p21-Activated Kinase 1 (Pak1) as an Element in Functional and Dysfunctional Interplay Among the Myocardium, Adipose Tissue, and Pancreatic Beta Cells

Abstract

This review focuses on p21-activated kinase 1 (Pak1), a multifunctional, highly conserved enzyme that regulates multiple downstream effectors present in many tissues. Upstream signaling via Ras-related small G-proteins, Cdc42/Rac1 promotes the activity of Pak1. Our hypothesis is that this signaling cascade is an important element in communication among the myocardium, adipose tissue, and pancreatic β-cells. Evidence indicates that a shared property of these tissues is that structure/function stability requires homeostatic Pak1 activity. Increases or decreases in Pak1 activity may promote dysfunction or increase susceptibility to stressors. Evidence that increased levels of Pak1 activity may be protective provides support for efforts to develop therapeutic approaches activating Pak1 with potential use in prevalent disorders associated with obesity, diabetes, and myocardial dysfunction. On the other hand, since increased Pak1 activity is associated with cancer progression, there has been a significant effort to develop Pak1 inhibitors. These opposing therapeutic approaches highlight the need for a deep understanding of Pak1 signaling in relation to the development of effective and selective therapies with minimal or absent off-target effects.

Keywords: Hippo signaling; adipokines; arrythmias; cardiokines; heart failure; obesity.

FIGURE 1

Activation of p21‐activated kinase (Pak1). In the transition to an active kinase, there is a stepwise conformational rearrangement from an autoinhibited dimeric state. (A) Inhibition is stabilized by the interaction of an inhibitory switch peptide (KI) binding tightly to a cleft in the kinase domain. (B) The dimeric configuration is disrupted by the binding of GTP‐Cdc42 or GTP‐Rac to a protein‐binding domain (PDB). This results in unfolding into an oval‐like structure with the withdrawal of the kinase inhibitory segment (KI). (C) With the phosphorylation of Thr423, there is activation of the enzyme followed by autophosphorylation of Ser sites in a stretch of 250 amino acids in the C‐terminal domain. Shown in the figure are major substrates demonstrated to be controlled by Pak1.

FIGURE 2

Major effectors of signaling upstream and downstream to activation of p21‐activated kinase (Pak1). This scheme of Pak1 signaling includes paths of homeostatic signaling and vulnerabilities in metabolic syndrome, obesity, Type 2 diabetes, and myocardial dysfunction. See text for further discussion.

FIGURE 3

An example of the complexity of p21‐activated kinase (Pak1) signaling in a ventricular cardiac myocyte. Illustrated are the multiple pathways of signaling with the activation of Pak1. Included are signaling to actin polymerization via cofilin and LIMK (LIM kinase) and to integrins in the costamere complex, including vinculin (Vin), paxillin (Pax), focal adhesion kinase (FAK), contiguous with the extracellular matrix (ECM), and signaling networks (via tensin and tailin). Noted also is the dystroglycan/sarcoglycan complex, which is linked to these networks and dysferlin/caveolin at the t‐tubules. Other mechano‐sensitive elements are shown in a network of stress sensors, including microtubules and Z‐disk (with docked signaling molecules, one of which is Pak1 in the inactivated state). Major proteins controlling sarcomere mechanics are cardiac myosin‐binding protein C (c‐MyBP‐C) and troponin I (cTnI). The illustration depicts membrane (LTCC, L‐type Ca‐channels) and sarcoplasmic reticulum ryanodine release channels (RYR), the sarco‐endoplasmic Ca‐Pump (SERCA2a) and phospholamban (PLB) all of which control excitation and Ca‐fluxes to and from the myofilaments and area regulated by protein kinase A and protein phosphatase 2A. Green arrows indicate activating and inhibiting pathways to these cellular organelles. Also indicated is Pak1 nuclear signaling via Akt, MKK7, JNK, SRF, calcineurin, and in the Hippo pathway via YAP (yes associated protein). See text for discussion of these regulatory mechanisms and an expanded section on Hippo signaling.

FIGURE 4

Scheme illustrating ON and OFF states of the canonical Hippo signaling pathway. The right panel of the scheme depicts evidence summarized in the text. Demonstrating in the basal homeostatic OFF state, there is relative suppression or restraint of phosphorylations in the Hippo pathway, which permits the nuclear transcriptional activator complex Yap (yes associated protein) and Taz (transcriptional coactivator with PDZ‐binding motif) to move from a cytoplasmic location into the nucleus, relieving transcriptional repression by TEAD (targeting transcriptional enhanced associate domains). As more fully discussed in the text and summarized in Figure 5, this transcriptional activation is important in the homeostasis of the physiologically stable state of the myocardium, adipose tissue, and pancreatic β‐cells, as indicated by the scroll at the right side of the scheme. Evidence indicates that in dysfunction in these tissues, there is a shift in the balance to the Hippo “ON” or unrestrained state, shown in the left panel, in which phosphorylated Yap is retained in the cytoplasm in a process that depends on a cascade of upstream phosphorylation signals. Activation of Mst1/2 (sterile 20‐like kinase 1 and 2) in combination with Sav (scaffolding protein Salvador) results in the activation of Lats1/2 (large tumor suppressor 1 and 2) that, in combination with MOB1A and 1B (Mps one binder 1A and B) promote phosphorylation of Yap/Taz, preventing its entry into the nucleus. In a dominant ON state, there is suppression of nuclear signaling promoting dysfunction, as listed as indicated in Figure 5. As indicated in Figure 1, the cytoplasmic Yap may sequestrate in the cytoplasm or be removed by ubiquitin‐related mechanisms involving specific forms of 14‐3‐3 proteins.

FIGURE 5

Scheme illustrating signals modulating ON and OFF states of the canonical Hippo signaling pathway. As shown and discussed in the text, signals engaging the Hippo pathway include GPCR activation by ligands, mechano‐signaling via the actin cytoskeleton/stress fibers, and the state of tight and adherens junctions. Other elements of Hippo mechano‐signaling include integrins, the ECM including collagen isoforms, and glycoproteins (fibronectin and laminins).

FIGURE 6

Scheme illustrating potential mechanisms by which activation of the p21 activated kinase (Pak1) may shift Hippo signaling to an OFF state suppressing pathological signaling by modifying upstream signals to Yap/Taz. As shown, signals engaging the Hippo pathway include GPCR activation by ligands and mechano‐signaling that shift from ON to OFF states. Shown are the effects of Pak1 activation to shift to an OFF state in which homeostasisis restored.