Targeting the secreted RGDKGE collagen fragment reduces PD‑L1 by a proteasome‑dependent mechanism and inhibits tumor growth

Structural alterations of collagen impact signaling that helps control tumor progression and the responses to therapeutic intervention. Integrins represent a class of receptors that include members that mediate collagen signaling. However, a strategy of directly targeting integrins to control tumor growth has demonstrated limited activity in the clinical setting. New molecular understanding of integrins have revealed that these receptors can regulate both pro‑ and anti‑tumorigenic functions in a cell type‑dependent manner. Therefore, designing strategies that block pro‑tumorigenic signaling, without impeding anti‑tumorigenic functions, may lead to development of more effective therapies. In the present study, evidence was provided for a novel signaling cascade in which β3‑integrin‑mediated binding to a secreted RGDKGE‑containing collagen fragment stimulates an autocrine‑like signaling pathway that differentially governs the activity of both YAP and (protein kinase‑A) PKA, ultimately leading to alterations in the levels of immune checkpoint molecule PD‑L1 by a proteasome dependent mechanism. Selectively targeting this collagen fragment, reduced nuclear YAP levels, and enhanced PKA and proteasome activity, while also exhibiting significant antitumor activity in vivo. The present findings not only provided new mechanistic insight into a previously unknown autocrine‑like signaling pathway that may provide tumor cells with the ability to regulate PD‑L1, but our findings may also help in the development of more effective strategies to control pro‑tumorigenic β3‑integrin signaling without disrupting its tumor suppressive functions in other cellular compartments.

An RGDKGE-Containing Cryptic Collagen Fragment Regulates Phosphorylation of Large Tumor Suppressor Kinase-1 and Controls Ovarian Tumor Growth by a Yes-Associated Protein-Dependent Mechanism

The growth and spread of malignant tumors, such as ovarian carcinomas, are governed in part by complex interconnected signaling cascades occurring between stromal and tumor cells. These reciprocal cross-talk signaling networks operating within the local tissue microenvironment may enhance malignant tumor progression. Understanding how novel bioactive molecules generated within the tumor microenvironment regulate signaling pathways in distinct cellular compartments is critical for the development of more effective treatment paradigms. Herein, we provide evidence that blocking cellular interactions with an RGDKGE-containing collagen peptide that selectively binds integrin β3 on ovarian tumor cells enhances the phosphorylation of the hippo effector kinase large tumor suppressor kinase-1 and reduces nuclear accumulation of yes-associated protein and its target gene c-Myc. Selectively targeting this RGDKGE-containing collagen fragment inhibited ovarian tumor growth and the development of ascites fluid in vivo. These findings suggest that this bioactive collagen fragment may represent a previously unknown regulator of the hippo effector kinase large tumor suppressor kinase-1 and regulate ovarian tumor growth by a yes-associated protein-dependent mechanism. Taken together, these data not only provide new mechanistic insight into how a unique collagen fragment may regulate ovarian cancer, but in addition may help provide a useful new alternative strategy to control ovarian tumor progression based on selectively disrupting a previously unappreciated signaling cascade.

Abstract 6206: The xl313 cryptic collagen epitope regulates ovarian tumor growth by a yap dependent mechanism

Structural changes in the extra cellular matrix (ECM) can lead to the exposure of cryptic regulatory sites that play important roles in angiogenesis, inflammation, tumor growth, and tumor metastasis. Previously, we discovered that a unique cryptic regulatory site (XL313 cryptic epitope) containing the amino acid sequence RGDKGE, is generated during tumor development, and we found that this collagen epitope promotes angiogenesis and inflammation in vivo. Here, we provide the first evidence that ovarian carcinoma cells can bind the XL313 collagen epitope through integrin alpha-V beta-3 and that selectively blocking cellular interactions with the XL313 epitope significantly inhibits ovarian tumor growth and the formation of ascites fluid in vivo. To understand the cellular mechanism(s) by which antagonists of the XL313 epitope inhibit these pathological processes, we examined down-stream signaling events that were altered following blocking ovarian tumor cell interaction with the XL313 collagen epitope. Importantly, selectively targeting the XL313 epitope significantly reduced nuclear accumulation of the transcriptional co-activator YAP. In addition, blocking cellular interaction with the XL313 epitope resulted in enhanced YAP phosphorylation at serine-127. Interestingly, specific targeting of the XL313 epitope also enhanced LATS1 phosphorylation, which is a key effector molecule in the hippo- signaling pathway. Taken together, our data provide new cellular and molecular insight for understanding the role of the XL313 cryptic collagen epitope in controlling ovarian tumor growth, and suggest that selectively targeting the XL313 epitope-integrin-signaling cascade may represent a novel strategy to inhibit ovarian cancer progression.

Citation Format: Xianghua Han, Jennifer M. Caron, Peter C. Brooks. The xl313 cryptic collagen epitope regulates ovarian tumor growth by a yap dependent mechanism [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6206.

Cryptic collagen elements as signaling hubs in the regulation of tumor growth and metastasis

Structural remodeling of the extracellular matrix is a well-established process associated with tumor growth and metastasis. Tumor and stromal cells that compose the tumor mass function cooperatively to promote the malignant phenotype in part by physically interacting with intact and structurally altered matrix proteins. To this end, collagen represents the most abundant component of the extracellular matrix and is known to control the behavior of histologically distinct tumor types as well as a diversity of stromal cells. Although a significant molecular understanding has been established concerning how cellular interactions with intact collagen govern signaling pathways that control tumor progression, considerably less is known concerning how interactions with cryptic or hidden regions within remodeled collagen may selectively alter signaling cascades, or whether inhibition of these cryptic signaling pathways may represent clinically effective therapeutic strategies. Here, we review the emerging evidence concerning the possible mechanisms for the selective generation of cryptic or hidden elements within collagen and their potential cell surface receptors that may facilitate signal transduction. We discuss the concept that cellular communication links between cell surface receptors and these cryptic collagen elements may serve as functional signaling hubs that coordinate multiple signaling pathways operating within both tumor and stromal cells. Finally, we provide examples to help illustrate the possibility that direct targeting of these unique cryptic signaling hubs may lead to the development of more effective therapeutic strategies to control tumor growth and metastasis.

The HU177 Collagen Epitope Controls Melanoma Cell Migration and Experimental Metastasis by a CDK5/YAP-Dependent Mechanism

Stromal components not only help form the structure of neoplasms such as melanomas, but they also functionally contribute to their malignant phenotype. Thus, uncovering signaling pathways that integrate the behavior of both tumor and stromal cells may provide unique opportunities for the development of more effective strategies to control tumor progression. In this regard, extracellular matrix-mediated signaling plays a role in coordinating the behavior of both tumor and stromal cells. Here, evidence is provided that targeting a cryptic region of the extracellular matrix protein collagen (HU177 epitope) inhibits melanoma tumor growth and metastasis and reduces angiogenesis and the accumulation of α-SMA-expressing stromal cell in these tumors. The current study suggests that the ability of the HU177 epitope to control melanoma cell migration and metastasis depends on the transcriptional coactivator Yes-associated protein (YAP). Melanoma cell interactions with the HU177 epitope promoted nuclear accumulation of YAP by a cyclin-dependent kinase-5-associated mechanism. These findings provide new insights into the mechanism by which the anti-HU177 antibody inhibits metastasis, and uncovers an unknown signaling pathway by which the HU177 epitope selectively reprograms melanoma cells by regulating nuclear localization of YAP. This study helps to define a potential new therapeutic strategy to control melanoma tumor growth and metastasis that might be used alone or in combination with other therapeutics.

Abstract 3166: The HU177 cryptic collagen epitope selectively regulates melanoma cell behavior by a CDK5 and PTPN12 associated mechanism

Previous studies indicate that selectively inhibiting cellular interactions with the HU177 cryptic collagen epitope, which can be generated in vivo during structural remodeling of the extracellular matrix (ECM), inhibits tumor growth and metastasis. However, a detailed understanding of the mechanisms by which antagonists of the HU177 collagen epitope inhibit these pathological processes is not fully understood. Uncovering the molecular mechanisms by which cellular interactions with this collagen epitope regulate tumor cell behavior may allow the development of more effective clinical strategies targeting tumor cell-ECM interactions. Here we provide evidence for the first time, that specific targeting of the HU177 cryptic collagen epitope reduces phosphorylation of cyclin-dependent kinase (CDK5) and in addition, reduces the expression of the phosphatase PTPN12 in melanoma cells attached to denatured collagen. Importantly, both CDK5 < PTPN12 play a functional role in the ability of the HU177 collagen epitope to regulate migration of melanoma cells as reducing expression of these molecules inhibited the anti-migratory activity of the anti-HU177 antibody. Surprisingly, reduction in either CDK5 or PTPN12 also inhibited nuclear accumulation of YAP. These novel findings help define a previously unknown signaling mechanism by which both CDK5 < PTPN12 may play a selective role in controlling nuclear accumulations of the transcriptional co-activator YAP following melanoma cell interactions with the HU177 cryptic collagen epitope. Taken together, these studies provide new molecular insight into the role of the HU177 collagen epitope in controlling melanoma cell behavior. Given that direct targeting of integrin receptors that mediate tumor cellular interactions with the ECM have demonstrated only limited anti-tumor activity in human clinical trials, a more detailed molecular understanding of how generation of biologically relevant cryptic elements of the ECM control melanoma cell behavior will likely facilitate the development of more effective new clinical strategies to control tumor progression.

Citation Format: Xianghua Han, Jennifer M. Caron, Peter C. Brooks. The HU177 cryptic collagen epitope selectively regulates melanoma cell behavior by a CDK5 and PTPN12 associated mechanism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3166.

Abstract 5940: The HU177 collagen neo-epitope regulates nuclear YAP accumulation and melanoma tumor growth and metastasis in vivo

The extracellular matrix (ECM) plays a central role in controlling cellular behavior and integrin receptors play key roles in this process. Significant molecular insight is available on how structurally intact forms of collagen regulate integrin-signaling, however unique collagen neo-epitopes can be generated during tumor development and these neo-epitopes may differentially bind to integrin receptors. These findings raise an interesting question as to whether signaling initiated by collagen neo-epitopes as compared to intact collagen, differentially modify the final signaling out-put. Integrin A10B1 is a receptor for the HU177 collagen neo-epitope, however, little is known about the down stream signaling events stimulated by this receptor-ligand interaction. We provide evidence that melanoma cell interaction with denatured collagen results in enhanced adhesion and migration. Talin is a molecule that facilitates integrin-associated linkages to the cytoskeleton and helps govern adhesive cellular processes. Melanoma cell interactions with denatured as compared to intact collagen, resulted in enhanced Talin phosphorylation and surprisingly increased the levels of Cyclin Dependent Kinase 5 (CDK5), a molecule implicated in Talin phosphorylation and motility. Melanoma cell interactions with denatured collagen also led to elevated nuclear accumulation of the Yes-associated protein YAP. Given these results, we examined the effects of anti-HU177 antibody on adhesion and migration. Targeting the HU177 neo-epitope inhibited cell adhesion and migration on denatured collagen and also inhibited YAP nuclear accumulation. Knocking down YAP in melanoma cells resulted in the generation of cells that were resistant to the inhibitory effects of anti-HU177 antibody, suggesting that part of the inhibitory activity of anti-HU177 antibody may be associated with its ability to alter YAP activity. We next examined the role of the HU177 neo-epitope and its receptor, A10B1, on melanoma tumor growth and metastasis. Targeting the HU177 neo-epitope inhibited tumor-angiogenesis, accumulation of ASMA positive stromal cells in melanomas and tumor growth in vivo. Importantly, knock down of A10B1, or direct targeting of the HU177 neo-epitope inhibited experimental metastasis of melanoma cells. Taken together, these studies provide new insight into the role of the HU177 collagen neo-epitope in melanoma tumor growth and metastasis and provide a novel strategy to inhibit melanoma progression through selective targeting of a A10B1 ligand, rather than the integrin itself.

Citation Format: Xianghua Han, Jennifer M. Caron, Peter C. Brooks. The HU177 collagen neo-epitope regulates nuclear YAP accumulation and melanoma tumor growth and metastasis in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5940. doi:10.1158/1538-7445.AM2017-5940

Inhibition of Ovarian Tumor Growth by Targeting the HU177 Cryptic Collagen Epitope

Evidence suggests that stromal cells play critical roles in tumor growth. Uncovering new mechanisms that control stromal cell behavior and their accumulation within tumors may lead to development of more effective treatments. We provide evidence that the HU177 cryptic collagen epitope is selectively generated within human ovarian carcinomas and this collagen epitope plays a role in SKOV-3 ovarian tumor growth in vivo. The ability of the HU177 epitope to regulate SKOV-3 tumor growth depends in part on its ability to modulate stromal cell behavior because targeting this epitope inhibited angiogenesis and, surprisingly, the accumulation of α-smooth muscle actin-expressing stromal cells. Integrin α10β1 can serve as a receptor for the HU177 epitope in α-smooth muscle actin-expressing stromal cells and subsequently regulates Erk-dependent migration. These findings are consistent with a mechanism by which the generation of the HU177 collagen epitope provides a previously unrecognized α10β1 ligand that selectively governs angiogenesis and the accumulation of stromal cells, which in turn secrete protumorigenic factors that contribute to ovarian tumor growth. Our findings provide a new mechanistic understanding into the roles by which the HU177 epitope regulates ovarian tumor growth and provide new insight into the clinical results from a phase 1 human clinical study of the monoclonal antibody D93/TRC093 in patients with advanced malignant tumors.

Abstract 5131: The XL313 cryptic collagen epitope regulates immune checkpoint molecules by a αVβ3-integrin-associated mechanism

Proteolytic remodeling of extracellular matrix (ECM) results in structural changes that facilitate the generation of cryptic regulatory sites that promotes angiogenesis, tumor growth and metastasis. While alterations in the biophysical characteristic of the ECM can help create a tumor permissive microenvironment, little is known concerning whether structural changes in the ECM contributes to the ability of tumors to escape immune control. Molecular insight into the signaling pathways operating in stromal cells has contributed to the development of new cancer therapies. While clear progress has been made, as indicated by the recent approvals of new therapies such as immune checkpoint inhibitors, the overall survival of patients with metastatic disease remains alarmingly low. Accumulating evidence suggests that stromal components of the tumor microenvironment may contribute to the development of multiple resistance mechanisms including adaptive immune resistance. Thus, there is an urgent need for a more detailed understanding of how immune and inflammatory mechanisms govern tumor progression in order to enhance long-term durable responses with current therapies in a larger percentage of patients.

Tumor-associated macrophages (TAMs) have been suggested to play roles in tumor growth and metastasis by multiple mechanisms including structural remodeling of the ECM. TAMs may also contribute to the development of resistance to current anti-cancer therapies. Our studies indicate that distinct subsets of macrophage may facilitate the generation of the RGDKGE containing XL313 cryptic collagen epitope that promotes angiogenesis and inflammation in vivo. Here we provide the first evidence that cellular interactions with the XL313 collagen epitope may regulate immune checkpoint molecules by a αVβ3 integrin-associated mechanism. Cellular interactions with the XL313 epitope and denatured forms of collagen, which are present within the tumor microenvironment, enhanced the levels of the immune checkpoint molecules PD-L1 and LAG-3. Selective targeting of the XL313 collagen epitope with a monoclonal antibody inhibited tumor growth and metastasis and tumors from these mice exhibited reduced levels of immune checkpoint molecules. Importantly, anti-XL313 epitope antibody significantly enhanced the anti-tumor activity of anti-PD-L1 therapy in vivo. These data suggest that the XL313 epitope may play a functional role in promoting immune suppression in tumors and that selective targeting of this cryptic collagen epitope may reduce immune suppression and significantly enhance the efficacy of immune checkpoint inhibitors. Taken together, our studies are consistent with the possibility that the endogenously generated XL313 epitope may regulate tumor growth in part by facilitating the escape of tumors from immune control.

Citation Format: Jennifer M. Caron, Liangru Contois, Jacquelyn Ames, Peter C. Brooks. The XL313 cryptic collagen epitope regulates immune checkpoint molecules by a αVβ3-integrin-associated mechanism. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 5131.

Identification of an Endogenously Generated Cryptic Collagen Epitope (XL313) That May Selectively Regulate Angiogenesis by an Integrin Yes-associated Protein (YAP) Mechano-transduction Pathway

Extracellular matrix (ECM) remodeling regulates angiogenesis. However, the precise mechanisms by which structural changes in ECM proteins contribute to angiogenesis are not fully understood. Integrins are molecules with the ability to detect compositional and structural changes within the ECM and integrate this information into a network of signaling circuits that coordinate context-dependent cell behavior. The role of integrin αvβ3 in angiogenesis is complex, as evidence exists for both positive and negative functions. The precise downstream signaling events initiated by αvβ3 may depend on the molecular characteristics of its ligands. Here, we identified an RGD-containing cryptic collagen epitope that is generated in vivo. Surprisingly, rather than inhibiting αvβ3 signaling, this collagen epitope promoted αvβ3 activation and stimulated angiogenesis and inflammation. An antibody directed to this RGDKGE epitope but not other RGD collagen epitopes inhibited angiogenesis and inflammation in vivo. The selective ability of this RGD epitope to promote angiogenesis and inflammation depends in part on its flanking KGE motif. Interestingly, a subset of macrophages may represent a physiologically relevant source of this collagen epitope. Here, we define an endothelial cell mechano-signaling pathway in which a cryptic collagen epitope activates αvβ3 leading to an Src and p38 MAPK-dependent cascade that leads to nuclear accumulation of Yes-associated protein (YAP) and stimulation of endothelial cell growth. Collectively, our findings not only provide evidence for a novel mechano-signaling pathway, but also define a possible therapeutic strategy to control αvβ3 signaling by targeting a pro-angiogenic and inflammatory ligand of αvβ3 rather than the receptor itself.

Block Magnetic Excitations in the Orbitally Selective Mott Insulator BaFe_{2}Se_{3}

Iron pnictides and selenides display a variety of unusual magnetic phases originating from the interplay between electronic, orbital, and lattice degrees of freedom. Using powder inelastic neutron scattering on the two-leg ladder BaFe_{2}Se_{3}, we fully characterize the static and dynamic spin correlations associated with the Fe_{4} block state, an exotic magnetic ground state observed in this low-dimensional magnet and in Rb_{0.89}Fe_{1.58}Se_{2}. All the magnetic excitations of the Fe_{4} block state predicted by an effective Heisenberg model with localized spins are observed below 300 meV and quantitatively reproduced. However, the data only account for 16(3)μ_{B}^{2} per Fe^{2+}, approximatively 2/3 of the total spectral weight expected for localized S=2 moments. Our results highlight how orbital degrees of freedom in iron-based magnets can conspire to stabilize an exotic magnetic state.

Abstract 1480: Regulation of ovarian tumor growth and stromal cell infiltration by the HU177 biomechanical ECM switch

Accumulating evidence indicates that the malignant behavior of tumors depends not only on tumor cells themselves, but also on the stromal cells that comprise the malignant tumor mass. Experimental findings suggest that stromal cells such as cancer-associated fibroblast (CAF) may play important roles in promoting tumor growth and metastasis as well as regulating the efficacy of certain chemotherapeutic drugs. However, developing novel clinical strategies that selectively and simultaneously impacts tumor and stromal cells remains challenging. Alterations in the integrity and molecular composition of the extracellular matrix (ECM) are hallmarks of tumor progression. Our previous studies have shown that structural remodeling of the ECM can result in localized triggering of what we have termed “biomechanical ECM switches” or changes in the three-dimensional structure of pre-existing ECM molecules. A humanized antibody (TRC093/D93) specifically directed to the HU177 cryptic collagen epitope that is selectively exposed following triggering of a biomechanical ECM switch has been developed, and a human phase-I clinical trial was recently completed with encouraging results. Here we provide evidence that the HU177 biomechanical ECM switch is triggered within human ovarian tumors resulting in the exposure of the HU177 cryptic collagen epitope. The relative exposure of the HU177 cryptic site was significantly (P<0.05) enhanced in biopsies of malignant ovarian tumors as compared to benign ovarian lesions. Selective targeting of the HU177 cryptic collagen epitope by Mab D93 significantly (P<0.05) inhibited SKOV-3 tumor growth by approximately 70% as compared to controls. Tumors from these mice exhibited reduced angiogenesis, elevated levels of apoptosis and a significant reduction in infiltration of alpha smooth muscle cell actin (αSMC-actin) positive stromal fibroblasts. Importantly, while Mab D93 inhibited SKOV-3 tumor cell adhesion to denatured collagen type-I and enhanced the expression of the cyclin dependent kinase inhibitor P27KIP1, it also selectively inhibited (80%) fibroblast migration on denatured collagen type-I that was induced by either FGF-2 or SKOV-3 condition medium. Collectively these studies provide evidence that specific targeting of the HU177 cryptic collagen epitope may represent a highly selective strategy to inhibit ovarian tumor growth in part by limiting stromal cell invasion.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1480. doi:1538-7445.AM2012-1480

Insulin-like growth factor binding protein-4 differentially inhibits growth factor-induced angiogenesis

An in-depth understanding of the molecular and cellular complexity of angiogenesis continues to advance as new stimulators and inhibitors of blood vessel formation are uncovered. Gaining a more complete understanding of the response of blood vessels to both stimulatory and inhibitory molecules will likely contribute to more effective strategies to control pathological angiogenesis. Here, we provide evidence that endothelial cell interactions with structurally altered collagen type IV may suppress the expression of insulin-like growth factor binding protein-4 (IGFBP-4), a well documented inhibitor of the IGF-1/IGF-1R signaling axis. We report for the first time that IGFBP-4 differentially inhibits angiogenesis induced by distinct growth factor signaling pathways as IGFBP-4 inhibited FGF-2- and IGF-1-stimulated angiogenesis but failed to inhibit VEGF-induced angiogenesis. The resistance of VEGF-stimulated angiogenesis to IGFBP-4 inhibition appears to depend on sustained activation of p38 MAPK as blocking its activity restored the anti-angiogenic effects of IGFBP-4 on VEGF-induced blood vessel growth in vivo. These novel findings provide new insight into how blood vessels respond to endogenous inhibitors during angiogenesis stimulated by distinct growth factor signaling pathways.

Abstract 1549: Regulation of inflammation and tumor growth by the novel XL313 cryptic ECM element

Experimental evidence suggests that multiple cell types within the tumor microenvironment such as fibroblast, endothelial cells and inflammatory cells contribute to tumor growth. Recent evidence indicates that biomechanical alterations in the structural integrity of ECM proteins may result in exposure of cryptic regulatory elements that play roles in differentially controlling cell signaling pathways. Therefore, exposure of cryptic regulatory sites may function as biomechanical switches, allowing various cell populations to respond in distinct ways to localized structural alterations within the tumor microenvironment. Thus, identifying and characterizing biomechanical ECM switches may provide selective new therapeutic targets to regulate tumor progression. In this regard, triple helical collagen has numerous RGD containing cryptic integrin binding sites. However, it is not known whether all these cryptic RGD elements are functionally redundant or whether distinct flanking sequences may convey differential control of cellular behavior.

To examine potential functional differences between distinct RGD containing cryptic sites, we generated a series of Mabs directed to synthetic RGD containing peptides found within type-I collagen. One Mab termed XL313 selectively recognized a cryptic element containing the amino acid sequence RGDKGE, but failed to bind other RGD containing sites. Mab XL313 failed to recognize intact triple helical collagen, but did bind proteolytically cleaved collagen. Importantly, Mab XL313 significantly (P<0.05) inhibited B16F10 melanoma tumor growth by greater than 50%. Given the possibility that inflammation may contribute to tumor development, we examined the impact of the XL313 epitope on growth factor induced inflammation. Our studies suggest that the XL313 cryptic site may regulate inflammation as Mab XL313 significantly (P<0.05) inhibited inflammation / CAM thickening in the chick embryo model. Collectively, our studies suggest that the XL313 biomechanical ECM switch may represent an important new therapeutic target for the control of tumor growth and inflammation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1549. doi:10.1158/1538-7445.AM2011-1549

Abstract 3485: Insulin-like growth factor binding protein-4 (IGFBP-4) differentially inhibits growth factor induced angiogenesis in vivo

Angiogenesis contributes to the development and progression of malignant tumors. An expanding body of evidence also suggests that the response of both vascular and tumor cells to growth factor stimulation depends in part on cellular communication with the extracellular matrix (ECM), and studies have documented important roles for the integrin family of ECM receptors in modulating growth factor signaling. Therefore, uncovering molecular mechanisms by which integrin-mediated cellular communication with the ECM modulate the specificity of growth factor signaling during pathological angiogenesis may provide unique insight into malignant tumor progression.

Here we provide evidence that disruption of αvβ3-dependent interaction with the ECM enhance expression of Insulin-like growth factor binding protein-4 (IGFBP-4), a well-known regulator of IGF-1 signaling. Interestingly, IGFBP-4 significantly (P<0.05) inhibited IGF-1 and bFGF-induced angiogenesis by greater than 50%. In contrast, IGFBP-4 alone failed to inhibit VEGF-induced angiogenesis in chick CAM. Levels of phosphorylated p38 MAP kinase were elevated in chick CAM tissues treated with VEGF as compared to those in tissues stimulated with either IGF-1 or bFGF. Surprisingly, while the p38 MAP kinase inhibitor SB 202190 alone had little effect on VEGF-induced angiogenesis, a combination of IGFBP-4 and the p38 MAP kinase inhibitor significantly (P<0.05) inhibited VEGF-induced angiogenesis. These novel findings are consistent with a role for elevated levels of activated p38 MAP kinase in modulating the anti-angiogenic activity of IGFBP-4 in vivo. Taken together, our experimental findings provide additional molecular insight into how integrin-mediated interactions with the local ECM microenvironment may modulate the responses of cells to distinct growth factor signaling pathways during angiogenesis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3485. doi:10.1158/1538-7445.AM2011-3485

Single site alpha-tubulin mutation affects astral microtubules and nuclear positioning during anaphase in Saccharomyces cerevisiae: possible role for palmitoylation of alpha-tubulin

We generated a strain of Saccharomyces cerevisiae in which the sole source of alpha-tubulin protein has a cys-to-ser mutation at cys-377, and then we examined microtubule morphology and nuclear positioning through the cell cycle. During G1 of the cell cycle, microtubules in the C377S alpha-tubulin (C377S tub1) mutant were indistinguishable from those in the control (TUB1) strain. However, mitotic C377S tub1 cells displayed astral microtubules that often appeared excessive in number, abnormally long, and/or misoriented compared with TUB1 cells. Although mitotic spindles were always correctly aligned along the mother-bud axis, translocation of spindles through the bud neck was affected. In late anaphase, spindles were often not laterally centered but instead appeared to rest along the sides of cells. When the doubling time was increased by growing cells at a lower temperature (15 degrees C), we often found abnormally long mitotic spindles. No increase in the number of anucleate or multinucleate C377S mutant cells was found at any temperature, suggesting that, despite the microtubule abnormalities, mitosis proceeded normally. Because cys-377 is a presumptive site of palmitoylation in alpha-tubulin in S. cerevisiae, we next compared in vivo palmitoylation of wild-type and C377S mutant forms of the protein. We detected palmitoylated alpha-tubulin in TUB1 cells, but the cys-377 mutation resulted in approximately a 60% decrease in the level of palmitoylated alpha-tubulin in C377S tub1 cells. Our results suggest that cys-377 of alpha-tubulin, and possibly palmitoylation of this amino acid, plays a role in a subset of astral microtubule functions during nuclear migration in M phase of the cell cycle.

Amino-terminal cysteine residues of RGS16 are required for palmitoylation and modulation of Gi- and Gq-mediated signaling

RGS proteins (Regulators of G protein Signaling) are a recently discovered family of proteins that accelerate the GTPase activity of heterotrimeric G protein alpha subunits of the i, q, and 12 classes. The proteins share a homologous core domain but have divergent amino-terminal sequences that are the site of palmitoylation for RGS-GAIP and RGS4. We investigated the function of palmitoylation for RGS16, which shares conserved amino-terminal cysteines with RGS4 and RGS5. Mutation of cysteine residues at residues 2 and 12 blocked the incorporation of [3H]palmitate into RGS16 in metabolic labeling studies of transfected cells or into purified RGS proteins in a cell-free palmitoylation assay. The purified RGS16 proteins with the cysteine mutations were still able to act as GTPase-activating protein for Gialpha. Inhibition or a decrease in palmitoylation did not significantly change the amount of protein that was membrane-associated. However, palmitoylation-defective RGS16 mutants demonstrated impaired ability to inhibit both Gi- and Gq-linked signaling pathways when expressed in HEK293T cells. These findings suggest that the amino-terminal region of RGS16 may affect the affinity of these proteins for Galpha subunits in vivo or that palmitoylation localizes the RGS protein in close proximity to Galpha subunits on cellular membranes.

Posttranslational modification of tubulin by palmitoylation: I. In vivo and cell-free studies

It is well established that microtubules interact with intracellular membranes of eukaryotic cells. There is also evidence that tubulin, the major subunit of microtubules, associates directly with membranes. In many cases, this association between tubulin and membranes involves hydrophobic interactions. However, neither primary sequence nor known posttranslational modifications of tubulin can account for such an interaction. The goal of this study was to determine the molecular nature of hydrophobic interactions between tubulin and membranes. Specifically, I sought to identify a posttranslational modification of tubulin that is found in membrane proteins but not in cytoplasmic proteins. One such modification is the covalent attachment of the long chain fatty acid palmitate. The possibility that tubulin is a substrate for palmitoylation was investigated. First, I found that tubulin was palmitoylated in resting platelets and that the level of palmitoylation of tubulin decreased upon activation of platelets with thrombin. Second, to obtain quantities of palmitoylated tubulin required for protein structure analysis, a cell-free system for palmitoylation of tubulin was developed and characterized. The substrates for palmitoylation were nonpolymerized tubulin and tubulin in microtubules assembled with the slowly hydrolyzable GTP analogue guanylyl-(alpha, beta)-methylene-diphosphonate. However, tubulin in Taxol-assembled microtubules was not a substrate for palmitoylation. Likewise, palmitoylation of tubulin in the cell-free system was specifically inhibited by the antimicrotubule drugs Colcemid, podophyllotoxin, nocodazole, and vinblastine. These experiments identify a previously unknown posttranslational modification of tubulin that can account for at least one type of hydrophobic interaction with intracellular membranes.

Posttranslational modification of tubulin by palmitoylation: II. Identification of sites of palmitoylation

As shown in the companion article, tubulin is posttranslationally modified in vivo by palmitoylation. Our goal in this study was to identify the palmitoylation sites by protein structure analysis. To obtain quantities of palmitoylated tubulin required for this analysis, a cell-free system for enzymatic [3H]palmitoylation was developed and characterized in our companion article. We then developed a methodology to examine directly the palmitoylation of all 451 amino acids of alpha-tubulin. 3H-labeled palmitoylated alpha-tubulin was cleaved with cyanogen bromide (CNBr). The CNBr digest was resolved according to peptide size by gel filtration on Sephadex LH60 in formic acid:ethanol. The position of 3H-labeled palmitoylated amino acids in peptides could not be identified by analysis of the Edman degradation sequencer product because the palmitoylated sequencer products were lost during the final derivatization step to phenylthiohydantoin derivatives. Modification of the gas/liquid-phase sequencer to deliver the intermediate anilinothiozolinone derivative, rather than the phenylthiohydantoin derivative, identified the cycle containing the 3H-labeled palmitoylated residue. Therefore, structure analysis of peptides obtained from gel filtration necessitated dual sequencer runs of radioactive peptides, one for sequence analysis and one to identify 3H-labeled palmitoylated amino acids. Further cleavage of the CNBr peptides by trypsin and Lys-C protease, followed by gel filtration on Sephadex LH60 and dual sequencer runs, positioned the 3H-labeled palmitoylated amino acid residues in peptides. Integration of all the available structural information led to the assignment of the palmitoyl moiety to specific residues in alpha-tubulin. The palmitoylated residues in alpha-tubulin were confined to cysteine residues only. The major site for palmitoylation was cysteine residue 376.

Transcriptional regulation of the albumin gene in cultured rat hepatocytes. Role of basement-membrane matrix

The expression of tissue-specific functions by hepatocytes in primary culture is enhanced in the presence of an extracellular matrix. A basement membrane-like substratum, derived from the Engelbreth-Holm-Swarm mouse sarcoma (EHS) and termed EHS gel, supports synthesis and secretion of albumin for at least three weeks, in contrast to a conventional substratum (plastic or collagen-coated plastic), on which cells rapidly lose this function. The presence of an EHS matrix (as a substratum or added to the medium as a dilute gel) supports transcriptional activity at 30 to 35% and specific mRNA at 70 to 80% of initial values after five days of culture, at a time when transcription in cells plated in conventional culture is undetectable. For examining the cis elements required for transcriptional regulation by EHS matrix, we are utilizing recombinant adenoviruses to introduce DNA into hepatocytes, as an alternative to transfection of DNA fragments. Initial studies are presented, in which hepatocytes are cultured on either collagen-coated plastic or on EHS gel. At various times after plating, the cultures are infected with an adenovirus containing the proximal 5' regulatory region (to -441 base-pair) of the albumin gene. The results indicate no effect of EHS gel on this proximal promoter region, implying that matrix-responsive element(s) lie further upstream, possibly within the previously described enhancer at about -10,000 base-pairs.

Alteration of microtubule physiology in hepatocytes by insulin

Incubation of primary cultures of rat hepatocytes with insulin caused an alteration in several parameters of microtubule physiology. There was a transient decrease in total cellular tubulin levels and, at the same time, an apparent stabilization of microtubule polymers which was also transient. The level of microtubule polymer, however, was not altered, which indicated that an insulin-inducible decrease in the level of nonpolymerized tubulin was responsible for the decrease in total cellular tubulin. These changes were followed by an increase in tubulin mRNA levels and, shortly thereafter, by an increase in tubulin protein synthesis. This study demonstrates that microtubule physiology in hepatocytes is modulated by insulin and, further, suggests a model by which insulin-induced changes in microtubule physiology may play a role in regulating other insulin responses.

Dynamic interactions between microtubules and artificial membranes

We report that extensive adsorption of microtubule protein to liposomes occurs above the transition temperature of the phospholipid bilayer, occurs to phosphatidylcholine (PC) or phosphatidylserine (PS) vesicles, and is not affected by preincubation of microtubule protein with colchicine. Most importantly, we show that 51-63% of the tubulin adsorbed onto neutral (PC) phospholipid vesicles can be desorbed to form microtubules when buffer conditions are adjusted to favor microtubule assembly. By contrast, no microtubule assembly occurred with preadsorption of microtubule protein onto acidic (PS) phospholipid vesicles, suggesting irreversible binding.

Autoregulation of tubulin synthesis in hepatocytes and fibroblasts

Microtubule polymer levels in mouse 3T6 fibroblasts and primary cultures of rat hepatocytes can be manipulated by treatment of cells with long term, low doses of colcemid. Such treatment produces a rather uniform population of cells with microtubules of reduced lengths. Using this system, we demonstrate (a) that the rate of tubulin synthesis is sensitive to small changes (10%) in microtubule polymer mass and (b) that the percent of inhibition of synthesis is proportional to the level of soluble tubulin. Experiments with hepatocytes indicate that not only synthesis but the stability of tubulin protein was also regulated to maintain a specific level of tubulin. Treatment of hepatocytes with colcemid or other microtubule-depolymerizing drugs reduced the half-life of tubulin from 50 to 2 h, whereas taxol, which stabilizes microtubules, increased the half-life. To assess the consequences of altering microtubule polymer mass, we have analyzed the effect of controlled depolymerization of microtubules in rat hepatocytes on the processing of endocytosed ligands and found it sensitive to small changes in microtubule polymer levels.

Autoregulation of tubulin synthesis in enucleated cells

The effects on tubulin messenger RNA levels and tubulin protein synthesis of treating cells with microtubule-depolymerizing drugs or directly microinjecting cells with tubulin have suggested that non-polymerized tubulin depresses its own synthesis. The precise level of this control is unclear. It has been shown that enucleated cells, termed cytoplasts, retain many properties of the original cell, including maintenance of cell shape, pinocytic activity and locomotion as well as biosynthetic activities such as protein synthesis and replication of cytoplasmic viruses. Furthermore, cytoplasts retain most of the components of the cytoskeleton including the centrioles. If cytoplasmic activities alone are responsible for regulating tubulin biosynthesis, cytoplasts should contain the necessary components. To distinguish between regulation which would occur in the nucleus, that is, alterations in mRNA synthesis or modifications of the mRNA, from alterations in mRNA stability and/or translatability which would take place in the cytoplasm, we examined the autoregulation of tubulin synthesis in enucleated cells. Here, we report that enucleated mouse fibroblasts retain the ability to turn off tubulin protein synthesis in response to microtubule depolymerization, the reduction in tubulin synthesis being accompanied by a corresponding decrease in tubulin mRNA levels. Thus, transcription, processing and transport of tubulin mRNA from the nucleus are not likely to be the loci of regulation. Instead, tubulin must reduce, either directly or indirectly, the translatability of its own mRNA.

Interaction of microtubule proteins with phospholipid vesicles

We have examined the interaction of unilamellar dimyristoyl phosphatidylcholine liposomes with the high-speed supernate of brain homogenate and with tubulin purified through one or two cycles of microtubule assembly-disassembly. Tubulin and certian high molecular weight proteins are selectively adsorbed from these mixtures onto liposomes. The composition of adsorbed proteins is similar to that obtained during corresponding cycles of microtubule assembly, suggesting the equivalency of these processes. Adsorption induces stacking and/or fusion of liposomes into multilamellar structures indicating strong protein-lipid interaction. In addition, liposome-adsorbed tubulin forms extensive intermolecular disulfide bridges that are inert to reducing agents in the aqueous medium. The observations form a basis for further study of the distribution, function, and properties of membrane-bound tubulin.