Faculty Bibliography

B-cell lymphoma is the most common immune system malignancy. TCL1 transgenic mice (TCL1-tg), in which TCL1 is ectopically expressed in mature lymphocytes, develop multiple B- and T-cell leukemia and lymphoma subtypes, supporting an oncogenic role for TCL1 that probably involves AKT and MAPK-ERK signaling pathway augmentation. Additional, largely unknown genetic and epigenetic alterations cooperate with TCL1 during lymphoma progression. We examined DNA methylation patterns in TCL1-tg B-cell tumors to discover tumor-associated epigenetic changes, and identified hypermethylation of sprouty2 (Spry2). Sprouty proteins are context-dependent negative or positive regulators of MAPK-ERK pathway signaling, but their role(s) in B-cell physiology or pathology are unknown. Here we show that repression of Spry2 expression in TCL1-tg mouse and human B-cell lymphomas and cell lines is associated with dense DNA hypermethylation and was reversed by inhibition of DNA methylation. Spry2 expression was induced in normal splenic B cells by CD40/B-cell receptor costimulation and regulated a negative feedback loop that repressed MAPK-ERK signaling and decreased B-cell viability. Conversely, loss of Spry2 function hyperactivated MAPK-ERK signaling and caused increased B-cell proliferation. Combined, these results implicate epigenetic silencing of Spry2 expression in B lymphoma progression and suggest it as a companion lesion to ectopic TCL1 expression in enhancing MAPK-ERK pathway signaling

The ubiquitin ligase Siah2 has been shown to regulate prolyl hydroxylase 3 (PHD3) stability with concomitant effect on HIF-1alpha availability. Because HIF-1alpha is implicated in tumorigenesis and metastasis, we used SW1 mouse melanoma cells, which develop primary tumors with a propensity to metastasize, in a syngeneic mouse model to assess a possible role for Siah2 in these processes. Inhibiting Siah2 activity by expressing a peptide designed to outcompete association of Siah2-interacting proteins reduced metastasis through HIF-1alpha without affecting tumorigenesis. Conversely, inhibiting Siah2 activity by means of a dominant-negative Siah2 RING mutant primarily reduced tumorigenesis through the action of Sprouty 2, a negative regulator of Ras signaling. Consistent with our findings, reduced expression of PHD3 and Sprouty2 was observed in more advanced stages of melanoma tumors. Using complementary approaches, our data establish the role of Siah2 in tumorigenesis and metastasis by HIF-dependent and -independent mechanisms

PURPOSE: Elevated phospholipase D (PLD) activity provides a survival signal in several human cancer cell lines and suppresses apoptosis when cells are subjected to the stress of serum withdrawal. Thus, targeting PLD survival signals has potential to suppress survival in cancer cells that depend on PLD for survival. Honokiol is a compound that suppresses tumor growth in mouse models. The purpose of this study was to investigate the effect of honokiol on PLD survival signals and the Ras dependence of these signals. EXPERIMENTAL DESIGN: The effect of honokiol upon PLD activity was examined in human cancer cell lines where PLD activity provides a survival signal. The dependence of PLD survival signals on Ras was investigated, as was the effect of honokiol on Ras activation. RESULTS: We report here that honokiol suppresses PLD activity in human cancer cells where PLD has been shown to suppress apoptosis. PLD activity is commonly elevated in response to the stress of serum withdrawal, and, importantly, the stress-induced increase in PLD activity is selectively suppressed by honokiol. The stress-induced increase in PLD activity was accompanied by increased Ras activation, and the stress-induced increase in PLD activity in MDA-MB-231 breast cancer cells was dependent on a Ras. The PLD activity was also dependent on the GTPases RalA and ADP ribosylation factor. Importantly, honokiol suppressed Ras activation. CONCLUSION: The data provided here indicate that honokiol may be a valuable therapeutic reagent for targeting a large number of human cancers that depend on Ras and PLD for their survival

Novel second-generation taxoids with systematic modifications at the C2, C10, and C3'N positions were synthesized and their structure-activity relationships studied. A number of these taxoids exhibited exceptionally high potency against multidrug-resistant cell lines, and several taxoids exhibited virtually no difference in potency against the drug-sensitive and drug-resistant cell lines. These exceptionally potent taxoids were termed 'third-generation taxoids'. 19 (SB-T-1214), 14g (SB-T-121303), and 14i (SB-T-1213031) exhibited excellent activity against paclitaxel-resistant ovarian cancer cell lines with mutations in beta-tubulin as well, wherein the drug resistance is mediated by the beta-tubulin mutation. These taxoids were found to possess exceptional activity in promoting tubulin assembly, forming numerous very short microtubules similar to those formed by discodermolide. Taxoids 19 and 14g also showed excellent cytotoxicity against four pancreatic cancer cell lines, expressing three to four multidrug-resistant genes. Moreover, taxoid 19 exhibited excellent in vivo efficacy against highly drug-resistant CFPAC-1 pancreatic as well as DLD-1 human colon tumor xenografts in mice

The kinetics of Ras activation by Son of sevenless (SOS) changes profoundly when Ras is tethered to membranes, instead of being in solution. SOS has two binding sites for Ras, one of which is an allosteric site that is distal to the active site. The activity of the SOS catalytic unit (SOS(cat)) is up to 500-fold higher when Ras is on membranes compared to rates in solution, because the allosteric Ras site anchors SOS(cat) to the membrane. This effect is blocked by the N-terminal segment of SOS, which occludes the allosteric site. We show that SOS responds to the membrane density of Ras molecules, to their state of GTP loading and to the membrane concentration of phosphatidylinositol-4,5-bisphosphate (PIP2), and that the integration of these signals potentiates the release of autoinhibition

Ras proteins are small GTPases that play key roles in the regulation of several cellular processes such as growth, differentiation, and transformation. Although Ras signaling was thought to occur uniformly on the inner leaflet of the plasma membrane, a growing body of evidence indicates that Ras activation happens dynamically within defined plasma membrane microdomains and at other specific intracellular compartments, thus ensuring the generation of distinct signal outputs. Yet the mechanisms that control the spatiotemporal segregation of Ras proteins remain poorly characterized. We have recently shown that the differential modification of Ras proteins by ubiquitination is a crucial factor that controls Ras intracellular trafficking and signaling potential. To better understand the process of Ras ubiquitination, it is important to establish assays that not only provide information about the nature of the ubiquitin modification involved, but also enable the monitoring of the dynamics of this process. In this chapter, we will describe biochemical and biophysical methodologies, namely immunoprecipitation, nickel-chelate affinity chromatography, and bioluminescence resonance energy transfer (BRET), for monitoring the ubiquitination of Ras proteins. Although the description focuses on Ras, the assays described can in principle be applied to the study of a range of proteins of interest that may be subject to ubiquitination, and the use of the different methods in parallel should provide new insights into the nature and dynamics of protein ubiquitination