A multifaceted study of the receptor tyrosine kinase Axl with focus on Renal Cell Carcinoma

University dissertation from Division of Clinical Chemistry and Pharmacology, Faculty of Medicine, Lund University

Abstract: The present investigation is a multifaceted study of the receptor tyrosine kinase Axl, whose overexpression and activation is implicated in several human malignancies, with focus on renal cell carcinoma (RCC).
At first, our main interest concerned the intracellular Axl interactor named Tensin2 (Tns2) and its family member Tensin3 (Tns3). Tns2 and Tns3 are structural focal adhesion proteins, linking the extracellular matrix and cytoskeleton via integrin and actin binding, and are involved in cellular signaling. Tns2 inhibits the PI3K and Akt/PKB pathway and decreases the level of PtdIns(3,4,5)P3 located at the cellular membrane.
Our primary goal was to further characterize the Axl and Tns2 interaction, and thereafter, to explore our hypothesis that Tns2 and Tns3 might act as PTEN-like lipid phosphatases. We verified the Axl and Tns2 interaction to occur through its carboxy-terminal region as originally reported, and excluded other Tns2 domains to take part in the interaction (Paper IV). In addition, we realized that a stretch of 100 amino acids with no known tertiary structure seemed invariant for the interaction. Thereafter, we continued with our exploration of the putative phosphatase activity of Tns2 and Tns3, but despite extensive efforts we were not able to show any direct in vitro activity (Paper III).
We continued our Axl exploration concerning the role of Axl and its ligand Gas6 in RCC. RCC is a disease present with poor prognosis and elusive etiology, and with great need for proper molecular prognostic markers.
With the opportunity of having a large RCC patient material, we could conclude Axl and Gas6 to correlate with patient survival outcome (Paper I). Furthermore, we found Axl expression in the primary tumor, at time of nephrectomy, to have the potential to be a prognostic marker, where low Axl expression was beneficial for the patient outcome (Paper I). Remaining time until present, we attempted to discover the biological role of Axl and Gas6 in a clear cell RCC (ccRCC) experimental system. Although more can be done, we could see that Gas6 stimulation of Axl in RCC cells had inhibitory effects on migration, and that Gas6 rendered the cells less viable (Paper II). Moreover, Axl expression was inversely correlated to the expression of the tumor suppressor VHL, and also downregulated by Gas6 stimulation (Paper II).
Altogether, the results from Paper I and II opens up for further exciting research of the complex role of the Gas6/Axl system in RCC, and for validation of the putative role of Axl as an important prognostic marker in the diagnosis of RCC.