Transcriptional regulation of autophagy-lysosomal function in BRAF-driven melanoma progression and chemoresistance

Abstract

Autophagy maintains homeostasis and is induced upon stress. Yet, its mechanistic interaction with oncogenic signaling remains elusive. Here, we show that in BRAF V600E -melanoma, autophagy is induced by BRAF inhibitor (BRAFi), as part of a transcriptional program coordinating lysosome biogenesis/function, mediated by the TFEB transcription factor. TFEB is phosphorylated and thus inactivated by BRAF V600E via its downstream ERK independently of mTORC1. BRAFi disrupts TFEB phosphorylation, allowing its nuclear translocation, which is synergized by increased phosphorylation/inactivation of the ZKSCAN3 transcriptional repressor by JNK2/p38-MAPK. Blockade of BRAFi-induced transcriptional activation of autophagy-lysosomal function in melanoma xenografts causes enhanced tumor progression, EMT-transdifferentiation, metastatic dissemination, and chemoresistance, which is associated with elevated TGF-β levels and enhanced TGF-β signaling. Inhibition of TGF-β signaling restores tumor differentiation and drug responsiveness in melanoma cells. Thus, the “BRAF-TFEB-autophagy-lysosome” axis represents an intrinsic regulatory pathway in BRAF-mutant melanoma, coupling BRAF signaling with TGF-β signaling to drive tumor progression and chemoresistance.

Human ZKSCAN3 and Drosophila M1BP are functionally homologous transcription factors in autophagy regulation

“…Finally, as ZKSCAN3 dysregulation contributes to numerous human cancers (Chi et al, 2018; Kawahara et al, 2016; Kim et al, 2016; Lee et al, 2018; Li et al, 2019; Yang et al, 2008; Yang et al, 2011) and that metabolic reprogramming is a hallmark of the tumour microenvironment (reviewed in (Reina-Campos et al, 2017)), the knowledge that Drosophila M1BP and vertebrate ZKSCAN3 are functionally homologous proteins controlling autophagy and metabolic gene expression will allow use of the powerful Drosophila model system to study ZKSCAN3 function in metabolism, autophagy control and cancerogenesis.…”

Non-Apoptotic Cell Death Signaling Pathways in Melanoma

“…Available data suggest, however, that not all melanoma patients may benefit from inhibition of autophagy. Mechanistic study on induction of autophagy upon inhibition of BRAF V600E has shown that BRAFi disrupted ERK-mediated phosphorylation of transcription factor EB (TFEB), allowing its nuclear translocation and activation of a cellular program that controls lysosome biogenesis [119]. This was, surprisingly, followed by tumor suppression and decreased metastasis in a mouse model of melanoma [119].…”

The Complex Role of Autophagy in Melanoma Evolution: New Perspectives From Mouse Models

“…However, the mechanistic studies unraveling the real cause-effect relation between autophagy loss and melanoma initiation are still incomplete and in need of further investigation. Interestingly, Li and collaborators have recently shown that autophagy is restrained in melanoma cells at a transcriptional level in a BRAF V600E -dependent fashion in a syngeneic mouse model of melanoma (26,27). Briefly, they have found that the master regulator of the expression of autophagy/lysosomal genes, TFEB, is under the control of the BRAF-oncogenic pathway, via its ERK-mediated phosphorylation and inactivation.…”

“…This has been associated with TGFβ-signaling activation and induction of a metastatic phenotype that can be rescued by autophagy activation through the block of TFEB ERK-mediated phosphorylation and activation. This phenomenon was reverted (back to the metastatic phenotype) upon BRAF inhibition (26). Of note, BRAF V600E activity has also been shown to inhibit AMPK by compromising its interaction with LKB1, thereby promoting melanoma cells proliferation (28).…”

“…Moreover, it is noteworthy that, although the master regulators of melanocyte development MiTF, TFEB, and TFE3 belong to the same MiT/TFE transcription factor family (30), their activity correlates with the expression of diverse lysosomal and autophagy genes (31). Indeed, while it is known that MAPK/ERK signaling is involved in MiTF turnover and activation, with a crucial contribution of BRAFmediated transcriptional and post-transcriptional regulation (32,33), the recently discovered ERK-mediated signaling pathway that controls TFEB in BRAF V600E -driven melanoma does not work in the same way for either MiTF or TFE3 (26). To further complicate this scenario, the MiT/TFE transcription factor family has been also reported to finely regulate the lysosomal activity of mTORC1, the master regulator of cell growth as well as autophagy inhibitor (10), through direct regulation of RagD GTPase expression (34).…”