Differential sensitivity of melanoma cell lines with differing B-Raf mutational status to the new oncogenic B-Raf kinase inhibitor UI-152

Systemic review on B-RafV600E mutation as potential therapeutic target for the treatment of cancer

Cancer is one of the major public catastrophes worldwide and as per WHO, cancer is the leading cause of death universally after CVS disorders accounting for 9.6 million deaths in 2018. WHO statistics revealed five dangerous types of cancer viz. lung, breast, colorectal, prostate and skin. In male, lung cancer causes highest death, while in female, breast cancer causes the most. Alteration in MAPK signalling pathway plays a significant role in majority of cancer cases. Raf protein is activated by phosphorylation via downstream regulation of the MAPK pathway. Raf composed of 3 subtypes, viz. A-Raf, B-Raf, and C-Raf. B-Raf kinase plays a significant role in healthy cell growth in the MAPK pathway and the problem associated with B-Raf mutation leads to the development of cancer and other diseases. The progression of mutant B-Raf (B-Raf^V600E) protein is higher in cancer as compare to other diseases. In 2002, B-Raf^V600E mutation was identified for the first time in the development of cancer. The frequency of B-Raf^V600E mutation is higher in melanoma, thyroid, colorectal and ovarian cancer. We have covered small molecule B-Raf^V600E inhibitors reported in various literatures; from 2002 to 2020 and also covered clinical trial data. To widen the scope of readers, we compiled details of small molecules, specifically inhibiting B-Raf^V600E mutant and showing anti-proliferative activity against various cancer cell lines along with in-vivo data. We believe that the information covered here will be important in signifying the potentials of B-Raf^V600E mutation and its inhibitors as potent anticancer agents.

Upregulation of S100A9 contributes to the acquired resistance to BRAF inhibitors

BACKGROUNDS

Acquired resistance is a significant clinical challenge in targeted therapy of melanomas using BRAF inhibitors. We previously identified that downregulation of miR-92a-1-5p confers acquired resistance to BRAF inhibition using an miRNA array platform.

OBJECTIVE

In this study, we investigated the target genes of miR-92a-1-5p and their functional significance in BRAF inhibitor resistance.

METHODS

The miRNA target prediction data were combined with RNA-Seq data to identify possible target genes for miR-92a-1-5p. Cellular effects of target genes were further examined using siRNA knockdown, WST-1 assay, and immunoblotting analysis.

RESULTS

We selected S100 calcium-binding protein A9 (S100A9) as a possible target gene for functional validation. S100A9 knockdown abrogated resistance to PLX4720 in A375P/Mdr cells. This result was similar to those described earlier for miR-92a-1-5p, indicating that miR-92a-1-5p inhibits cell viability by targeting S100A9. S100A9 overexpression partially conferred PLX4720 resistance to A375P cells. We also demonstrated that MAPK re-activation does not contribute to the promotion of BRAF inhibitor resistance by S100A9.

CONCLUSION

Taken together, our results indicate that S100A9 might be functionally involved in development of resistance to BRAF inhibitors and might be a target for melanoma therapy in the future.

Differential Gene Expression Common to Acquired and Intrinsic Resistance to BRAF Inhibitor Revealed by RNA-Seq Analysis

Melanoma cells have been shown to respond to BRAF inhibitors; however, intrinsic and acquired resistance limits their clinical application. In this study, we performed RNA-Seq analysis with BRAF inhibitor-sensitive (A375P) and -resistant (A375P/Mdr with acquired resistance and SK-MEL-2 with intrinsic resistance) melanoma cell lines, to reveal the genes and pathways potentially involved in intrinsic and acquired resistance to BRAF inhibitors. A total of 546 differentially expressed genes (DEGs), including 239 up-regulated and 307 down-regulated genes, were identified in both intrinsic and acquired resistant cells. Gene ontology (GO) analysis revealed that the top 10 biological processes associated with these genes included angiogenesis, immune response, cell adhesion, antigen processing and presentation, extracellular matrix organization, osteoblast differentiation, collagen catabolic process, viral entry into host cell, cell migration, and positive regulation of protein kinase B signaling. In addition, using the PANTHER GO classification system, we showed that the highest enriched GOs targeted by the 546 DEGs were responses to cellular processes (ontology: biological process), binding (ontology: molecular function), and cell subcellular localization (ontology: cellular component). Ingenuity pathway analysis (IPA) network analysis showed a network that was common to two BRAF inhibitor-resistant cells. Taken together, the present study may provide a useful platform to further reveal biological processes associated with BRAF inhibitor resistance, and present areas for therapeutic tool development to overcome BRAF inhibitor resistance.

Upregulation of MicroRNA-1246 Is Associated with BRAF Inhibitor Resistance in Melanoma Cells with Mutant BRAF

Purpose

Intrinsic and acquired resistance limit the therapeutic benefits of inhibitors of oncogenic BRAF in melanoma. To identify microRNAs (miRNAs) associated with resistance to a BRAF inhibitor, we compared miRNA expression levels in three cell lines with different BRAF inhibitor sensitivity.

Materials and Methods

miRNA microarray analysis was conducted to compare miRNA expression levels. Real-time quantitative reverse-transcription polymerase chain reaction (qRT-PCR) was performed to confirm the expression of differentially expressed miRNAs. The cellular effects of miR-1246 were further examined by MTT assay, immunoblotting analysis, cell cycle analysis, flow cytometric assay of apoptosis, and autophagy assay.

Results

The miRNA microarray analysis and qRT-PCR identified five miRNAs (miR-3617, miR-92a-1, miR-1246, miR-193b-3p, and miR-17-3p) with expression that was consistently altered in two BRAF inhibitor-resistant cell lines. Among the five miRNAs, a miR-1246 mimic significantly reduced the antiproliferative effects of the BRAF inhibitor PLX4720 in BRAF inhibitor–resistant A375P (A375P/Mdr) cells, suggesting that miR-1246 upregulation confers acquired resistance to BRAF inhibition. In particular, apoptosis was identified as a major type of cell death in miR-1246–transfected cells; however, necrosis predominated in mimic-control-transfected cells, indicating that the resistance to PLX4720 in miR-1246 mimic-transfected cells is predominantly due to a reduction in necrosis. Furthermore, we found that miR-1246 promoted G₂/M arrest through autophagy as a way to escape cell death by necrosis and apoptosis in response to PLX4720. The promotion of BRAF inhibitor resistance by miR-1246 was associated with lowered levels of p-ERK.

Conclusion

These results suggest that miR-1246 may be a potential therapeutic target in melanoma with acquired resistance to BRAF inhibitors.

Induction of Resistance to BRAF Inhibitor Is Associated with the Inability of Spry2 to Inhibit BRAF-V600E Activity in BRAF Mutant Cells

The clinical benefits of oncogenic BRAF inhibitor therapies are limited by the emergence of drug resistance. In this study, we investigated the role of a negative regulator of the MAPK pathway, Spry2, in acquired resistance using BRAF inhibitor-resistant derivatives of the BRAF-V600E melanoma (A375P/Mdr). Real-time RT-PCR analysis indicated that the expression of Spry2 was higher in A375P cells harboring the BRAF V600E mutation compared with wild-type BRAF-bearing cells (SK-MEL-2) that are resistant to BRAF inhibitors. This result suggests the ability of BRAF V600E to evade feedback suppression in cell lines with BRAF V600E mutations despite high Spry2 expression. Most interestingly, Spry2 exhibited strongly reduced expression in A375P/Mdr cells with acquired resistance to BRAF inhibitors. Furthermore, the overexpression of Spry2 partially restored sensitivity to the BRAF inhibitor PLX4720 in two BRAF inhibitor-resistant cells, indicating a positive role for Spry2 in the growth inhibition induced by BRAF inhibitors. On the other hand, long-term treatment with PLX4720 induced pERK reactivation following BRAF inhibition in A375P cells, indicating that negative feedback including Spry2 may be bypassed in BRAF mutant melanoma cells. In addition, the siRNA-mediated knockdown of Raf-1 attenuated the rebound activation of ERK stimulated by PLX4720 in A375P cells, strongly suggesting the positive role of Raf-1 kinase in ERK activation in response to BRAF inhibition. Taken together, these data suggest that RAF signaling may be released from negative feedback inhibition through interacting with Spry2, leading to ERK rebound and, consequently, the induction of acquired resistance to BRAF inhibitors.

Low inducible expression of p21Cip1 confers resistance to paclitaxel in BRAF mutant melanoma cells with acquired resistance to BRAF inhibitor

The therapeutic efficacy of oncogenic BRAF inhibitor is limited by the onset of acquired resistance. In this study, we investigated the potential therapeutic effects of the mitotic inhibitor paclitaxel on three melanoma cell lines with differing sensitivity to the BRAF inhibitor. Of the two BRAF inhibitor-resistant cell lines, A375P/Mdr cells harboring the BRAF V600E mutant were resistant and the wild-type BRAF SK-MEL-2 cells were sensitive to paclitaxel. In particular, paclitaxel caused the growth inhibition of SK-MEL-2 cells to a much greater extent than it caused growth inhibition of A375P cells. Paclitaxel exhibited no significant effect on the phosphorylation of MEK-ERK in any cell lines tested, regardless of both the BRAF mutation and the drug resistance, implying that paclitaxel activity is independent of MEK-ERK inhibition. In A375P cells, paclitaxel treatment resulted in a marked emergence of apoptotic cells after mitotic arrest, concomitant with a remarkable induction of p21(Cip1). However, paclitaxel only moderately increased the levels of p21(Cip1) in A375P/Mdr cells, which exhibited a strong resistance to paclitaxel. The p21(Cip1) overexpression partially conferred paclitaxel sensitivity to A375P/Mdr cells. Interestingly, we found an extremely low background expression level of p21(Cip1) in SK-MEL-2 cells lacking normal p53 function, which caused much greater G2/M arrest than that seen in A375P cells. Taken together, these results suggest that paclitaxel may be an effective anticancer agent through regulating the expression of p21(Cip1) for the treatment of BRAF mutant melanoma cells resistant to BRAF inhibitors.

Textile industry manufacturing by-products induce human melanoma cell proliferation via ERK1/2 activation

OBJECTIVES

Textiles used to make clothing can represent a source, often ignored, of chemicals potentially noxious to both skin and the whole organism. Among the most frequently produced potentially noxious chemical manufacturing by-products are formaldehyde (FA), nickel (Ni) and hexavalent chromium (Cr); they are of potential clinical interest as all are known to be carcinogenic to humans and to be potent skin sensitizers. The aim of this study was to investigate, in vitro, effects of these potentially dangerous compounds on two different melanoma cell lines. In particular, attention was focused on A375P, a poorly metastatic and low invasive cell line and SK-MEL-28, a highly metastatic cell line.

MATERIALS AND METHODS

Effects of these compounds was evaluated on A375P and SK-MEL-28 cells. FA (1-5 × 10(-5)  m), NiSO4 (10(-6) -10(-3)  m), K2 Cr2 O7 (10(-7) -10(-6)  m) effects on cell proliferation were evaluated by cell counting, while ERK pathway involvement was evaluated by Western blot analysis.

RESULTS

Low concentrations of the chemicals, covering a range that corresponds to commonly accepted limits in textile production, induced a significant increase in cell proliferation concomitant with transient activation of phosphorylated ERK expression.

CONCLUSIONS

Data obtained suggest that increasing attention must be focused on these by-products' potentially harmful effects in chemical manufacturing of clothes and accessories, that remain for long periods of time, in contact with human skin.

The siRNA-mediated downregulation of N-Ras sensitizes human melanoma cells to apoptosis induced by selective BRAF inhibitors

The clinical benefit of selective BRAF inhibitor therapies is limited by the emergence of drug resistance. Here, we investigated the molecular basis underlying the acquired resistance to a BRAF inhibitor by comparing the signaling pathways in the parental A375P cells and the resistant subline (A375P/Mdr). We demonstrate that MAPK re-activation does not contribute to the mechanism of resistance to UAI-201 of A375P/Mdr cells. The relative quantitative analysis using the 2(-ΔΔCt) method revealed that the BRAF inhibitor resistance observed in A375P/Mdr cells is not mediated through the overexpression of MDR proteins. In particular, we found that the expression of N-Ras was upregulated in BRAF inhibitor-resistant A375P/Mdr cells compared with A375P cells. In fact, siRNA-mediated N-Ras knockdown partially conferred UAI-201 sensitivity to A375P/Mdr cells, implying that N-Ras upregulation confers acquired resistance to BRAF inhibition. Notably, the flow cytometric analysis of the N-Ras-knockdown A375P/Mdr cells revealed that UAI-201 causes a significant accumulation of cells in the G 0/G 1 phase with a concomitant decrease in the number of cells in the S and G 2/M phases. However, platelet-derived growth factor receptor β (PDGFRβ) knockdown failed to sensitize A375P/Mdr cells to growth suppression by UAI-201, although a remarkable increase in PDGFRβ was observed in the A375P cells after UAI-201 treatment. Taken together, our results suggest that N-Ras is worth targeting to improve the therapeutic outcome of melanomas with acquired resistance to BRAF inhibitors.

Autophagy-Dependent Survival of Mutant B-Raf Melanoma Cells Selected for Resistance to Apoptosis Induced by Inhibitors against Oncogenic B-Raf

Most patients with mutant B-Raf melanomas respond to inhibitors of oncogenic B-Raf but resistance eventually emerges. To better understand the mechanisms that determine the long-term responses of mutant B-Raf melanoma cells to B-Raf inhibitor, we used chronic selection to establish B-Raf (V600E) melanoma clones with acquired resistance to the new oncogenic B-Raf inhibitor UI-152. Whereas the parental A375P cells were highly sensitive to UI-152 (IC₅₀<0.5 μM), the resistant sub-line (A375P/Mdr) displayed strong resistance to UI-152 (IC₅₀>20 μM). Immunofluorescence analysis indicated the absence of an increase in the levels of P-glycoprotein multidrug resistance (MDR) transporter in A375P/Mdr cells, suggesting that resistance was not attributable to P-glycoprotein overexpression. In UI-152-sensitive A375P cells, the anti-proliferative activity of UI-152 appeared to be due to cell-cycle arrest at G₀/G₁ with the induction of apoptosis. However, we found that A375P/Mdr cells were resistant to the apoptosis induced by UI-152. Interestingly, UI-152 preferentially induced autophagy in A375P/Mdr cells but not in A375P cells, as determined by GFP-LC3 puncta/cell counts. Further, autophagy inhibition with 3-methyladenine (3-MA) partially augmented growth inhibition of A375P/Mdr cells by UI-152, which implies that a high level of autophagy may protect UI-152-treated cells from undergoing growth inhibition. Together, our data implicate high rates of autophagy as a key mechanism of acquired resistance to the oncogenic B-Raf inhibitor, in support of clinical studies in which combination therapy with autophagy targeted drugs is being designed to overcome resistance.

RAF inhibition overcomes resistance to TRAIL-induced apoptosis in melanoma cells

Mutated BRAF represents a critical oncogene in melanoma, and selective inhibitors have been approved for melanoma therapy. However, the molecular consequences of RAF inhibition in melanoma cells remained largely elusive. Here, we investigated the effects of the pan-RAF inhibitor L-779,450, which inhibited cell proliferation both in BRAF-mutated and wild-type melanoma cell lines. It furthermore enhanced apoptosis in combination with the death ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and overcame TRAIL resistance in melanoma cells. Enhanced apoptosis coincided with activation of mitochondrial pathways, seen by loss of mitochondrial membrane potential and release of cytochrome c, Smac (second mitochondria-derived activator of caspases), and apoptosis-inducing factor (AIF). Subsequently, caspase-9 and -3 were activated. Apoptosis induction by L-779,450/TRAIL was prevented by Bcl-2 overexpression and was dependent on Bax. Thus, activation of Bax by L-779,450 alone was demonstrated by Bax conformational changes, whereas Bak was not activated. Furthermore, the BH3-only protein Bim was upregulated in response to L-779,450. The significant roles of Smac, Bax, and Bim in this setting were proven by small interfering RNA (siRNA)-mediated knockdown experiments. L-779,450 also resulted in morphological changes indicating autophagy confirmed by the autophagy marker light chain 3-II (LC3-II). The pro-apoptotic effects of L-779,450 may explain the antitumor effects of RAF inhibition and may be considered when evaluating RAF inhibitors for melanoma therapy.

Matrine activates PTEN to induce growth inhibition and apoptosis in V600EBRAF harboring melanoma cells

Here, we report a natural chemical Matrine, which exhibits anti-melanoma potential with its PTEN activation mechanism. Matrine effectively inhibited proliferation of several carcinoma cell lines, including melanoma ^V600EBRAF harboring M21 cells. Flow cytometry analysis showed Matrine induced G₀/G₁ cell cycle arrest in M21 cells dose-dependently. Apoptosis in M21 cells induced by Matrine was identified by Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) analysis and Annexin-V/FITC staining. Molecular mechanistic study suggested that Matrine upregulated both mRNA level and protein expression level of phosphatase and tensin homolog deleted on chromosome ten (PTEN), leading to inhibition of the PI3K/Akt pathway. Downregulation of phosphor-Akt^ser473 by Matrine activated p21 and Bax, which contributed to G₀/G₁ cell cycle and apoptosis. Besides, Matrine enhanced the PI3K/Akt inhibition effects to inhibit the cell proliferation with PI3K inhibitor, LY2940002. In summary, our findings suggest Matrine is a promising antitumor drug candidate with its possible PTEN activation mechanisms for treating cancer diseases, such as melanomas.