Although commitment to epidermal differentiation is generally considered to be irreversible, differentiated keratinocytes (KCs) have been shown to maintain a regenerative potential and to reform skin epithelia when placed in a suitable environment. To obtain insights into the mechanism of reinitiation of this proliferative response in differentiated KCs, we examined the reversibility of commitment to Ca 2؉ -induced differentiation. Lowering Ca 2؉ concentration to micromolar levels triggered culture-wide morphological and biochemical changes, as indicated by derepression of cyclin D1, reinitiation of DNA synthesis, and acquisition of basal cell-like characteristics. These responses were inhibited by Goedecke 6976, an inhibitor of protein kinase D (PKD) and PKC␣, but not with GF109203X, a general inhibitor of PKCs, suggesting PKD activation by a PKCindependent mechanism. PKD activation followed complex kinetics with a biphasic early transient phosphorylation within the first 6 h, followed by a sustained and progressive phosphorylation beginning at 24 h. The second phase of PKD activation was followed by prolonged ERK1/2 signaling and progression to DNA synthesis in response to the low Ca 2؉ switch. Specific knockdown of PKD-1 by RNA interference or expression of a dominant negative form of PKD-1 did not have a significant effect on normal KC proliferation and differentiation but did inhibit Ca 2؉ -mediated reinitiation of proliferation and reversion in differentiated cultures. The present study identifies PKD as a major regulator of a proliferative response in differentiated KCs, probably through sustained activation of the ERK-MAPK pathway, and provides new insights into the process of epidermal regeneration and wound healing.Terminal differentiation defines cells that permanently exit the cell cycle in the process of acquiring specialized function. Epidermis is a continuously renewing stratified epithelium, which forms a protective barrier from the environment. In epidermis, proliferating KCs 2 reside in the basal layer. As cells exit the proliferative compartment and begin their migration toward the skin surface, they withdraw from the cell cycle and commit to terminal differentiation in the spinous layer before completing their differentiation program in granular and cornified layers and sloughing from the skin surface (1). Under normal steady state conditions, proliferation is strictly restricted to the basal layer, although aberrant suprabasal proliferation is observed during wound healing and in some diseased states when normal epidermal proliferation and differentiation are perturbed (2, 3). Regulation of epidermal cell growth and terminal differentiation has been extensively studied in primary cultures of mouse KCs through modulation of extracellular Ca 2ϩ levels (4, 5). Cultivation of primary KCs in low Ca 2ϩ conditions (0.05 mM) promotes expansion of a proliferative population of undifferentiated, basal-like cells. Raising extracellular Ca 2ϩ levels (Ͼ1 mM; high Ca 2ϩ conditions) in confluent cultures t...