In humans, there are three PIM family genes, the two first of which were originally identified from mice as proviral integration sites for Moloney leukemia virus. They encode serine/threonine kinases that are aberrantly expressed in a variety of hematological malignancies and solid tumors. PIM kinases contribute to cell proliferation, cell survival and cell motility by phosphorylating multiple downstream substrates. PIM kinases have emerged as attractive anti-cancer drug targets, and their distinct structures enable the generation of selective inhibitors.

The aim of this PhD study was to functionally validate three novel PIM targets that affect cancer cell signalling, namely actin capping proteins (CPs) that restrict elongation of actin fibers, liver kinase B1 (LKB1) that suppresses cell growth, and lactate dehydrogenase A (LDHA) that regulates cell metabolism. For this purpose, CRISPR/Cas9-based genome editing and structurally unrelated pharmacological PIM-selective inhibitors were used. PIM-targeted sites were identified by mass spectrometry and mutagenesis, after which the functional impacts of phosphorylation were studied in cultured cells or using a chick embryo xenograft model. In line with the pro-migratory function of PIM kinases, phosphorylation of CPs was shown to reduce their ability to bind to the plus ends of actin filaments and thereby to promote cell adhesion and migration. Catalytic activity of LKB1 towards its substrates such as AMPK was shown to be inhibited by PIM-dependent phosphorylation. These studies also indicated that the oncogenic effects of PIMs and the tumor-suppressive effects of LKB1 are tightly controlled at the cellular level. In the case of LDHA, PIM-dependent phosphorylation was observed to prevent nuclear LDHA from being degraded via K48-mediated ubiquitination, highlighting the connection between PIM kinases and the regulation of glycolytic enzymes. Altogether, the results from these studies help to better understand the mechanisms, through which PIM kinases stimulate cancer cell signalling.