Mitogen‐activated protein kinase expression and activation does not differentiate benign from malignant mesothelial cells

Abstract

BACKGROUND In vitro studies of malignant mesothelioma (MM) cells have suggested activation of mitogen‐activated protein kinase (MAPK) in response to asbestos exposure. The objective of this study was to investigate protein expression (level) and phosphorylation status (activity) of the extracellular‐regulated kinase (ERK), the c‐Jun amino‐terminal kinase (JNK), and the high‐osmolarity glycerol response kinase (p38) in vivo through the analysis of fresh frozen reactive mesothelium (RM) and MM specimens. METHODS MAPK levels were analyzed in 36 fresh‐frozen MM specimens (32 effusions, 4 biopsies) and in 14 RM specimens (all effusions) using immunoblotting with antibodies detecting the total (pan‐) and activated (phospho‐) fraction (p‐) of ERK, JNK, and p38. Values for pan‐MAPK and p‐MAPK expression and the p‐MAPK/pan‐MAPK ratio in MM and RM specimens were compared. Results were corroborated using immunocytochemistry for p‐ERK, p‐JNK, and p‐38 in selected specimens. RESULTS Pan‐ERK, pan‐JNK, and pan‐p38 expression was found frequently in both MM specimens (35 of 36 specimens) and RM specimens (14 of 14 specimens) using immunoblotting, with comparable findings for activated p‐p38 (34 of 36 MM specimens, 13 of 14 RM specimens). Activation of p‐ERK (27 of 36 MM specimens, 10 of 14 RM specimens) and p‐JNK (25 of 36 MM specimens, 10 of 14 RM specimens) was less frequent. Pan‐ERK (P = 0.016), pan‐JNK (P = 0.004), pan‐p38 (P = 0.012), and p‐ERK (P = 0.02) expression levels were higher in MM specimens from female patients. Pan‐p38 expression levels also were higher in peritoneal MM specimens (P = 0.019). MM and RM showed similar MAPK expression, activation, and activation ratios (Mann–Whitney test; P > 0.05). Immunocytochemistry localized MAPK to MM and RM cells. CONCLUSIONS The current results provided the first evidence of in vivo activation of MAPK in clinical MM and RM. The similar values in these two cell types suggest that MAPK may not be involved in the transformation of benign to malignant mesothelium, thus bringing into question the validity of using MAPKs as molecular therapeutic targets in patients with MM. Cancer 2005. © 2005 American Cancer Society.