Plakophilin 1 (PKP1) belongs to the desmosome family as an anchoring junction protein in cellular junctions. It localizes at the interface of the cell membrane and cytoplasm. Although PKP1 is a non-transmembrane protein, it may become associated with the cell membrane via transmembrane proteins such as desmocollins and desmogleins. Homozygous deletion of PKP1 results in ectodermal dysplasia-skin fragility syndrome (EDSF) and complete knockout of PKP1 in mice produces comparable symptoms to EDSF in humans, although mice do not survive more than 24 h. PKP1 is not limited to expression in desmosomal structures, but is rather widely expressed in cytoplasm and nucleus, where it assumes important cellular functions. This review will summarize distinct roles of PKP1 in the cell membrane, cytoplasm, and nucleus with an overview of relevant studies on its function in diverse types of cancer.

Plakophilin (PKP1) 1 is a member of the arm-repeat family of catenins and acts as a structural component of desmosomes, which are important stabilizers of cell-cell adhesion. Besides this, PKP1 also occurs in a non-junctional, cytoplasmic form contributing to post-transcriptional regulation of gene expression. Moreover, PKP1 is expressed in the prostate epithelium but its expression is frequently downregulated in prostate cancers with a more aggressive phenotype. This observation may imply a tumour-suppressive role of PKP1. We found that, in prostatic adenocarcinomas with PKP1 deficiency, the occurrence of T-cells, B-cells, macrophages and neutrophils were significantly increased. In a PKP1-deficient prostatic cancer cell line expressing IL8, these levels were statistically meaningfully reduced upon PKP1 re-expression. When analysing prostatic PKP1 knockdown cell lines, the mRNA and protein levels of additional cytokines, namely CXCL1 and IL6, were upregulated. The effect was rescued upon re-expression of a PKP1 RNAi-resistant form. The corresponding mRNAs were co-precipitated with cytoplasmic PKP1, indicating that they are components of PKP1-containing mRNA ribonucleoprotein particles. Moreover, the mRNA half-lives of CXCL1, IL8 and IL6 were significantly increased in PKP1-deficient cells, showing that these mRNAs were stabilized by PKP1. In an in vitro migration assay, the higher cytokine concentrations led to higher migration rates of THP1 and PBMC cells. This finding implies that PKP1 loss of expression in vivo correlates with the recruitment of immune cells into the tumour area to set up a tumour-specific environment. One may speculate that this newly established tumour environment has tumour-suppressive characteristics and thereby accelerates tumour progression and metastasis.

On August 6, 2015, a male infant with ectodermal dysplasia/skin fragility syndrome at 6 hours of birth was admitted to the Burn Department of Tongren Hospital of Wuhan University & Wuhan Third Hospital. The ulcerous skin tissue in thoracic area was harvested. The histopathological change of wound tissue was observed with hematoxylin-eosin staining. The result showed that the epidermal muscle cell layer was slightly released, there were bullae under the epidermis, the dermal papilla layer disappeared, and a small amount of inflammatory cells infiltrated in the dermis. The expression of plakophilin 1 (PKP1) in wound tissue was observed with immunohistochemical staining. The result showed that the PKP1 expression was completely absent. The PKP1 gene mutation site was identified by target sequencing. The result showed that the PKP1 gene had a homozygous mutation at intron (PKP1: c.203-1G>A). Most of the wounds of the pediatric patient healed after 35 days of treatment, with many scattered residual wounds visible, and new blisters and skin lesions continue to appear.
2015年8月6日，武汉大学同仁医院暨武汉市第三医院烧伤科收治1例出生6 h的外胚层发育不良/皮肤脆性综合征男性患儿。取患儿前躯破溃处皮肤组织，采用苏木精－伊红染色观察皮肤组织病理学改变，结果显示患儿表皮肌细胞层轻度松解，表皮下方有大疱，真皮乳头层消失，真皮层内少量炎性细胞浸润；采用免疫组织化学染色检测患儿皮肤组织中斑菲素蛋白1(PKP1)的表达情况，结果显示PKP1表达完全缺失；靶向测序鉴定PKP1基因突变位点显示，在PKP1基因上有1个内含子纯合突变PKP1：c.203－1G>A。患儿经过35 d的治疗，全身大部分创面已经愈合，可见多处散在残余创面，不断有新的水疱及皮肤破损出现。.

Desmosomes are intercellular cadherin-mediated adhesion complexes that anchor intermediate filaments to the cell membrane and are required for strong adhesion for tissues under mechanical stress. One specific component of desmosomes is plakophilin 1 (PKP1), which is mainly expressed in the spinous layer of the epidermis. Loss-of-function autosomal recessive mutations in PKP1 result in ectodermal dysplasia-skin fragility (EDSF) syndrome, the initial inherited Mendelian disorder of desmosomes first reported in 1997.
To investigate two new cases of EDSF syndrome and to perform a literature review of pathogenic PKP1 mutations from 1997 to 2019.
Sanger sequencing of PKP1 identified two new homozygous frameshift mutations: c.409_410insAC (p.Thr137Thrfs*61) and c.1213delA (p.Arg411Glufs*22). Comprehensive analyses were performed for the 18 cases with confirmed bi-allelic PKP1 gene mutations, but not for one mosaic case or 6 additional cases that lacked gene mutation studies. All pathogenic germline mutations were loss-of-function (splice site, frameshift, nonsense) with mutations in the intron 1 consensus acceptor splice site (c.203-1>A or G>T) representing recurrent findings. Skin fragility and nail involvement were present in all affected individuals (18/18), with most cases showing palmoplantar keratoderma (16/18), alopecia/hypotrichosis (16/18) and perioral fissuring/cheilitis (12/15; not commented on in 3 cases). Further observations in some individuals included pruritus, failure to thrive with low height/weight centiles, follicular hyperkeratosis, hypohidrosis, walking difficulties, dysplastic dentition and recurrent chest infections.
These data expand the molecular basis of EDSF syndrome and help define the spectrum of both the prototypic and variable manifestations of this desmosomal genodermatosis.

Plakophilin 1 (PKP1) is an important plaque component of desmosomes, major intercellular adhesive junctions that act as anchorage points for intermediate filaments. Abnormal expression of PKP1 was observed in various types of cancer, however so far its function in lung cancer has not yet been elucidated.
The expression of PKP1 was analyzed by RT-PCR and western blotting in lung cancer cell lines. The protein expression of PKP1 was evaluated by immunohistochemistry in tissue microarray. The epigenetic mechanism of PKP1 was explored by demethylation test, bisulfite sequencing and Methylation-Specific-PCR. The function of PKP1 was investigated by stable transfection with an expression vector.
We found that PKP1 was downregulated in 6 out of 8 lung cancer cell lines, and downregulation of PKP1 was associated with DNA hypermethylation. In advanced primary lung tumor samples, higher expression of PKP1 was significantly associated with favorable clinical outcome (p = .003). Ectopic expression of PKP1 inhibited cell proliferation, colony formation, migration/invasion and enhanced apoptosis. These phenomena are accompanied by increased caspase 3/7 activities and cleaved PARP-1 as well as decreased extracellular signal-regulated kinase (ERK) activity.
Taken together, our data suggest that PKP1 is a novel tumor suppressor and its protein expression might be a potential prognostic marker for patients with advanced lung cancer.