L-type Amino Acid Transporter 1 as a Therapeutic Target in Pancreatic Cancer

Abstract

Metabolic rewiring is a key feature of cancer cells to support the demands of growth and proliferation. The metabolism of amino acids is altered in many cancers, including pancreatic cancer. The cellular uptake of amino acids is regulated by amino acid transporters, such as L-type amino acid transporter 1 (LAT1). Accumulating evidence suggests that LAT1 is overexpressed in pancreatic cancer and confers a poor prognosis. Here we discuss the prospects of utilizing LAT1 as a novel target for pancreatic cancer therapy.

Introduction

Pancreatic cancer is an almost uniformly lethal disease with a 5 year survival rate <5%. ¹ The aggressive tumor biology and the predisposition for early metastasis, low resectability rates, high recurrence rates and resistance to conventional therapy, all contribute to the grim prognosis. Despite intensive research efforts, effective treatments targeting specific molecular vulnerabilities in pancreatic cancer cells are still inaccessible in the clinical setting. Pancreatic cancer cells demonstrate different metabolic requirements than normal cells and consume additional nutrients and divert those nutrients into macromolecular synthesis pathways in order to support their high rates of proliferation. ² This metabolic reprogramming of pancreatic cancer cells creates metabolic vulnerabilities that can be harnessed for therapeutic gain.

L-type amino acid transporter 1 (LAT1) is a sodium-independent transporter of large neutral amino acids across the cell membrane. LAT1 is mainly expressed in the blood-brain barrier and placenta. However, LAT1 is also expressed in many types of cancer, including pancreatic cancer. ³ The LAT1-mediated uptake of essential amino acids plays a key role in tumor growth and proliferation by activating multiple signaling pathways, including the mTOR pathway. The proto-oncogene c-Myc has been identified as an important regulator of LAT1 expression. ⁴ It has also been revealed that the YAP/TAZ signaling pathway can induce LAT1 expression. ⁵ Mutationally activated KRAS represents an initiating genetic event for pancreatic cancer. More than 90% of pancreatic tumors harbor a KRAS mutation, which has been linked to metabolic reprogramming of pancreatic cancer cells. ⁶ Growing evidence suggests that KRAS-mutant cancers upregulate LAT1 to meet the nutritional demands, while genetic deletion of LAT1 alters the amino acid pool, reduces protein synthesis and slows tumor growth and metastasis. ⁷ However, the complete molecular mechanisms governing LAT1 upregulation in pancreatic cancer cells are yet to be fully elucidated.

Several studies have suggested that LAT1 expression may serve as a prognostic marker in pancreatic cancer. Kaira et al ⁸ investigated a total of 97 patients with surgically resected pancreatic cancers using immunohistochemistry. LAT1 was highly expressed in 52.6% of cases and was significantly associated with poor survival. Yanagisawa et al ⁹ evaluated 66 resected pancreatic cancer specimens with a mean LAT1-score of 3.5. LAT1 scores of 6-9 were classified as high and correlated with reduced survival. Altan et al ¹⁰ analyzed 110 resected pancreatic cancers, of whom 64.1% had high LAT1 expression. The patients with high LAT1 expression presented worse outcome. The prognostic utility of LAT1 was further confirmed using the TCGA cohort. Interestingly, high LAT1 expression was also found to predict response to chemotherapy and LAT1 was closely correlated to hypoxia-induced genes (eg, PTGES, PYGL, and KPNA2). In a systematic review and meta-analysis, Lu et al ¹¹ demonstrated that LAT1 is a prognostic marker across multiple solid cancers based on 4579 cases and 35 studies. High LAT1 expression was associated with poor prognosis for all tumors, and in subgroup analysis for lung cancer, biliary tract cancer and pancreatic cancer.

The cancer specificity of LAT1 expression has been shown by both immunohistochemistry and PET using a LAT1-specific probe. The LAT1-selective PET tracer may enable differentiation between malignant tumors and inflammation, and also allow selection of patients who may benefit from LAT1-targeting anticancer drug treatment. ¹²

LAT1 has been proposed as a molecular target for cancer therapy (Figure 1). Strategies to target LAT1 may include LAT1 inhibitors that deprive pancreatic cancer cells of large amino acids functioning as substrates and building blocks in cellular processes stimulating growth, proliferation and survival. In experimental models, it was shown that JPH203 (nanvuranlat), a selective inhibitor of LAT1, suppressed pancreatic cancer cell proliferation and tumor growth, alone and in combination with gemcitabine.^13,14 The results indicated that the inhibition of LAT1 downregulated global translation in pancreatic cancer cells through the amino acid signaling mediated by mTORC1 and general amino acid control (GAAC) pathways. In the clinical setting, JPH203 was tested in a phase 1 study involving 17 patients with solid tumors, including 4 with pancreatic cancer. ¹⁵ 1 pancreatic cancer patient discontinued treatment before efficacy was evaluated. All 3 remaining pancreatic cancer patients showed progressive disease. The authors also analyzed phenotypes of NAT2, which is involved in the metabolism of JPH203. The analysis showed a higher disease control rate in the slow metabolizer group (50%) compared to the rapid metabolizer group (12.5%). 2/3 pancreatic cancer patients belonged to the rapid metabolizer group. NAT2 testing was used for classification in a subsequent randomized phase 2 trial for biliary tract cancer, where JPH203 demonstrated a statistically significant improvement in progression-free survival. ¹⁶

Figure 1.

LAT1 as a therapeutic target in pancreatic cancer cells.

Plasma branched chain amino acids (BCAAs) have also been found to predict the efficacy of JPH203. ¹⁷ Thus, development of predictive biomarkers seems to be crucial for identification of patients that may derive benefit from LAT1 inhibitor treatment. While the clinical trial ¹⁵ demonstrated little beneficial effects on the few pancreatic cancer patients evaluated, future possible means to apply LAT1 inhibitors for pancreatic cancer treatment may apart from predictive biomarkers for patient selection, include the use of LAT1 inhibitors in combination with other systemic therapy as part of multimodal management.

Additional therapeutic strategies based on LAT1 include polyamine transport inhibitors, such as compound 10, which functions as a LAT1-efflux agonist that can inhibit cell growth by reducing cellular levels of amino acids methionine and leucine. ¹⁸ The effect of gemcitabine coupled to the neutral amino acid threonine was evaluated in pancreatic cancer cells demonstrating improved gemcitabine uptake in LAT1-overexpressing cells. ¹⁹ Other drugs coupled to a LAT1-targeting molecule include 211At-AAMTl, composed of 211At, a radionuclide for targeted α-therapy, bound to α-methyl-l-tyrosine (AMT), which is transported by LAT1 with high affinity, which showed effect in vitro and in vivo. ²⁰ Another example of a LAT1-targeted cytotoxic substance is tyrosine modified irinotecan-loaded liposomes capable of simultaneously targeting LAT1 and the amino acid transporter B^0,+ (ATB^0,+) with improved anti-tumor effects when compared to commercially available liposomes loaded CPT-11 (Onivyde). ²¹

In summary, LAT1 is upregulated in pancreatic cancer, which contributes to the increased uptake of essential amino acids to meet the demand of the growing tumor. High expression of LAT1 is a predictor of poor survival in pancreatic cancer. Inhibiting LAT1 may potentially interfere with the nutritional supply of pancreatic cancer cells. Multiple strategies to use LAT1 as a therapeutic target have been investigated experimentally, but clinical data are still limited in this area.