Differential Sensitivity of Wild-Type and BRAF-Mutated Cells to Combined BRAF and Autophagy Inhibition

Emerging dimensions of autophagy in melanoma

Macroautophagy/autophagy has previously been regarded as simply a way for cells to deal with nutrient emergency. But explosive work in the last 15 years has given increasingly new knowledge to our understanding of this process. Many of the functions of autophagy that are unveiled from recent studies, however, cannot be reconciled with this conventional view of cell survival but, instead, point to autophagy being integrally involved at a deeper level of cell biology, playing a critical role in maintaining homeostasis and promoting an integrated stress/immune response. The new appreciation of the role of autophagy in the evolutionary trajectory of cancer and cancer interaction with the immune system provides a mechanistic framework for understanding the clinical benefits of autophagy-based therapies. Here, we examine current knowledge of the mechanisms and functions of autophagy in highly plastic and aggressive melanoma as a model disease of human malignancy, while highlighting emerging dimensions indicating that autophagy is at play beyond its classical face.Abbreviation: AMBRA1: autophagy and beclin 1 regulator 1; AMPK: AMP-activated protein kinase; ATF4: activating transcription factor 4; ATG: autophagy related; BRAF: B-Raf proto-oncogene, serine/threonine kinase; CAFs: cancer-associated fibroblasts; CCL5: C-C motif chemokine ligand 5; CQ: chloroquine; CRISPR: clustered regularly interspaced short palindromic repeats; CTLA4: cytotoxic T-lymphocyte associated protein 4; CTL: cytotoxic T lymphocyte; DAMPs: danger/damage-associated molecular patterns; EGFR: epidermal growth factor receptor; EIF2A/eIF2α: eukaryotic translation initiation factor 2A; EIF2AK3/PERK: eukaryotic translation initiation factor 2 alpha kinase 3; ER: endoplasmic reticulum; FITM2: fat storage inducing transmembrane protein 2; HCQ: hydroxychloroquine; ICB: immune checkpoint blockade; ICD: immunogenic cell death; LDH: lactate dehydrogenase; MAPK: mitogen-activated protein kinase; MTORC1: mechanistic target of rapamycin kinase complex 1; NDP52: nuclear dot protein 52; NFKB/NF-κ B: nuclear factor kappa B; NBR1: the neighbor of BRCA1; NK: natural killer; NRF1: nuclear respiratory factor 1; NSCLC: non-small-cell lung cancer; OPTN: optineurin; PDAC: pancreatic ductal adenocarcinoma; PDCD1/PD-1: programmed cell death 1; PPT1: palmitoyl-protein thioesterase 1; PTEN: phosphatase and tensin homolog; PTK2/FAK1: protein tyrosine kinase 2; RAS: rat sarcoma; SQSTM1/p62: sequestosome 1; STK11/LKB1: serine/threonine kinase 11; TAX1BP1: Tax1 binding protein 1; TFEB: transcription factor EB; TGFB/TGF-β: transforming growth factor beta; TMB: tumor mutational burden; TME: tumor microenvironment; TSC1: TSC complex subunit 1; TSC2: TSC complex subunit 2; ULK1: unc-51 like autophagy activating kinase 1; UVRAG: UV radiation resistance associated.

Atg5 knockout induces alternative autophagy via the downregulation of Akt expression

Autophagy play contradictory roles in cellular transformation. We previously found that the knockout (KO) of autophagy-related 5 (Atg5), which is essential for autophagy, leads to the malignant transformation of NIH 3T3 cells. In this study, we explored the mechanism by which autophagy contributes to this malignant transformation using two transformed cell lines, Atg5 KO and Ras-NIH 3T3. Monomeric red fluorescent protein-green fluorescent protein-light chain 3 reporter and Cyto-ID staining revealed that Ras-NIH 3T3 cells exhibited higher basal autophagy activity than NIH 3T3 cells. Additionally, transformed cells, regardless of their Atg5 KO status, were more sensitive to autophagy inhibitors (SBI-0206965, chloroquine, and obatoclax) than the untransformed NIH 3T3 cells, suggesting that the transformed cells are more autophagy-dependent than the normal cells. Loss of Atg5 improved the cell viability and mobility, especially in Ras-NIH 3T3 cells. Furthermore, we discovered that autophagy was alternatively induced in a Rab9-dependent manner in Ras-NIH 3T3 and NIH 3T3/Atg5 KO cells. In particular, Atg5 KO cells showed reduced mTOR-mediated phosphorylation of Akt (pAkt S473), indicating the mTOR-independent occurrence of alternative autophagy in Atg5 KO cells. Therefore, our study provides evidence that alternative autophagy may contribute to tumorigenesis in cells with an impaired Atg5-dependent autophagy pathway.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43188-023-00191-3.

Autophagy Paradox: Strategizing Treatment Modality in Melanoma

OPINION STATEMENT

The primordial autophagy process, originally identified as a starvation response in baker's yeast, has since been shown to have a wide spectrum of functions other than survival. In many cases, it is accepted that autophagy operates as a key tumor suppressor mechanism that protects cells from adverse environmental cues by enforcing homeostasis and maintaining the functional and structural integrity of organelles. Paradoxically, heightened states of autophagy are also seen in some cancers, leading to the prevailing view that the pro-survival aspect of autophagy might be hijacked by some tumors to promote their fitness and pathogenesis. Notably, recent studies have revealed a broad range of cell-autonomous autophagy in reshaping tumor microenvironment and maintaining lineage integrity and immune homeostasis, calling for a renewed understanding of autophagy beyond its classical roles in cell survival. Here, we evaluate the increasing body of literature that argues the "double-edged" consequences of autophagy manipulation in cancer therapy, with a particular focus on highly plastic and mutagenic melanoma. We also discuss the caveats that must be considered when evaluating whether autophagy blockade is the effector mechanism of some anti-cancer therapy particularly associated with lysosomotropic agents. If autophagy proteins are to be properly exploited as targets for anticancer drugs, their diverse and complex roles should also be considered.

Increase in the sensitivity to PLX4720 through inhibition of transcription factor EB-dependent autophagy in BRAF inhibitor-resistant cells

Long-term treatment with oncogenic BRAF inhibitors confers resistance to BRAF inhibitor monotherapy. In this study, a combination treatment strategy with autophagy inhibitors was proposed to increase the sensitivity of BRAF mutant containing A375P melanoma cells that have developed resistance to BRAF inhibitors. We found that the A375P/Multi-drug resistance (A375P/Mdr) cells, which are resistant to both BRAF inhibitors and MEK inhibitors, exhibited a higher basal autophagic flux compared to their parental A375P cells, as determined by tandem mRFP-GFP-tagged LC3 imaging assay and LC3 conversion. In addition, transcription factor EB (TFEB), which acts as a transcription factor regulating the transcription of autophagy-related genes, was much more localized in the nucleus in A375P/Mdr cells than in A375P cells, indicating that the increase in basal autophagic flux was TFEB-dependent. In particular, the overexpression of an activated form of TFEB (TFEBAA) caused a modest increase in PLX4720 resistance in A375P/Mdr cells. Interestingly, treatment with early stage autophagy inhibitors reversed BRAF inhibitor-induced resistance, whereas late autophagy inhibition did not. In contrast, inhibition of ER stress by 4-phenylbutyric acid suppressed basal autophagic flux. Moreover, ER stress inhibition significantly remarkably inhibited the nuclear localization of TFEB, resulting in an increase in the sensitivity of A375P/Mdr cells to PLX4720. Taken together, these results suggest that autophagy may be an important mechanism of acquired resistance to BRAF inhibitors. Thus, targeting autophagy may be suitable for the treatment of tumors resistant to BRAF inhibitor.