Differentiation-associated expression of the proto-oncogene pim-1 in cultured human keratinocytes

The proto-oncogene pim-1 was expressed in nine human tissues (including epidermis) examined by Northern blotting. Expression of pim-1 was also observed in a number of carcinoma-derived keratinocyte lines in addition to strains derived from normal epidermis. With the exception of a squamous carcinoma line that exhibits little differentiated character in culture (SCC-4), where it was not detected, pim-1 expression was substantially higher after confluence than during log-phase growth in each case. The differentiation marker keratinocyte transglutaminase showed the same pattern of expression as pim-1 in relation to confluence in each of the cell lines and strains studied. The influences on pim-1 mRNA levels of several known effectors of keratinocyte differentiation were studied in the squamous carcinoma line SCC-9. pim-1 mRNA was stimulated by hydrocortisone and suppressed by the tumor promoter tetradecanoyl phorbol acetate. pim-1 mRNA was also regulated by calcium ion concentration in the culture medium, with expression being threefold higher in 0.15 mM than in 0.03 mM calcium ion. Keratinocyte transglutaminase was regulated similarly by these effectors. Thus pim-1 expression was associated with keratinocyte differentiation in these cultured cells.