Microarray phosphatome profiling of breast cancer patients unveils a complex phosphatase regulatory role of the MAPK and PI3K pathways in estrogen receptor-negative breast cancers

Phosphatases are proteins with the ability to dephosphorylate different substrates and are involved in critical cellular processes such as proliferation, tumor suppression, motility and survival. Little is known about their role in the different breast cancer (BC) phenotypes. We carried out micro-array phosphatome profiling in 41 estrogen receptor-negative (ER(−)) BC patients, as determined by immunohistochemistry (IHC), containing both ERBB2(+) and ERBB2(−) in order to characterize the differences between these two groups. We characterized and confirmed the distinct phosphatome of the two main ER(−) BC subgroups (in two independent microarrays series) and that of ER(+) BC (in three large independent series). Our findings point to the importance of the MAPK and PI3K pathways in ER(−) BCs as some of the most differentially expressed phosphatases (like DUSP4 and DUSP6) sharing ERK as substrate, or regulating the PI3K pathway (INPP4B, PTEN). It was possible to identify a selective group of phosphatases upregulated only in the ER(−) ERBB2(+) subgroup and not in ER(+) (like DUSP6, DUSP10 and PPAPDC1A among others), suggesting a role of these phosphatases in specific BC subtypes, unlike other differentially expressed phosphatases (DUSP4 and ENPP1) that seemed to have a role in multiple BC subtypes. Significant correlation was found at the protein level by IHC between the expression of DUSP6 and phospho-ERK (p=0.04) but not of phospho-ERK with DUSP4. To show the potential prognostic relevance of phosphatases as a functional group of genes, we derived and validated in two large independent BC microarray series a multiphosphatase signature enriched in differentially expressed phosphatases, to predict distant metastasis-free survival (DMFS). ER(−) ERBB2(+), ER(−) ERBB2(−) and ER(+) BC patients have a distinct pattern of phosphatase RNA expression with a potential prognostic relevance. Further studies of the most relevant phosphatases found in this study are warranted.