Signaling pathways in malignant transformation of cells

Signaling pathways in malignant transformation of cells

During normal development secreted signals such as Wnt (wingless), TGF-β(Transforming growth factor-β), and Hedgehog (Hh) are frequently used to direct cells to particular developmental fates, which may include the property of rapid mitosis. The effects of such signals must be regulated so that growth is limited to the right time and place.Cellular signaling pathways are not isolated from each other butare interconnected to form complex signaling networks. Cells receive information from many different growth factor receptors and from cell-matrix and cell-cell contacts.

They must then integrate this information to regulate diverse processes, such asprotein synthesis and cell growth, motility, cell architecture andpolarity, differentiation, and programmed cell death. The same signaling molecules are used to control different processes within different signaling complexes or at different intracellular locations. Moreover, signaling pathways are subject to developmental regulation and generate different outcomes in differentcell types; the activation of a signaling molecule may have Distinct consequences, depending on the cellular context.

Understanding how these extraordinarily complex signalling networks function in vivo and how they are altered in cancer cells represents a major intellectual challenge. The ability of intracellular signalling networks to integrate and distribute regulatory information requires that individual signalling proteins must act as nodes, responding to multiple inputs and regulating multiple effector outputs. One of the major advances in the last decade has been the recognition that many signalling proteins contain modular protein domains that mediate protein-protein interactions. These interaction modules serve to target signaling proteins to their substrates or to specific intracellular locations, to respond to posttranslational modifications, such as phosphorylation, acetylation and methylation,and to link polypeptides into multi-protein signaling complexes and pathways.

The intricacy of cellular signaling networks has major implications for our understanding of tumor cell behavior and for ourability to use this knowledge for cancer therapy. Cell proliferation,motility, and survival are regulated by multiple pathways,and the changes that occur in cancer cells are the result of multiple alterations in cellular signaling machinery.

The following section gives an account of the most prominent signaling pathways involved in malignant transformation of cells.

1) Mitogen-activated protein kinases/extracellular signal-regulated kinases (MAPK/ ERK) pathway

The MAPK/ERK pathway (also known as the Ras-Raf-MEK-ERK pathway) is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell.The signal starts when a signaling molecule binds to the receptor on the cell surface and ends when the DNA in the nucleus expresses a protein and produces some change in the cell, such as cell division. The pathway includes many proteins, including MAPK (mitogen-activated protein kinases, originally called ERK, extracellular signalregulated kinases), which communicate by adding phosphate groups to a neighboring protein, which acts as an “on” or “off” switch.

Aberrant activation of the pathway is frequently seen in human cancers. RAF–MAPK/(MEK)–ERK signaling cascade has been the most extensively studied, its role in variety of human cancers is well established particularly those associated with activation of RAS protein (ras is an oncogene) which bind to and activate the RAF kinase, triggering engagement of this pathway.Upon RAS activation, RAF is recruited to the cell membrane where subsequent changes in RAF phosphorylation status result in stimulation of its serine-threonine kinase activity. Activated RAF triggers sequential phosphorylation and activation of the MEK1/MEK2 dual-specificity proteinkinases and ERK, which translocates to the nucleus where they regulate the activity of several transcription factors that induce the expression of multiple genes required for survival and proliferation. Mutation in any protein of this signaling pathway leads to development of many cancers.

Signaling pathways in malignant transformation of cells
MAPK/ ERK Pathway

2) Epidermal growth factor receptor (EGFR) and Human Epidermal Growth Factor (HER2/ErbB-2) pathway

The epidermal growth factor receptor (EGFR; ErbB-1; HER1 in humans) is the cell-surface receptor for members of the epidermal growth factor family (EGFfamily) of extracellular protein ligands. Growth factor peptides and their receptors are involved in cell proliferation, differentiation and survival and are often over expressed in human cancer cells. The EGFR is a 170-kDa transmembrane protein able to bind several ligands, such as endothelial growth factor (EGF), transforming growth factor-a (TGF-α), heparin-binding EGF, amphiregulin, betacellulin, epiregulin and neuregulin G2b. Ligand binding is followed by receptor dimerization and Tyrosine Kinase (TK) auto-activation which in turn, triggers a cascade of intracellular signaling pathways.

Activation of EGFR or HER2 signaling triggers multiple and integrated biological responses, including mitogenesis, apoptosis, cellular motility, angiogenesis and regulation of differentiation. Deregulation of these tightly regulated ErbB receptor signalingpathways contributes to malignant transformation. Several mechanisms lead to aberrant receptor activation, including receptor overexpression, gene amplification, activatingmutations, overexpression of receptor ligands, and/orloss of negative regulatory mechanisms. One of the most studiedgrowth factor receptor systems is the HER (also defined ErbB)family.Amplification or overexpression of this oncogene has been shown to play an important role in the development and progression of certain aggressive types of breast cancer. In recent years the protein has become an important biomarker and target of therapy for approx. 30% of breast cancer patients.

3) Ras

The signal transduction cascade activated by growth factors receptors, cytokines (Interleukin 2, 3), and hormones(insulin, Insulin-like growth factor-IGF), involves the 21-kDaguanine–nucleotide-binding proteins encoded by the rasproto-oncogene.Ras (abbreviated form of Rat sarcoma) is the name given to a family of related proteins which are ubiquitously expressed in all cell lineages and organs. All Ras protein family members belong to a class of protein called small GTPase, and are involved in transmitting signals within cells The aberrant activation of Ras proteins is implicatedin facilitating virtually all aspects of the malignantphenotype, including cellular proliferation, transformation,invasion and metastasis. Ras activity is regulated by cycling between inactive GDP-bound and active GTP-bound forms.Once in its GTP-bound form, RAS activates several effector pathways that mediate cell proliferation andsuppression of apoptosis.

Hydrolysis of GTP by Ras is facilitated by GTPase-activating proteins (GAPs) such asp120GAP and NF1. Point mutations in the ras gene (at residues 12, 13 or 61) areoncogenic because they render Ras insensitive to GAP stimulation,resulting in a permanently active GTP-bound Ras form,which continuously activates the downstream pathways inthe absence of any upstream stimulation resulting in unchecked cell proliferation leading to development of cancer. Mutation of the ras gene is involved inmany tumor types, including roughly half of all colon cancers and 90% of pancreatic carcinomas (Goodsell, 1999).

4) Protein Kinase C (PKC)

PKC belongs to a class of serine–threonine kinases composed of 12 closely related isozymes that have distinct and, in some cases, opposing roles in cell growth and differentiation.Based on their structural and activation characteristics, this protein family can be further classified into three subfamilies: conventional or classic PKC isozymes (cPKCs: α, β and γ), novel (nPKCs: δ, ε, ηand θ) and atypical (aPKCs: ζ, ι and λ).Activation of classical enzymes (cPKC) depends on Ca+2 anddiacylglycerol (DAG), novel enzymes (nPKC) areactivated by diacylglycerol (DAG), and atypicalenzyme (aPKC) activation takes place independentlyof calcium or DAG, but they may be activated byother PKC.

PKC isozymes are involved in multiple signal transduction systems that respond to a variety of external stimulators, including hormones, growth factors, and other membrane receptor ligands. In general, PKCs are involved in various physiologicalprocesses of cells. Short-term activation of PKC is often associated with short-term events such assecretion and ion-influx. In contrast, sustained activationis suggested to induce long-term effects such as proliferation, differentiation, apoptosis, migration, ortumorigenesis. PKC isoenzymes have been shown to display variable expression profiles during cancer progression depending on a particular cancer type. The most common isoenzymes displaying alterations inexpression during cancer progression are α, β, andδ, but abnormal expression of other isoenzymes mayalso take place.

5) Protein kinase B (PKB)/Akt

Protein kinase B (PKB), also known as Akt, is a serine/threonine-specific protein kinase that plays a key role in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, transcription and cell migration. It is an important pathway that regulates the signaling of multiple essential biological processes. Akt is an indirect positive regulator of the Mammalian target of rapamycin (mTOR), a central controller ofeukaryotic cell growth and proliferation, through the phosphorylationand inactivation of mTOR inhibitors, such as tuberin(TSC2).

The activation of Akt provides cells with a survival signalthat allows them to withstand apoptotic stimuli, through phosphorylation/inactivation of pro-apoptotic proteins, such as BAD and Caspase 9, and transcriptional factors. Moreover,Akt is involved in the regulation of cell metabolism through inhibition of glycogen synthase kinase 3 (GSK3). AKT and its regulators play an important role in cancer pathogenesis: Akt is overexpressed in a variety of human cancer types and it conferschemo resistance.

6) Transforming growth factor beta (TGF-β)

Transforming growth factor-beta (TGF-β) is a multifunctional regulatory polypeptide that is the member of a large family of cytokines that controls many aspects of cellular function, including cellular proliferation, differentiation, migration, apoptosis, adhesion, angiogenesis, immune surveillance, and survival. The actions of TGF-β are dependent on several factors including cell type, growth conditions, and the presence of other polypeptide growth factors. One of the biological effects of TGF-β is the inhibition of proliferation of most normal epithelial cells using an autocrine mechanism of action, and this suggests a tumor suppressor role for TGF-β. Loss of autocrine TGF-β activity and/or responsiveness to exogenous TGF-β provide epithelial cells with a growth advantage leading to malignant progression. This suggests a pro-oncogenic role for TGF-beta in addition to its tumor suppressor role.

In normal cells, TGF-β, acting through its signaling pathway, stops the cell cycle at the G1 stage to stop proliferation, induce differentiation, or promote apoptosis. When a cell is transformed into a cancer cell, parts of the TGF-β signaling pathway are mutated, and TGF- β no longer controls the cell. These cancer cells proliferate. The surrounding stromal cells (fibroblasts) also proliferate. Both cells increase their production of TGF- β. This TGF-β acts on the surrounding stromal cells, immune cells, endothelial and smooth-muscle cells. It causes immunosuppression and angiogenesis, which makes the cancer more invasive. TGF- β also converts effector T-cells, which normally attack cancer with an inflammatory (immune) reaction, into regulatory (suppressor) Tcells, which turn off the inflammatory reaction.