Skin immunosenescence: decreased receptor for activated C kinase-1 expression correlates with defective tumour necrosis factor-α production in epidermal cells

Skin immunosenescence accounts for increased susceptibility in the elderly to cutaneous infections and malignancies, and decreased contact hypersensitivity and response to vaccination. We have recently shown in immune cells that decreased expression of the receptor for activated C kinase (RACK)-1 underlies defective protein kinase C (PKC) activation and functional immune impairment with ageing. This study was designed to determine if an age-related decline in skin RACK-1 expression was present and whether it correlated with defective tumour necrosis factor (TNF)-alpha production. PKC isoforms and RACK-1 expression were evaluated by Western blot analysis and by immunofluorescence in skin obtained from Sprague-Dawley rats of different ages. TNF-alpha release by epidermal cells induced by lipopolysaccharide, 12-O-tetradecanoyl-phorbol-13-acetate and the contact allergen dinitrochlorobenzene was assessed by the L929 biological assay. Skin obtained from old rats (> 18 months) showed decreased RACK-1 immunoreactivity if compared with young rats (< 3 months). RACK-1 preferentially interacts with PKC beta. Despite a similar total skin content of this isoform, the reduced expression of RACK-1 was associated with a decreased translocation of PKC beta in the membrane compartment. The defective PKC beta translocation associated with ageing correlated with decreased TNF-alpha release from epidermal cells following treatment with different inflammatory stimuli. Overall, we demonstrated for the first time a decrease in RACK-1 expression, defective PKC beta translocation and reduced TNF-alpha release in epidermal cells with ageing. These alterations might be mechanistically significant, and provide a new understanding of the consequences of ageing on skin immunology.