Role for protein kinase C-alpha in keratinocyte growth arrest.

Multiple protein kinase C (PKC) isoforms have been associated with the epidermal keratinocyte (KC) granular layer differentiation program. Here we show PKCalpha membrane localization and substrate phosphorylation in the first suprabasal KCs of normal human epidermis, suggesting activation in vivo in the lower spinous layers where terminal differentiation-associated growth arrest occurs. To determine if PKCalpha is sufficient for KC growth arrest, we expressed a constitutively active PKCalpha (PKCalpha Delta22-28) in normal human KCs and observed growth arrest and accumulation of cells in G1.

The protein kinase C delta catalytic fragment targets Mcl-1 for degradation to trigger apoptosis.

Proteolytic cleavage and subsequent activation of protein kinase C (PKC) delta is required for apoptosis induced by a variety of genotoxic agent, including UV radiation. In addition, overexpression of the constitutively active PKCdelta catalytic fragment (PKCdelta-cat) is sufficient to trigger Bax activation, cytochrome c release, and apoptosis. While PKCdelta is a key apoptotic effector, the downstream target(s) responsible for the mitochondrial apoptotic cascade are not known.

Bax activation and induction of apoptosis in human keratinocytes by the protein kinase C delta catalytic domain.

The constitutively active catalytic domain of protein kinase C (PKC)delta is an apoptotic effector generated by caspase-3 cleavage of full-length PKCdelta in response to a wide variety of apoptotic stimuli, including UV radiation. The PKCdelta catalytic domain induces apoptosis when ectopically expressed, however, the mechanism of apoptosis induction is unclear. We constructed a chimeric protein encoding the PKCdelta catalytic domain fused to a mutated estrogen receptor ligand-binding domain in order to selectively activate the PKCdelta catalytic domain.

Protein kinase C-delta is commonly expressed in multiple myeloma cells and its downregulation by rottlerin causes apoptosis.

The growth and proliferation of multiple myeloma (MM) cells are influenced by various cytokines produced by bone marrow stromal cells. As cytokine interaction between malignant plasma cells and neighbouring stromal cells is important in the pathogenesis of MM, the understanding of intracellular signalling events elicited by this interaction is of central importance. Recent reports have shown that protein kinase C (PKC) is directly involved in modulating apoptosis in different cells types, including those of haematopoietic neoplasms.

Activation of protein kinase C triggers irreversible cell cycle withdrawal in human keratinocytes.

Irreversible cell cycle withdrawal occurs as normal keratinocytes detach from the basement membrane and initiate their terminal differentiation program. To investigate which signaling pathways regulate this permanent cell cycle withdrawal, we added inhibitors of kinases implicated in integrin signaling and keratinocyte differentiation to normal human keratinocytes induced to differentiate in suspension culture, and assayed the growth capacity of the recovered cells. Keratinocytes suspended in methylcellulose for 24 h underwent approximately 1000-fold loss of proliferative capacity.

Activation of caspase-9 is required for UV-induced apoptosis of human keratinocytes.

UV radiation from the sun activates both the membrane death receptor and the intrinsic or mitochondrial apoptotic signaling pathways in epidermal keratinocytes, triggering apoptosis and affording protection against skin cancer formation. We have investigated the involvement of caspase-9 in the UV death effector pathway in human keratinocytes, since this is the initiating caspase in the mitochondrial pathway required for UV-induced apoptosis in some, but not all, cell types. UV radiation triggered activation of caspase-3, caspase-9, and caspase-8 with similar kinetics, although the rank order of activation was caspase-3 > caspase-9 > caspase-8.