Trastuzumab regulates IGFBP-2 and IGFBP-3 to mediate growth inhibition: implications for the development of predictive biomarkers for trastuzumab resistance

Activation of insulin-like growth factor-I receptor (IGF-IR) signaling is an important mechanism for trastuzumab resistance. IGF-binding proteins (IGFBP) modulate IGF-IR signaling and play important roles in the control of breast cancer progression. In this article, we report that trastuzumab treatment enhances the expression and secretion of IGFBP-3 in SKBR3 cells, a trastuzumab-sensitive breast cancer cell line, and that this upregulation of IGFBP-3 induced by trastuzumab correlates with trastuzumab-mediated growth inhibition. We describe a new role for IGFBP-3 in the regulation of IGF-I-mediated cross-talk between IGF-IR and ErbB2 signaling pathways. In particular, treatment of SKBR3 cells with recombinant IGFBP-3 blocks IGF-I-induced activation of IGF-IR and ErbB2, and stable expression of IGFBP-3 inhibits SKBR3 cell growth. We find an inverse relationship in the levels of secreted IGFBP-3 such that high levels of IGFBP-3 are associated with trastuzumab-sensitive breast cancer cells (SKBR3 and BT-474), whereas low levels of IGFBP-3 are found in trastuzumab-resistant cells (clone 3 and JIMT-1). In contrast to IGFBP-3, the secretion and expression of IGFBP-2 are upregulated in trastuzumab-resistant SKBR3 cells. Furthermore, we show that IGFBP-2 stimulates activation of ErbB2 and that trastuzumab reduces IGFBP-2-stimulated ErbB2 activation. Based on our data, we propose a novel mechanism of action whereby trastuzumab enhances the expression and secretion of IGFBP-3, which interferes with IGF-I-mediated mitogenic signaling via autocrine and paracrine mechanisms and reduces IGFBP-2-induced ErbB2 activation to mediate growth inhibition. Changes in secretion profiles of IGFBP-2 and IGFBP-3 in trastuzumab-sensitive and trastuzumab-resistant cells may promote the development of IGFBP-2 and IGFBP-3 as predictive biomarkers for trastuzumab resistance.

Identification and characterization of a major liver lysophosphatidylcholine acyltransferase

Phosphatidylcholine (PC) is synthesized through the Kennedy pathway, but more than 50% of PC is remodeled through the Lands cycle, i.e. the deacylation and reacylation of PC to attain the final and proper fatty acids within PC. The reacylation step is catalyzed by lysophosphatidylcholine acyltransferase (LPCAT), and we report here the identification of a novel LPCAT, which we named LPCAT3. LPCAT3 belongs to the membrane-bound O-acyltransferase (MBOAT) family and encodes a protein of 487 amino acids with a calculated molecular mass of 56 kDa. Membranes from HEK293 cells overexpressing LPCAT3 showed significantly increased LPCAT activity as assessed by thin layer chromatography analysis with substrate preference toward unsaturated fatty acids. LPCAT3 is localized within the endoplasmic reticulum and is primarily expressed in metabolic tissues including liver, adipose, and pancreas. In a human hepatoma Huh7 cells, RNA interference-mediated knockdown of LPCAT3 resulted in virtually complete loss of membrane LPCAT activity, suggesting that LPCAT3 is primarily responsible for hepatic LPCAT activity. Furthermore, peroxisome proliferator-activated receptor alpha agonists dose-dependently regulated LPCAT3 in liver in a peroxisome proliferator-activated receptor alpha-dependent fashion, implicating a role of LPCAT3 in lipid homeostasis. Our studies identify a long-sought enzyme that plays a critical role in PC remodeling in metabolic tissues and provide an invaluable tool for future investigations on how PC remodeling may potentially impact glucose and lipid homeostasis.

Differential Targeting of G -Subunit Signaling with Small Molecules

G protein betagamma subunits have potential as a target for therapeutic treatment of a number of diseases. We performed virtual docking of a small-molecule library to a site on Gbetagamma subunits that mediates protein interactions. We hypothesized that differential targeting of this surface could allow for selective modulation of Gbetagamma subunit functions. Several compounds bound to Gbetagamma subunits with affinities from 0.1 to 60 muM and selectively modulated functional Gbetagamma-protein-protein interactions in vitro, chemotactic peptide signaling pathways in HL-60 leukocytes, and opioid receptor-dependent analgesia in vivo. These data demonstrate an approach for modulation of G protein-coupled receptor signaling that may represent an important therapeutic strategy.

Structural and molecular characterization of a preferred protein interaction surface on G protein beta gamma subunits

G protein betagamma subunits associate with many binding partners in cellular signaling cascades. In previous work, we used random-peptide phage display screening to identify a diverse family of peptides that bound to a common surface on Gbetagamma subunits and blocked a subset of Gbetagamma effectors. Later studies showed that one of the peptides caused G protein activation through a novel Gbetagamma-dependent, nucleotide exchange-independent mechanism. Here we report the X-ray crystal structure of Gbeta(1)gamma(2) bound to this peptide, SIGK (SIGKAFKILGYPDYD), at 2.7 A resolution. SIGK forms a helical structure that binds the same face of Gbeta(1) as the switch II region of Galpha. The interaction interface can be subdivided into polar and nonpolar interfaces that together contain a mixture of binding determinants that may be responsible for the ability of this surface to recognize multiple protein partners. Systematic mutagenic analysis of the peptide-Gbeta(1) interface indicates that distinct sets of amino acids within this interface are required for binding of different peptides. Among these unique amino acid interactions, specific electrostatic binding contacts within the polar interface are required for peptide-mediated subunit dissociation. The data provide a mechanistic basis for multiple target recognition by Gbetagamma subunits with diverse functional interactions within a common interface and suggest that pharmacological targeting of distinct regions within this interface could allow for selective manipulation of Gbetagamma-dependent signaling pathways.

A docking site for G protein βγ subunits on the parathyroid hormone 1 receptor supports signaling through multiple pathways

The G protein-coupled receptor for PTH and PTH-related protein (PTH1R) signals via many intracellular pathways. The purpose of this work is to investigate a G protein binding site on an intracellular domain of the PTH1R. The carboxy-terminal, cytoplasmic tail of the PTH1R fused to glutathione-S-transferase interacts with Gi/o proteins in vitro. All three subunits of the heterotrimer interact with the receptor C-tail. Activation of the heterotrimeric complex with GTPgammaS has no effect on Gbetagamma interactions, but markedly disrupts binding of the Galphai/o subunits to the receptor tail, suggesting that direct Gbetagamma binding indirectly links Galpha subunits to this region of the receptor. Gbetagamma subunits alone bind the C-tail with an affinity that is comparable to the heterotrimeric G protein complex. G protein complexes consisting of Galphashis6-beta1gamma2 and Galphaqhis6-beta1gamma2 also interact with the PTH1R tail in vitro. The Gbetagamma interaction domain is located on the juxta-membrane region of the tail between amino acids 468 and 491. Mutations that disrupt Gbetagamma interactions block PTH signaling via phospholipase Cbeta/[Ca2+]i and MAPK and markedly reduce signaling via adenylyl cyclase/cAMP. Herein, we define a domain on the PTH1R that is capable of binding G protein heterotrimeric complexes via direct Gbetagamma interactions.

Ric-8 Enhances G Protein βγ-Dependent Signaling in Response to βγ-Binding Peptides in Intact Cells

Peptides derived from a random-peptide phage display screen with purified Gbeta(1)gamma(2) subunits as the target promote the dissociation of G protein heterotrimers in vitro and activate G protein signaling in intact cells. In vitro, one of these peptides (SIRKALNILGYPDYD; SIRK) promotes subunit dissociation by binding directly to Gbetagamma subunits and accelerating the dissociation of GalphaGDP without catalyzing nucleotide exchange. The experiments described here were designed to test whether the mechanism of SIRK action in vitro is in fact the mechanism of action in intact cells. We created a mutant of Gbeta(1) subunits (beta(1)W332A) that does not bind SIRK in vitro. Transfection of Gbeta(1)W332A mutant into Chinese hamster ovary cells blocked peptide-mediated activation of extracellular signal-regulated kinase (ERK), but it did not affect receptor-mediated Gbetagamma subunit-dependent ERK activation, indicating that Gbetagamma subunits are in fact the direct target in cells responsible for ERK activation. To determine whether free Galpha subunits were released from G protein heterotrimers upon peptide treatment, cells were transfected with Ric-8A, a guanine nucleotide exchange factor for free GalphaGDP, but not heterotrimeric G proteins. Ric-8A-transfected cells displayed enhanced myristoyl-SIRKALNILGYPDYD (mSIRK)-dependent inositol phosphate (IP) release and ERK activation. Ric-8A also enhanced ERK activation by the G(i)-linked G protein coupled receptor agonist lysophosphatidic acid. Inhibitors of Gbetagamma subunit function blocked Ric-8-enhanced activation of ERK and IP release. These results suggest that one potential function of Ric-8 in cells is to enhance G protein Gbetagamma subunit signaling. Overall, these experiments provide further support for the hypothesis that mSIRK promotes G protein subunit dissociation to release free betagamma subunits in intact cells.

Regulatory interactions between the amino terminus of G-protein betagamma subunits and the catalytic domain of phospholipase Cbeta2

We previously identified a 10-amino acid region from the Y domain of phospholipase Cbeta2 (PLCbeta2) that associates with G-protein betagamma subunits (Sankaran, B., Osterhout, J., Wu, D., and Smrcka, A. V. (1998) J. Biol. Chem. 273, 7148-7154). We mapped the site for cross-linking of a synthetic peptide (N20K) corresponding to this Y domain region to Cys(25) within the amino-terminal coiled-coil domain of Gbetagamma (Yoshikawa, D. M., Bresciano, K., Hatwar, M., and Smrcka, A. V. (2001) J. Biol. Chem. 276, 11246-11251). Here, further experiments with a series of variable length cross-linking agents refined the site of N20K binding to within 4.4-6.7 angstroms of Cys(25). A mutant within the amino terminus of the Gbeta subunit, Gbeta(1)(23-27)gamma(2), activated PLCbeta2 more effectively than wild type, with no significant change in the EC(50), indicating that this region is directly involved in the catalytic regulation of PLCbeta2. This mutant was deficient in cross-linking to N20K, suggesting that a binding site for the peptide had been eliminated. Surprisingly, N20K could still inhibit Gbeta(1)(23-27)gamma(2)-dependent activation of PLC, suggesting a second N20K binding site. Competition analysis with a peptide that binds to the Galpha subunit switch II binding surface of Gbetagamma indicates a second N20K binding site at this surface. Furthermore, mutations to the N20K region within the Y-domain of full-length PLCbeta2 inhibited Gbetagamma-dependent regulation of the enzyme, providing further evidence for aGbetagamma binding site within the catalytic domain of PLCbeta2. The data support a model with two modes of PLC binding to Gbetagamma through the catalytic domain, where interactions with the amino-terminal coiled-coil domain are inhibitory, and interactions with the Galpha subunit switch II binding surface are stimulatory.