Phospholipase C mimics the differential effects of phorbol-12-myristate-13-acetate on the colony formation and cornification of cultured normal and transformed human keratinocytes.

Phorbol ester tumour promoters like phorbol-12-myristate-13-acetate (PMA) and serum-derived factors inhibit growth and induce terminal differentiation in normal human and mouse keratinocytes but have a much reduced effect on their transformed counterparts. These observations may be relevant to potency of PMA and wounding as tumour promoters in mouse epidermis. Since some serum factors produced during wounding are thought to exert their effects through the production of diacylglycerol (DAG-the proposed physiological ligand for the phorbol ester receptor) from phospholipids by the activation of phospholipase C (PC) we have compared the effects of PC with PMA in cultures of normal and transformed human keratinocytes. The addition of PC from Clostridium perfringens (0.1-3.0 units/ml) to the culture medium of normal human keratinocytes produced similar morphological changes to PMA and also mimicked the effects of the phorbol ester on cloning efficiency and cornified envelope formation. Most importantly PC, like PMA, had a very weak effect on the human squamous cell carcinoma lines SCC-12B and SCC-15. All the effects of PC were abolished by boiling the enzyme. These results are discussed in relation to the proposed role of serum factors in tumour promotion by deep skin wounding and their mechanistic relationship to phorbol esters.