Tyrosines in the MUC1 cytoplasmic tail modulate transcription via the extracellular signal-regulated kinase 1/2 and nuclear factor-kappaB pathways.

Much of the ability of the MUC1 oncoprotein to foster tumorigenesis and tumor progression likely originates from the interaction of its cytoplasmic tail with proteins involved in oncogenic signaling. Many of these interactions are regulated by phosphorylation, as the cytoplasmic tail contains seven highly conserved tyrosines and several serine/threonine phosphorylation sites. We have developed a cell line-based model system to study the effects of tyrosine phosphorylation on MUC1 signaling, with particular emphasis on its effects on gene transcription. COS-7 cells, which lack endogenous MUC1, were stably infected with wild-type MUC1 or a MUC1 construct lacking all seven tyrosines (MUC1 Y0) and analyzed for effects on transcription mediated by the extracellular signal-regulated kinase 1/2 (ERK1/2) and nuclear factor-kappaB (NF-kappaB) pathways. COS.MUC1 Y0 cells showed heightened active ERK1/2 with increased activator protein-1 (AP-1) and signal transducer and activator of transcription 3 (STAT3) transcriptional activity; there was also a simultaneous decrease in NF-kappaB transcriptional activity and nuclear localization. These changes altered the phenotype of COS.MUC1 Y0 cells, as this line displayed increased invasion and enhanced [(3)H]thymidine incorporation. Analysis of the three lines also showed significant differences in their cell cycle profile and bromodeoxyuridine incorporation when the cells were serum starved. These data support the growing evidence that MUC1 is involved in transcriptional regulation and link MUC1 for the first time to the NF-kappaB pathway.