Different expression of the alpha2-macroglobulin receptor/low-density lipoprotein receptor-related protein in human keratinocytes and fibroblasts

We compared, using a combination of different immunological methods and by competitive PCR, the expression of the alpha2-macroglobulin receptor/low-density lipoprotein receptor-related protein (alpha2-M-R/LRP) in human keratinocytes and fibroblasts. This receptor has previously been found in skin only in dermal cells associated with fibroblasts and dendritic cells. For immunodetection we used mouse monoclonal antibodies against the two subunits of the receptor and against the receptor-associated protein (RAP), known as the regulatory protein of the receptor activity. The alpha2-M-R/LRP was found to be predominantly located intracellularly in keratinocytes whereas a distinct labelling of the outer membrane surface was found in fibroblasts. RAP is abundant in fibroblasts but is less expressed in keratinocytes. In frozen skin sections receptor immunoreactivity was detected in the epidermis with increased reactivity of basal keratinocytes, as well as in the dermis in association with dermal fibroblasts. By immunoprecipitation of biotinylated cell extracts, polypeptides were identified corresponding to the alpha-subunit and beta-subunit of the receptor as well as to the coprecipitating RAP. Competitive PCR revealed the presence of 67.9 and 2049.7 ag of alpha2-M-R/LRP mRNA per cell in keratinocytes and fibroblasts, respectively. The results demonstrate that both cell types express alpha-M-R/LRP mRNA and contain receptor protein as well as RAP but in different quantities and subcellular localizations.