**3.3 Basal-like subtype**

The name of "basal-like" subtype comes from the similarity in gene expression to that of the basal epithelial cells. This subtype shows lower expression of the luminal and HER2 gene clusters. Therefore, these tumors are typically ER-, PR-, and HER2-negative on clinical assays. Because of this reason, the name "triple negative" is also used to describe them. However, while most triple negative tumors are basal-like, and most basal-like tumors are triple negative, there is significant inconsistency (up to 30 percent) between these two classifications. Although any subtype can be triple negative on clinical assays, an interesting subtype found in non-basal triple negative breast cancers is the more newly described claudin-low subtype, which is uncommon but interesting because of its expression of epithelial-mesenchymal transition genes and characteristics reminiscent of stem cells (9).

Recently, many studies have focused on finding molecular pathways that play some roles in breast cancer pathogenesis. Mutation in oncogenes, pro-oncogenes and tumor suppressor genes has been remarked as potential elements in breast cancer. DNA amplication (mostly in proto -oncogenes, growth factors and their receptors) and DNA deletion (in tumorsuppressor genes) are repeatedly observed in breast tumors. Berouk him et al. found 76 amplications and 82 deletions in 243 breast tumors, in regions containing new possible sensitive genes, such as MCL1 and BCL2L1 (apoptosis), Interleukin-1 receptor-associated

Breast Cancer from Molecular Point of View: Pathogenesis and Biomarkers 107

tumor cells are unable to proliferate, There are still viable and bears pathological tumor dormancy. Interesting point is that inhibition of integrin-mediated FAK signaling will also shows the similar pathological features. ß4integrin, other member of integrin family, has shown a clear role cell proliferation and invasion through association with Erb B2. Not all integrins, however, have a role in bearing cancer. Deficiency in ß3 or/an d ß 5 integrins did not produce much difference in tumor growth, tumor numbers or lung metastasis in the PyV MT mouse model , only a little increase in tumor onset was observed. Taken together, these observations give promising data for targeting integrin receptors and their associated

Activation of the phosphatidyl inositol-3 kinase is also important in mammary tumor progression. Association of PI-3K links to PyV mT through its binding to phosphotyrosine residues (Tyr 315/322) within the PyV mT coding sequences. Association with Neu happens through recruitment to ErbB3 (ErbB, is derived from the name of a viral oncogene to which these receptors are homologous: Erythroblastic Leukemia Viral Oncogene). Activation of PI-3K and resultant production of phosphoinotide-3 lipids stimulates several members of serine kinase family. The final of these cascades will be the stimulation a number of

Because of the wide range of activities of transcription factor NF- κB in apoptosis and cell survival and cell proliferation pathways as well as cell adhesion and angiogenesis it plays a

Regulatory influence of NF- κ B on the expression of various tumor-promoting molecules such as MMP, cycloxygenase 2, inducible nitric oxide synthase, chemokines, and inflammatory cytokines explain its significant effect on bearing cancer. NF- κB increased the expression of these molecules, all of which enhance tumoral cell invasion and angiogenesis. Other aspect of the role of NF- κB in tumorigeneses includes increasing expression of proto-

Adapter proteins do not exert any kinase activity, but they regulate protein – protein interaction and help the formation of protein complex which participate in signal transduction pathways. GRB2-associated-binding protein 2 (Gab2) is one of the adapter proteins which is overexpressed in breast cancer. It promotes signaling pathways by recruiting SH2 containing proteins such as PI3K, Shc, and Shp2 downstream of tyrosine kinase receptors. Although elevated expression of Gab2 in the mammary epithelium is unable to induce tumor development, it has been shown that tumor onset time will decrease

Activation of the Ras signaling pathway is commonly observed in mammary tumor progression. Adapter proteins such as Shc and Grb2 create some specific complexes with activated forms of Neu and PyV mT. The co-operation of Grb2 and Shc with these activated oncoproteins will result in stimulation of Ras signaling. In contrast to PyV mT, which signals to Ras only through its association with Shc, Neu can activate Ras through Grb2, Shc

oncogenes such as c- myc and cyclin D1 which directly stimulate proliferation. (14)

signaling pathways as a new treatment of breast cancer (11).

**4. Role of NF- κB** 

**4.1 Adapter proteins** 

in presence of Gab2 (16,17)

**4.2 Activation of the Ras signaling pathway** 

remarkable role in tumorigeneses.

antiapoptotic signaling molecules such as nuclear factor-kB (NF- κB) (14,15)

kinase1 (IRAK1), TNF receptor associated factor (TRAF) 6, IKBKG which codes NF-kappa-B essential modulator (NEMO) protein and IKBKB which codes inhibitor of nuclear factor kappa-B kinase subunit beta (IKK-β) protein in NK- kB signaling pathway. PIK 3CA, the gene encoding the catalytic subunit of phosphatidylinositol 3-kinase (PI3K), is mutated in about 20 – 30% of breast tumors. TP53 mutations are found in about 30 – 35% of cases (10).

Two newly identified genes, BRCA1 (Breast Cancer gene A1) and BRCA 2 (Breast Cancer gene A2), have been identied and categorized as human tumor suppressor genes. Mutations in these two genes have been found in the majority of hereditary breast cancer cases. Until the age of 70 women with mutated BRCA1 or BRCA2 genes faces to 45-85% increase in the risk of developing breast cancer. Several studies have demonstrated that patients with mutation in BRCA1 usually bear triple-negative kind breast tumors. In contrast, pathologic characteristics of BRCA2-mutant cases did not seem to be very different with non-carriers. Both these two genes play important roles in DNA repair in a common pathway. BRCA 1 is necessary for mammary stem cell differentiation, a function that could explain its tissue-specificity.

Mutations usually result in dysregulation of signal transduction pathways. Increased expression of specific receptor tyrosine kinases (RTKs) has been implicated in the genesis of a significant proportion of sporadic human breast cancers. Increased activity of some of tyrosine kinases can result in aberrant cell proliferation. This phenomenon may result in cell transformation. For example, amplification and overexpression of neu/erbB2 protooncogene is observed in 20–30% human breast cancer, and is inversely correlated with the survival of the patient.

The epidermal growth factor receptor (EGFR) family is a member of growth factor receptors which consists of four members: EGFR, ErbB2/Neu, ErbB 3, and ErbB 4. Increase ErbB2 expression, has been further associated with poor clinical outcome, is observed in 20 – 30% of sporadic breast tumors. The main reason is ErbB2 gene amplification (11). Increased level of tyrosine phosphorylated ErbB3 has been also reported. The important point is that ErbB3 is a bridge which links the phosphatidyl inositol-3 kinase (PI-3K) signaling molecule to Neu which has attracted much attention because of its potent transforming properties. This oncogene activates a number of common signaling pathways by providing specific binding sites for a variety of signaling molecules that include either Src Homology 2 (SH2) or phosphotyrosine binding/interacting domains. Co-expression of ErbB2 and ErbB3 RTKs is usually observed in common tumor progression (11,12).

 Mammary epithelial expression of Polyoma virus middle T (PyV mT) antigen, another tyrosine kinase involved in murine mammary tumorigenesis and metastasis, results in the rapid induction of multifocal metastatic mammary tumors. Since these tumors occur during early steps of mammary gland development and involve whole of the gland, expression of PyV mT will result in transformation of the primary mammary epithelium. This molecule is also associated with many signaling pathways via Src Homology 2 (SH2) or phosphotyrosine binding/interacting domains (13).

It has been shown that Activated growth factor receptors can interact with integrin receptors and control their biological function in cancerous cells. An example is the stimulation of a6ß1 integrin through association with activated members of the EGFR family which conversely results in activation of EGFR family phosphorylation. Induction of tumor by the PyV M T oncogene is also dependent on the presence of functional ß1-integrin. Lack of functional ß1-integrin makes tumor cells unable to enter the cell cycle. Although, these tumor cells are unable to proliferate, There are still viable and bears pathological tumor dormancy. Interesting point is that inhibition of integrin-mediated FAK signaling will also shows the similar pathological features. ß4integrin, other member of integrin family, has shown a clear role cell proliferation and invasion through association with Erb B2. Not all integrins, however, have a role in bearing cancer. Deficiency in ß3 or/an d ß 5 integrins did not produce much difference in tumor growth, tumor numbers or lung metastasis in the PyV MT mouse model , only a little increase in tumor onset was observed. Taken together, these observations give promising data for targeting integrin receptors and their associated signaling pathways as a new treatment of breast cancer (11).

Activation of the phosphatidyl inositol-3 kinase is also important in mammary tumor progression. Association of PI-3K links to PyV mT through its binding to phosphotyrosine residues (Tyr 315/322) within the PyV mT coding sequences. Association with Neu happens through recruitment to ErbB3 (ErbB, is derived from the name of a viral oncogene to which these receptors are homologous: Erythroblastic Leukemia Viral Oncogene). Activation of PI-3K and resultant production of phosphoinotide-3 lipids stimulates several members of serine kinase family. The final of these cascades will be the stimulation a number of antiapoptotic signaling molecules such as nuclear factor-kB (NF- κB) (14,15)
