Induction of immediate early transcription factors (ITF) represents the first transcriptional program controlling mitogen-stimulated cell cycle progression in cancer. Here, we examined the transcriptional mechanisms regulating the ITF protein c-Myc and its role in pancreatic cancer growth in vitro and in vivo.

Expression of ITF proteins was examined by reverse-transcription polymerase chain reaction and immunoblotting, and its implications in cell cycle progression and growth was determined by flow cytometry and [³H]-thymidine incorporation. Intracellular Ca²⁺ concentrations, calcineurin activity, and cellular nuclear factor of activated T cells (NFAT) distribution were analyzed. Transcription factor complex formations and promoter regulation were examined by immunoprecipitations, reporter gene assays, and chromatin immunoprecipitation. Using a combination of RNA interference knockdown technology and xenograft models, we analyzed the significance for pancreatic cancer tumor growth.

Serum promotes pancreatic cancer growth through induction of the proproliferative NFAT/c-Myc axis. Mechanistically, serum increases intracellular Ca²⁺ concentrations and activates the calcineurin/NFAT pathway to induce c-Myc transcription. NFAT binds to a serum responsive element within the proximal promoter, initiates p300-dependent histone acetylation, and creates a local chromatin structure permissive for the inducible recruitment of Ets-like gene (ELK)-1, a protein required for maximal activation of the c-Myc promoter. The functional significance of this novel pathway was emphasized by impaired c-Myc expression, G1 arrest, and reduced tumor growth upon NFAT depletion in vitro and in vivo.

Our study uncovers a novel mechanism regulating cell growth and identifies the NFAT/ELK complex as modulators of early stages of mitogen-stimulated proliferation in pancreatic cancer cells.