The Ras/Raf/MEK/Extracellular Signal-Regulated Kinase Pathway Induces Autocrine-Paracrine Growth Inhibition via the Leukemia Inhibitory Factor/JAK/STAT Pathway

Abstract

Sustained activation of the Ras/Raf/MEK/extracellular signal-regulated kinase (ERK) pathway can lead to cell cycle arrest in many cell types. We have found, with human medullary thyroid cancer (MTC) cells, that activated Ras or c-Raf-1 can induce growth arrest by producing and secreting an autocrine-paracrine factor. This protein was purified from cell culture medium conditioned by Raf-activated MTC cells and was identified by mass spectrometry as leukemia inhibitory factor (LIF). LIF expression upon Raf activation and subsequent activation of JAK-STAT3 was also observed in small cell lung carcinoma cells, suggesting that this autocrine-paracrine signaling may be a common response to Ras/Raf activation. LIF was sufficient to induce growth arrest and differentiation of MTC cells. This effect was mediated through the gp130/JAK/STAT3 pathway, since anti-gp130 blocking antibody or dominant-negative STAT3 blocked the effects of LIF. Thus, LIF expression provides a novel mechanism allowing Ras/Raf signaling to activate the JAK-STAT3 pathway. In addition to this cell-extrinsic growth inhibitory pathway, we find that the Ras/Raf/MEK/ERK pathway induces an intracellular growth inhibitory signal, independent of the LIF/JAK/STAT3 pathway. Therefore, activation of the Ras/Raf/MEK/ERK pathway can lead to growth arrest and differentiation via at least two different signaling pathways. This use of multiple pathways may be important for “fail-safe” induction and maintenance of cell cycle arrest.

ACKNOWLEDGMENTS

We thank S. Baylin, J. Shaper, and members of our lab for helpful discussions, B. Vogelstein for use of chromatography apparatus, J. Darnell, Jr., and R. Arceci for STAT3 genes, N. Reich and R. Jove for STAT3 reporter constructs, L. Cheng for recombinant LIF protein, L. Parada for Ras V12 and Raf BXB adenoviruses, R. Cole from the Johns Hopkins Mass Spectrometry/Proteomics Facility for mass spectrometry analysis, and L. Meszler from the Cell Imaging Core Facility at Johns Hopkins for excellent technical assistance.

This work was supported by NCI R01-CA47480 and R01-CA85567 (to B.D.N.) and NCI R01-CA70244 (to D.W.B.).