Deciphering the Role of Oncogenic MITFE318K in Senescence Delay and Melanoma Progression.

Background: MITF encodes an oncogenic lineage-specific transcription factor in which a germline mutation ( MITFE318K ) was identified in human patients predisposed to both nevus formation and, among other tumor types, melanoma. The molecular mechanisms underlying the oncogenic activity of MITF E318K remained uncharacterized. Methods: Here, we compared the SUMOylation status of endogenous MITF by proximity ligation assay in melanocytes isolated from wild-type (n = 3) or E318K (n = 4) MITF donors. We also used a newly generated Mitf E318K knock-in (KI) mouse model to assess the role of Mitf E318K (n = 7 to 13 mice per group) in tumor development in vivo and performed transcriptomic analysis of the tumors to identify the molecular mechanisms. Finally, using immortalized or normal melanocytes (wild-type or E318K MITF, n = 2 per group), we assessed the role of MITF E318K on the induction of senescence mediated by BRAF V600E . All statistical tests were two-sided. Results: We demonstrated a decrease in endogenous MITF SUMOylation in melanocytes from MITF E318K patients (mean of cells with hypoSUMOylated MITF, MITF E318K vs MITF WT , 94% vs 44%, difference = 50%, 95% CI = 21.8% to 67.2%, P = .004). The Mitf E318K mice were slightly hypopigmented (mean melanin content Mitf WT vs Mitf E318K/+ , 0.54 arbitrary units [AU] vs 0.36 AU, difference = -0.18, 95% CI = -0.36 to -0.007, P = .04). We provided genetic evidence that Mitf E318K enhances BRaf V600E -induced nevus formation in vivo (mean nevus number for Mitf E318K , BRaf V600E vs Mitf WT , BRaf V600E , 68 vs 44, difference = 24, 95% CI = 9.1 to 38.9, P = .006). Importantly, although Mitf E318K was not sufficient to cooperate with BRaf V600E alone in promoting metastatic melanoma, it accelerated tumor formation on a BRaf V600E , Pten-deficient background (median survival, Mitf E318K/+ = 42 days, 95% CI = 31 to 46 vs Mitf WT = 51 days, 95% CI = 50 to 55, P < .001). Transcriptome analysis suggested a decrease in senescence in tumors from Mitf E318K mice. We confirmed this hypothesis by in vitro experiments, demonstrating that Mitf E318K impaired the ability of human melanocytes to undergo BRAF V600E -induced senescence. Conclusions: We characterized the functions of melanoma-associated MITF E318K mutations. Our results demonstrate that MITF E318K reduces the program of senescence to potentially favor melanoma progression in vivo.

FANCD2 functions as a critical factor downstream of MiTF to maintain the proliferation and survival of melanoma cells.

Proteins involved in genetic stability maintenance and safeguarding DNA replication act not only against cancer initiation but could also play a major role in sustaining cancer progression. Here, we report that the FANC pathway is highly expressed in metastatic melanoma harboring the oncogenic microphthalmia-associated transcription factor (MiTF). We show that MiTF downregulation in melanoma cells lowers the expression of several FANC genes and proteins. Moreover, we observe that, similarly to the consequence of MiTF downregulation, FANC pathway silencing alters proliferation, migration and senescence of human melanoma cells. We demonstrate that the FANC pathway acts downstream MiTF and establish the existence of an epistatic relationship between MiTF and the FANC pathway. Our findings point to a central role of the FANC pathway in cellular and chromosomal resistance to both DNA damage and targeted therapies in melanoma cells. Thus, the FANC pathway is a promising new therapeutic target in melanoma treatment.

SIRT1 promotes proliferation and inhibits the senescence-like phenotype in human melanoma cells.

SIRT1 operates as both a tumor suppressor and oncogenic factor depending on the cell context. Whether SIRT1 plays a role in melanoma biology remained poorly elucidated. Here, we demonstrate that SIRT1 is a critical regulator of melanoma cell proliferation. SIRT1 suppression by genetic or pharmacological approaches induces cell cycle arrest and a senescence-like phenotype. Gain and loss of function experiments show that M-MITF regulates SIRT1 expression, thereby revealing a melanocyte-specific control of SIRT1. SIRT1 over-expression relieves the senescence-like phenotype and the proliferation arrest caused by MITF suppression, demonstrating that SIRT1 is an effector of MITF-induced proliferation in melanoma cells. Interestingly, SIRT1 level and activity are enhanced in the PLX4032-resistant BRAF(V600E)-mutated melanoma cells compared with their sensitive counterpart. SIRT1 inhibition decreases melanoma cell growth and rescues the sensibility to PLX4032 of PLX4032-resistant BRAF(V600E)-mutated melanoma cells. In conclusion, we provide the first evidence that inhibition of SIRT1 warrants consideration as an anti-melanoma therapeutic option.

Secretome from senescent melanoma engages the STAT3 pathway to favor reprogramming of naive melanoma towards a tumor-initiating cell phenotype.

Here, we showed that the secretome of senescent melanoma cells drives basal melanoma cells towards a mesenchymal phenotype, with characteristic of stems illustrated by increased level of the prototype genes FN1, SNAIL, OCT4 and NANOG. This molecular reprogramming leads to an increase in the low-MITF and slow-growing cell population endowed with melanoma-initiating cell features. The secretome of senescent melanoma cells induces a panel of 52 genes, involved in cell movement and cell/cell interaction, among which AXL and ALDH1A3 have been implicated in melanoma development. We found that the secretome of senescent melanoma cells activates the STAT3 pathway and STAT3 inhibition prevents secretome effects, including the acquisition of tumorigenic properties. Collectively, the findings provide insights into how the secretome of melanoma cells entering senescence upon chemotherapy treatments increases the tumorigenicity of naïve melanoma cells by inducing, through STAT3 activation, a melanoma-initiating cell phenotype that could favor chemotherapy resistance and relapse.

Aurora B is regulated by the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling pathway and is a valuable potential target in melanoma cells.

Metastatic melanoma is a deadly skin cancer and is resistant to almost all existing treatment. Vemurafenib, which targets the BRAFV600E mutation, is one of the drugs that improves patient outcome, but the patients next develop secondary resistance and a return to cancer. Thus, new therapeutic strategies are needed to treat melanomas and to increase the duration of v-Raf murine sarcoma viral oncogene homolog B1 (BRAF) inhibitor response. The ERK pathway controls cell proliferation, and Aurora B plays a pivotal role in cell division. Here, we confirm that Aurora B is highly expressed in metastatic melanoma cells and that Aurora B inhibition triggers both senescence-like phenotypes and cell death in melanoma cells. Furthermore, we show that the BRAF/ERK axis controls Aurora B expression at the transcriptional level, likely through the transcription factor FOXM1. Our results provide insight into the mechanism of Aurora B regulation and the first molecular basis of Aurora B regulation in melanoma cells. The inhibition of Aurora B expression that we observed in vemurafenib-sensitive melanoma cells was rescued in cells resistant to this drug. Consistently, these latter cells remain sensitive to the effect of the Aurora B inhibitor. Noteworthy, wild-type BRAF melanoma cells are also sensitive to Aurora B inhibition. Collectively, our findings, showing that Aurora B is a potential target in melanoma cells, particularly in those vemurafenib-resistant, may open new avenues to improve the treatment of metastatic melanoma.

Senescent cells develop a PARP-1 and nuclear factor-{kappa}B-associated secretome (PNAS).

Melanoma cells can enter the process of senescence, but whether they express a secretory phenotype, as reported for other cells, is undetermined. This is of paramount importance, because this secretome can alter the tumor microenvironment and the response to chemotherapeutic drugs. More generally, the molecular events involved in formation of the senescent-associated secretome have yet to be determined. We reveal here that melanoma cells experiencing senescence in response to diverse stimuli, including anti-melanoma drugs, produce an inflammatory secretory profile, where the chemokine ligand-2 (CCL2) acts as a critical effector. Thus, we reveal how senescence induction might be involved in therapeutic failure in melanoma. We further provide a molecular relationship between senescence induction and secretome formation by revealing that the poly(ADP-ribose) polymerase-1 (PARP-1)/nuclear factor-κB (NF-κB) signaling cascade, activated during senescence, drives the formation of a secretome endowed with protumoral and prometastatic properties. Our findings also point to the existence of the PARP-1 and NF-κB-associated secretome, termed the PNAS, in nonmelanoma cells. Most importantly, inhibition of PARP-1 or NF-κB prevents the proinvasive properties of the secretome. Collectively, identification of the PARP-1/NF-κB axis in secretome formation opens new avenues for therapeutic intervention against cancers.

Microphthalmia-associated transcription factor controls the DNA damage response and a lineage-specific senescence program in melanomas.

Apoptosis and senescence are cellular failsafe programs that counteract excessive mitogenic signaling observed in cancer cells. Melanoma is known for its notorious resistance to apoptotic processes; therefore, senescence, which remains poorly understood in melanomas, can be viewed as a therapeutic alternative. Microphthalmia-associated transcription factor (MITF), in which its M transcript is specifically expressed in melanocyte cells, plays a critical role in melanoma proliferation, and its specific inhibition is associated with G(0)-G(1) growth arrest. Interestingly, decreased MITF expression has been described in senescent melanocytes, and we have observed an inhibition of MITF expression in melanoma cells exposed to chemotherapeutic drugs that induce their senescence. All these observations thereby question the role of MITF in controlling senescence in melanoma cells. Here, we report that long-term depletion of MITF in melanoma cells triggers a senescence program characterized by typical morphologic and biochemical changes associated with a sustained growth arrest. Further, we show that MITF-silenced cells engage a DNA damage response (DDR) signaling pathway, leading to p53 upregulation, which is critically required for senescence entry. This study uncovers the existence of a lineage-restricted DDR/p53 signaling pathway that is inhibited by MITF to prevent senescence and favor melanoma cell proliferation.