Normal keratinocytes, SV40-transformed keratinocytes (SVK14), and various
squamous carcinoma cell (SCC) lines have been used as an in vitro model system to study the properties of
phorbol ester receptor and
protein kinase C expression during keratinocyte differentiation. The cell lines used exhibit a decreasing capacity to differentiate in the order of keratinocytes approximately SVK14 greater than SCC-12F2 greater than SCC-15 greater than SCC-4; moreover, all cell lines respond to a low external Ca2+ concentration by a decreased capacity to differentiate. Normal keratinocytes exhibited the highest number of
phorbol ester receptors as compared to the other cell lines, while each individual cell line exhibited a higher number of
phorbol ester receptors during growth under normal Ca2+ conditions as compared to cells grown under low Ca2+ conditions. The apparent dissociation constant (Kd) demonstrated only small variations in the various cell lines. In contrast, the cytoplasmic
protein kinase C activity, was found to be higher in cells grown under low Ca2+ conditions than in cells grown under normal Ca2+ conditions, indicating the absence of a causal relationship between cytoplasmic
protein kinase C activity and
phorbol ester receptor expression. Therefore the properties of
protein kinase C have been determined in more detail in normal keratinocytes and SCC-15 cells. These studies revealed differences between
protein kinase C properties from the two cell lines grown under normal and low Ca2+ conditions. The differences included the effect of
phorbol 12-myristate 13-acetate (PMA) on the redistribution pattern of
protein kinase C between the cytoplasmic and particulate fractions as well as the activating effect of
diolein in vitro on
protein kinase C activity, partly purified from particulate or cytoplasmic fractions. These observations demonstrate that the functional
protein kinase C activity of keratinocytes is determined by various endogenous and exogenous activators and that these activators are modulated differently in various cell lines, under various growth conditions (low Ca2+ versus normal Ca2+).