Differences in low density lipoprotein receptor expression in the suprabasal layer of normal and psoriatic epidermis

Previous morphological experiments on the distribution of binding sites for low density lipoprotein (LDL) on normal and psoriatic epidermis in situ, done with the LDL-gold technique [Mommaas-Kienhuis AM, et al. J Invest Dermatol 89: 513-517, 1987.] showed an unequivocal correlation between the ability to bind LDL-gold complexes and the state of keratinocyte differentiation. To determine the involvement of the LDL receptor in this phenomenon, we applied immunoelectronmicroscopical methods in conjunction with a monoclonal anti-LDL receptor antibody. Biopsy specimens of normal and psoriasis skin were fixed before being embedded in Lowicryl K4M. Ultrathin sections were incubated first with the anti-LDL receptor antibody, and then with a second antibody conjugated to colloidal gold. On basal cells of both normal and psoriatic epidermis the LDL receptor was distributed evenly between the cell surface and the cytoplasm. No obvious differences in the density of LDL receptors were observed. However, cells from the suprabasal layer showed two striking differences in the localization of the LDL receptor: 1) normal epidermis showed fewer LDL receptor molecules, whereas in psoriasis epidermis the number increased relative to those on basal cells; and 2) in normal suprabasal cells most of the LDL receptors were located inside the cell, but in psoriasis the majority was found on the cell surface. Both phenomena are discussed and we postulate that the higher expression of LDL receptors in psoriasis suprabasal cells and the high expression of the receptor on the cell surface is connected with the hyperproliferative state of the disorder.