Using siRNA knockdown in HaCaT cells to study transcriptional control of epidermal proliferation potential

c-Src mediated tyrosine phosphorylation of plakophilin 3 as a new mechanism to control desmosome composition in cells exposed to oxidative stress

Plakophilins (PKP1 to PKP3) are essential for the structure and function of desmosomal junctions as demonstrated by the severe skin defects observed as a result of loss-of-function mutations in mice and men. PKPs play additional roles in cell signaling processes, such as those controlling the cellular stress response and cell proliferation. A key post-translational process controlling PKP function is phosphorylation. We have discovered that reactive oxygen species (ROS) trigger the c-Src kinase-mediated tyrosine (Tyr)-195 phosphorylation of PKP3. This modification is associated with a change in the subcellular distribution of the protein. Specifically, PKP3 bearing phospho-Tyr-195 is released from the desmosomes, suggesting that phospho-Tyr-195 is relevant for the control of desmosome disassembly and function, at least in cells exposed to ROS. Tyr-195 phosphorylation is transient under normal physiological conditions and seems to be strictly regulated, as the activation of particular growth factor receptors results in a modification at this site only when tyrosine phosphatases are inactivated by pervanadate. We have identified Tyr-195 of PKP3 as a phosphorylation target of epidermal growth factor receptor signaling. Interestingly, this PKP3 phosphorylation also occurs in certain poorly differentiated adenocarcinomas of the prostate, suggesting a possible role in tumor progression. Our study thus identifies a new mechanism controlling PKP3 and hence desmosome function in epithelial cells.

Involvement of RAGE, MAPK and NF-κB pathways in AGEs-induced MMP-9 activation in HaCaT keratinocytes

Advanced glycation end products (AGEs) exert divergent effects on the pathogenesis of diabetes complications. Excessive expression of matrix metalloproteinases-9 (MMP-9) is deleterious to the cutaneous wound-healing process in the context of diabetes. However, the effect of AGEs on MMP-9 induction in skin cells and the exact molecular mechanisms involved are still poorly understood. In this study, we investigated the effect of AGEs on the production of MMP-9 in HaCaT keratinocytes and characterized the signal transduction pathways activated by AGEs that are involved in MMP-9 regulation. We showed that AGE-BSA increased MMP-9 expression in HaCaT cells at both the protein and mRNA levels. The stimulatory effect of AGE-BSA on MMP-9 was attenuated by inhibitors of extracellular-signal-regulated kinase (ERK1/2, U0126), p38 mitogen-activated protein kinase (MAPK, SB203580) and NF-κB, but not c-Jun N-terminal kinase. Furthermore, receptor for advanced glycation end products (RAGE) was expressed in keratinocytes, and incubation with AGE-BSA resulted in a significant upregulation of RAGE expression in a dose-dependent manner. Silencing of the RAGE gene prevented AGE-BSA-induced MMP-9 activation and the phosphorylation of ERK1/2 and p38 MAPK. We also observed the involvement of NF-κB in AGE-BSA-induced MMP-9 activation, which was not blocked by U0126 and SB203580. These results suggest that AGEs may play an important role in the impairment of diabetic wound healing by upregulating MMP-9 expression in keratinocytes via the RAGE, ERK1/2 and p38 MAPK pathways; activation of NF-κB is also involved in this process. These pathways may represent potential targets for drug interventions to improve diabetic wound healing, a process in which MMP-9 plays a critical role.

Transcriptional regulation of epidermal barrier formation

The mammalian epidermis is a self-renewing stratified squamous epithelium. Its basal cell layer contains proliferating keratinocytes that exit the cell cycle when they move into the suprabasal compartment. These cells activate a gene differentiation program aimed at building a protective epidermal barrier as they move toward the surface, successively going through the spinous and granular layers. At the completion of this process, the keratinocytes become enucleated and form the cornified layer, the surface layer of the skin. The highly cross-linked protein-lipid envelope and extracellular lipids in the cornified layer along with cell-cell adhesions in the granular layer are required for an effective epidermal barrier. Transcriptional mechanisms are critical for the formation of the epidermal barrier, and in this chapter, we describe methods to evaluate the role of a transcription factor (TF) in epidermal differentiation. To identify direct target genes of a TF, we propose a combination of bioinformatics and experimental approaches. The ultimate goal of these approaches is to understand the mechanisms whereby a TF regulates epidermal barrier formation.