Protein kinase A and local signaling in cancer

Abstract

Protein kinase A (PKA) is a basophilic kinase implicated in the modulation of many cell-signaling and physiological processes. PKA also contributes to cancer-relevant events such as growth factor action, cell cycle control, cell migration and tumor metabolism. Germline and somatic mutations in PKA, gene amplifications, and chromosome rearrangements that encode kinase fusions, are linked to a growing number of malignant neoplasms. Mislocalization of PKA by exclusion from A-Kinase Anchoring Protein (AKAP) signaling islands further underlies cancer progression. This article highlights the influence of AKAP signaling and local kinase action in selected hallmarks of cancer. We also feature the utility of kinase inhibitor drugs as frontline and future anti-cancer therapies.

Keywords: A Kinase Anchoring Proteins; cancer; fibrolamellar carcinoma; mislocalization; protein kinase A.

Figure 1.. Anchored kinases and pathological signaling.

(A) Receptor tyrosine kinases (RTKs) contain an intracellular kinase domain that anchors Growth factor receptor bound protein 2 (Grb2) via the Grb2 SH2 domain. The N terminal SH3 domain of Grb2 interacts with Son of Sevenless (SOS), a Ras-guanine exchange factor. This interaction results in the catalysis of Ras from an inactive to active form, resulting in the activation of downstream MAPK signaling (both indicated). Kinase inhibitor drug Osimertinib targets tumors with activating EGFR mutations through irreversibly binding to the Cys797 residue. This results in inhibition of Ras and downstream ERK signaling. (B) G protein coupled receptors (GPCRs) are localized to the plasma membrane and mobilize heterotrimeric G proteins to catalyze production of cAMP by adenylyl cyclases. A-kinase anchoring proteins (AKAPs) constrain PKA within local pools of cAMP to elicit downstream responses. (Inset) Several points of this cascade are implicated in pathological signaling in the context of cancer. (1) Overstimulation of GPCRs, (2) point mutations in the stimulatory G protein GNAS, (3) amplification of downstream kinase signaling through aberrant PKA signaling, (4) inactivation of the autoinhibitory domains of PKA regulatory subunits, and (5) mutations to PKAc that impact association with AKAPs to mis-localize the kinase. (C) In fibrolamellar carcinoma, a genetic deletion results in the fusion PKAc with the J domain of Hsp40, denoted DNAJ. This results in mis-localization from AKAPs and pathological signaling. (D) A gene fusion of exons 1–8 of AKAP9 with exons 9-18 BRAF results in elevated kinase activity and subsequent thyroid cancer. (Created in BioRender. Rosenthal, K. (2024) BioRender.com/b49b923).

Figure 2.. Oncogenic kinase signaling in fibrolamellar carcinoma.

(A) In normal livers, PKA is recruited to AKAP signaling islands. (B) In FLC, most of PKAc is fused with DNAJ, which facilitates the exclusion of the fusion kinase from AKAP signaling islands. The fusion recruits the co-chaperone BAG2 to the complex via Heat shock protein 70 (Hsp70). Recruitment of BAG2 to the complex activates Bcl2, which inhibits normal cell death pathways and contributes to aspects of FLC pathology including chemotherapeutic resistance. (Created in BioRender. Rosenthal, K. (2024) BioRender.com/s12j016).

Figure 3.. AKAPs involved in cytoskeletal rearrangement and membrane dynamics that impact metastasis.

(A) AKAP2, a cytoskeletal AKAP, anchors PKA and protein phosphatase 1 (PP1) at the actin cytoskeleton. This brings PP1 in proximity to cofilin. PP1 mediated dephosphorylation promotes an invasive phenotype in prostate cancer cells through increased actin turnover. (B) Ezrin, a member of the ERM proteins, acts as a linker between the actin cytoskeleton and the cell membrane. Amplification of Ezrin promotes membrane protrusion to facilitate local invasion and subsequent metastasis. (Created in BioRender. Rosenthal, K. (2024) BioRender.com/t80z735).

Figure 4.. Depletion of dAKAP1 drives a glycolytic phenotype and invasive potential.

(A) In primary breast cancer, dAKAP1 is expressed at the outer mitochondrial membrane. Anchored PKA at dAKAP1 phosphorylates the GTPase effector protein Drp1, a mitochondrial fission regulator. (B) In invasive breast cancer cells, dAKAP1 is depleted, which results in decreased Drp1 phosphorylation and fragmented mitochondria through fission. This results in movement of mitochondria to the leading edge of the cell and increased glycolytic potential to produce local pools of ATP at the leading edge and increased motility. (Lower panels) Control and dAKAP1 siRNA treated cells showing changes in mitochondrial morphology (Mito tracker staining) upon depletion of the anchoring protein. (Created in BioRender. Rosenthal, K. (2024) BioRender.com/c07x768).