A perspective on HPK1 as a novel immuno-oncology drug target.

In this perspective review, the role Hematopoietic Progenitor Kinase 1 (HPK1) in tumor immunity will be reviewed, with special emphasis on how T cells are negatively-regulated at different junctures of cancer-immunity cycle by this regulatory kinase. The review will highlight the strengths and weaknesses of HPK1 as a candidate target for novel immuno-oncology (IO) drug development that is centered on the use of small molecule kinase inhibitor to modulate the immune response against cancer. Such a therapeutic approach, if proven successful, could supplement the cancer cell-centric standard of care therapies in order to fully meet the therapeutic needs of cancer patients.

HPK1 Influences Regulatory T Cell Functions.

Hematopoietic progenitor kinase 1 (HPK1) is a negative regulator of TCR-initiated signal transduction. Both the mice and the genetically engineered mice with a point mutation that disrupts the catalytic activity of HPK1 possess enhanced antitumor immunity, especially when these mice are treated with anti-PD-L1 immune checkpoint Ab. Because CD4FOXP3 regulatory T cells (Tregs) play an important role in suppressing tumor immunity, we investigated whether the loss of HPK1 expression could result in the reduction of Treg functions.

Multiple conformational states of the HPK1 kinase domain in complex with sunitinib reveal the structural changes accompanying HPK1 trans-regulation.

Hematopoietic progenitor kinase 1 (HPK1 or MAP4K1) is a Ser/Thr kinase that operates via the c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) signaling pathways to dampen the T-cell response and antitumor immunity. Accordingly, selective HPK1 inhibition is considered a means to enhance antitumor immunity. Sunitinib, a multi-receptor tyrosine kinase (RTK) inhibitor approved for the management of gastrointestinal stromal tumors (GISTs), renal cell carcinoma (RCC), and pancreatic cancer, has been reported to inhibit HPK1 In this report, we describe the crystal structures of the native HPK1 kinase domain in both nonphosphorylated and doubly phosphorylated states, in addition to a double phosphomimetic mutant (T165E,S171E), each complexed with sunitinib at 2.

The Structure of HPK1 Kinase Domain: To Boldly Go Where No Immuno-Oncology Drugs Have Gone Before.

In this issue of Structure, Wu et al. (2018) report several apo and small-molecule inhibitor-bound structures of the kinase domain of hematopoietic progenitor kinase 1, a ser/thr kinase that functions as an inhibitor of T cell activation. The studies reveal that the HPK1 kinase domain exists as a domain-swapped dimer.

HPK1 as a novel target for cancer immunotherapy.

Identifying the appropriate drug targets for the development of a novel anti-tumor immunotherapy is one of the most risky steps in the drug development cycle. We have identified a hematopoietic cell-restricted serine/threonine kinase, hematopoietic progenitor kinase 1 (HPK1), as a possible target for therapeutic intervention. Targeted disruption of HPK1 alleles confers T cells with an elevated Th1 cytokine production in response to TCR engagement.

Detecting tyrosine-phosphorylated proteins by Western blot analysis.

The development of monoclonal antibodies (mAbs) that recognize nearly all of the phosphorylated tyrosine residues, irrespective of the surrounding sequences, enables researchers to detect the phosphorylation state of proteins through the use of anti-phosphotyrosine western blotting. The availability of this simple, reliable, nonradioactive and yet sensitive method created a boom in signal transduction research. While the methodology of how to perform an anti-phosphotyrosine western blot remains unchanged since the procedure became widely used in the early part of 1990s, steady improvements in reagents and detection technologies have allowed researchers to detect tyrosine phosphorylation quantitatively, at unprecedented sensitivity.

Hematopoietic progenitor kinase 1 is a critical component of prostaglandin E2-mediated suppression of the anti-tumor immune response.

Lung cancer is the leading cause of cancer-related mortality in the world, resulting in over a million deaths each year. Non-small cell lung cancers (NSCLCs) are characterized by a poor immunogenic response, which may be the result of immunosuppressive factors such as prostaglandin E2 (PGE(2)) present in the tumor environment. The effect of PGE(2) in the suppression of anti-tumor immunity and its promotion of tumor survival has been established for over three decades, but with limited mechanistic understanding.

Hematopoietic progenitor kinase 1 is a negative regulator of dendritic cell activation.

Hematopoietic progenitor kinase 1 (HPK1) is a hematopoietic cell-restricted member of the Ste20 kinases that acts as a negative regulator of T cell functions through the AP-1, NFAT, and NFkappaB pathways. Using HPK1-deficient (HPK1(-/-)) mice, we report in this study a novel role for HPK1 in dendritic cells (DCs). Specifically, we observed that matured HPK1(-/-) bone marrow-derived DCs (BMDCs) are superior to their wild-type (WT) counterpart in stimulating T cell proliferation in vivo and in vitro.

Prostaglandin E2 activates HPK1 kinase activity via a PKA-dependent pathway.

Hematopoietic progenitor kinase 1 (HPK1) is a hematopoietic cell-restricted member of the Ste20 serine/threonine kinase super family. We recently reported that the immunosuppressive eicosanoid, prostaglandin E(2) (PGE(2)), is capable of activating HPK1 in T cells. In this report, we demonstrate that unlike the TCR-induced activation of HPK1 kinase activity, the induction of HPK1 catalytic activity by PGE(2) does not require the presence of phosphotyrosine-based signaling molecules such as Lck, ZAP-70, SLP-76, and Lat.

Hematopoietic progenitor kinase 1 (HPK1) negatively regulates prostaglandin E2-induced fos gene transcription.

Prostaglandin E(2) (PGE(2)) is the predominant eicosanoid product released by macrophages at the site of inflammation. Binding of PGE(2) to its cognate 7 transmembrane-spanning G protein-coupled receptors (GPCRs) activates signaling pathways, leading to the synthesis of the Fos transcription factor. Because the Ste20 serine/threonine protein kinase (S/TPK) is a critical signal transducer for the G protein-coupled pheromone receptor in Saccharomyces cerevisiae, we postulated that the PGE(2) GPCRs may activate one of the Ste20 mammalian orthologs.

A novel Src homology 2 domain-containing molecule, Src-like adapter protein-2 (SLAP-2), which negatively regulates T cell receptor signaling.

We have cloned a novel adapter protein containing Src homology 2 and Src homology 3 domains similar to the Src family of tyrosine kinases. This molecule lacks a catalytic tyrosine kinase domain and is related to a previously identified protein, Src-like adapter protein (SLAP), and is therefore designated SLAP-2. Northern blot analysis indicates that SLAP-2 is predominantly expressed in the immune system.