Cyclosporin A inhibits phorbol ester-induced cellular proliferation and tumor promotion as well as phosphorylation of a 100-kd protein in mouse epidermis.

Mouse epidermis and HEL30 keratinocytes contain cellular binding sites for the immunosuppressive agent cyclosporin A (CsA). Phorbol ester-induced DNA synthesis and tumor promotion in mouse skin is strongly suppressed by CsA. The anti-tumor effect of CsA is not the consequence of destruction of initiated cells. CsA selectively inhibits the phosphorylation in vitro of a protein with the relative molecular mass of 100 kd in mouse epidermis and HEL30 cytosol. The CsA-suppressible phosphorylation in vitro does not depend on the addition of Ca2+, phospholipid and the phorbol ester TPA.