Cadherin function is required for normal keratinocyte intercellular adhesion and stratification. In the present study, we have investigated whether cadherin-cadherin interactions may also modulate keratinocyte differentiation, as evidenced by alterations in the levels of several differentiation markers. Confluent keratinocyte cultures, propagated in low Ca(2+) medium in which cadherins are not active, were pre-incubated with antibodies that block the function of E-cadherin and/or P-cadherin; Ca(2+ )was then elevated to 1 mM to activate the cadherins and induce differentiation. In control cultures (incubated with no antibody or with antibodies to other cell surface molecules), Ca(2+) elevation induced an increase in type 1 transglutaminase, profilaggrin, and loricrin, as measured by western blotting and in agreement with previous results. However, the concurrent addition of antibodies against both E- and P-cadherin prevented this increase in transglutaminase 1 protein. Incubation with either antibody alone had no consistent effect. Profilaggrin and loricrin, which are later markers of keratinocyte differentiation, responded differently from transglutaminase 1 to addition of antibodies. In the presence of anti-E-cadherin antibody, both loricrin and profilaggrin levels were dramatically enhanced compared to the high Ca(2+) control cells, while addition of antibody to P-cadherin slightly attenuated the Ca(2+)-induced increase. In the presence of both antibodies, loricrin and profilaggrin protein levels were intermediate between those observed in the presence of either antibody alone. The expression of involucrin, however, was unaffected by addition of antibodies. In addition, effects of the anti-cadherin antibodies were not secondary to alterations in proliferation or programmed cell death, as determined by several independent assays of these processes. Thus, the consequences of cadherin inhibition depend upon both the particular cadherin and the differentiation marker under study. Taken together, these data suggest that E-cadherin and P-cadherin contribute to the orderly progression of terminal differentiation in the epidermis in multiple ways.