Page 45 - Read Online

Page 20 of 38 J Cancer Metastasis Treat 2020;6:5 I http://dx.doi.org/10.20517/2394-4722.2020.13

degradation products. Among the genes involved in hepatic cancer, transmembrane 4 L six family member
5 (TM4SF5), which is structurally similar to the tetraspanins with 4 transmembrane domains, is shown
to be highly expressed in cancerous liver tissues and correlated with many hepatic metabolism genes.
However, it is unknown whether and how TM4SF5 is involved in arginine metabolism in livers.

Experimental procedure: We examined whether TM4SF5 expression can be involved in the mTORC1
signaling pathway, since mTORC1 signaling is a central hub of various cellular metabolism processes.

Results: First, we found shuttling of TM4SF5 between plasma membrane and lysosomal (late endosomal)
membrane, depending on availability of amino acids. Further, upon resupply of arginine to arginine-
starved cells, lysosomal TM4SF5 was associated with mTOR, leading to an increased S6K phosphorylation.
The association between TM4SF5 and mammalian target of rapamycin (mTOR) appeared to require
the C-terminal regions of TM4SF5 and kinase activity of mTOR. In addition, an endosomal arginine
transporter (SLC38A9) and a cytosolic arginine sensor (Castor1) were found to be associated with
TM4SF5, indicating TM4SF5 as an arginine sensor on late endosomal (lysosomal) membrane. Interestingly,
the association of Castor1 with TM4SF5 was negatively regulated by L-arginine, but concomitantly the
association between mTOR and TM4SF5 increased. Furthermore, certain residues in the extracellular loop
2 of TM4SF5 bound to arginine. Thus, association of TM4SF5 with mTOR, SLC38A9, and arginine on
lysosomal membrane might allow TM4SF5 to propagate arginine response to mTORC1 by directly sensing
arginine in the lysosome as well as to elevate the cytosolic arginine pool for cellular homeostasis.

Conclusion: Therefore, these observations suggest TM4SF5 as an arginine sensor on late endosomal
membrane and as a promising therapeutic target candidate for the arginine auxotroph of hepatic cancers.

28. Targeting cancer stem cells in malignant mesothelioma

1
1
3,4
Richard L. Eckert 1,2,4 , Gautam Adhikary , Daniel Grun , Joseph S. Friedberg , Wen Xu , Jeffrey
1
5
3
Keillor , Sivaveera Kandasamy , H. Richard Alexander 6

1 Department of Biochemistry and Molecular Biology,University of Maryland School of Medicine, Baltimore,
MD 21201, USA.
2 Department of Dermatology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
3 Department of Surgery and Division of General and Surgical Oncology, University of Maryland School of
Medicine, Baltimore, MD 21201, USA.
4 Greenebaum Comprehensive Cancer Center, University of Maryland School of Medicine, Baltimore, MD
21201, USA.
5 Department of Surgery, Robert Wood Johnson Medical School, New Brunswick, New Jersey, NJ 08901, USA.
6 Department of Chemistry, University of Ottawa, Ottawa, ON K1N 6N5, Canada.

Background and aim: Mesothelioma is an aggressive, treatment resistant, and fatal cancer of the mesothelial
lining of the pleural and peritoneal cavities that is initiated by exposure to asbestos or nanotubes. Surgical
reduction and chemotherapy are first line treatments, but recurrence of highly aggressive and drug-resistant
disease is common. Disease recurrence is associated with expansion of mesothelioma cancer stem cells
(MCS cells). Thus, new treatments are needed for this disease that target the cancer stem cell population.

Experimental procedure: We used genetic gene expression and knockdown approaches, signaling studies,
xenograft tumor studies, transcriptome analysis, and protein structure studies to characterize the role of the
transglutaminase 2 (TG2) cancer stem cell survival factor in enhancing MSC cell survival and function.