The human immune response is incredible. Unlike the adaptive immune response, the innate response acts quickly and is considered the first line of defense against pathogens and foreign DNA such as tumor cells. To be effective, the innate immune response relies on several cell types and a multitude of proteins such as interferons (INFs). This group of cytokines consists of three types and facilitates the removal of viral pathogens. Activation of interferon regulatory factor 3 (IRF3) leads to type I interferon production, which typically fights microbial infection-causing pathogens.¹ Immunotherapy has become an effective and well-liked means to treat cancer. Numerous drugs are on the market using this strategy. And, researchers are constantly discovering more proteins and pathways with the potential to treat cancer through immunotherapy.
The Ectonucleotide Capacity of ENPP1
Ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1) is an enzyme that breaks down nucleotides to nucleosides. Additionally, it is responsible for metabolizes 2’3’-cGAMP into its corresponding nucleotide (AMP and GMP) by degrading phosphodiester bonds.² The cyclic di-nucleotide is formed in response to double-stranded DNA (dsDNA) in the cytosol of a host cell, alerting the innate immune system to dangerous intracellular pathogens and damaged or cancerous cells.¹
ENPP1 Regulates STING pathway
Experts believe the STING pathway is the main innate immune sensing pathway within the tumor microenvironment and have found that STING is even epigenetically silenced in some cancers.² The STING (stimulator of interferon genes) pathway ultimately leads to the phosphorylation and activation of IRF3; therefore, initiating antiviral activity. ENPP1 regulates this process by degrading 2’3’-cGAMP, inhibiting the binding to STING, which impedes the successive steps to IRF3 activation.² Stalling ENPP1 might be a great approach to stimulate the innate response by means of keeping the STING pathway open for business so to speak.
The therapeutic potential in ENPP1 is vast. One area is breast cancer since ENPP1 has been found to be highly expressed in some breast cancers and its level has been correlated with poor prognosis.¹ As is the case with most proteins with biological function, there are some considerable hurdles scientists will face before finding the ideal drug though. Although analogs of 2’3’-cGAMP are resistant to ENPP1, the multiple distinct functions of the enzyme cause development challenges for specificity.²
ENPP1 Inhibitors Could Be An Option
Finding ENPP1 inhibitors that functionally stop the degradation of cGAMP could effectively be the avenue of choice. Biochemical activity assays provide a tool to sift through vast compound libraries on a quest for a molecule with the best properties. The Transceener AMP/GMP Assay could provide one possibility. The assay can readily measure the AMP or GMP produced when ENPP1 degrades cGAMP, effectively determining its enzymatic ability in the presence of various compounds. As a simple mix-and-read assay system, Transcreener provides a simple yet powerful method that is HTS compatible and a proven fit for hit-to-lead discovery.
1. Jacqueline A Carozza, et al. 2’3′-cGAMP is an immunotransmitter produced by cancer cells and regulated by ENPP1. bioRxiv 539312; doi: https://doi.org/10.1101/539312
2. Kaadige MR. Development of Enpp1 Inhibitors as a Strategy to Activate Stimulator of Interferon Genes (STING) in Cancers and Other Diseases. Int J Cell Sci Mol Biol. 2018;5(1):1-5. doi:10.19080/ijcsmb.2018.05.555655