The BRAC1 network (from Nature Reviews, http://goo.gl/Nz1Dy)

The BRCA1 (breast cancer gene 1) tumour suppressor gene has been reported as a potential predictive biomarker of response to chemotherapy to antimicrotubule agents including vinorelbine, vincristine, paclitaxel and docetaxel. High levels of BRCA1 mRNA expression have been linked to better response rates and improved progression-free survival.

The authors here correlated the BRCA1 levels and the apoptotic response to vinorelbine and found:

1. a strong correlation between vinorelbine sensitivity and BRCA1 expression levels
2. emergence of resistance to vinorelbine by ablating BRCA1 using siRNA
3. a dramatic downregulation of BRCA1 following selection for vinorelbine resistance
4. re-expression of BRCA1 in resistant cells significantly augments their apoptotic response

Furthermore, the analysis of 144 primary mesothelioma specimens showed a loss of BRCA1 expression in ~40% of samples.

Altogether, the data presented highlights that BRCA1-immunonegativity can effectively predict resistance to vinorelbine and microtubule-directed chemotherapy .

Of note, it has been shown that BRCA1-deficiency correlates with a better outcome following platinum chemotherapy and clinical trials report on significant antitumor activity following PARP inhibitor treatment in BRCA-deficient patients.
Accordingly, mesotheliomas lacking expression of BRCA1, resistant to vinorelbine and microtubule-directed chemotherapy, might also represent a molecularly defined subgroup of tumors with sensitivity to PARP inhibition and platinum therapy.

Full article here.

PRC2 is shown with PRC1 and various recruiting molecules at a CpG-rich site where it mediates transcriptional repression. The actual mechanism of repression is not yet understood (http://goo.gl/HXwf5).

Polycomb group proteins (PcG), proteins that act to regulate transcription, have been implicated in numerous aspects of malignancies especially the ones with an aggressive phenotype and in stem cell biology. Misregulation of Polycomb protein levels often leads to either a block or unscheduled activation of developmental pathways, thereby enhancing the proliferation capability of a cell. Polycomb proteins form at least two distinct complexes, the Polycomb-repressive complexes 1 and 2 (PRC1 and PRC2). There is increasing evidence that PRC complexes have a role in tumor progression and development by blocking differentiation and promoting stem cell self-renewal. A recent study demonstrated that enhanced EZH2 levels led to growth of Ewing tumors and a consequential inhibition of endothelial and neuroectodermal differentiation. Here the authors find that EZH2, a core component of polycomb repressor complex-2 (PRC-2) is overexpressed in the majority of pleural mesotheliomas and that elevated EZH2 correlates with decreased patient survival.

Inhibition of PRC-2 expression by knock-down or with 3-deazaneplanocin A (DZNep), an S-adenosylhomocysteine hydrolase inhibitor, significantly inhibited proliferation, migration, clonogenicity and tumorigenicity of pleural mesothelioma cells. Furthermore, authors identified a loss of miR-101 or miR-26 in primary MPM specimens, which may explain the EZH2 overexpression. Authors do not conclusively show that DZNep inhibited the malignant phenotype by blocking PRC-2. Nevertheless, these findings warrant further analysis of PcG protein expression in mesotheliomas, and the development of agents targeting PRC-2 expression/activity for treatment.

Read the article here.

Vandetanib mechanism of action

Many targeted agents, such as anti-VEGF and anti-EGFR blockers, have not shown promise against mesothelioma, at least when used as single agents. However these same targeted agents might be effective when used in combination with chemotherapy or when used in a selected subpopulation. This study tested a variety of targeted agents together with cisplatinum-pemetrexed against 4 mesothelioma lines and found vandetanib, an inhibitor of EGFR/VEGFR2/RET, to be the most effective. Although the agent has multiple effects (see figure), the effect seemed to be explained by the effect on EGFR and was associated with suppression of phosphorylated Akt. The promise from this and other studies is that clinical trials using targeted agents may be designed in the future in subpopulations with evidence of dependence on that target (“addiction”). The targeted agent may then be effective on its own or by decreasing resistance to standard chemotherapy.

Read the article here.

BAP1 may target histones and cell cycle checkpoints.  Reintroduction of BAP1 in BAP1-null cells resulted in a modest increase in proliferation, quite a paradoxical result for a gene that supports a tumor suppression factor. Several groups have shown that BAP1 also has a role in cell cycle progression and that re-expression of BAP1 causes growth suppression in vitro and in vivo. So it is possible that, in mesothelioma, inactivation of BAP1 allows for a slower cell proliferation, with a more permissive G1/S checkpoint, causing cells to grow more slowly but uncontrollably.

This paper, published on Nature Genetics in July 2011, has been followed by another paper on BAP1 germline mutations in mesothelioma.  Together, these articles shed more light on the pathogenesis of mesothelioma and the implication of transcriptional deregulation in this highly lethal cancer.

Read the article here.

Structure of the new SRC inhibitors

In this short communication, the authors focus on the SRC pathway, which is hyperactivated in MM specimens and cell lines compared with normal mesothelial cells (Menges et al., 2010). Inhibition of the SRC pathway with the multi-targeted inhibitor dasatinib  has been shown to result in apoptosis, cell cycle arrest and decreased migration and invasion in mesothelioma cell lines (Tsao et al., 2007).
Here the authors test a panel of newly synthesized pyrazolo[3,4-d]pyrimidine derivative SRC kinase inhibitors, which bind the ATP pocket of the SRC kinase and inhibit SRC activity at nanomolar concentrations.

They found that two of these molecules, in particular, were indeed able to inhibit SRC activation and reduce cell viability by induction of apoptosis, p27 stabilization and nuclear localization.
Moreover, they observed a decrease in AKT activity and cyclin D1 levels.

These inhibitors may hold promise against mesothelioma, although in vivo and clinical trials will be needed to assess efficacy and toxicity.

Read the article here.

Read the paper here.

The authors show that the HDAC inhibitors, depakine (valproic acid) and vorinostat (SAHA), and a demethylating agent, decitabine (5-azaCdR), increase tumor antigen expression both in vitro and in vivo which promotes lymphocyte infiltration and recognition of mesothelioma cells by specific CD8+ cytotoxic T-cells.

These results show the potential promise of the combined treatment of mesothelioma with HDAC inhibitors and demethylating agents.

Read paper here.

Examples of different staining of mesothelioma tissues for CXCR4, which was ultimately found in almost all tumors.

The expression of chemokines and chemokine receptors has been linked to disease prognosis of various cancers.  The expression and role of chemokines and their receptors in mesothelioma is not well known.  In this article by Tong Li et.al, the authors report that the chemokine CXCL12 (stromal derived factor 1) and the receptor CXCR4 were overexpressed in mesothelioma samples from 41 patients and in cell lines.  CXCR4 was found in almost all mesotheliomas (97%) and CXCL12 in 78%; another receptor CXCR7 was only weakly expressed. CXCL12 binding to CXCR4 may mediate cell adhesion, migration and proliferation in tumor cells. In cell lines with high expression, an antagonist of CXCR4, AMD3100, reduced proliferation.  In conclusion, this paper proposed CXCR4-CXCL12 axis as a potential therapeutic target in mesothelioma.

Read the article here.

Electron microscopy of NCI-H226 mesothelioma spheroids.

The chemoresistance of solid tumors such as mesothelioma may be better studied in vitro 3D models because multicellular spheroids acquire multicellular resistance that may recapitulate the stubborn chemoresistance seen in vivo.

Dr. Xiang et al. recently used multicellular spheroids to test the efficacy of the recombinant anti-mesothelin immunotoxin SS1P in spheroids. The authors compared 2D (monolayer) and 3D (multicellular spheroid) mesothelioma cultures of the cell line NCI-H226 and found that, regardless of equal mesothelin expression in 2D and in 3D, SS1P was at least 100 times less cytotoxic in spheroids compared to monolayers. Spheroids were found to have increased tight junctions and E-cadherin expression, limiting the diffusion of SS1P into the interior of the spheroid. These results confirm the findings of Daubriac et al. (Cell Death Differ. 2009 Aug;16(8)), who described how resistance to loss of anchorage-anoikis of mesothelioma cellular aggregates relies on calcium-dependent intercellular junctions. Interestingly, other authors (Barbone et al. J Biol Chem. 2008 May 9;283(19) and Yang et al. Am J Respir Cell Mol Biol. 2009 Jul;41(1)) found no difference in the penetration of agents within mesothelioma and lung cancer spheroids, suggesting that treatments may display different diffusion kinetics in 3D. E-cadherin silencing by RNA interference or by blocking-antibodies sensitized NCI-H226 spheroids to SSP1, supporting antibody-based therapies in conjunction with strategies targeting the tumor microenvironment.

This paper further highlights the value of using 3D models for studing mechanisms of  chemoresistance of solid tumors.

Read the article here.

Coactivation of multiple RTKs in mesothelioma cell lines analyzed by phospho-RTK array

Although inhibition of EGFR has not shown promise in clinical trials of mesothelioma, broader inhibition of growth pathways might be more effective.  In this study, mesothelioma cells were propagated from human mesothelioma tumors of different histologies and found to have activation of several receptor tyrosine kinases.  In the figure, one sees an array from one mesothelioma cell line with evidence of activation of EGF-R, ERBB3, IGF1R, AXL, and MET, not seen in the normal mesothelial cells.  (The two spots in each corner are positive controls)  Inhibition of a chaperone protein (HSP90) involved in maintenance of proper protein folding was more effective in inhibiting all active RTK than was individual or combined inhibition of the receptors themselves.  The HSP90 inhibitor (geldanamycin, 17-AAG) was also effective at decreasing mesothelioma proliferation, perhaps by virtue of its inhibition of the RTK or perhaps by its other effects.   The 17-AAG inhibitor is now in clinical trials for several tumors.  The authors of this study propose that it be considered for use in clinical trials for mesothelioma.

Read the article here.