Leptin potentiates antiproliferative action of cAMP elevation via protein kinase A down‐regulation in breast cancer cells

Abstract

Previously, we have shown that leptin potentiates the antiproliferative action of cAMP elevating agents in breast cancer cells and that the protein kinase A (PKA) inhibitor KT-5720 prevented the antiproliferative effects induced by the leptin plus cAMP elevation. The present experiments were designed to gain a better understanding about the PKA role in the antitumor interaction between leptin and cAMP elevating agents and on the underlying signaling pathways. Here we show that exposure of MDA-MB-231 breast cancer cells to leptin resulted in a strong phosphorylation of both ERK1/2 and STAT3. Interestingly, intracellular cAMP elevation upon forskolin pretreatment completely abrogated both ERK1/2 and STAT3 phosphorylation in response to leptin and was accompanied by a consistent CREB phosphorylation. Notably, leptin plus forskolin cotreatments resulted in a strong decrease of both PKA regulatory RIα and catalytic subunits protein levels. Importantly, pretreatment with the PKA inhibitor KT-5720 blocked the forskolin-induced CREB phosphorylation and prevented both the inhibition by forskolin of leptin-induced ERK1/2 and STAT3 phosphorylation and the PKA subunits down-regulation induced by the combination of leptin and forskolin. Altogether, our results indicate that leptin-dependent signaling pathways are influenced by cAMP elevation and identify PKA as relevantly involved in the pharmacological antitumor interaction between leptin and cAMP elevating drugs in MDA-MB-231 cells. We propose a molecular model by which PKA confers its effects. Potential therapeutic applications by our data will be discussed.

The angiogenesis suppressor gene AKAP12 is under the epigenetic control of HDAC7 in endothelial cells

“…Accordingly, kinases such as JAK2 have been previously reported as implicated in phosphorylating the Y705 residue. PKA and PKC also showed the ability to phosphorylate STAT3 [18,19]. Both kinases were particularly relevant because AKAP12 is known to participate in their scaffolding.…”

High-Content pSTAT3/1 Imaging Assays to Screen for Selective Inhibitors of STAT3 Pathway Activation in Head and Neck Cancer Cell Lines

“…Although 39 compounds (76.5%) were confirmed to inhibit IL-6-induced pSTAT3 activation by ‡ 50%, only three compounds (3.9%) also exhibited < 30% inhibition of IFNg-induced pSTAT1 activation at both compound exposures (Table 5): azelastine, etomidate and a-methyl-norepinephrine. Twenty (51%) of the 39 compounds that were confirmed to inhibit pSTAT3 and/or pSTAT1 activation elevate cellular cAMP levels either because they are G s -coupled GPCR agonists 49 (17 b-AR agonists, and 2 dopamine receptor 1 agonists) or they directly activate adenylate cyclase 50 (e.g., forskolin; Table 5). Pre-exposure of Cal33 cells to 20 mM forskolin for 3 or 24 h inhibited the subsequent activation of both the pSTAT3 and pSTAT1 pathways by their respective cytokines (Fig.…”

“…The fitted C parameter was the IC 50 and defined as the concentration giving a response half way between the fitted top (B) and bottom (A) of the curve. For normalized data, the A and B parameters were locked as 0 and 100, respectively.…”

“…49 In addition, forskolin, another of the 39 confirmed actives (Table 5), elevates cellular cAMP levels by direct activation of adenylate cyclase. 50 To determine whether forskolin or the beta-adrenoreceptor (b-AR) agonist isoproterenol could inhibit IL-6-induced pSTAT3 activation and/or IFNg-induced pSTAT1 activation, serum-starved Cal33 cells were exposed to 20 mM forskolin or isoproterenol for either 3 or 24 h and then treated with either medium, 50 ng/mL of IL-6, or 30 ng/mL IFNg for 15 min (Fig. 6).…”

The Roles of the RIIβ Linker and N-terminal Cyclic Nucleotide-binding Domain in Determining the Unique Structures of the Type IIβ Protein Kinase A

“…Like all protein kinases, PKA acts as a molecular switch at critical biochemical junctions, phosphorylating ion channels and receptors at the plasma/mitochondrial membranes, as well as other substrates in response to increasing cellular cAMP levels, thus altering their functions. Not surprisingly, aberrant PKA activity is implicated in a variety of diseases including Carney complex (5), Cushing syndrome (6 -10), breast and liver cancer (11)(12)(13)(14), myocardial hypertrophy, atrial fibrillation, and long QT syndrome (15)(16)(17), as well as several neurodegenerative disorders (18 -20).…”