Palladin expression is a conserved characteristic of the desmoplastic tumor microenvironment and contributes to altered gene expression

Abstract

The stroma surrounding solid tumors contributes in complex ways to tumor progression. Cancer-associated fibroblasts (CAFs) are the predominant cell type in the tumor stroma. Previous studies have shown that the actin-binding protein palladin is highly expressed in the stroma of pancreas tumors, but the interpretation of these results is complicated by the fact that palladin exists as multiple isoforms. In the current study, the expression and localization of palladin isoform 4 was examined in normal specimens and adenocarcinomas of human pancreas, lung, colon, and stomach samples. Immunohistochemistry with isoform-selective antibodies revealed that expression of palladin isoform 4 was higher in adenocarcinomas versus normal tissues, and highest in CAFs. Immunohistochemistry staining revealed that palladin was present in both the cytoplasm and the nucleus of CAFs, and this was confirmed using immunofluorescence staining and subcellular fractionation of a pancreatic CAF cell line. To investigate the functional significance of nuclear palladin, RNA Seq analysis of palladin knockdown CAFs versus control CAFs was performed, and the results showed that palladin regulates the expression of genes involved in the biosynthesis and assembly of collagen, and organization of the extracellular matrix. These results suggested that palladin isoform 4 may play a conserved role in establishing the phenotype of CAFs in multiple tumor types.

The role of stromal cancer-associated fibroblasts in pancreatic cancer

“…Some have been established, but a true, unique marker is yet to be discovered. Native fibroblasts stain for vimentin and, once activated, stain positive for alpha-smooth muscle actin (αSMA) [31]. Other CAF markers previously reported include stromal cell-derived factor-1α (SDF1A), fibroblast activation protein (FAP), and fibroblast-specific protein-1 (FSP-1) [28, 32].…”

“…More recent studies, such as that by Cannon et al, found that palladin, an actin-binding protein highly expressed in cancers, actually precedes αSMA expression in cells undergoing activation to CAFs. The group further elucidated the role of palladin isoform 4 in CAF activation including its location in the cytoplasm and nucleus, and its role in expression of genes associated with collagen formation and MMP activation pathways demonstrated by shRNA knockdown [31]. …”

Crosstalk between cancer‐associated fibroblasts and immune cells in cancer

“…NG2 and PDGFR‐β are also expressed in normal pericytes, and podoplanin is expressed in lymphatic endothelial cells . Furthermore, Cannon et al found that paladin, a highly expressed actin binding protein in cancer, was expressed earlier than α‐SMA when activated in CAFs . Cytokeratin and CD31 are regarded as negative markers because CAFs had no epithelial or endothelial characteristics .…”

“…38 Furthermore, Cannon et al found that paladin, a highly expressed actin binding protein in cancer, was expressed earlier than α-SMA when activated in CAFs. 39 Cytokeratin and CD31 are regarded as negative markers because CAFs had no epithelial or endothelial characteristics. 37,40 Because of the lack of specific CAF markers, combinations of markers may help identify CAFs.…”

Coordinated Regulation of Palladin and α–Smooth Muscle Actin by Transforming Growth Factor–β in Human Corneal Fibroblasts

“…22,[29][30][31] Palladin is expressed in myofibroblasts of the stroma of solid tumors, and it is thought to promote the migration of these cells and thereby to facilitate tumor cell invasion. 32,33 Indeed, palladin may serve as a marker for cancer-associated myofibroblasts. 34 These observations suggest that palladin might also play an important role in corneal myofibroblasts, in which actin stress fibers have been shown to be abundant both in vitro and in vivo.…”