MEK inhibitor CI-1040 reduces proliferation in lung adenomas but has no influence on apoptosis or lung specific differentiation. (A) The number of adenoma foci per mm2 was reduced after treatment with CI-1040 but not with BAY 43-9006. (B) CI-1040 reduced the PCNA positive cells in lung adenomas by half but not BAY 43-9006. (C, D) Ki-67 and Bmi-1 positive cells were reduced to a third in lung adenomas after CI-1040 treatment. (E) Neither BAY 43-9006 nor CI-1040 activated the caspase-3 mediated apoptotic pathway in lung adenomas. (F) BAY 43-9006 or CI-1040 treatment did not change cell differentiation in lung adenomas, which was assessed by expression of pro-SP-C, a marker for alveolar type II cells.