Desmosomal plakophilin 2 as a differentiation marker in normal and malignant tissues

Abstract

Plakophilin 2 (PKP2) is a widespread protein which shows a remarkable dual location: On the one hand, it appears as a constitutive karyoplasmic protein and on the other it is a desmosomal plaque component of most, probably all, desmosome-possessing tissues and cell culture lines. Here we report on its desmosomal occurrence as revealed by immunocytochemical results obtained with three PKP2-specific murine monoclonal antibodies (mAbs) PP2-62, PP2-86 and PP2-150. These mAbs detect PKP2 in characteristic desmosomes of most normal cells, including simple and stratified epithelia as well as non-epithelial tissues such as myocardium and lymph node follicles. In addition, however, several normal tissues consistently display a differentiation-related PKP2 distribution, for example an absence of immunostaining in the "keratinizing" local specializations of the thymic epithelial reticulum, i. e. Hassall's corpuscles, and the restriction of PKP2 to the stratum basale of most stratified squamous epithelia, in contrast to its absence in upper strata, which contain PKP1-or PKP3-rich desmosomes instead. Taking advantage of the reactivity of mAb PP2-150 with formalin-fixed, paraffin-embedded material, a series of human carcinomas (n=37) has also been analyzed. The results suggest that mAbs to PKP2 may serve as markers for the identification and characterization of carcinomas derived from -or corresponding to -simple or complex epithelia. Thus consistent PKP2 immunostaining has been observed in all 18 cases of adenocarcinomas tested, but more variable and heterogeneous staining has been noted in squamous cell carcinomas, depending on the specific tumor type. The potential value of such mAbs for cell typing in normal and embry-onic tissues and for detecting cell subpopulations with different degrees of differentiation is discussed with respect to their possible application in tumor diagnosis.& b d y :

Nuclear particles containing RNA polymerase III complexes associated with the junctional plaque protein plakophilin 2

“…Moreover, in a wide range of different kinds of cells lacking desmosomes including fibroblasts, lymphocytes, and chicken erythrocytes, plakophilins have been detected exclusively in the nucleus (6,7,10,11). This finding suggests that plakophilins are constitutive nuclear proteins that, in certain pathways of cell differentiation, are recruited to a second specific location in desmosomal plaques (6)(7)(8)(9).…”

“…[18][19][20][21]. Among them one was identified as a Ϸ100-kDa polypeptide corresponding in size to plakophilin 2 (6)(7)(8). Indeed, immunoblot analysis (Fig.…”

“…Confocal laser scanning immunofluorescence images were acquired with a Zeiss LSM 410 microscope (Zeiss; ref. [6][7][8].…”

“…Plakophilins 2a (92.75 kDa) and 2b (97.41 kDa) are of special importance, because they display the most widespread occurrence in desmosomes of all simple and complex epithelia, in lower layers of many stratified epithelia, in all carcinomas, in heart muscle, in reticulum cells of lymph node follicles, and in early embryos (6)(7)(8). Therefore, we have decided to characterize the plakophilin 2-containing structures of nuclei.…”

“…Among these, the plakophilins (PKPs) are characteristic of desmosomes and also occur constitutively in the nucleoplasm of a wide range of cells (2)(3)(4)(5)(6)(7)(8)(9)(10)(11). These positively charged proteins (isoelectric points ranging from 9.3 to 10.1) represent the products of three genes, PKP1, PKP2, and PKP3, and at least PKP1 and PKP2 occur in two splice variants a and b (3)(4)(5)(6)(7)(8)(9)(10)(11)(12). Both variants can also be detected in the nuclei of various cells devoid of desmosomes and after cDNA transfection have also been shown to accumulate in the nuclei (9).…”