Possible role of phosphatidylglycerol-activated protein kinase C-βII in keratinocyte differentiation

Background: The epidermis is a continuously regenerating tissue maintained by a balance between proliferation and differentiation, with imbalances resulting in skin disease. We have previously found that in mouse keratinocytes, the lipid-metabolizing enzyme phospholipase D2 (PLD2) is associated with the aquaglyceroporin, aquaporin 3 (AQP3), an efficient transporter of glycerol. Our results also show that the functional interaction of AQP3 and PLD2 results in increased levels of phosphatidylglycerol (PG) in response to an elevated extracellular calcium level, which triggers keratinocyte differentiation. Indeed, we showed that directly applying PG can promote keratinocyte differentiation. Objective: We hypothesized that the differentiative effects of this PLD2/AQP3/PG signaling cascade, in which AQP3 mediates the transport of glycerol into keratinocytes followed by its PLD2-catalyzed conversion to PG, are mediated by protein kinase CβII (PKCβII), which contains a PG-binding domain in its carboxy-terminus. Method: To test this hypothesis we used quantitative RT-PCR, western blotting and immunocytochemistry. Results: We first verified the presence of PKCβII mRNA and protein in mouse keratinocytes. Next, we found that autophosphorylated (activated) PKCβII was redistributed upon treatment of keratinocytes with PG. In the unstimulated state phosphoPKCβII was found in the cytosol and perinuclear area; treatment with PG resulted in enhanced phosphoPKCβII localization in the perinuclear area. PG also induced translocation of phosphoPKCβII to the plasma membrane. In addition, we observed that overexpression of PKCβII enhanced calcium- and PG-induced keratinocyte differentiation without affecting calcium-inhibited keratinocyte proliferation. Conclusion: These results suggest that the PG produced by the PLD2/AQP3 signaling module may function by activating PKCβII.