Receptor tyrosine kinase (RTK)-mediated signals play major roles in the regulation of various cellular processes such as control of cell growth and differentiation.1
The essential and diverse roles of RTKs are evident in various developmental abnormalities and tumors that occur due to overexpression of RTK proteins or abnormal stimulation by autocrine growth factor loops contributing to constitutive RTK signaling irregularities.2
Members of large groups of RTK proteins have been classified on the basis of their structural and ligand affinity properties. The RTK family includes several well-known subfamilies; some of them proved highly expressed in leiomyomas (), including the epidermal growth factor receptors (EGFR or ErbB), fibroblast growth factor receptors (FGFR), the insulin and the insulin-like growth factor receptors (IR and IGFR), the platelet-derived growth factor receptors (PDGFRα, β), the vascular endothelial growth factor receptors (VEGFR), and the hepatocyte growth factor receptors (HGFR).1,3
Lesser known RTKs possibly also involved in leiomyoma growth that our laboratory has identified to be overly expressed in fibroids compared with normal myometrium, include EphA1–7, EphB2,4,6, Axl, c-Ret, Tie-1, 2, Ror1, 2, Dtk, and others ().
Differential expression of phosphorylated growth factor receptor tyrosine kinases (RTKs) in leiomyoma (L) and myometrial (M) tissues (*p<0.02 to 0.03; L vs M).
Uterine leiomyomas express many types of growth factors that may promote leiomyoma growth through local paracrine and/or autocrine mechanisms.4
Several studies have shown that growth factors and their receptor-mediated signaling pathways are important in uterine leiomyoma growth. One such growth factor, epidermal growth factor (EGF), is mitogenic and expressed more in leiomyomas than in myometrial tissue during the luteal phase of the menstrual cycle;5
in addition, its receptor, EGFR, is more sensitive to regulation by sex steroids in leiomyomas than those in the myometrium.6
Leiomyoma cell growth is effectively blocked by TKS050, a new EGFR inhibitor.7
The presence of large quantities of bFGF stored in the extracellular matrix (ECM) of the fibroids and the more intense expression of FGF receptor in leiomyomas than in the myometrium suggest a role for FGF in proliferation of smooth muscle cells in leiomyomas.8,9
The mRNA expression of another potent mitogen, PDGF, has been found in leiomyomas, and PDGF receptor sites per cell are increased in leiomyomas compared with the myometrium.10
PDGF also acts with other growth factors such as transforming growth factor beta (TGF-β), EGF, and the IGFs to enhance proliferation. The mRNA expression and protein levels of IGF-I have been reported to be higher in leiomyomas than in the myometrium. The levels of IGF-I receptor in leiomyomas have also been reported to exceed those of the myometrium,11
which suggest that IGF-I and the IGF-IR signaling pathway may be of major significance in the growth of uterine leiomyomas.
Based on the studies just cited, it appears there may be different families of RTKs and their ligands involved in leiomyoma growth and development. To fully understand the association between upregulation of various growth factor RTKs and leiomyoma development, studies were conducted in our laboratory to assess RTK expression profiles in human uterine leiomyoma and matched myometrial tissues by using a RTK array technique. We found that 39 out of 42 RTKs evaluated were highly expressed in the leiomyomas compared with myometrial samples ().
During the past decade, several studies have identified upregulation of lesser known RTKs in leiomyomas, as we have noted in our RTK array studies. These include Dtk, a RTK reported to be involved in the regulation of hematopoiesis, particularly during embryonic stages of blood cell development, and also possibly involved in tumorigenesis. Upregulation of this receptor may increase the chances of fibroid growth and survival through its hematopoietic effects. If a tumor can somehow increase its blood supply internally through increased hematopoiesis, and externally through increased angiogenesis, it would be more likely to increase in size faster, and prevent necrosis of nutrient and oxygen starved regions.12
Another RTK not often reported in uterine leiomyomas is stem cell factor receptor (SCF-R). SCF-R is thought to play an essential role in erythroid cell development. Stem cell factor (SCF) and erythropoietin (Epo) synergistically activate mitogen-activated protein kinases (MAPKs) that correlate cell growth. This receptor may play a role synergistically with some of the factors that are responsible for increased hematopoiesis, by increasing survival of those erythroid progenitor cells developed in the tumor through upregulation of hematopoietic factors.8
Fit-3 is a hematopoietic growth factor and may act in concert with some of the other hematopoietic growth factors to increase tumor blood supply.
Macrophage colony-stimulating factor (M-CSF) is a growth factor of the mononuclear phagocytic lineage that participates in immunological and inflammatory reactions, bone metabolism, and pregnancy. M-CSF/M-CSF-R could potentially participate in the physiology of infectious, inflammatory, and neoplastic diseases.13
Another RTK is RET, which is the receptor for glial-derived neurotrophic factor growth factors. Like other RTKs, once activated, RET recruits a variety of signaling molecules that mediate a myriad of biological responses.14
Ror1 is related to muscle-specific kinase (MuSK), a muscle-specific tyrosine kinase. The Ror family of proteins are expressed in many tissue types during development and are thought to play a role in cell migration and in orientation of cell polarity.15
Mice lacking one of the two Ror RTK gene products display defects in bone and heart formation. MuSK seems to be involved at the neuromuscular junction with mediation of the synapse-inducing role of motor neuron-derived agrin (Agrin is a motor neuron-derived factor that directs formation of the postsynaptic apparatus of the neuromuscular junction).16
EphA, a RTK expressed in leiomyomas is involved in developmental and pathological angiogenesis. Particularly A-class Eph RTKs have recently emerged as critical regulators of tumor angiogenesis. Overexpression of Eph RTKs has been observed in several types of cancers in mouse models and humans. Expression of the ligand, ephrin-A1, has been predominantly detected in tumor cells, while the majority of EphA2 RTK protein is localized to tumor-associated endothelium, suggesting that ephrin-A1 might serve as a proangiogenic signal to attract EphA2-positive endothelial cells. Ephrin-A1 expression is seen in normal human endometrial epithelial cells and may contribute to normal endometrial angiogenesis.17
There are also interactions between VEGF and EphA, and because VEGF contributes to angiogenesis, this interaction might promote tumor growth and expand the blood supply during proliferation.3
The upregulation of these different families of phosphorylated growth factor RTK proteins in tumors and in leiomyoma samples found in the preceding studies and in our RTK array thus indicates that multiple growth factors are important in the pathogenesis and growth of fibroids.
Another RTK overexpressed in leiomyomas is the Ax1 receptor. This is the RTK for the ligand Gas6, the protein product of growth arrest-specific gene 6. Ax1 has functions in developmental processes and plays roles in the function of the hematopoietic and nervous systems and in tumorigenesis. Gas6 is a growth-potentiating factor for thrombin-induced proliferation of vascular smooth muscle cells. Levels of Ax1 receptor and Gas6 are reportedly higher in uterine leiomyoma tissues compared with normal myometrial tissues. The signal transduction of Gas6 and Ax1 might be important factors involved in the development and growth of uterine leiomyomas.18