Regulation of CEACAM1 Protein Expression in Braf-Mutant Human Metastatic Melanoma Cells

Melanoma is among the most aggressive types of cancers and is responsible for 80% of skin cancer–related deaths worldwide.

BRAF, a key component of MAPK pathway, is mutated in 50%-70% of melanoma cases, with V600E responsible for the majority of cases.

FDA approved BRAF inhibitor achieves high and rapid response rate. However, secondary resistance develops within varying periods of time.

CEACAM1 is a transmembrane glycoprotein that protects melanoma cells from immune attack. CEACAM1 expression in primary lesions strongly predicts the development of metastasis and poor survival.

Our results show strong correlation between BRAF mutation status and CEACAM1 expression in melanoma cells derived from patients. Cells treated with MAPK inhibitors (MAPKi) showed down-regulation in CEACAM1 expression, only among BRAF^V600E melanoma lines in a dose- and exposure time-dependent manner. Furthermore, mRNA of CEACAM1 decreased following MAPKi treatment, suggesting a transcriptional regulation. MAPKi-resistant cell system showed an increase of membrane CEACAM1 expression and mRNA, compared to control. Investigation of CEACAM1 transcription regulation show significant reduction in pCEACAM1 activity following treatment with MAPKi. ETS1 is a downstream effector of the MAPK pathway and a regulator of the pCEACAM1. Our results indicate a reduction of pCEACAM1 activity following ETS1 binding site deletion from pCEACAM1. ETS1 overexpression enhanced the activity of pCEACAM1 and mutant ETS1^T38A showed dominant negative effect. Thus we hypothesize that CEACAM1 is regulated at transcriptional level through MAPK pathway activated by BRAF^V600E mutation via ETS1. Finally, CEACAM1 down-regulation by MAPKi rendered the cells more sensitive to immune attack. These results provide a new view on a potential immunological mechanism of action of MAPKi in melanoma, as well as on the aggressive phenotype observed in drug-resistant cells.