PTEN Loss Confers BRAF Inhibitor Resistance to Melanoma Cells through the Suppression of BIM Expression

Abstract

This study addresses the role of PTEN loss in intrinsic resistance to the BRAF inhibitor PLX4720. Immunohistochemical staining of a tissue array covering all stages of melanocytic neoplasia (n ¼ 192) revealed PTEN expression to be lost in >10% of all melanoma cases. Although PTEN expression status did not predict for sensitivity to the growth inhibitory effects of PLX4720, it was predictive for apoptosis, with only limited cell death observed in melanomas lacking PTEN expression (PTENÀ). Mechanistically, PLX4720 was found to stimulate AKT signaling in the PTENÀ but not the PTENþ cell lines. Liquid chromatography multiple reaction monitoring mass spectrometry (LC-MRM) was performed to identify differences in apoptosis signaling between the two cell line groups. PLX4720 treatment significantly increased BIM expression in the PTENþ (>14-fold) compared with the PTENÀ cell lines (four-fold). A role for PTEN in the regulation of PLX4720-mediated BIM expression was confirmed by siRNA knockdown of PTEN and through reintroduction of PTEN into cells that were PTENÀ. Further studies showed that siRNA knockdown of BIM significantly blunted the apoptotic response in PTENþ melanoma cells. Dual treatment of PTENÀ cells with PLX4720 and a PI3K inhibitor enhanced BIM expression at both the mRNA and protein level and increased the level of apoptosis through a mechanism involving AKT3 and the activation of FOXO3a. In conclusion, we have shown for the first time that loss of PTEN contributes to intrinsic BRAF inhibitor resistance via the suppression of BIM-mediated apoptosis. Cancer Res; 71(7); 2750-60. Ó2011 AACR.

Targeted agents and immunotherapies: optimizing outcomes in melanoma

“…• Clinical therapeutics for advanced-stage melanoma have improved dramatically with the development of BRAF and MEK inhibitors, cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) and programmed cell-death protein 1 (PD-1) blocking antibodies, and a modified oncolytic herpes virus that is delivered intratumourally • The overall survival of patients with advanced-stage melanoma has improved from ~9 months before 2011 to an as yet undefined timeframe, with a subset of patients having ongoing long-term tumour control • Melanoma, particularly cutaneous melanoma, is amendable to immunotherapy for various reasons, including extensive tumour infiltration by T cells, a high mutational load, and crosstalk between oncogenic signalling pathways and immunobiology • Resistance mechanisms to BRAF-targeted treatments and immunotherapies are being elucidated; reactivation of the MAPK pathway is common after BRAF inhibition, whereas the effectiveness of both approaches might be limited by loss of tumour antigen presentation and T-cell trafficking • To move the field of clinical therapeutics forward, a greater focus on specific patient populations (based on serum lactose dehydrogenase levels, ECOG performance status, and number of metastases), as well as on landmark progression-free and overall survival measures, will be required in clinical trials occur with or without BRAF, NRAS, or NF1 alterations, but are known to cooperate in driving resistance to BRAF inhibitors in BRAF-mutant melanoma cell lines 19 and biopsy samples of resistant tumours 20,21 .…”

Role of the pro-survival molecule Bfl-1 in melanoma

“…Additionally, loss of PTEN activity in BRaf mutant cells can lead to lower levels of proapoptotic Bim through hyperactivated Akt, which can result in resistance to BRaf inhibitors 84 .…”

The CDK4/6 Inhibitor LY2835219 Overcomes Vemurafenib Resistance Resulting from MAPK Reactivation and Cyclin D1 Upregulation

“…The most common resistant mechanism is MAPK pathway reactivation, which is caused by genetic mutation of MEK or different Ras isoforms (7)(8)(9), upstream activation of receptor tyrosine kinases (RTK) such as FGFR3 and c-Met (10,11), expression of B-RAF V600E splice variants that dimerize in presence of the B-RAF inhibitor (12), amplification of B-RAF (13,14), and upregulation of MAP3Ks such as COT or C-RAF (15,16). Alternatively, activation of MAPKredundant pathways such as PI3K/Akt as a consequence of PTEN loss (17) or overexpression of RTKs such as PDGFRb and IGF1R have also been reported to induce resistance in B-RAF V600E melanoma (7,18,19). In addition, secretion of growth factors such as HGF or FGF has also been implicated in resistance to B-RAF inhibition (10,11,20,21).…”