Purpose: We sought to gain insight into the mechanisms of heregulin-beta1 (HRG) action on breast epithelial cells by identifying and characterizing HRG-regulated proteins.
Experimental design: Differential display mRNA screening of human breast cancer cells grown in the presence or absence of HRG was used to identify HRG-regulated genes. Biochemical and functional studies were undertaken to examine the impact of HRG and the therapeutic antibody herceptin on protein expression, localization, and function.
Results: We identified the ATPase subunit 4 (S4) of the 26S proteasome as a HRG-regulated target. Both S4 mRNA and protein levels were increased by HRG; however, this HRG-stimulated increase was blocked by the therapeutic antibody herceptin. S4 expression was significantly increased in primary human breast tumors and in estrogen receptor-negative tumors. Coimmunoprecipitation, immunofluorescence, and ATPase activity assays suggested that HRG also induced S4 activity and formation of a functional proteasome complex.
Conclusions: This is the first demonstration of growth factor-regulated expression, localization, and activity of the S4 subunit of the 26S proteasome in human breast cancer cells. These findings now provide a potential mechanistic rationale for the use of proteasome inhibitors in breast cancers with active HRG signaling.