HCK

Tyrosine-protein kinase HCK is an enzyme that in humans is encoded by the HCK gene.

Structure
HCK comprises five distinct domains which include two terminal domains and three SH domains. The N-terminal domain is important for lipid modifications and a C-terminal domain includes a regulatory tyrosine residue. Next, HCK comprises three highly conserved SH domains: SH1, SH2, and SH3. The catalytic SH1 domain houses the kinase's active site. The regulatory SH3 and SH2 domains are tightly bound together when HCK is in an inactive state.

Signaling
HCK is localized in the cytoplasm where it executes its functions as a kinase. In a steady state, HCK remains in an inactive conformation. Upon interaction with stimuli, such as TLR4 or IL-2, C-terminal tyrosine residues of HCK are dephosphorylated by phosphatases, e.g. CD45, and the inactive conformation of HCK is disrupted resulting in HCK activation. Activated HCK can then phosphorylate downstream molecules such as Bcr/Abl, PI3K/AKT, MAPK/ERK or STAT5 which then participate in myeloid cell polarization, proliferation and migration. A case study of a patient with a loss of C-terminal tyrosine residue in HCK showed that the patient suffered from severe pneumonia and vasculitis. This was due to increased HCK activity which led to increased myeloid cell migration and effector functions, such as the production of pro-inflammatory cytokines IL1b, IL-6, IL-8, and TNF-a, and the production of reactive oxygen species. These abnormal functions manifested as the infiltration of inflammatory leukocytes into the lungs and skin, resulting in pneumonia and vasculitis.

Function
HCK plays a key role during inflammation as it participates in actin-dependent processes like phagocytosis, membrane remodeling, and cell migration. It has also been shown that HCK participates in NLRP3 inflammasome formation and LPS-induced inflammatory response in mice. However, the mechanism of action is yet to be elucidated. HCK not only participates in inflammation-associated processes but also in cancerous processes. It has been shown, that HCK is part of a CXCL12/CXCR4 signaling axis that is partially responsible for the migration of leukemic cells in the bone marrow of patients with acute myeloid leukemia. This finding proposes HCK to be a novel target for the treatment of acute myeloid leukemia. HCK and the Src family kinases have also been implicated in driving cell survival in drug-tolerant cancer cells.

Interactions
HCK has been shown to interact with:


 * ADAM15
 * BCR gene,
 * Cbl gene,
 * ELMO1,
 * Granulocyte colony-stimulating factor receptor,
 * RAPGEF1,
 * RAS p21 protein activator 1, and
 * RASA3.