Identification of proteins that are abnormally regulated in differentiated cultured human keratinocytes

Comparison of the protein expression patterns of proliferating normal primary human keratinocytes plated in serum‐free medium (SFKM), supplemented with epidermal growth factor (EGF) and bovine pituitary extract (BPE), and similar cultures induced to differentiate by the addition of Dulbecco's modified Eagle medium (DMEM), containing 10% fetal calf serum (FCS), revealed several known and unknown polypeptides that are abnormally regulated in the differentiated cells. Upregulated proteins included keratins (keratins 6, 10/11, 14 and 16), members of the S100 protein family psoriasin, MRP8, MRP14 and S100c), actin‐binding proteins (gelsolin and tropomyosin 9220), annexins (annexins IV and VIII), hsp28, the fatty acid binding protein 5 (FABP5), the squamous cell carcinoma (SCC) antigen, members of the 14‐3‐3 family, involucrin, E‐cadherin, cystatin A, desmoglein and integrins α2 and β1, as well as several proteins of as yet unknown identity. The highest upregulated proteins corresponded to psoriasin (124.0 times), MRP8 (42.4 times), MRP14 (14.9 times), tropomyosin 9220 (11.5 times), involucrin (11.1 times), and FABP5 (9.1 times). FABP5, hsp28, and tropomyosin 9220 were also highly upregulated in quiescent keratinocytes indicating that their increased levels in the differentiated cell may be due to loss of proliferative activity. Highly downregulated proteins included PAI‐2, tropomyosins 9213, 9121 and 9122, keratin 5, calnexin, 14‐3‐3 beta and eta, nucleoside diphosphate kinase A, Rho GDIs, hsp60, hnRNPs H and C2, α‐enolase, eIF‐4D, thioredoxin, annexins III and V, moesin, nucleolar protein B23, GST π and PCNA/cyclin. Both the high expression of keratin 6 and 16 – which are markers for an alternative pathway of keratinocyte differentiation – as well as the extremely high upregulation of some members of the S100 protein family indicate that the cells have differentiated via an abnormal pathway.