Breast cancer resistance protein/ABCG2 is differentially regulated downstream of extracellular signal-regulated kinase

Breast cancer resistance protein (BCRP)/ABCG2 is a drug efflux pump responsible for multidrug resistance in cancer cells. We report that dephosphorylation of extracellular signal-regulated kinase (ERK) by treatment with mitogen-activated protein kinase/ERK kinase (MEK) inhibitors causes two opposing effects, transcriptional upregulation and prompted protein degradation of endogenous BCRP in breast cancer MCF-7 cells. Endogenous BCRP was eventually found to be upregulated. Conversely, treatment with epidermal growth factor was associated with its downregulation in the cells. MEK inhibitors also caused prompted degradation of exogenous BCRP in MCF-7 and gastric cancer NCI-N87 cells that express exogenous BCRP without affecting its transcriptional levels, and potentiated anticancer agents in the cells. A lysosomal inhibitor abolished this prompted degradation of exogenous BCRP, but a proteasome inhibitor did not. Inhibition of p90 ribosomal protein S6 kinase (RSK), one of the downstream effectors of ERK, resulted in transcriptional upregulation of endogenous BCRP but did not affect the protein degradation of exogenous BCRP. The data suggest that BCRP expression is differentially regulated downstream of the MEK-ERK pathway, transcriptionally upregulated through the inhibition of the MEK-ERK-RSK pathway, and post-transcriptionally downregulated through the inhibition of the MEK-ERK-non-RSK pathway. Although the immediate downstream effector of ERK remains to be elucidated, the data provide new insights into regulatory mechanisms of BCRP activity and may assist the development of BCRP-specific expression modulators. (Cancer Sci 2009; 100: 1118-1127).