Protein kinase N3 (PKN3; also known as PKNβ and PRK3) is a member of a subfamily of three isozymes within the AGC serine/threonine protein kinase family (Biochem. Soc. Trans. 49, 217–235; 2021). PKN1 and PKN2 are expressed ubiquitously, while the expression of PKN3 is enriched in some cancer cell lines and there is interest in its potential as an anticancer target.

Members of the PKN family are effectors of the Rho family of small GTPases, which are important in processes including cytoskeletal rearrangement, cell migration and cell adhesion. Activation of these kinases is complex, involving a combination of Rho protein binding, oligomerization, autophosphorylation and lipid binding (Biochem. Soc. Trans. 49, 217–235; 2021).

Growing evidence indicates an important role for PKN3 in some cancers, in particular in metastasis; for example, knockout of PKN3 in mice has been shown to inhibit melanoma metastasis (Sci. Rep. 6, 18979; 2016). PKN3-knockout mice are viable and develop normally (Sci. Rep. 6, 18979; 2016), and its position downstream of phosphoinositide 3-kinase (PI3K) means that targeting PKN3 might be a promising strategy for inhibiting this signalling pathway with fewer unwanted side effects than direct PI3K inhibitors (Fig. 1a). Evidence for the role of PKN3 in neovascularization has also led to speculation that it could be a target for diseases such as age-related macular degeneration (Sci. Rep. 6, 18979; 2016).

Fig. 1 | PKN3 as an anticancer drug target. a | Simplified overview of the potential role of PKN3 signalling in cancer. b, c | Structures of the selective PKN3 inhibitors JZ128 and UNC-CA94 (part b) and visualizations of these inhibitors docked into the structure of PKN3 (part c). JZ128 is on the left and UNC-CA94 is on the right.

A liposomal siRNA formulation known as Atu027 that silences PKN3 mRNA expression in the vascular endothelium has been developed as a potential cancer therapeutic (Cancers 12, 3130; 2020). Atu027 prevented liver and lung metastases in mouse models, and the combination of Atu027 with gemcitabine was safe and well tolerated in a phase Ib/IIa study involving 23 patients with pancreatic adenocarcinoma (Cancers 12, 3130; 2020).

The chemical tool inhibitors for investigating PKN3 that have been reported in the literature are limited. This is due in part to PKN3 not having been included in major kinome screening panels in the past. Nevertheless, several PKN3 inhibitors have been identified, mainly through unbiased screening approaches looking at the expressed kinome and the repurposing of existing kinase inhibitors.

Initial subfamily selectivity towards PKN3 was established by screening a small library of kinase inhibitors using a mobility shift assay. This identified several potent inhibitors — PP1 (Ki = 1.3 nM), SB202190 (Ki = 4.0 nM), H-8 (Ki = 10 nM) and fasudil (Ki = 110 nM) — albeit with limited wider kinome selectivity screening (Biosci. Rep. 34, e00097; 2014).

More recently, an unbiased chemoproteomic screen identified JZ128 as a covalent modifier of PKN3 with moderate functional inhibition (IC50 = 120 nM) and a narrow kinome inhibition spectrum (Fig. 1b) (J. Am. Chem. Soc. 141, 191–203; 2019). Furthermore, progress towards a reversible chemical probe for PKN3 has been achieved by in-depth annotation of an existing kinase inhibitor library to identify UNC-CA94, which potently inhibits PKN3 in a biochemical assay (IC50 = 14 nM) and with micromolar activity in cells (IC50 = 1.3 μM) (Fig. 1b) (ChemMedChem. e202200161; 2022). JZ128 and UNC-CA94 bind to PKN3 differently (Fig. 1c), and taken together these tools could be useful to further elucidate the role and therapeutic potential of PKN3 in cancer and other diseases.

Acknowledgements

This article is part of a series from the NIH Common Fund Illuminating the Druggable Genome (IDG) program. The goal of IDG is to catalyse research on understudied proteins from druggable gene families by providing reagents, phenotypes and a mineable database; focusing on GPCRs, kinases and ion channels. For more information, see https://druggablegenome.net/

The authors declare no competing interests.