AIM: To investigate the expression of mitogen-activated protein kinases (MAPKs) and its upstream protein kinase in human gastric cancer and to evaluate the relationship between protein levels and clinicopathological parameters.

METHODS: Western blot was used to measure the expression of extracellular signal-regulated kinase (ERK)-1, ERK-2, ERK-3, p38 and mitogen or ERK activated protein kinaseMEK-1 proteins in surgically resected gastric carcinoma, adjacent normal mucosa and metastatic lymph nodes from 42 patients. Immunohistochemistry was employed for their localization.

RESULTS: Compared with normal tissues, the protein levels of ERK-1 (integral optical density value 159526±65760 vs 122807±65515, P= 0.001), ERK-2 (168471±95051 vs 120469±72874, P< 0.001), ERK-3 (118651±71513 vs 70934±68058, P< 0.001), P38 (104776±51650 vs 82930±40392, P= 0.048) and MEK-1 (116486±45725 vs 101434±49387, P= 0.027) were increased in gastric cancer tissues. Overexpression of ERK-3 was correlated to TNM staging [average ratio of integral optic density (IOD) tumor: IOD normal in TNM I, II, III, IV tumors was 1.43±0.34, 5.08±3.74, 4.99±1.08, 1.44±1.02, n= 42, P= 0.023] and serosa invasion (4.31±4.34 vs 2.00±2.03, P= 0.037). In poorly differentiated cancers (n= 33), the protein levels of ERK-1 and ERK-2 in stage III and IV tumors were higher than those in stage I and II tumors (2.64±3.01 vs 1.01±0.33, P= 0.022; 2.05±1.54 vs 1.24±0.40, P= 0.030). Gastric cancer tissues with either lymph node involvement (2.49±2.91 vs 1.03±0.36, P= 0.023; 1.98±1.49 vs 1.24±0.44, P= 0.036) or serosa invasion (2.39±2.82 vs 1.01±0.35, P= 0.022; 1.95±1.44 vs 1.14±0.36, P= 0.015) expressed higher protein levels of ERK-1 and ERK-2. In Borrmann II tumors, expression of ERK-2 and ERK-3 was increased compared with Borrmann III tumors (2.57±1.86 vs 1.23±0.60, P= 0.022; 5.50±5.05 vs 1.83±1.21, P= 0.014). Borrmann IV tumors expressed higher p38 protein levels. No statistically significant difference in expression of MAPKs was found when stratified to tumor size or histological grade (P> 0.05). Protein levels of ERK-2, ERK-3 and MEK-1 in metastatic lymph nodes were 2-7 folds higher than those in adjacent normal mucosa. The immunohistochemistry demonstrated that ERK-1, ERK-2, ERK-3, p38 and MEK-1 proteins were mainly localized in cytoplasm. The expression of MEK-1 in gastric cancer cells metastasized to lymph nodes was higher than that of the primary site.

CONCLUSION: MAPKs, particularly ERK subclass are overexpressed in the majority of gastric cancers. Overexpression of ERKs is correlated to TNM staging, serosa invasion, and lymph node involvement. The overexpression of p38 most likely plays a prominent role in certain morphological subtypes of gastric cancers. MEK-1 is also overexpressed in gastric cancer, particularly in metastatic lymph nodes. Upregulation of MAPK signal transduction pathways may play an important role in tumorigenesis and metastatic potential of gastric cancer.