Signaling disorders causing hyperproliferation of tumor cells

Introduction
Cell signaling is a set of complex  cascade reactions that are mediated by an interconnected network of kinases, phosphatases, transcription factors, G proteins, and enzymes.

The simplified scheme represents the binding of a ligand to a receptor, the activation of this receptor, the signal transduction in the cell (or its amplification - most often by a kinase cascade) and the activation of transcription factors that regulate the expression of certain genes. Expression of a particular gene means the synthesis of a protein with a specific function to proliferate, differentiate a cell, initiate or block apoptosis or the cell cycle. Individual proteins in the cascade recognize the phosphorylated domains of the previous member of the cascade using their own -SH2 domains.

Signaling pathways in the cell are important for carcinogenesis at the molecular level. Disorders in these cascades - usually overexpression or inhibition of a particular intermediate in the cascade - can cause changes in the cell that give it a proliferative advantage, and the cell may be subject to clonal selection.

Signaling pathways for growth factors
An example is the cascade via MAPK protein and PI3K/Akt
 * They use a wide range of signal transducers
 * Binding of growth factor to the receptor induces dimerization of the receptor, thereby activating the receptor tyrosine kinase activity and autophosphorylating it.
 * This is followed by a cascade of phosphorylation and dephosphorylation of individual members of the signaling pathway

MAPK cascade (mitogen activated protein kinase)

 * By phosphorylating the receptor, the signal is transmitted through the Grb2 and SOS proteins to the Ras protein. The Ras protein is a G protein that exhibits GTPase activity. Following GTP binding to the RAS, the''' Raf protein is activated
 * Raf protein activates other kinases in the cascade, namely MEK and Erk
 * Phosphorylated Erk enters the nucleus and there activates the transcription factors responsible for cell growth and differentiation
 * In addition to the above, stress and proinflammatory cytokines can trigger the activation of the Ras protein after activation of the Ras protein, which cascades through MEKK, MKK and SAPK / JNK, which activate genes responsible for growth, differentiation and apoptosis in the nucleus. MKK can also be activated via ASK 1, which responds directly to oxidative stress
 * MAPK kascade scheme


 * Pathologically, the following may occur:
 * For growth factor receptor overexpression (EGFR family)
 * To mutate in k-ras, the GTPase activity of the Ras protein is eliminated and it becomes permanently stimulated
 * To lose the lipid bond that holds Ras on the membrane, whereby the protein loses its function (however, this disorder inhibits signaling, is used therapeutically)
 * To overexpress some of the kinases


 * The whole MAPK signaling cascade is much more complex, in addition, the individual pathways are interconnected, so it is difficult to clearly separate them from each other. The principle of all variants of the MAPK cascade is the same, their application depends on the type of stimulus

PI3K / Nude track

 * PI3K is activated either by autophosphorylation of the growth factor receptor (EGFR family) or by activation of the cytokine receptor
 * PI3K produces PIP3 from membrane proteins that activate PDK1 which in turn phosphorylates Akt protein – kinase, which has a broad spectrum of activity
 * For example, it phosphorylates IKKalpha, triggering the NF kappaB signaling pathway
 * Activates glycolysis and inhibits glycogen synthesis
 * It acts on the MDM2 protein, which controls the activity of the p53 protein by inhibiting it
 * It activates the 14-3-3 protein, which phosphorylates the Bad protein, releasing it from binding to the Bcl-2 protein, which is thus activated
 * It activates the mTOR protein signaling pathway, which ultimately triggers cellular proteosynthesis and proliferation.
 * Inhibits cell cycle blockers
 * schéma PI3K/Akt act


 * Pathologically, the following may occur:
 * To increase receptor activity (similar to the MAPK pathway)
 * A mutation in the PI3K gene, resulting in uncontrollable activity of this kinase
 * For overexpression of Akt protein - for example ovarian tumors
 * To a mutation in which the function of PTEN phosphatase, which inhibits the formation of PIP3, a product of PI3K, is lost

JAK/STAT track

 * It begins with a receptor for cytokines (also interleukins), which dimerizes upon substrate binding. It is a receptor with associated tyrosine kinase activity mediated by JAK kinases. By dimerizing the receptor, they activate, phosphorylate each other and, in addition, also activate Grb2 and Ras, PI3K and STAT proteins by phosphorylation.
 * Phosphorylated STAT proteins dimerize and enter the nucleus, where they function as transcription factors for the expression of c-Myc, cytokines and other transcription factors
 * Activated transcription negatively inhibits the signaling cascade through the SOCS protein, which inhibits the activity of JAK kinases
 * JAK/STAT path diagram


 * Patologically
 * The upregulation of JAK kinases occurs most often, which occurs in some lymphoproliferative and myeloproliferative diseases
 * SOCS gene downregulation may also occur (eg by DNA hypermethylation)

NF kappaB

 * It responds to the action of TNF, growth factors, cytokines.
 * TNF factor, upon binding to the receptor, triggers a cascade that acts on IKK
 * IKK kinase phosphorylates the IkappaB complex and it breaks down into two complexes:
 * IkappaBalfa, NFkappaB1,p50,ReIA
 * IkappaBbeta, NFkappaB2,p52,ReIA,p65
 * The IkappaB inhibitor, which is degraded in the proteasome, is separated from each complex. The above proteins combine into the p65, ReIA, p50, p52, NFkappaB complex, which enters the nucleus and acts there as a transcription factor
 * NFkappaB is responsible for cell proliferation, is involved in the inflammatory response and regulation of immunity. It is also involved in B cell survival and differentiation. Disorders of this pathway can be observed in some hematological malignancies
 * The pathway is also activated after the action of proinflammatory cytokines
 * NFkappaB signaling pathway

This pathway can be suppressed by the administration of salicylates, flavonoids, glucocorticoids.

TGF

 * TGF acts on its TGF receptors, which activate Smad2 protein by activating it
 * Smad2 phosphorylation activates Smad4, which acts as a transcription factor in the nucleus and is responsible for cell activation, cell growth, increased cell motility and migration potential
 * TGF plays a role in angiogenesis and metastasis, where it potentiates cell migration by impairing ECM matrix consistency and increasing vascular permeability
 * Its production is stimulated, for example, by the activation of hypoxia-sensitive HIF protein in the tumor cell, which triggers the synthesis of ECM proteases, growth factors and cytokines needed for angiogenesis.
 * TGFbeta signaling scheme
 * application of TGF in angiogenesis

Wnt/Beta catenin

 * It is a pathway that mediates signal transduction through Beta catenin (a protein involved in the intercellular adhesion junction), which is activated by the binding of the Wnt protein to its receptor.
 * Beta catenin subsequently acts as a transcription factor in the nucleus
 * Deregulation of this pathway leads in some cases to the development of malignant disease


 * Wnt/beta catenin signaling pathway

Notch track

 * This signaling pathway mediates juxtacrine (ligands are the membrane proteins of the neighboring cell that transmits the signal) communication between neighboring cells in very close contact with each other.
 * It is used in the development of the nervous, cardiovascular and endocrine systems
 * Receptor activation triggers a cascade by modifying the receptor protein, which in the end acts as a transcription factor in the nucleus
 * Mutations in Notch receptors are involved in the development of some types of T leukemias


 * The enzyme gamma secretase, which is a possible therapeutic target of so-called targeted therapy is also used in the cascade.


 * Notch signaling scheme

Cell cycle regulation

 * It is regulated at two main checkpoints, where DNA integrity and quality are assessed before replication, followed by regulatory and corrective mechanisms to monitor whether errors have occurred after DNA replication and whether DNA has replicated in its entirety.
 * The main role in regulation is played by two proteins, the products of tumor suppressor genes, whose disorders can cause uncontrolled cell hyperproliferation.

pRB

 * It is normally bound to the transcription factor E2F and blocks the synthesis of cyclin E. When the cyclin D / CDK4,6 complex is activated, pRb is phosphorylated, releasing it from binding to E2F and causing the synthesis of cyclin E (G1 / G0 checkpoint is overcome ), which will allow the synthesis of cyclin A and the entry into the S phase.
 * Cyclin D synthesis is inhibited by proteins p15, p16 and p27, which inhibits Cdk2, which is required to enter the S phase of the cycle. These inhibitory proteins are inactivated upon mitogenic stimulation by growth factors (eg TGF) or cell cycle regulating proteins (eg c-Myc products)

p53

 * It is a protein that is responsible for checking the integrity of the genome ("genome guardian")
 * It induces the production of proteins that inhibit cell cycle progression until DNA is checked or the cell is stimulated.
 * Its level is kept low thanks to the mdm2 protein, which ensures ubiquitinylation of p53 (p53 after ubiquitinylation travels to the proteasome, where it is degraded).
 * However, DNA damage activates the ATM/ATR, kinase, which phosphorylates p53 and phosphorylated p53 cannot interact with mdm2.
 * This in turn induces the GADD45 protein (DNA repair), the BRCA1 tumor suppressor, p21, which blocks Cdk and thus stops the cell cycle, and last but not least, the Bax protein, which triggers the internal pathway of apoptosis. At the same time, Bcl2 protein is inhibited.


 * G1/S checkpoint
 * G2/M checkpoint

List of abbreviations

 * Akt - protein kinase B
 * ASK - kinase 1 regulating the apoptosis signal
 * ATM / ATR - ataxia telangiectasia mutated protein kinase / ATM and Rad3 related protein kinase
 * Bad - a member of the Bcl-2 family of proapoptosis proteins
 * Bax - proapoptosis factor
 * c-Myc - oncogene, transcription factor of mitosis signaling pathways
 * Erk - extracellular signal regulated protein kinase
 * Grb2 - growth factor receptor 2 adapter protein
 * HIF - a factor induced by hypoxia
 * IKK - IkappaB kinase
 * Ikappa - NFkappaB inhibitor
 * JAK - Janus activation tyrosine kinase pathway for cytokines and growth factors
 * MAPK - mitogen-activated protein kinase
 * MDM2 - mouse doubleminute 2
 * MEK (MKK) - MAPK / ERK kinase
 * MEKK - Erk activating kinase via MEK
 * mTOR - mammalian rapamycin
 * PDK1 - phosphatidyl dependent kinase
 * PI3K - phosphatidyl-inositol-3-kinase
 * PIP3 - phosphatidyl-inositol-3,4,5, -phosphate
 * Rac - cytosolic Ras
 * Raf-activated Ras-activated cytoplasmic serine / threonine kinase
 * Ras - GDP / GTP binding protein
 * SAPK - stress activated protein kinase
 * Smad – Smad a Mad related proteins
 * SOCS - cytokine signaling suppressor
 * SOS - son of sevenles
 * STAT - signal transducer and transcription activator