Expression of mutationally activated RAS is a feature common to the vast majority of human pancreatic
adenocarcinomas. RAS elicits its effects through numerous signaling pathways including the RAF-->
mitogen-activated
protein (MAP)/
extracellular signal-regulated kinase (ERK)
kinase [MEK]-->
ERK MAP kinase pathway. To assess the role of this pathway in regulating cell proliferation, we tested the effects of pharmacologic inhibition of
MEK on human
pancreatic cancer cell lines. In eight cell lines tested,
MEK inhibition led to a cessation of cell proliferation accompanied by G0-G1 cell cycle arrest. Concomitant with cell cycle arrest, we observed induced expression of p27Kip1, inhibition of
cyclin/
cyclin-dependent kinase 2 (cdk2) activity, accumulation of hypophosphorylated pRb, and inhibition of E2F activity. Using both antisense and RNA interference techniques, we assessed the role of p27Kip1 in the observed effects of
MEK inhibition on
pancreatic cancer cell proliferation. Inhibition of p27Kip1 expression in Mia PaCa-2 cells restored the activity of
cyclin/cdk2, phosphorylation of pRb, and E2F activity and partially relieved the effects of
U0126 on
pancreatic cancer cell cycle arrest. Consistent with the effects of p27Kip1 on
cyclin/cdk2 activity, inhibition of CDK2 expression by RNA interference also led to G0-G1 cell cycle arrest. These data suggest that the expression of p27Kip1 is downstream of the RAF-->MEK-->ERK pathway and that the regulated expression of this
protein plays an important role in promoting the proliferation of
pancreatic cancer cells. Moreover, these data suggest that pharmacologic inhibition of the RAF-->
MEK-->ERK signaling pathway alone might tend to have a
cytostatic, as opposed to a cytotoxic, effect on
pancreatic cancer cells.