Crosstalk between TGF-ß and HER2 (ErbB2) signaling in mammary tumorigenesis
Mammary tumorigenesis is a multi-event process that occurs in a dynamic microenvironment. Unlike normal differentiated cells, whose functions are under tight regulation, tumor cells are multitasking as a result of the hyperactivation of multiple intracellular signaling pathways and loss of tumor suppressors. In tumor cells, these pathways do not respond to normal regulatory signals but are manipulated simultaneously by more than one oncogenic signal. Besides these tumor intracellular events, modification of the extracellular matrix (ECM) and transformation of the stromal tissues also are involved in tumor progression and metastasis. Therefore, it is rational to suppress tumor progression via: 1) simultaneous targeting of key oncogenic pathways expressed in cancer cells; and 2) targeting microenvironmental modifications that result from the crosstalk between oncogenic signaling networks in cancer cells.
Gene amplification/overexpression or somatic mutation of the receptor tyrosine kinase (RTK) HER2 (ErbB2) have been observed in numerous human cancers including about 20 percent of breast cancers. Therapeutic strategies targeting HER2, such as the humanized monoclonal IgG1 trastuzumab (Herceptin), are currently applied in these patients; however, the majority of patients with HER2-overexpressing advanced disease do not respond clinically to trastuzumab, and most that initially respond eventually relapse with antibody-resistant disease. The transforming growth factor (TGF) ß, a tumor suppressor in normal epithelia, is a tumor-promoting factor in established cancers. TGF-ß signaling impacts survival and invasion-related pathways and cooperates with oncogenes such as Ras, Wnt and ErbB2 in cancer progression. In cancer patients, high levels of TGF-ß at tumor sites correlate with high histological grade, risk of metastasis, poor response to chemotherapy and poor patient prognosis. We have previously shown that overexpression of HER2 alters the readouts of TGF-ß signaling in human mammary epithelial cells (HMEC). Currently, we are testing the hypothesis that the crosstalk between TGF-ß and HER2 is reciprocal, temporal and spatial, and involves not only intra- and intercellular mechanisms but also microenvironmental events. Our ongoing studies suggest that in HER2-mediated transformation, TGF-ß can: 1) deregulate epithelial cell junctions and polarity; 2) dynamically modulate the subcellular distribution of HER2 and the repertoire of HER2 substrates and signal transducers; 3) regulate the magnitude and duration of HER2 signaling through indirectly modulating receptor phosphorylation and cellular trafficking; 4) modify the ECM and stroma to build a microenvironment that facilitates tumor growth and metastasis; and 5) alter the response to therapeutic inhibitors of HER2.
Role of TGF-ß in the functional network of oncogenes and onco-microRNAs during breast cancer progression
MicroRNAs (miRNAs) represent a class of naturally occurring small, non-coding RNA molecules of 20 to 22 nucleotides. In animals, miRNA binds through partial sequence homology to the target mRNAs and causes either block of translation or mRNA degradation. Changes in the expression level or mutations of miRNAs have been detected in many types of human tumors. Certain miRNAs can function as tumor suppressors (miR-15a and miR-16-1) or oncogenes (miR-155 and the miR-17-92 cluster). Chromosomal rearrangements, genomic amplifications or deletions and transcriptional regulation of gene promoters that are common in the deregulation of oncogenes and tumor suppressor genes also influence the activity of miRNAs. Deregulation of different yet overlapping subsets of miRNA genes have been recently reported in various human cancers. Some of the cancer-related miRNAs target oncogenes or components of oncogenic signaling pathways, such as Ras, Myc, RhoC and Bcl2. The rationale of this current project in our laboratory is based on our preliminary studies showing that TGF-ß regulates the transcription of a subset of miRNAs that are characteristically deregulated in human breast cancer. We hypothesize that these miRNAs contribute to the cancer-promoting function of TGF-ß through targeting crucial components of oncogene signaling pathways and interfering with their biological functions and/or targeting other aspects, such as the tumor microenvironment, to exert indirect impacts on oncogene-driven tumor cells. The following research aims are being pursued in our laboratory: 1) to determine the target miRNA genes regulated by TGF-ß and whether the TGF-ß-induced Smad family of transcription factors is responsible for the regulation of TGF-ß target miRNAs; and 2) to determine how the TGF-ß target miRNAs contribute to breast cancer pathogenesis and how they may influence the signaling cascades initiated by oncogenes.
Role of TGF-ß in coordinating tumor progression and microenvironmental modifications
Metastasis involves the coordination of several signal transduction pathways that allow cancer cells to proliferate, remodel their surrounding environment, invade and migrate through new tissues and differentiate. Metastatic cancer cells develop altered affinity and avidity for their ECM, which is mediated by alterations in integrin expression and function, deposition of ECM molecules and release of proteases (e.g., MMPs) that remodel the ECM. All these processes are regulated by TGF-ß. Our studies indicate that, during cancer cell invasion, activated TGF-ß receptors localize themselves and relocalize RTK signaling molecules such as HER2 to lamellipodia that attach to the ECM. This allows the crosstalk of several signaling pathways, including TGF-ß, HER2 and integrin signaling to occur in this compartment. Meanwhile, TGF-ß also stimulates the ECM deposition by cancer cells in an oncogene-dependent fashion, which in turn activates corresponding integrin signaling, resulting in a dynamic interaction between tumor and microenvironment.
Dissect and simulate the profound effects of TGF-ß on tumor growth and invasion using a mathematical model
The family of TGF-ß ligands have both tumor-suppressive and tumor-promoting potentials and affect multiple physiological events such as cell proliferation, lineage determination, extracellular matrix production, cell motility, apoptosis and modulation of immune function. As a result of oncogenic stimuli or other unknown mechanisms, human cancers attenuate TGF-ß-induced suppression but exploit TGF-ß-mediated autocrine and paracrine mechanisms conducive to tumor progression. In the natural progression of different cancers from preneoplasia to advanced metastatic states, the timing at which the role of TGF-ß switches from tumor suppressor to promoter is unclear. The aim of this project is to decipher the complex effects of TGF-ß on various critical biological parameters using a mathematical model as collaboration with our mathematician colleagues. The model can be validated experimentally and will underlie realistic computer simulations of the dynamic functions of TGF-ß during tumor progression. Based on the Fisher-Kolmogorov equation, we presently have established a mathematical model that quantifies the proliferation and motility effects of TGF-ß. More parameters, including apoptosis and ECM deposition, are being integrated into our current model.
