Protein kinase Cδ regulates keratinocyte death and survival by regulating activity and subcellular localization of a p38δ-extracellular signal-regulated kinase 1/2 complex

Protein kinase Cδ (PKCδ) is an important regulator of apoptosis in epidermal keratinocytes. However, little information is available regarding the downstream kinases that mediate PKCδ-dependent keratinocyte death. This study implicates p38δ mitogen-activated protein kinase (MAPK) as a downstream carrier of the PKCδ-dependent death signal. We show that coexpression of PKCδ with p38δ produces profound apoptosis-like morphological changes. These morphological changes are associated with increased sub-G₁ cell population, cytochrome c release, loss of mitochondrial membrane potential, caspase activation, and PARP cleavage. This death response is specific for the combination of PKCδ and p38δ and is not produced by replacing PKCδ with PKCα or p38δ with p38α. A constitutively active form of MEK6, an upstream activator of p38δ, can also produce cell death when coupled with p38δ. In addition, concurrent p38δ activation and extracellular signal-regulated kinase 1/2 (ERK1/2) inactivation are required for apoptosis. Regarding this inverse regulation, we describe a p38δ-ERK1/2 complex that may coordinate these changes in activity. We further show that this p38δ-ERK1/2 complex relocates into the nucleus in response to PKCδ expression. This regulation appears to be physiological, since H₂O₂, a known inducer of keratinocyte apoptosis, promotes identical PKCδ and p38δ-ERK1/2 activity changes, leading to similar morphological changes.