epidermal growth factor (HB-EGF) activation of keratinocyte erbB receptors has been proposed to mediate retinoid-induced epidermal hyperplasia. We have recently shown that intermediate size HA fragments (HAFi) reverse skin atrophy by a CD44-dependent mechanism.Methodology and Principal FindingsTreatment of primary mouse keratinocyte cultures with retinaldehyde (RAL) resulted in the most significant increase in keratinocyte proliferation when compared with other retinoids, retinoic acid, retinol or retinoyl palmitate. RAL and HAFi showed a more significant increase in keratinocyte proliferation than RAL or HAFi alone. No proliferation with RAL was observed in CD44−/− keratinocytes. HA synthesis inhibitor, 4-methylumbelliferone inhibited the proliferative effect of RAL. HB-EGF, erbB1, and tissue inhibitor of MMP-3 blocking antibodies abrogated the RAL- or RAL- and HAFi-induced keratinocyte proliferation. Topical application of RAL or RAL and HAFi for 3 days caused a significant epidermal hyperplasia in the back skin of wild-type mice but not in CD44−/− mice. Topical RAL and HAFi increased epidermal CD44 expression, and the epidermal and dermal HA. RAL induced the expression of active HB-EGF and erbB1. However, treatment with RAL and HAFi showed a more significant increase in pro-HB-EGF when compared to RAL or HAFi treatments alone. We then topically applied RAL and HAFi twice a day to the forearm skin of elderly dermatoporosis patients. After 1 month of treatment, we observed a significant clinical improvement.Conclusions and SignificanceOur results indicate that (i) RAL-induced in vitro and in vivo keratinocyte proliferation is a CD44-dependent phenomenon and requires the presence of HA, HB-EGF, erbB1 and MMPs, (ii) RAL and HAFi show a synergy in vitro and in vivo in mouse skin, and (iii) the combination of RAL and HAFi seems to have an important therapeutic effect in dermatoporosis.