Regulated synthesis of low-molecular-weight antigens in keratinocyte cell cultures

Proteins from mouse epidermis cytosol extracts react on immunoblots with a polyclonal rabbit antiserum raised against rat skin calcium-binding protein (SCaBP), a parvalbumin of the panniculus carnosus. Three mouse epidermal proteins with molecular weights between 10-12K, which are distinct from SCaBP, are recognized by the antiserum. The synthesis of these proteins in keratinocyte culture is modulated by Ca++, as is the differentiation of the keratinocytes. Proliferating mouse keratinocytes in medium containing 0.07 mM Ca++ (low Ca++) undergo terminal differentiation when the Ca++ concentration is elevated to 1.8 mM (high Ca++). Synthesis of the 3 antigens can be demonstrated when soluble extracts of keratinocytes labeled with [35S]methionine in low Ca++ medium are immunoprecipitated with anti-SCaBP serum. These antigens are not synthesized in cultures of dermal fibroblasts. When keratinocytes are switched to high Ca++ medium, synthesis of these antigens is greatly diminished over the course of 48-72 h. However, the antigens persist in differentiating cells. When proliferating keratinocytes in low Ca++ medium are exposed to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), differentiation is induced in a subpopulation of cells, and specific antigen synthesis is transiently inhibited. The inhibition correlates with the time when many cells are differentiating in response to TPA. When proliferating keratinocytes are pulse-labeled with 32PO4, the 11K antigen is phosphorylated and the phosphorylation is not enhanced by TPA exposure. All 3 antigens are synthesized in a reticulocyte lysate preparation with added newborn mouse epidermis messenger RNA or mRNA from keratinocytes cultured in low Ca++ medium. Thus, these antigens are likely to represent unique proteins rather than processed or degraded ones. The coordinately regulated expression of these antigens associated with the differentiation state of the keratinocytes suggests that these proteins are important in keratinocyte proliferation and differentiation.