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 embryonic tissues and for detecting cell subpopulations
with different degrees of differentiation is discussed with respect to their possible
application in tumor diagnosis.