Keratinocyte apoptosis on type I collagen fibrils is prevented by Erk1/2 activation under high calcium condition

Keratinocytes adhere and proliferate well on collagen-coated surfaces, but they undergo apoptosis without differentiation on collagen gels according to our past research. In the current studies, we investigated the necessary conditions for keratinocyte survival on fibrous collagen gels. We found that keratinocytes survived on collagen gels when the medium contains elevated levels (1.8 mM) of calcium. Under this high calcium condition, cells formed multicellular colonies and differentiated. Akt was not activated in cells cultured on collagen gels regardless of the calcium concentration, whereas it was activated in cells cultured on nonfibrous collagen. On the other hand, Erk1/2, key kinases of MAPK pathway, were phosphorylated in cells cultured under high calcium condition but not in cells cultured on collagen gels under low calcium condition. The necessity of Erk1/2 activation for keratinocyte survival on collagen gel was confirmed with experiment using U0126, an inhibitor for Erk1/2. These studies show that activation of Akt depends on collagen assembly, whereas activation of Erk1/2 is induced by increased extracellular calcium concentration. Thus, activation of the Erk1/2 by increasing calcium concentration in the incubation medium may compensate for the loss of Akt activation, allowing keratinocyte survival on collagen gels.