Epidermal growth factor-like domain-containing protein 7 (Egfl7) as a new potential target for therapy in gynecologic oncology

Karolina Król1, Jacek Jan Sznurkowski2 Received: 04.10.2016 Accepted: 09.10.2016 Published: 30.11.2016 © Curr Gynecol Oncol 2016, 14 (3), p. 162–167 DOI: 10.15557/CGO.2016.0019 Czynnik podobny do czynnika wzrostu naskórka z domeną 7 (Egfl7) nowym potencjalnym celem terapeutycznym w ginekologii onkologicznej Epidermal growth factor-like domain-containing protein 7 (Egfl7) as a new potential target for therapy in gynecologic oncology


THE ROLE OF Egfl7 IN THE HUMAN BODY
E pidermal growth factor-like domain-containing protein 7 (Egfl7) was originally described as a protein with molecular weight of 30 kDa, expressed exclusively in endothelial cells (1) . Currently, it is known to be produced also e.g. by neurons of adult mice, suggesting its functions to span beyond the vascular system (2) . Analysis of human Egfl7 has shown three isoforms of the protein to exist, transcribed from different promoters (3) . In vertebrates, Egfl7 gene encodes two biologically active forms of miRNA, namely miR-126 and miR-126*, that play an important role both in development of the cardiovascular system and in some pathologic conditions, including cardiovascular diseases and malignant neoplasms (4) . The concentration of Egf l7 remains relatively stable throughout an organism's life, with measurable concentrations of the protein persisting in subsets of lung, heart, kidney, liver, and uterine vessels. Increased Egfl7 expression may be related to physiologic conditions (e.g. blood vessels in the pregnant uterus) as well as pathologic ones, including damage of arterial vessels, ischemic injury (5) , or solid cancer tumors (6) . Egfl7 is involved in the process of angiogenesis. There are two mechanisms that contribute to the development of blood vessels: vasculogenesis, i.e. the formation of blood vessels de novo from progenitor cells, and angiogenesis, that is the formation of new blood vessels from pre-existing ones (7) . Angiogenesis is a very ordered, carefully controlled, multi-staged process. The prerequisite for normal formation of functional blood vessels is the cooperation of endothelial cells of vascular walls and smooth muscles (8) . The relationship between these two cell types is controlled indirectly by growth factors, and directly by interactions on cell-to-cell and cell-to-extracellular matrix levels. Egfl7, secreted by endothelial cells, plays an important regulatory role in recruiting smooth muscle cells and endothelial cells for tubulogenesis, or the formation of a blood vessel and its lumen (1,6,9) . It promotes migration of endothelial cells as a stand-alone factor, or via complexes containing particles of extracellular matrix which it is closely associated with, and whose molecular composition affects the morphogenesis of blood vessels (10) . Egfl7 is crucial in angiogenesis in normal and pathologic conditions (11) . Increased Egfl7 expression is present in the vessels of proliferating tissues, whereas decreased -in the majority of developed vessels supplying healthy, mature tissues (6) . Egfl7 provides optimal microenvironment for normal migration of endothelial cells. The important role of Egfl7 in the process of angiogenesis was confirmed by a study examining the development of blood vessels in the tissue of mice with mutated Egfl7 gene. It demonstrated that w tkankach myszy ze zmutowanym genem Egfl7. Wykazano, że u takich osobników zasięg unaczynienia w poszczególnych tkankach był zmniejszony lub osiągany z opóźnieniem, a morfogeneza naczyń -nieprawidłowa (10) . Udział Egfl7 w procesie kiełkowania naczyń krwionośnych przedstawiono schematycznie na ryc. 1 (6) .
in such organisms the extent of vascularization in given tissues was decreased, or achieved later than normal, and vascular morphogenesis was defective (10) . The role of Egfl7 in the process of vascular sprouting has been schematically presented in Fig. 1 (6) .

Egfl7 AND NEOPLASMS
In tissues of several malignant solid tumors, such as colorectal cancer, glioblastoma, breast cancer, and liver cancer, cancer endogenous expression of Egfl7 has been notified, with a trace level of the protein exhibited in the endothelium of the abnormal, cancer vessels (12) . The association of Egfl7 with clinicopathological features of the disease, and patients' prognosis is not clear. For colorectal cancer, two independent analyses have demonstrated Egfl7 expression to be deregulated. The elevated level of Egfl7 transcript was correlated with more advanced disease stage and the presence of lymph node metastases. However, no correlation with overall patient survival and progression-free survival was found (13) . Egfl7 is expressed in cells of primary hepatocellular carcinoma, and high levels of this protein were associated with reduced patient survival time (14) .
In the glioblastoma group, high Egfl7 levels in cancer tissue were correlated with the tumor's grade, Ki67 index, and intratumoral microvascular density, yet they were of no prognostic value (15) . Summarizing the few available studies mentioned above, it may be concluded that increased Egfl7 expression, despite correlating with multiple adverse clinical and pathologic factors, such as disease advancement, metastases to lymph nodes, or high tumor grade, has not been demonstrated to be of negative prognostic value, except for the small group of patients suffering from primary hepatocellular carcinoma (13)(14)(15)(16)(17) . Contradictory results are reported in one paper on breast cancer noting the correlation of endogenous Egfl7 expression with low grade of the tumor and a better prognosis for the patients (18) . Moreover, it has demonstrated Egfl7 expression in breast cancer to be higher for smaller lesions and in situ lesions without stromal response (18) . In cancers with elevated Egfl7 expression, smaller tendency for metastasizing to lymph nodes was noted. Presumably, Egfl7 expression does not provide breast cancer cells growth advantage.
The potential protective effect of Egfl7 in breast cancer certainly requires verification by independent studies, similarly to the biomarker's negative correlation with survival time in patients suffering from hepatocellular cancer (14) . The majority of existing studies suggest some role of Egfl7 in the progression of cancer, yet its direct impact remains unclear. The only common features noted in all studies regarding the presence of Egfl7 in cancer tissue regardless of its type include the unexpected presence of this biomarker in cancer cells (endogenous Egfl7 deregulation in cancer) and its poor representation in endothelial cells of stromal blood vessels. The latest reports in the field of basic research seemingly suggest Egfl7 to be an endogenous regulator of activation of endothelial cells, expressed in neoplastic cells to protect the tumor against the host's immune response (19) . In vitro research has shown that in mice cells of breast and lung cancer Egfl7 inhibits ICAM-1 and VCAM-1 expression, thereby adversely influencing the process of adhesion and lymphocyte migration through vessel wall. Hence, Egfl7 may hypothetically contribute to the cancer's escape from the host's immunosurveillance by inhibiting diapedesis in the tumor (19) . Such mechanism's indirect exponent may be the negative correlation existing between Egfl7 expression in cancer cells and the number of tumor-infiltrating lymphocytes (TILs).

Egfl7 AND GYNECOLOGIC CANCERS
The effect of Egfl7 on diapedesis in cancer may have huge therapeutic potential, particularly in gynecologic cancers. In ovarian, endometrial, and uterine cancer a strong correlation has been demonstrated to exist between patient survival time and the intensity of CD8+ cell infiltration