Differential expression of Hela-type caldesmon in tumour neovascularization: a new marker of angiogenic endothelial cells.

Caldesmon (CaD) is a major actomyosin-binding protein found in various cell types. There are at least two high-molecular-weight isoforms (h-CaD) and four low-molecular-weight isoforms (l-CaD) produced by alternative splicing. The alternatively spliced variants of the l-CaD class are further differentiated by inclusion (Hela l-CaD) or exclusion (WI-38 l-CaD) of exon 1. Currently, nothing is known about differential expression of the Hela l-CaD in tumour neovascularization. In a previous study, expression of the Hela-type transcripts was found in glioma blood vessels but not in the normal cerebral vasculature. To investigate whether the differentially expressed transcripts are translated into protein, a specific antibody against the peptide encoded by exon 1 was raised. Initially, exclusive expression of the protein in glioma vasculature was confirmed. To determine further whether these findings are generalizable to neovascularization in a wide variety of other tumour types, a large cohort of cancers derived from various organs, including breast, lung, kidney, colon, stomach, ovary, uterus, prostate, thyroid, liver, giving a total of 180 cases, were examined. Expression of the Hela l-CaD was restricted to tumour vasculature and was not found in normal blood vessels. Hela l-CaD was preferentially expressed in the early stage of tumour neovascularization and the Hela l-CaD+ endothelial cells (ECs) were frequently enlarged, multinucleated, and developed elongated cell projections or free fragments of cytoplasm, correlating with the features of motile cells. In the Hela l-CaD+ ECs, disassembly of focal adhesion and the formation of podosome-like structures was observed. Therefore, the findings support the notion that quiescent ECs undergo activation of motility, necessary for ubiquitous tumour-associated neovascularization. The data indicate that Hela l-CaD can be considered as a marker for angiogenic ECs during the early stages of tumour neovascularization.