R-253 disrupts microtubule networks in multiple tumor cell lines

PURPOSE

The design and development of synthetic small molecules to disrupt microtubule dynamics is an attractive therapeutic strategy for anticancer drug discovery research. Loss of clinical efficacy of many useful drugs due to drug resistance in tumor cells seems to be a major hurdle in this endeavor. Thus, a search for new chemical entities that bind tubulin, but neither are a substrate of efflux pump, P-glycoprotein 170/MDR1, nor cause undesired side effects, would potentially increase the therapeutic index in certain cancer treatments.

EXPERIMENTAL DESIGN

A high-content cell-based screen of a compound library led to the identification of a new class of compounds belonging to a thienopyrimidine series, which exhibited significant antitumor activities. On structure-activity relationship analysis, R-253 [N-cyclopropyl-2-(6-(3,5-dimethylphenyl)thieno[3,2-d]pyrimidin-4-yl)hydrazine carbothioamide] emerged as a potent antiproliferative agent (average EC(50), 20 nmol/L) when examined in a spectrum of tumor cell lines.

RESULTS

R-253 is structurally unique and destabilizes microtubules both in vivo and in vitro. Standard fluorescence-activated cell sorting and Western analyses revealed that the effect of R-253 on cell growth was associated with cell cycle arrest in mitosis, increased select G(2)-M checkpoint proteins, and apoptosis. On-target activity of R-253 on microtubules was further substantiated by immunofluorescence studies and selected counter assays. R-253 competed with fluorescent-labeled colchicine for binding to tubulin, indicating that its binding site on tubulin could be similar to that of colchicine. R-253 neither is a substrate of P-glycoprotein 170/MDR1 nor is cytotoxic to nondividing human hepatocytes.

CONCLUSION

Both biochemical and cellular mechanistic studies indicate that R-253 could become a promising new tubulin-binding drug candidate for treating various malignancies.

Critical role of the ubiquitin ligase activity of UHRF1, a nuclear RING finger protein, in tumor cell growth

Early cellular events associated with tumorigenesis often include loss of cell cycle checkpoints or alteration in growth signaling pathways. Identification of novel genes involved in cellular proliferation may lead to new classes of cancer therapeutics. By screening a tetracycline-inducible cDNA library in A549 cells for genes that interfere with proliferation, we have identified a fragment of UHRF1 (ubiquitin-like protein containing PHD and RING domains 1), a nuclear RING finger protein, that acts as a dominant negative effector of cell growth. Reduction of UHRF1 levels using an UHRF1-specific shRNA decreased growth rates in several tumor cell lines. In addition, treatment of A549 cells with agents that activated different cell cycle checkpoints resulted in down-regulation of UHRF1. The primary sequence of UHRF1 contains a PHD and a RING motif, both of which are structural hallmarks of ubiquitin E3 ligases. We have confirmed using an in vitro autoubiquitination assay that UHRF1 displays RING-dependent E3 ligase activity. Overexpression of a GFP-fused UHRF1 RING mutant that lacks ligase activity sensitizes cells to treatment with various chemotherapeutics. Taken together, our results suggest a general requirement for UHRF1 in tumor cell proliferation and implicate the RING domain of UHRF1 as a functional determinant of growth regulation.

A novel role for p21-activated protein kinase 2 in T cell activation

To identify novel components of the TCR signaling pathway, a large-scale retroviral-based functional screen was performed using CD69 expression as a marker for T cell activation. In addition to known regulators, two truncated forms of p21-activated kinase 2 (PAK2), PAK2DeltaL(1-224) and PAK2DeltaS(1-113), both lacking the kinase domain, were isolated in the T cell screen. The PAK2 truncation, PAK2DeltaL, blocked Ag receptor-induced NFAT activation and TCR-mediated calcium flux in Jurkat T cells. However, it had minimal effect on PMA/ionomycin-induced CD69 up-regulation in Jurkat cells, on anti-IgM-mediated CD69 up-regulation in B cells, or on the migratory responses of resting T cells to chemoattractants. We show that PAK2 kinase activity is increased in response to TCR stimulation. Furthermore, a full-length kinase-inactive form of PAK2 blocked both TCR-induced CD69 up-regulation and NFAT activity in Jurkat cells, demonstrating that kinase activity is required for PAK2 function downstream of the TCR. We also generated a GFP-fused PAK2 truncation lacking the Cdc42/Rac interactive binding region domain, GFP-PAK2(83-149). We show that this construct binds directly to the kinase domain of PAK2 and inhibits anti-TCR-stimulated T cell activation. Finally, we demonstrate that, in primary T cells, dominant-negative PAK2 prevented anti-CD3/CD28-induced IL-2 production, and TCR-induced CD40 ligand expression, both key functions of activated T cells. Taken together, these results suggest a novel role for PAK2 as a positive regulator of T cell activation.

Systematic identification of regulatory proteins critical for T-cell activation

BACKGROUND

The activation of T cells, mediated by the T-cell receptor (TCR), activates a battery of specific membrane-associated, cytosolic and nuclear proteins. Identifying the signaling proteins downstream of TCR activation will help us to understand the regulation of immune responses and will contribute to developing therapeutic agents that target immune regulation.

RESULTS

In an effort to identify novel signaling molecules specific for T-cell activation we undertook a large-scale dominant effector genetic screen using retroviral technology. We cloned and characterized 33 distinct genes from over 2,800 clones obtained in a screen of 7 x 108 Jurkat T cells on the basis of a reduction in TCR-activation-induced CD69 expression after expressing retrovirally derived cDNA libraries. We identified known signaling molecules such as Lck, ZAP70, Syk, PLC gamma 1 and SHP-1 (PTP1C) as truncation mutants with dominant-negative or constitutively active functions. We also discovered molecules not previously known to have functions in this pathway, including a novel protein with a RING domain (found in a class of ubiquitin ligases; we call this protein TRAC-1), transmembrane molecules (EDG1, IL-10R alpha and integrin alpha2), cytoplasmic enzymes and adaptors (PAK2, A-Raf-1, TCPTP, Grb7, SH2-B and GG2-1), and cytoskeletal molecules (moesin and vimentin). Furthermore, using truncated Lck, PLC gamma 1, EDG1 and PAK2 mutants as examples, we showed that these dominant immune-regulatory molecules interfere with IL-2 production in human primary lymphocytes.

CONCLUSIONS

This study identified important signal regulators in T-cell activation. It also demonstrated a highly efficient strategy for discovering many components of signal transduction pathways and validating them in physiological settings.

Functional cloning of Src-like adapter protein-2 (SLAP-2), a novel inhibitor of antigen receptor signaling

In an effort to identify novel therapeutic targets for autoimmunity and transplant rejection, we developed and performed a large-scale retroviral-based functional screen to select for proteins that inhibit antigen receptor-mediated activation of lymphocytes. In addition to known regulators of antigen receptor signaling, we identified a novel adaptor protein, SLAP-2 which shares 36% sequence similarity with the known Src-like adaptor protein, SLAP. Similar to SLAP, SLAP-2 is predominantly expressed in hematopoietic cells. Overexpression of SLAP-2 in B and T cell lines specifically impaired antigen receptor-mediated signaling events, including CD69 surface marker upregulation, nuclear factor of activated T cells (NFAT) promoter activation and calcium influx. Signaling induced by phorbol myristate acetate (PMA) and ionomycin was not significantly reduced, suggesting SLAP-2 functions proximally in the antigen receptor signaling cascade. The SLAP-2 protein contains an NH2-terminal myristoylation consensus sequence and SH3 and SH2 Src homology domains, but lacks a tyrosine kinase domain. In antigen receptor-stimulated cells, SLAP-2 associated with several tyrosine phosphorylated proteins, including the ubiquitin ligase Cbl. Deletion of the COOH terminus of SLAP-2 blocked function and abrogated its association with Cbl. Mutation of the putative myristoylation site of SLAP-2 compromised its inhibitory activity and impaired its localization to the membrane compartment. Our identification of the negative regulator SLAP-2 demonstrates that a retroviral-based screening strategy may be an efficient way to identify and characterize the function of key components of many signal transduction systems.

[High alkaline phosphatase in subacute thyroiditis]

Subacute thyroiditis may be hard to diagnose, therefore patients are sometimes misdiagnosed and subjected to unnecessary work-up. We report a 37-year-old man with subacute thyroiditis and a high concentration of serum alkaline phosphatase. After aspirin treatment there was clinical improvement and decrease in rapid ESR, and in high serum thyroxin and alkaline phosphatase. The increased alkaline phosphatase, seen in as many as 50% of patients, is of hepatic origin, and is not caused by high serum thyroxin. Awareness of this relationship may help in diagnosis and may prevent unnecessary diagnostic procedures, which may be invasive.