Tyrosine Sulfation: A New Player and Potential Target in Pancreatic Cancer

Pancreatic ductal adenocarcinoma remains one of the deadliest human malignancies, with the 5-year survival rate currently at 12%.¹ Limited progress towards its treatment has been achieved over the past several decades. Most patients present with local invasion or metastatic cancer. Even patients with local disease, treated with surgical resection, often relapse with treatment resistant disease. Pancreatic cancer has limited response to current chemotherapy (Folfirinox or Gem/Abraxane) and radiation therapy regimens.² Although the advent of novel inhibitors of oncogenic KRAS—a key genetic driver for pancreatic cancer—might revolutionize pancreatic cancer treatment,³ development of resistant disease is likely over time. A thorough understanding of the biology of this disease is necessary to optimize our approaches to treatment and identify opportunities for combination therapy.

In the current issue of the Journal of Cellular and Molecular Gastroenterology and Hepatology, Cai and colleagues explored the role of tyrosine sulfation in pancreatic cancer.⁴ Tyrosine sulfation is a posttranslational modification that has been associated with a growing list of protein substrates. Most notably, tyrosine sulfation in the vasculature is important for leukocyte adhesion and transmigration. Further, several chemokine receptors undergo tyrosine sulfation (for review see⁵). Tyrosine sulfation is catalyzed by 2 enzymes, the tyrosyl protein sulfotransferases TPST-1 and TPST-2, both membrane-bound, Golgi-associated enzymes. Our understanding of tyrosine sulfation in cancer is limited, but this process has been associated with cancer-related growth factors, such as those of the Wnt family.⁶ TPST enzymes use 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as a sulfate donor.⁵ In turn, PAPS is transported from the cytosol into the lumen of the Golgi—where sulfation modification of proteins and proteoglycans occurs—by solute carrier family 35 (adenosine 3'-phospho 5'-phosphosulfate transporter 1) member B2 (SLC35B2).⁷

An association between SLC35B transporters, TPST enzymes, and cancer is slowly emerging. The expression of SLC35B2 is elevated in colorectal carcinoma, and siRNA-mediated silencing of SLC35B2 inhibits proliferation of colorectal carcinoma because of reduced sulfation.⁸ Moreover, suppression of heparan sulfation sensitizes YAP1-driven melanoma to MAPK pathway inhibitors.⁹ Whether SLC35B transporters are expressed in pancreatic cancer and whether they might regulate cancer growth remain unknown.

In the current study, the authors show that SLC35B2 expression is elevated in human pancreatic cancer and negatively correlates with patient survival.⁴ They inactivated SLC35B2 expression using siRNAs in 2 human pancreatic cancer cell lines. For both lines, loss of SLC35B2 expression resulted in reduced cell proliferation and invasion in vitro. Given that SLC35B2 is a PAPS transporter, which in turn is the substrate for TPST, Cai and colleagues posited that inactivation of TPST2 would have a similar growth reduction effect. Indeed, TPST2 inactivation inhibited growth and invasion of human pancreatic cancer cells in vitro. Further, pancreatic cancer cells lacking TPST had reduced ability to form tumors in nude mice. To further characterize how the SLC35B2-TPST2 axis contributes to pancreatic cancer, the authors generated Tpst2-knockdown mouse pancreatic ductal adenocarcinoma cells derived from Kras^LSL-G12D/+; Trp53^LSL-R172/+; Pdx1-Cre (KPC)¹⁰ mice, also known as KPC, by CRISPR-Cas9 technology. KPC cells were then orthotopically transplanted in the pancreas of syngeneic, immune-competent mice, or injected in the tail vein for metastasis seeding assays. Both in the pancreas and the lungs (the primary site of seeding of tail vein-injected cancer cells), loss of TPST2 reduced tumor growth, consistent with the notion that TPST and, by extension, tyrosine sulfation, is an important mediator of pancreatic cancer growth. Mechanistically, the authors linked the effect of TPST inactivation to decreased protein expression of Integrin β4 (ITGB4). Further analysis revealed ITGB4 to be a novel tyrosine sulfation substrate of TPST2.

Overall, this study provides new evidence that tyrosine sulfation is important in pancreatic cancer. Although the authors focus on ITGB4 sulfation as a key mechanism and an exciting novel target of TPST2, it is likely that sulfation occurs on other biologically important targets, warranting future studies.

Pancreatic cancer is characterized by an extensive stroma, including cancer-associated fibroblasts and abundant immune cells, mostly suppressive in nature.¹¹ Whether tyrosine sulfation occurs and whether it plays a role in the tumor stroma remain to be investigated. In colorectal carcinoma, immunohistochemistry staining revealed SLC35B2 expression in cancer-associated fibroblasts,⁸ supporting the notion that tyrosine sulfation might occur in this compartment. Tyrosine sulfation is known to regulate chemokine signaling⁵; whether this process mediates immune suppression in pancreatic cancer remains to be investigated. Lastly, it remains to be elucidated whether cancer cells have a higher requirement for tyrosine sulfation than normal tissues; if so, targeting sulfation might be a new tool for combination treatment of pancreatic cancer.