Activity of deacetylase inhibitor panobinostat (LBH589) in cutaneous T‐cell lymphoma models: Defining molecular mechanisms of resistance

Abstract

Panobinostat (LBH589) is a highly potent deacetylase inhibitor that has demonstrated clinical efficacy in patients with advanced cutaneous T-cell lymphoma (CTCL). To gain a better understanding of the compound activity in this tumor type, we investigated the cellular and molecular effects of panobinostat using both in vitro and in vivo models of CTCL. All 4 tested CTCL cell lines exhibited very high sensitivity to panobinostat-induced growth inhibition. However, only 2 of 4 lines exhibited significant response to the cytotoxic activity of panobinostat. In a CTCL xenograft mouse tumor model, panobinostat treatment resulted in complete tumor regression. The difference in cell sensitivity to panobinostat-induced death enabled us to further investigate potential mechanisms responsible for tumor sensitivity or resistance. In CTCL cell lines that were insensitive to panobinostat-induced apoptosis, constitutively activated NF-jB and high levels of Bcl-2 were observed. Inhibition of Bcl-2 sensitized cells to the cytotoxic activity of panobinostat. Conversely, knockdown of Bax diminished the CTCL cell sensitivity. Interestingly, panobinostat could induce cytotoxicity in vorinostat-resistant CTCL cells by downregulating phosphorylated STAT3 and STAT5 proteins. These studies suggest distinct mechanisms responsible for resistance to different deacetylase inhibitors. We show that the intrinsic apoptotic signaling plays an essential role in mediating panobinostat anticancer activity. Moreover, cancer cell sensitivity to panobinostat treatment may be further improved by combination with inhibition of anti-apoptotic factors. These data provide preclinical support that panobinostat, as a single agent or in combination with other anticancer agents, is a promising therapy for CTCL.Cutaneous T-cell lymphomas (CTCLs) are lymphoproliferative disorders characterized by clonal expansion and localization of neoplastic T lymphocytes to the skin, resulting in immune dysregulation and tumor growth. 1 The most common form of CTCL is mycosis fungoides (MF), which is typically indolent but may evolve into the leukemic variant, Sézary syndrome (SS) or large-cell lymphoma. Traditionally, a number of skin-directed and systemic therapies are applied to control CTCL progression. Patients with progressed or refractory CTCL generally have a poor prognosis. Subsequent relapses are common in treated MF patients, and few available treatments have impacted overall survival (OS) for CTCL patients, resulting in a serious unmet therapeutic need.Distinct subtypes of CTCL are currently delineated on the basis of clinical or morphological features, and neither underlying pathogenesis of cutaneous lymphomas nor the distinguishing molecular abnormalities have been identified. However, CTCL cells have been shown to have deregulation of apoptotic responses. 2,3 Thus, treatments that can overcome CTCL resistance to apoptosis may provide new therapeutic strategies. To this effect, several novel targeted agents have emerged, which show promise in treating CTCL.Deace...

Constitutive activation of STAT3 in Sézary syndrome is independent of SHP-1

“…Our data supports previous studies demonstrating constitutive expression of STAT3 in CTCL cell lines, [10][11][12][13][14][15][16][17][18][19] and confirm findings in a recent report that showed constitutive and sustained ex vivo expression of pSTAT3 over 24 h in PBMCs from a group of four SS/MF patients. 18 In contrast, an earlier study documented strong expression of pSTAT3 in only 2 out of 14 SS patients.…”

“…40 Our ex vivo findings of the induction of apoptosis following inhibition of STAT3 activation in Sézary cells, supports the hypothesis that pSTAT3 has a pivotal role in mediating the known resistance to apoptosis. These data confirm several in vitro studies using CTCL cell lines, which have induced apoptosis via STAT3 knockdown, 11,12 or STAT3 inhibition with different agents including Curcumin, 18 Cucurbitacin, 17 Panobinostat 15 and Avicin D. 16 Our data suggests that downregulation of the pSTAT3 anti-apoptotic genes BCL-2, Survivin and Bcl-XL contribute to the observed commitment to apoptosis. Furthermore, it is now recognised that unphosphorylated monomeric STATs, including STAT3, regulate gene transcription and have distinct target genes to their phosphorylated isoforms.…”

“…9 Constitutive activation of STAT3 has been demonstrated in studies of cell lines derived from MF and SS patients and anaplastic large cell lymphoma. [10][11][12][13][14][15][16][17][18][19] In lesional patient-derived material, strong expression of pSTAT3 has been identified by immunohistochemistry in tumour biopsy samples from advanced stage MF patients, but only weak expression was detected in early patch/plaque lesions. 11 In SS patients, the activation status of STAT3 is less clear, with one study demonstrating the strong expression of pSTAT3 in peripheral blood mononuclear cells (PBMCs) from only 2 of 14 patients.…”

Cutaneous T‐cell lymphoma: 2011 update on diagnosis, risk‐stratification, and management

“…The 18 human HDACs may be classified as either zinc-or NAD 1 -dependent and further subclassified into class I (HDAC1, 2, 3, and 8), class II (HDAC4, 5, 6, 7, 9, and 10), class III (including NAD 1 -dependent sirtuins), and class IV (HDAC11) HDACs. As HDACs regulate a wide variety of processes involved in carcinogenesis, multiple mechanisms may explain the clinical activity of HDAC inhibitors [249,250], including altered gene expression of cell-cycle and apoptotic regulatory proteins [251][252][253][254][255], acetylation of nonhistone proteins regulating cell growth and survival [256][257][258][259], angiogenesis [260,261], aggresome formation [262], and DNA repair [263]. In addition, HDAC inhibitors may have important effects on the tumor microenvironment via reactive oxygen species [264,265], enhanced antigen presentation [266] and downregulation of immunomodulatory cytokines, like IL-10 [267].…”

“…Further studies are needed to fully define the mechanisms of resistance to HDAC inhibition in CTCL [255,[279][280][281][282], enabling the development of rational therapeutic combinations incorporating HDAC inhibitors in CTCL [283,284].…”