Insulin-Increased Prolactin Gene Expression Requires Actin Treadmilling: Potential Role for P21 Activated Kinase

Abstract

Insulin-increased prolactin gene transcription in GH4 cells was enhanced by binding on fibronectin. This was mediated by receptor-like protein tyrosine phosphatase alpha, which activated Src, Rho, and phosphatidylinositol 3-kinase. It suggested that insulin signaling to gene transcription was partly dependent on actin rearrangement. This was confirmed through studies using inhibitors of actin treadmilling. Cytochalasin D, jasplakinolide, latrunculin B, and swinholide A altered the actin cytoskeleton of GH4 cells, as assessed by Alexa Fluor phalloidin staining, and inhibited insulin-increased prolactin gene transcription. These reagents did not affect the controls. Nor was it due to a gross defect of insulin signaling because activation/translocation of glycogen synthase kinase 3beta and mammalian target of rapamycin were not affected. Expression of wild-type and mutant actin treadmilling agents, Cdc42, TC10, neuronal Wiskott-Aldrich syndrome protein, and Nck, indicated that they were essential to insulin-increased prolactin gene expression, and suggested that activation of p21 associated kinase (PAK) might also be essential to this process. PAK expression also increased and PAK mutants decreased prolactin promoter activity in insulin-treated cells. The activation of PAK in the presence of inhibitors was also consistent with a role in activation of insulin-increased prolactin gene expression. Finally, small interfering RNA-mediated reduction of PAK decreased the effect of insulin on prolactin gene expression. Thus, it is likely that insulin activation of actin treadmilling through Cdc42/TC10 and neuronal Wiskott-Aldrich syndrome protein activates PAK and prolactin gene transcription.

Inhibition or Ablation of p21-activated Kinase (PAK1) Disrupts Glucose Homeostatic Mechanisms in Vivo

“…Interestingly, PAK1 has been shown to function downstream of insulin in multiple cell types (36,37), one of which being L6 myotubes (21). Consistent with our data showing normal AKT activation in insulin-stimulated PAK1 KO skeletal muscle extracts, Klip and colleagues showed PAK1 (termed PAK65) to be downstream of PI-3 kinase in the insulin signaling cascade (21).…”

Estrogen Regulates Protein Synthesis and Actin Polymerization in Hippocampal Neurons through Different Molecular Mechanisms

“…It has been previously reported that translocation of the translation machinery to dendritic spines is dependent on actin cytoskeletal dynamics and represent a key event in activity-dependent synaptic plasticity (31, 59). In addition, PAK has been previously implicated in insulin-mediated gene expression through the regulation of the actin cytoskeleton (60). In this study (60), actin depolymerizing agents did not affect nuclear mTOR activation induced by insulin, but reduced its activation in the cytoplasm, which indicates the specificity of these drugs to target specific populations of actin filaments.…”

“…In addition, PAK has been previously implicated in insulin-mediated gene expression through the regulation of the actin cytoskeleton (60). In this study (60), actin depolymerizing agents did not affect nuclear mTOR activation induced by insulin, but reduced its activation in the cytoplasm, which indicates the specificity of these drugs to target specific populations of actin filaments. In this regard, low concentrations of Lat A have been reported to specifically target actin filaments in dendritic shafts (where protein translation is generally thought to take place) but not in dendritic spines (31).…”

Swinholide J, a Potent Cytotoxin from the Marine Sponge Theonella swinhoei

“…Swinholide A ( 1 ) displays impressive biological properties including antifungal activity and potent cytotoxicity against a number of tumor cells. Its mechanism of action has been clarified in detail and its activity has been attributed to its ability to dimerize actin and disrupt the actin cytoskeleton [7–9]. Today, swinholide A is one of the better-characterized membrane permeable and specific inhibitors of actin filament networks and is actively used in cell biology studies [10].…”