Signaling pathway switch in breast cancer

Cancer Cell International Signaling pathway switch in breast cancer Arnaud Guille 0 Max Chaffanet 0 Daniel Birnbaum 0 0 Centre de Recherche en Cancerologie de Marseille, Oncologie Moleculaire, Equipe labellisee Ligue Contre le Cancer, UMR1068 Inserm, CNRS UMR7258, Institut Paoli-Calmettes, Aix-Marseille Universite , 27 bd. Lei Roure, BP 30059, Marseille 13273 , France Next generation sequencing studies have drawn the general landscape of breast cancers and identified hundreds of new, actual therapeutic targets. Two major signaling pathways seem to be altered in a vast proportion of breast cancers. The PI3 kinase/AKT pathway is activated and the JUN/MAPK pathway is repressed. Via the regulation of the cell cycle this metabolic switch impacts on the balance between self-renewal, proliferation and differentiation of the tumor-initiating cells Next generation sequencing; Breast cancer; Signaling pathways; Cell cycle; Kinases - Background Recent results from next generation sequencing (NGS) studies have established the repertoire of driver gene mutations and copy number alterations (CNA) in breast cancer [1-4]. Nearly 900 cancers representative of all major expression subtypes (basal, luminal A and B, ERBB2 and normal-like) have been studied. Many recurrent mutations have been uncovered. Mutations in TP53, PIK3CA, GATA3 and PTEN genes are among the most frequent. These studies have forever changed our understanding of mammary oncogenesis. Hypothesis Many studies will extend these pioneering ones but it is already possible to speculate further on the NGS data. Data analysis revealed that some alterations (CNA and/ or mutations) never occur in the same tumor, i.e. are mutually exclusive [1-4]. Two main signaling pathways seem to be targeted, the PI3K/AKT pathway and the JUN/ MAPK pathway [1-4]. Alterations in components of the PI3K/AKT pathway (PIK3CA, PIK3R1, AKTs, PTEN, INPP4B) are mutually exclusive but strikingly, amplification and upregulation of genes encoding receptor-type tyrosine kinases (RTKs) (IGF1R, EGFR, ERBB2) are also (globally) mutually exclusive with alterations of the PI3K/ AKT pathway. This suggests that the primary role of RTK amplification or mutation is to activate the PI3K/AKT pathway. Thus, in the normal mammary epithelium these RTKs are repressed or expressed at a low level and their signaling is primarily oriented toward the JUN/MAPK pathway, whereas when upregulated in tumor cells they stimulate the PI3K/AKT pathway. To obtain this dosage effect could be the reason for the amplification of ERBB2 and FGFR1 genes, although there could be other reasons [5]. It is known that the PI3K/AKT pathway is activated in tumors with mutated EGFR or overexpression of ERBB2 and determines the response to ERBB targeted inhibitors [6]. Within the JUN/MAPK pathway alterations of the components are also mutually exclusive [1]. Components of the JUN/MAPK pathway are inactivated by deletions and mutations, such as MAP2K4 and MAP3K1, or by amplifications, such as PAK1. Most importantly, alterations leading to the activation of the PI3K/AKT pathway and those leading to the inactivation of the JUN/MAPK pathway are mutually exclusive [1]. Finally, not only mutations and genomic rearrangements affect genes encoding components of the two pathways but opposite modifications in expression patterns of these genes could also participate to their switch in breast cancer. Our hypothesis is that one important consequence of mutations, CNA and modifications of expression is to shift cell signaling in the targeted mammary epithelial cell from an active JUN/MAPK pathway inactive PI3K/AKT pathway state to an active PI3K/AKT pathway inactive JUN/MAPK pathway state. Testing the hypothesis In the diagram shown in Figure 1 we have represented 19 KEGG metabolic and signaling pathways and cell processes altered by mutations in their components in Figure 1 Altered KEGG pathways in breast cancers. In the bar-plot depicting the proportion of cancer samples with altered pathways (19 pathways are listed in the x-axis) the y-axis represents the percentage of samples altered in a given pathway stratified by subtype (red for basal, purple for ERBB2, blue for luminal A, cyan for luminal B, and green for normal-like). The analysis was done in 602 samples from different studies in which the molecular subtype (using PAM50) was available (466 from ref. 1, 98 from ref. 2, 38 from ref. 3). the 602 breast cancer samples of the NGS studies for which the molecular subtype had been assigned [1-4]. All pathways and processes are altered, although to different extents. Many potential therapeutical targets can be found in these pathways. Two main signaling pathways are targeted (Figure 2), the JUN/MAPK pathway and the PI3K/AKT pathway [1-4]. Mutations in PTEN, amplifications and mutations in PIK3CA and AKT genes, the MAGI3-AKT3 gene fusion [3], activate the PI3K/AKT pathway whereas mutations in MAP3K1, MAP3K13 and MAP2K4 inactivate the JUN/MAPK pathway. The PI3K/AKT and JUN/MAPK pathways are intimately related and intricate. For example, AKT activation inhibits MAP2K4. This interaction and the mirror effect of the alterations on the two signaling pathways suggest that the PI3K/AKT pathway stimulates the growth of tumor cells whereas the JUN/MAPK pathway has an opposite effect and that the two pathways are the two sides of the same coin. The downstream effects of the activated PI3K/AKT and inhibited JUN/MAPK pathways are multiple, but at least two could be of primary importance for the behavior of the tumor-initiating cell that fuels the tumor growth. A first major effect could be on the cell cycle. During the G1 phase of the cell cycle, a checkpoint before entering S phase, also called the restriction (R) point, has been defined as an important cell cycle stage controling various cell fates [8] (Figure 3). The G1 phase of the cell cycle has been thus divided into an early, signaling factor-dependent subphase, controled by D cyclins and a late, factor-independent subphase, controled by E cyclins and a fully inactivated (hyperphosphorylated) RB protein. The JUN/MAPK signaling pathway plays a role in the early G1 subphase, where the cell may be induced into quiescence, senescence or committed to differentiation, depending on the presence of external factors [9]. When entering the late G1 subphase, quiescence, cell death or differentiation are no longer options and the cell Figure 2 Subnetwork representation of protein interactions based on 78 mutated genes in 602 breast cancers [1-3]. Gene networks were inferred using the Reactome FI Cytoscape Plugin [7]. A total of 182 genes with more than 7 mutations in 602 breast cancer samples were used. Among them, 78 (43%) were mapped in the subnetwork and subsequently clustered into 12 modules, six of which were greater than 4. Finally, modules functions were assessed with pathway enrichment analysis (FDR < 0.05). Three main pathways can be recognized: the P53 p (...truncated)