RET(MEN 2B) is active in the endoplasmic reticulum before reaching the cell surface

Molecular Basis and Natural History of Medullary Thyroid Cancer: It is (Almost) All in the RET

Medullary thyroid cancer (MTC) is a rare disease, which can be either sporadic (roughly 75% of cases) or genetically determined (multiple endocrine neoplasia type 2, due to REarranged during Transfection RET germline mutations, 25% of cases). Interestingly, RET pathogenic variants (mainly M918T) have also been reported in aggressive forms of sporadic MTC, suggesting the importance of RET signalling pathways in the pathogenesis of MTC. The initial theory of RET codon-related MTC aggressiveness has been recently questioned by studies suggesting that this would only define the age at disease onset rather than the aggressiveness of MTC. Other factors might however impact the natural history of the disease, such as RET polymorphisms, epigenetic factors, environmental factors, MET (mesenchymal-epithelial transition) alterations, or even other genetic alterations such as RAS family (HRAS, KRAS, NRAS) genetic alterations. This review will detail the molecular bases of MTC, focusing on RET pathways, and the potential mechanisms that explain the phenotypic intra- and interfamilial heterogeneity.

A Recent Update on Pathophysiology and Therapeutic Interventions of Alzheimer's Disease

AIM

Alzheimer's disease (AD) has been identified as a progressive brain disorder associated with memory dysfunction and the accumulation of β-amyloid plaques and neurofibrillary tangles of τ protein. Mitochondria is crucial in maintaining cell survival, cell death, calcium regulation, and ATP synthesis. Mitochondrial dysfunction and linked calcium overload have been involved in the pathogenesis of AD. CRM2 (Collapsin response mediator protein-2) is involved in endosomal lysosomal trafficking as well as autophagy, and their reduced level is also a primary culprit in the progression of AD. In addition, Cholinergic neurotransmission and neuroinflammation are two other mechanisms implicated in AD onset and might be protective targets to attenuate disease progression. The microbiota-gut-brain axis (MGBA) is another crucial target for AD treatment. Crosstalk between gut microbiota and brain mutually benefitted each other, dysbiosis in gut microbiota affects the brain functions and leads to AD progression with increased AD-causing biomarkers. Despite the complexity of AD, treatment is only limited to symptomatic management. Therefore, there is an urgent demand for novel therapeutics that target associated pathways responsible for AD pathology. This review explores the role of different mechanisms involved in AD and possible therapeutic targets to protect against disease progression.

BACKGROUND

Amidst various age-related diseases, AD is the most deleterious neurodegenerative disorder that affects more than 24 million people globally. Every year, approximately 7.7 million new cases of dementia have been reported. However, to date, no novel disease-modifying therapies are available to treat AD.

OBJECTIVE

The aim of writing this review is to highlight the role of key biomarker proteins and possible therapeutic interventions that could play a crucial role in mitigating the ongoing prognosis of Alzheimer's disease.

MATERIALS AND METHODS

The available information about the disease was collected through multiple search engines, including PubMed, Science Direct, Clinical Trials, and Google Scholar.

RESULTS

Accumulated pieces of evidence reveal that extracellular aggregation of β-amyloid plaques and intracellular tangles of τ protein are peculiar features of perpetuated Alzheimer's disease (AD). Further, the significant role of mitochondria, calcium, and cholinergic pathways in the pathogenesis of AD makes the respiratory cell organelle a crucial therapeutic target in this neurodegenerative disease. All currently available drugs either delay the clinical damage to cells or temporarily attenuate some symptoms of Alzheimer's disease.

CONCLUSION

The pathological features of AD are extracellular deposition of β-amyloid, acetylcholinesterase deregulation, and intracellular tangles of τ protein. The multifactorial heterogeneity of disease demands more research work in this field to find new therapeutic biological targets.

FLT3-ITD transduces autonomous growth signals during its biosynthetic trafficking in acute myelogenous leukemia cells

FMS-like tyrosine kinase 3 (FLT3) in hematopoietic cells binds to its ligand at the plasma membrane (PM), then transduces growth signals. FLT3 gene alterations that lead the kinase to assume its permanently active form, such as internal tandem duplication (ITD) and D835Y substitution, are found in 30–40% of acute myelogenous leukemia (AML) patients. Thus, drugs for molecular targeting of FLT3 mutants have been developed for the treatment of AML. Several groups have reported that compared with wild-type FLT3 (FLT3-wt), FLT3 mutants are retained in organelles, resulting in low levels of PM localization of the receptor. However, the precise subcellular localization of mutant FLT3 remains unclear, and the relationship between oncogenic signaling and the mislocalization is not completely understood. In this study, we show that in cell lines established from leukemia patients, endogenous FLT3-ITD but not FLT3-wt clearly accumulates in the perinuclear region. Our co-immunofluorescence assays demonstrate that Golgi markers are co-localized with the perinuclear region, indicating that FLT3-ITD mainly localizes to the Golgi region in AML cells. FLT3-ITD biosynthetically traffics to the Golgi apparatus and remains there in a manner dependent on its tyrosine kinase activity. Tyrosine kinase inhibitors, such as quizartinib (AC220) and midostaurin (PKC412), markedly decrease FLT3-ITD retention and increase PM levels of the mutant. FLT3-ITD activates downstream in the endoplasmic reticulum (ER) and the Golgi apparatus during its biosynthetic trafficking. Results of our trafficking inhibitor treatment assays show that FLT3-ITD in the ER activates STAT5, whereas that in the Golgi can cause the activation of AKT and ERK. We provide evidence that FLT3-ITD signals from the early secretory compartments before reaching the PM in AML cells.

RET Receptor Tyrosine Kinase: Role in Neurodegeneration, Obesity, and Cancer

Rearranged during transfection (RET) is the tyrosine kinase receptor that under normal circumstances interacts with ligand at the cell surface and mediates various essential roles in a variety of cellular processes such as proliferation, differentiation, survival, migration, and metabolism. RET plays a pivotal role in the development of both peripheral and central nervous systems. RET is expressed from early stages of embryogenesis and remains expressed throughout all life stages. Mutations either activating or inhibiting RET result in several aggressive diseases, namely cancer and Hirschsprung disease. However, the physiological ligand-dependent activation of RET receptor is important for the survival and maintenance of several neuronal populations, appetite, and weight gain control, thus providing an opportunity for the development of disease-modifying therapeutics against neurodegeneration and obesity. In this review, we describe the structure of RET, its signaling, and its role in both normal conditions as well as in several disorders. We highlight the differences in the signaling and outcomes of constitutive and ligand-induced RET activation. Finally, we review the data on recently developed small molecular weight RET agonists and their potential for the treatment of various diseases.

Small Molecules and Peptides Targeting Glial Cell Line-Derived Neurotrophic Factor Receptors for the Treatment of Neurodegeneration

Glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are able to promote the survival of multiple neuronal populations in the body and, therefore, hold considerable promise for disease-modifying treatments of diseases and conditions caused by neurodegeneration. Available data reveal the potential of GFLs for the therapy of Parkinson's disease, neuropathic pain and diseases caused by retinal degeneration but, also, amyotrophic lateral sclerosis and, possibly, Alzheimer's disease. Despite promising data collected in preclinical models, clinical translation of GFLs is yet to be conducted. The main reasons for the limited success of GFLs clinical development are the poor pharmacological characteristics of GFL proteins, such as the inability of GFLs to cross tissue barriers, poor diffusion in tissues, biphasic dose-response and activation of several receptors in the organism in different cell types, along with ethical limitations on patients' selection in clinical trials. The development of small molecules selectively targeting particular GFL receptors with improved pharmacokinetic properties can overcome many of the difficulties and limitations associated with the clinical use of GFL proteins. The current review lists several strategies to target the GFL receptor complex with drug-like molecules, discusses their advantages, provides an overview of available chemical scaffolds and peptides able to activate GFL receptors and describes the effects of these molecules in cultured cells and animal models.

Mislocalisation of Activated Receptor Tyrosine Kinases - Challenges for Cancer Therapy

Activating mutations in genes encoding receptor tyrosine kinases (RTKs) mediate proliferation, cell migration, and cell survival, and are therefore important drivers of oncogenesis. Numerous targeted cancer therapies are directed against activated RTKs, including small compound inhibitors, and immunotherapies. It has recently been discovered that not only certain RTK fusion proteins, but also many full-length RTKs harbouring activating mutations, notably RTKs of the class III family, are to a large extent mislocalised in intracellular membranes. Active kinases in these locations cause aberrant activation of signalling pathways. Moreover, low levels of activated RTKs at the cell surface present an obstacle for immunotherapy. We outline here why understanding of the mechanisms underlying mislocalisation will help in improving existing and developing novel therapeutic strategies.

Development of the urogenital system is regulated via the 3'UTR of GDNF

Mechanisms controlling ureter lenght and the position of the kidney are poorly understood. Glial cell-line derived neurotrophic factor (GDNF) induced RET signaling is critical for ureteric bud outgrowth, but the function of endogenous GDNF in further renal differentiation and urogenital system development remains discursive. Here we analyzed mice where 3′ untranslated region (UTR) of GDNF is replaced with sequence less responsive to microRNA-mediated regulation, leading to increased GDNF expression specifically in cells naturally transcribing Gdnf. We demonstrate that increased Gdnf leads to short ureters in kidneys located in an abnormally caudal position thus resembling human pelvic kidneys. High GDNF levels expand collecting ductal progenitors at the expense of ureteric trunk elongation and result in expanded tip and short trunk phenotype due to changes in cell cycle length and progenitor motility. MEK-inhibition rescues these defects suggesting that MAPK-activity mediates GDNF’s effects on progenitors. Moreover, Gdnf   ^hyper mice are infertile likely due to effects of excess GDNF on distal ureter remodeling. Our findings suggest that dysregulation of GDNF levels, for example via alterations in 3′UTR, may account for a subset of congenital anomalies of the kidney and urinary tract (CAKUT) and/or congenital infertility cases in humans and pave way to future studies.

Multiple Endocrine Neoplasia Syndromes from Genetic and Epigenetic Perspectives

Multiple endocrine neoplasia (MEN) syndromes are infrequent inherited disorders in which more than one endocrine glands develop noncancerous (benign) or cancerous (malignant) tumors or grow excessively without forming tumors. There are 3 famous and well-known forms of MEN syndromes (MEN 1, MEN 2A, and MEN 2B) and a newly documented one (MEN4). These syndromes are infrequent and occurred in all ages and both men and women. Usually, germ line mutations that can be resulted in neoplastic transformation of anterior pituitary, parathyroid glands, and pancreatic islets in addition to gastrointestinal tract can be an indicator for MEN1. The medullary thyroid cancer (MTC) in association with pheochromocytoma and/or multiple lesions of parathyroid glands with hyperparathyroidism can be pointer of MEN2 which can be subgrouped into the MEN 2A, MEN 2B, and familial MTC syndromes. There are no distinct biochemical markers that allow identification of familial versus nonfamilial forms of the tumors, but familial MTC usually happens at a younger age than sporadic MTC. The MEN1 gene (menin protein) is in charge of MEN 1 disease, CDNK1B for MEN 4, and RET proto-oncogene for MEN 2. The focus over the molecular targets can bring some hope for both diagnosis and management of MEN syndromes. In the current review, we look at this disease and responsible genes and their cell signaling pathway involved.

Small-Molecule Ligands as Potential GDNF Family Receptor Agonists

To find out potential GDNF family receptor α1 (GFRα1) agonists, small molecules were built up by molecular fragments according to the structure-based drug design approach. Molecular docking was used to identify their binding modes to the biological target GFRα1 in GDNF-binding pocket. Thereafter, commercially available compounds based on the best predicted structures were searched from ZINC and MolPort databases (similarity ≥ 80%). Five compounds from the ZINC library were tested in phosphorylation and luciferase assays to study their ability to activate GFRα1-RET. A bidental compound with two carboxyl groups showed the highest activity in molecular modeling and biological studies. However, the relative position of these groups was important. The meta-substituted structure otherwise identical to the most active compound 2-[4-(5-carboxy-1H-1,3-benzodiazol-2-yl)phenyl]-1H-1,3-benzodiazole-5-carboxylic acid was inactive. A weaker activity was detected for a compound with a single carboxyl group, that is, 4-(1,3-benzoxazol-2-yl)benzoic acid. The substitution of the carboxyl group by the amino or acetamido group also led to the loss of the activity.

A Novel Small Molecule GDNF Receptor RET Agonist, BT13, Promotes Neurite Growth from Sensory Neurons in Vitro and Attenuates Experimental Neuropathy in the Rat

Neuropathic pain caused by nerve damage is a common and severe class of chronic pain. Disease-modifying clinical therapies are needed as current treatments typically provide only symptomatic relief; show varying clinical efficacy; and most have significant adverse effects. One approach is targeting either neurotrophic factors or their receptors that normalize sensory neuron function and stimulate regeneration after nerve damage. Two candidate targets are glial cell line-derived neurotrophic factor (GDNF) and artemin (ARTN), as these GDNF family ligands (GFLs) show efficacy in animal models of neuropathic pain (Boucher et al., 2000; Gardell et al., 2003; Wang et al., 2008, 2014). As these protein ligands have poor drug-like properties and are expensive to produce for clinical use, we screened 18,400 drug-like compounds to develop small molecules that act similarly to GFLs (GDNF mimetics). This screening identified BT13 as a compound that selectively targeted GFL receptor RET to activate downstream signaling cascades. BT13 was similar to NGF and ARTN in selectively promoting neurite outgrowth from the peptidergic class of adult sensory neurons in culture, but was opposite to ARTN in causing neurite elongation without affecting initiation. When administered after spinal nerve ligation in a rat model of neuropathic pain, 20 and 25 mg/kg of BT13 decreased mechanical hypersensitivity and normalized expression of sensory neuron markers in dorsal root ganglia. In control rats, BT13 had no effect on baseline mechanical or thermal sensitivity, motor coordination, or weight gain. Thus, small molecule BT13 selectively activates RET and offers opportunities for developing novel disease-modifying medications to treat neuropathic pain.

Mechanisms of RET signaling in cancer: current and future implications for targeted therapy

De-regulation of RET signaling by oncogenic mutation, gene rearrangement, overexpression or transcriptional up-regulation is implicated in several human cancers of neuroendocrine and epithelial origin (thyroid, breast, lung). Understanding how RET signaling mechanisms associated with these oncogenic events are deregulated, and their impact in the biological processes driving tumor formation and progression, as well as response to treatment, will be crucial to find and develop better targeted therapeutic strategies. In this review we emphasie the distinct mechanisms of RET signaling in cancer and summarise current knowledge on small molecule inhibitors targeting the tyrosine kinase domain of RET as therapeutic drugs in RET-positive cancers.

Mitogen-activated protein kinase (MAPK) pathway regulates branching by remodeling epithelial cell adhesion

Although the growth factor (GF) signaling guiding renal branching is well characterized, the intracellular cascades mediating GF functions are poorly understood. We studied mitogen-activated protein kinase (MAPK) pathway specifically in the branching epithelia of developing kidney by genetically abrogating the pathway activity in mice lacking simultaneously dual-specificity protein kinases Mek1 and Mek2. Our data show that MAPK pathway is heterogeneously activated in the subset of G1- and S-phase epithelial cells, and its tissue-specific deletion results in severe renal hypodysplasia. Consequently to the deletion of Mek1/2, the activation of ERK1/2 in the epithelium is lost and normal branching pattern in mutant kidneys is substituted with elongation-only phenotype, in which the epithelium is largely unable to form novel branches and complex three-dimensional patterns, but able to grow without primary defects in mitosis. Cellular characterization of double mutant epithelium showed increased E-cadherin at the cell surfaces with its particular accumulation at baso-lateral locations. This indicates changes in cellular adhesion, which were revealed by electron microscopic analysis demonstrating intercellular gaps and increased extracellular space in double mutant epithelium. When challenged to form monolayer cultures, the mutant epithelial cells were impaired in spreading and displayed strong focal adhesions in addition to spiky E-cadherin. Inhibition of MAPK activity reduced paxillin phosphorylation on serine 83 while remnants of phospho-paxillin, together with another focal adhesion (FA) protein vinculin, were augmented at cell surface contacts. We show that MAPK activity is required for branching morphogenesis, and propose that it promotes cell cycle progression and higher cellular motility through remodeling of cellular adhesions.

RET revisited: expanding the oncogenic portfolio

The RET receptor tyrosine kinase is crucial for normal development but also contributes to pathologies that reflect both the loss and the gain of RET function. Activation of RET occurs via oncogenic mutations in familial and sporadic cancers - most notably, those of the thyroid and the lung. RET has also recently been implicated in the progression of breast and pancreatic tumours, among others, which makes it an attractive target for small-molecule kinase inhibitors as therapeutics. However, the complex roles of RET in homeostasis and survival of neural lineages and in tumour-associated inflammation might also suggest potential long-term pitfalls of broadly targeting RET.

Alternative splicing results in RET isoforms with distinct trafficking properties

The RET gene encodes a receptor tyrosine kinase that is alternatively spliced to two protein isoforms that differ in their C-terminal peptide sequences (RET9, RET51). These unique C-terminal tails produce distinct subcellular localizations and intracellular trafficking properties, which affect downstream signaling.

RET encodes a receptor tyrosine kinase that is essential for spermatogenesis, development of the sensory, sympathetic, parasympathetic, and enteric nervous systems and the kidneys, as well as for maintenance of adult midbrain dopaminergic neurons. RET is alternatively spliced to encode multiple isoforms that differ in their C-terminal amino acids. The RET9 and RET51 isoforms display unique levels of autophosphorylation and have differential interactions with adaptor proteins. They induce distinct gene expression patterns, promote different levels of cell differentiation and transformation, and play unique roles in development. Here we present a comprehensive study of the subcellular localization and trafficking of RET isoforms. We show that immature RET9 accumulates intracellularly in the Golgi, whereas RET51 is efficiently matured and present in relatively higher amounts on the plasma membrane. RET51 is internalized faster after ligand binding and undergoes recycling back to the plasma membrane. This differential trafficking of RET isoforms produces a more rapid and longer duration of signaling through the extracellular-signal regulated kinase/mitogen-activated protein kinase pathway downstream of RET51 relative to RET9. Together these differences in trafficking properties contribute to some of the functional differences previously observed between RET9 and RET51 and establish the important role of intracellular trafficking in modulating and maintaining RET signaling.

A direct role for Met endocytosis in tumorigenesis

Compartmentalization of signals generated by receptor tyrosine kinase (RTK) endocytosis has emerged as a major determinant of various cell functions. Here, using tumour-associated Met-activating mutations, we demonstrate a direct link between endocytosis and tumorigenicity. Met mutants exhibit increased endocytosis/recycling activity and decreased levels of degradation, leading to accumulation on endosomes, activation of the GTPase Rac1, loss of actin stress fibres and increased levels of cell migration. Blocking endocytosis inhibited mutants' anchorage-independent growth, in vivo tumorigenesis and metastasis while maintaining their activation. One mutant resistant to inhibition by a Met-specific tyrosine kinase inhibitor was sensitive to endocytosis inhibition. Thus, oncogenicity of Met mutants results not only from activation but also from their altered endocytic trafficking, indicating that endosomal signalling may be a crucial mechanism regulating RTK-dependent tumorigenesis.

Inhibition of the Ras/Raf/MEK/ERK and RET kinase pathways with the combination of the multikinase inhibitor sorafenib and the farnesyltransferase inhibitor tipifarnib in medullary and differentiated thyroid malignancies

PURPOSE

Ras/Raf/MAPK kinase/ERK and rearranged in transformation (RET) kinase pathways are important in thyroid cancer. We tested sorafenib, a B-Raf, RET, and vascular endothelial growth factor receptor kinase inhibitor, combined with tipifarnib, a farnesyltransferase inhibitor that inactivates Ras and other farnesylated proteins.

PATIENTS AND METHODS

We treated 35 patients with differentiated thyroid cancer (DTC) and medullary thyroid cancer (MTC) in a phase I trial. Sorafenib and tipifarnib were given for 21 d with 7 d rest in each 28-d cycle.

RESULTS

We enrolled 22 patients with metastatic DTC (16 papillary, five follicular, and one poorly differentiated) and 13 patients with MTC, of whom 15 with DTC and 10 with MTC reached first restaging. When tissue was available, eight of 15 DTC patients (53%) had B-Raf mutations; eight of 13 MTC (61.5%) patients had RET mutations. MTC partial response rate was 38% (five of 13) (duration = 9+, 12, 13, 16+, and 34+ months), stable disease of at least 6 months was 31% (four of 13). The DTC partial response rate was 4.5% (one of 22), and stable disease of at least 6 months was 36% (eight of 22). Median progression-free survival for all 35 patients was 18 months (95% confidence interval, 14.6 to not reached months). Median overall survival has not been reached, with a median follow-up of 24 months with 80% overall survival. Grade 1-2 toxicities were mainly rash, fatigue, and diarrhea. The most common grade 3-4 toxicities were rash, rise in amylase/lipase, and fatigue.

CONCLUSIONS

Inhibiting the Ras/Raf/MAPK kinase/ERK and RET kinase pathways with sorafenib and tipifarnib is well tolerated and active against thyroid cancer.

Chronic infusion of CDNF prevents 6-OHDA-induced deficits in a rat model of Parkinson's disease

Cerebral dopamine neurotrophic factor (CDNF) and mesencephalic astrocyte-derived neurotrophic factor (MANF) constitute a novel, evolutionarily conserved family of neurotrophic factors (NTF) expressed in vertebrates and invertebrates. The effects of two-week infusions of CDNF, MANF and glial cell line-derived neurotrophic factor (GDNF) were studied in a rat 6-hydroxydopamine (6-OHDA) hemiparkinsonian model. Degeneration of nigrostriatal dopamine nerve tract after toxin injection was assessed by measuring amphetamine-induced rotational behavior, and at the end of the experiment by quantifying tyrosine hydroxylase (TH)-positive neurons in the substantia nigra pars compacta (SNpc) and TH-positive fibers in the striatum. The diffusion of the NTFs into the brain tissue following chronic infusion was also studied. Finally, we examined the transportation of intrastriatally injected (125)I-CDNF within the brain. The amphetamine-induced rotational behavior was gradually normalized in rats treated with CDNF for two weeks following the intrastriatal 6-OHDA injection. CDNF was also able to inhibit 6-OHDA-induced loss of TH-immunoreactive cells of the SNpc and TH-positive fibers in the striatum. MANF and GDNF had no statistically significant effect in any of the above measures. The volume of distribution for MANF in the striatum was significantly larger than that of GDNF after 3-day infusions. Both (125)I-CDNF and (125)I-GDNF were retrogradely transported from the striatum to the SN. No behavioral signs of toxicity were observed during treatment with the three NTFs. These results imply that CDNF may have potential as a neuroprotective or even neurorestorative therapy of PD.

Physiology and pathology of proteostasis in the early secretory compartment

The endoplasmic reticulum (ER), the port of entry for proteins into the secretory pathway, is a multifunctional organelle emerging as a central integrator of numerous signalling pathways. The mechanisms that control proteostasis are integral part of this signalling network, providing cues for morphological and functional cell remodelling, proliferation, inflammation and cell death. The complexity of ER responses is exploited during physiological and pathological tissue development, cell differentiation and lifespan control. This essay outlines some of the mechanisms that link proteostasis within the early secretory compartment to signalling in development and disease.

Familial associations in medullary thyroid carcinoma with Hirschsprung disease: the role of the RET-C620 "Janus" genetic variation

INTRODUCTION

Hirschsprung disease (HSCR) is associated with the later development of multiple endocrine neoplasia (MEN2), because RET gene variations are associated with both conditions. Specifically, HSCR-MEN2 cosegregation mostly relates to the cysteine-rich area at the RET-620 (the "Janus gene").

AIM

The aim of this study was to explore the clinical and genetic associations of HSCR-MEN2 in a cohort of HSCR patients.

METHODS

RET gene variation was evaluated by heteroduplex single-strand conformational polymorphism analysis and validated with automated sequencing techniques in HSCR patients (including 18 kindreds). Those with RET C620 variations were subjected to familial evaluation for coexisting HSCR-MEN2.

RESULTS

A cohort of 118 patients with HSCR (n = 89) or medullary thyroid carcinoma (n = 29) were studied, including 3 families where a RET-620 point mutation was identified. No C618, C609, or C611 variations were detected. In 1 remarkable 6-generational family (family 3), HSCR in early generations seemed to be later replaced by MEN2A. In the other 2 families with total colonic aganglionosis, a relative with a medullary thyroid carcinoma was identified.

CONCLUSION

Gene mutation in the RET-620 position carries significant risk and may be part of a targeted investigation of high-risk areas in HSCR. We propose an alternative hypothesis of endoplasmic reticulum control to explain the changing phenotypic expression.

Dysregulated RET signaling in thyroid cancer

Numerous biologic processes and such diseases as cancer depend on activation of tyrosine kinase receptors. The RET tyrosine kinase receptor was discovered two decades ago as a transforming gene and was subsequently implicated in the formation of papillary and medullary thyroid carcinoma. This article examines the data about the mechanism of activation of downstream signal transduction pathways by RET oncoproteins. Collectively, these findings have advanced the understanding of the processes underlying thyroid carcinoma formation.