The Bax/Bcl-2 ratio determines the susceptibility of human melanoma cells to CD95/Fas-mediated apoptosis

Defective cytochrome c release and the resulting loss of caspase-3 activation was recently shown to be essential for the susceptibility of human melanoma cells to CD95/Fas-induced apoptosis. Cytochrome c release from mitochondria is regulated by the relative amounts of apoptosis-promoting and apoptosis-inhibiting Bcl-2 proteins in the outer membrane of these organelles. The assignment of Bax/Bcl-2 ratios by quantitative Western blotting in 11 melanoma cell populations revealed a relation to the susceptibility to CD95-mediated apoptosis. We could show that a low Bax/Bcl-2 ratio was characteristic for resistant cells and a high Bax/Bcl-2 ratio was characteristic for sensitive cells. Low Bax expression was not a consequence of mutations in the p53 coding sequence. The Bax/Bcl-2 ratio was also in clear correlation with sensitivity to another cell death inducer, N-acetylsphingosine. Furthermore, Bcl-2 overexpression abolished apoptosis triggered by both apoptotic stimuli, confirming the critical role of the Bax/Bcl-2 ratio as a rheostat that determines the susceptibility to apoptosis in melanoma cells by regulating mitochondrial function. Interestingly, some chemotherapeutics lead to the activation of death pathways by CD95L upregulation, ceramide generation, direct activation of upstream caspases, or upregulation of proapoptotic genes. Taken together, these signals enter the apoptotic pathway upstream of mitochondria, resulting in activation of this central checkpoint. We therefore assumed that apoptosis deficiency of malignant melanoma can be circumvented by drugs directly influencing mitochondrial functions. For this purpose we used betulinic acid, a cytotoxic agent selective for melanoma, straightly perturbing mitochondrial functions. In fact, betulinic acid induced mitochondrial cytochrome c release and DNA fragmentation in both CD95-resistant and CD95-sensitive melanoma cell populations, independent of the Bax/Bcl-2 ratio.     

Point mutations in the N-ras oncogene in malignant melanoma and congenital naevi

DNA from formalin-fixed and paraffin-processed samples from 100 melanocytic lesions (39 malignant melanomas, 18 cases of dysplastic naevi, and 43 congenital naevi) was extracted, and the sequences around codons 12/13 and 61 of the N-ras oncogene were amplified using the polymerase chain reaction. The amplified product was then analysed both by dot-blotting and by direct sequencing for point mutations. By the dot-blotting technique, mutations were seen in 18 of 100 lesions. These were in one of five distant metastases (20%), in one of three nodal metastases (33%), in four of 31 (13%) primary melanomas, in none of 18 dysplastic naevi, and in 12 of 43 (28%) congenital naevi, all at codon 61. On direct sequencing, nine of 18 mutations were confirmed, in two of 31 (6%) primary tumours, one distant metastasis, and six of 43 (14%) congenital naevi. Of the 23 superficial spreading melanomas examined, eight were on sun-exposed skin. A superficial spreading melanoma, in which the N-ras mutation at codon 61 was confirmed, was on non-exposed skin, and an unconfirmed mutation was from an exposed site. One of three nodular melanomas with a confirmed mutation was on a light-exposed site, and the other two nodular melanomas were from non-exposed areas. All four lentigo maligna melanomas were from exposed sites, and one of these had an unconfirmed mutation. The only acral lentiginous melanoma, which had no mutation, was from a sun-exposed area. Ten of the 43 congenital or early onset naevi were on sun-exposed sites; six unconfirmed and five confirmed mutations were from lesions on non-sun-exposed sites (5/33; 15%), and one confirmed mutation was from a lesion on a sun-exposed area (1/10; 10%). Our results do not, therefore, support the hypothesis that N-ras mutations are specifically associated with primary melanomas arising on continually sun-exposed skin. The overall pattern suggests that N-ras mutations in isolation in melanocytic lesions are not frequent. This is the first report of N-ras mutations in congenital/early onset melanocytic naevi, and shows a relatively high incidence in comparison with that observed in melanomas in this study.     

Expression of activated N-ras in a primary melanoma cell line counteracts growth inhibition by transforming growth factor-beta

One critical factor in melanoma progression is the change from radial growth phase to vertical growth phase. We previously showed a high incidence of ras mutations in progressing but not early human melanomas. We also found that stable expression of activated Ras in a primary human melanoma cell line (WM35) led to enhanced proliferation, anchorage-independent survival, migration and invasion in vitro and enhanced subcutaneous tumor formation in vivo, transforming the melanoma phenotype from the radial growth phase to the vertical growth phase. Inhibitory cytokines, especially transforming growth factor-beta, are important in homeostasis of normal human melanocytes. Proliferation of early melanoma cells can be inhibited by transforming growth factor-beta, whereas more aggressive stages lose this response. Using a transforming growth factor-beta activated luciferase reporter transiently transfected into WM35, WM35N-ras, and WM35H-ras (WM35 transfected with mutant N-ras or H-ras genes), we demonstrated significant decreases (p < 0. 04) in transforming growth factor-beta induced reporter expression in both ras transfected cell lines. Transforming growth factor-beta also induced significant decreases (p < 0.002) in the proportion of WM35 cells in S-phase of the cell cycle; this effect was not observed in WM35N-ras cells. Furthermore, we demonstrated that an important controlling factor in transforming growth factor-beta inhibition of cell cycle progression, the phosphorylation of the Rb protein, was altered in WM35N-ras; transforming growth factor-beta caused a marked relative increase in hypophosphorylated pRb in WM35 cells, but not in WM35N-ras. These data suggest that activated Ras plays an important part in melanoma progression from the radial growth phase to the vertical growth phase by counteracting inhibition by cytokines such as transforming growth factor-beta, thus providing a growth advantage.     

Regulation of human melanoma growth and metastasis by AGE-AGE receptor interactions

Advanced glycation end products (AGE), nonenzymatically glycated protein derivatives, have been implicated in the development and progression of diabetic angiopathies, including skin dermopathy. Nevertheless, the involvement of AGE in the development and progression of melanoma has not been fully elucidated. In this study we investigated the expression levels of their receptor for AGE (RAGE) in human melanoma and subsequently studied the effects of AGE on melanoma growth and migration. First, RAGE was detected in the cytoplasm of human melanoma cells (G361 and A375). Among the different types of AGE, glyceraldehyde- and glycolaldehyde-derived AGE significantly stimulated the growth and migration of human melanoma cells. Furthermore, tumor formation of melanoma cell xenografts in athymic mice was prevented by treatment with anti-RAGE neutralizing antibodies. In tumor-bearing mice, survival rates were prolonged, and spontaneous pulmonary metastases were inhibited by treatment using anti-RAGE neutralizing antibodies. In addition, all AGE were present in beds of human melanoma tumor, whereas they were barely detected in normal skin. These results suggest that AGE might be involved in the growth and invasion of melanoma through interactions with RAGE and represent promising candidates for assessing the future therapeutic potential of this therapy in treating patients with melanoma.     

白塞病患者外周血T细胞凋亡和Fas表达的相关性

目的：探讨Fas（CD95）在T淋巴细胞表达及其介导的凋亡与白塞病（BD）的关系。方法：通过Annexin V（AV）荧光探针,使用5色流式细胞仪检测了Fas介导的凋亡及其在BD患者CD4^＋和CD8^＋T细胞的表达。结果：与非活动期患者相比,BD活动期患者外周血AV^＋CD4^＋CD3^＋细胞和CD95^＋AV^＋CD4^＋CD3^＋细胞的百分数明显减少。结论：T细胞抵抗Fas介导的凋亡与BD病情加重有关,Fas介导的T细胞凋亡与BD病情减轻有关。     

Selective induction of apoptosis in melanoma cells by tyrosinase promoter-controlled CD95 ligand overexpression

Induction of apoptosis has been demonstrated previously by overexpression of CD95 ligand (CD95L) in cultured human melanoma cells. For in vivo approaches based on CD95L, however, targeted expression is a prerequisite and tyrosinase promoters have been considered for selection. Luciferase reporter gene assays performed for a representative panel of melanoma cell lines characterized by strong (SK-Mel-19), moderate (SK-Mel-13, MeWo), weak (A-375), and missing expression (M-5) of endogenous tyrosinase revealed high tyrosinase promoter activities in SK-Mel-19, SK-Mel-13, and MeWo, but only weak activities in A-375 and M-5 as well as in non-melanoma cell lines. After transfection of a CMV promoter CD95L expression construct, melanoma cells were found highly sensitive, as compared with non-melanoma cells. By applying a tyrosinase promoter CD95L construct, apoptosis was selectively induced in SK-Mel-19, SK-Mel-13, MeWo as well as in A-375, which was characterized by high CD95 surface expression and high sensitivity to agonistic CD95 activation. M5 and non-melanoma cell lines remained uninfluenced. Also, resistance to agonistic CD95 activation seen in MeWo characterized by weak CD95 surface expression was overcome by overexpression of CD95L. Our investigations provide evidence that tyrosinase promoter CD95L constructs may be of value for selective induction of apoptosis in therapeutic strategies for melanoma.     

Death receptor-independent apoptosis in malignant melanoma induced by the small-molecule immune response modifier imiquimod

Bypassing molecular mechanisms of apoptosis deficiency may be of great utility for the successful treatment of malignant tumors. We have discovered that imiquimod, a small-molecule immunomodulator, exerts rather tumor-selective direct pro-apoptotic activity in vivo and in vitro towards cutaneous metastases of malignant melanoma, an aggressive skin tumor. This pro-apoptotic activity was not detectable with resiquimod, a closely related structural analogue whose pro-inflammatory activity is even greater than that of imiquimod. Unresponsiveness of some melanoma metastases to imiquimod in vivo corresponded to resistance towards imiquimod-induced apoptosis in vivo and in vitro. At the molecular level, the pro-apoptotic activity of imiquimod was independent of membrane-bound death receptors, but depended on Bcl-2 expression as demonstrated by overexpression of Bcl-2 in melanoma cells. Imiquimod is the first topical compound with the potential to bypass molecular mechanisms of apoptosis deficiency, a concept that may be relevant for other tumors as well.     

金黄色葡萄球菌肠毒素A诱导黑素瘤B16细胞凋亡的体外研究

目的 探讨超抗原金黄色葡萄球菌肠毒素A(SEA)诱导黑素瘤B16细胞凋亡.方法 用不同浓度的SEA(0.001、0.01、0.1、1 μg/mL)作用于体外培养的B16细胞,采用荧光法观察SEA诱导B16细胞凋亡的形态学改变,用末端标记法(TUNEL)计算细胞凋亡率,用流式细胞仪分析B16细胞的凋亡周期.结果 三种方法均显示SEA对小鼠B16细胞有明显的诱导凋亡作用,SEA在0.01 μg/mL浓度时B16细胞的凋亡率最高.结论 超搞原SEA对体外培养的小鼠B16细胞有诱导凋亡的作用.     

Induction of apoptosis in melanoma cell lines by p53 and its related proteins.

Melanoma cells rarely contain mutant p53 and hardly undergo apoptosis by wild-type p53. By using recombinant adenoviruses that express p53 or p53-related p51A or p73beta, we tested their apoptotic activities in melanoma cells. Yeast functional assay revealed a mutation of p53 at the 258th codon (AAA [K] instead of GAA [E]) in one cell line, 70W, out of six human melanoma cell lines analyzed (SK-mel-23, SK-mel-24, SK-mel-118, TXM18, 70W, and G361). Adenovirus-mediated transfer of p53, p51A, and/or p73beta suppressed growth and induced apoptotic DNA fragmentation of SK-mel-23, SK-mel-118, and 70W cells. Interestingly, p51A induced DNA fragmentation in them more significantly than p53 and p73beta. By Western blotting we analyzed levels of apoptosis-related proteins in cells expressing p53 family members. Apoptotic Bax and antiapoptotic Bcl-2 were not significantly upregulated or downregulated by expression of p53, p51A, or p73beta, except for p53-expressing 70W cells, which contained a larger amount of Bax protein than LacZ-expressing cells. Activation of caspase-3 was demonstrated only in p51A-expressing SK-mel-118 cells. We show here that p51A can mediate apoptosis in both wild-type and mutant p53-expressing melanoma cells more significantly than p53 and p73beta. It is also suggested that in melanoma cells (i) cellular target protein(s) other than Bcl-2 and Bax might be responsible for induction of p51A-mediated apoptosis and (ii) caspase-3 is not always involved in the apoptosis by p53 family members.     

Regulation of pigmentation by substrate elasticity in normal human melanocytes and melanotic MNT1 human melanoma cells

The elasticity of the cellular microenvironment is a key regulator of cellular physiology in many cell types. To investigate the effects of substrate stiffness on the pigmentation process, we cultured normal human melanocytes (NHM) and MNT1 melanoma cells on laminin‐coated polydimethylsiloxane (PDMS) substrates of different stiffness. The dendricity of NHM and MNT1 cells was reduced as the substrate stiffness decreased, and the degree of melanosome transfer from NHM or MNT1 cells to normal human keratinocytes was decreased on softer substrates with the reduced dendricity. Gene and protein expressions of MITF, tyrosinase, TRP2, and gp100/PMEL17 exhibited a consistent decreasing trend with the decreasing stiffness. Because the stiffness sensing is mediated by focal adhesion complex through integrin receptors, we checked laminin specific integrin alpha 6 and p‐FAK for MNT1 cells to observe that the substrate adhesion was weakened as the substrate stiffness decreased. Weaker adhesion on a softer substrate was accompanied by dynamic shape changes in MNT1 cells with higher speed and larger scattering. Dendritic MNT1 cells cultured on a stiffer substrate exhibited lower migration with smaller root mean squared displacement. These results demonstrate the possibility that skin pigmentation can be influenced by mechanical properties of the cellular microenvironment and can increase when the skin becomes stiff.     

ERK1/2 is highly phosphorylated in melanoma metastases and protects melanoma cells from cisplatin-mediated apoptosis

Activation (phosphorylation) of mitogen-activated protein kinase (MAPK) signal transduction through BRAF and RAS causes a variety of functional effects including cell survival and cell death. In this study, we observed high extracellular signal-regulated kinase (ERK)1/2 phosphorylation levels in clinical melanoma metastases and various melanoma cell lines. Treatment of melanoma cell lines with cisplatin, a potent antitumor agent, increased the level of phosphorylated-ERK (P-ERK)1/2 and enhanced chemoresistance through activation of the cell survival protein 90-kDa ribosomal S6 kinase (RSK)1. The mitogen-activated protein kinase kinase (MEK) inhibitor (U0126) was able to block this effect and reduced cell viability and sensitized cells to cisplatin-induced apoptosis, as shown by PARP cleavage, caspase 3 expression, and annexin-V staining. In conclusion, the MAP kinase-ERK pathway is activated in melanoma and reduces the sensitivity of melanoma to cisplatin. Thus, inhibition of ERK1/2 in combination with selected chemotherapeutic agents may hold promise for more effective therapy of melanoma.     

p53-dependent apoptosis in melanoma cells after treatment with camptothecin

Cutaneous malignant melanoma is a life-threatening cancer with poor prognosis due to a high metastasis potential. The main obstacle in treatment of metastatic melanoma is the resistance to chemotherapy. Recent studies indicated that apoptosis is a common mechanism of action for various cytotoxic agents. As p53 plays an important part in apoptosis, we investigated the role of p53 in chemosensitivity of melanoma cells. Previously, we found that melanoma cell lines containing wild-type p53 have significantly higher response rates to chemotherapy than cell lines with a mutant p53 gene. To confirm the role of p53 in melanoma chemosensitivity further, we transfected an expression vector, pED1, which carries a mutant p53 gene, into a wild-type p53 melanoma cell line, MMAN. We examined the effect of mutant p53 on camptothecin-induced apoptosis and the expression of genes which are known to be involved in apoptosis or drug resistance, such as bcl-2, bax, bak, p21waf1, and P-glycoprotein. Our results indicate that overexpression of the mutant p53 increased the growth rate of MMAN cells, reduced the sensitivity to camptothecin, and lowered drug-induced apoptosis by 2-3-fold. Flow cytometry indicated that the camptothecin-induced apoptosis is not associated with G1 arrest. Furthermore, camptothecin treatment reduced bcl-2 and P-glycoprotein expression in wild-type p53 MMAN cells, but not cells overexpressing mutant p53. These results demonstrate that p53 mutational status is a determinant of melanoma chemosensitivity. p53 may downregulate bcl-2 and P-glycoprotein to induce apoptosis in melanoma cells after chemotherapy.     

HINT1对人黑素瘤A375细胞裸鼠皮下异种移植瘤模型肿瘤生长及凋亡的影响

目的 探讨组氨酸三聚体核苷结合蛋白1（HINT1）对人黑素瘤A375细胞裸鼠皮下异种移植瘤模型肿瘤生长及凋亡的影响。方法 三组裸鼠随机皮下接种HINT1-A375、neo-A375及未转染的A375细胞,观察各组移植瘤的生长情况,计算成瘤率。组织病理切片观察移植瘤组织形态学改变,并应用TUNEL法检测移植瘤组织中细胞凋亡情况。结果 三组裸鼠皮下异种移植瘤成瘤率均为100%。HINT1组移植瘤生长速度慢,接种后18 d移植瘤生长速度显著低于neo组及A375细胞组（P < 0.01）。HINT1组、neo组及A375细胞组肿瘤重量分别为（0.04 ± 0.00） g、（0.23 ± 0.00） g、（0.29 ± 0.03） g;肿瘤体积分别为（0.06 ± 0.04） cm3、（0.34 ± 0.15） cm3、（0.43 ± 0.19） cm3。HINT1组肿瘤重量及体积均显著低于neo组及A375细胞组（P值均 < 0.01）。组织病理结果显示,与neo组及A375细胞组相比,HINT1组肿瘤团块较小,细胞异形小,病理性核分裂象少见,瘤团内未见大片坏死灶。HINT1组中平均凋亡细胞百分比为12.87% ± 1.18%,显著高于neo组（3.22% ± 0.49%）及A375细胞组（3.00% ± 0.53%）（P值均 < 0.01）。结论 高表达HINT1可显著抑制A375细胞裸鼠皮下异种移植瘤模型肿瘤的生长,促进其凋亡,提示HINT1基因可能是人黑素瘤的抑癌基因之一。     

甲氧沙林脂质体对小鼠黑素瘤细胞黑素生成影响的研究

目的:探讨甲氧沙林脂质体(8-MOPL)对小鼠B16F黑素瘤细胞增殖、黑素含量、酪氨酸酶活性的影响。方法:采用体外培养的小鼠B16F黑素瘤细胞株,以等浓度的甲氧沙林(8-MOP)为对照,用四甲基偶氮唑蓝(MTT)法测定细胞增殖情况;NaOH裂解法测定黑素合成;多巴氧化法测定酪氨酸酶活性。结果:与等浓度8-MOP相比,低浓度8-MOPL(10-25μmol/L)能显著提高酪氨酸酶活性和黑素含量(P<0.05)。高浓度8-MOPL(100-200μmol/L)对细胞的抑制作用更显著(P<0.05)。结论:用脂质体作为8-MOP的载体可以显著提高8-MOP对靶细胞的作用,为8-MOPL制剂治疗白癜风的临床应用提供了实验依据。     

Hyperthermia induces endoplasmic reticulum-mediated apoptosis in melanoma and non-melanoma skin cancer cells

Hyperthermia has been revived as a promising approach for cancer treatment. To understand the underlying mechanisms of hyperthermic killing of cancer cells, we examined the cytotoxic effects of hyperthermia on various skin cancer cell lines using cell viability, morphological analyses, and caspase activation assays. Hyperthermia induced cytotoxicity in a time- and temperature-dependent manner. At middle dose/time combinations, heat-induced apoptosis, whereas at higher doses, necrosis was the mechanism of cell death. To investigate the mechanisms of hyperthermia-induced apoptosis, we examined the activation of extrinsic (Caspase 8) and intrinsic (Caspase 9) apoptotic pathways. Hyperthermia did not activate Caspases 8 or 9, but did activate Caspase 3/7, suggesting a non-conventional apoptotic pathway. Last, analysis of Grp78 expression and Caspase 12 or 4 activation indicated that hyperthermia induced endoplasmic reticulum-mediated apoptosis. Thus, hyperthermia induced apoptosis in two types of skin cancer cells through endoplasmic reticulum-mediated apoptosis and not through the classical intrinsic or extrinsic apoptosis pathways. Hyperthermia may be a promising treatment for basal cell carcinoma and melanoma, bypassing the antiapoptotic defenses concentrated in the intrinsic and extrinsic apoptosis pathways. These results also raise the possibility that heat may be combined with other approaches for induction of apoptosis to achieve synergistic killing of skin cancers.     

微小RNA-let-7a对黑素瘤细胞系A375细胞凋亡的影响

目的 研究微小RNA（microRNA）-let-7a对黑素瘤细胞系A375细胞凋亡的影响及其作用机制。方法 采用实时荧光定量PCR法（TaqMan探针法）检测microRNA-let-7a在A375细胞及黑素细胞的表达差异;然后用microRNA-let-7a的模拟物转染A375细胞,流式细胞仪检测其凋亡的改变;最后采用Western印迹检测转染后半胱天冬酶-3（caspase-3）蛋白的变化。结果 与黑素细胞相比,microRNA-let-7a在A375细胞中表达下降了0.462倍;采用microRNA-let-7a的模拟物转染后,与阴性对照（细胞凋亡率16.9%）相比,A375细胞凋亡（细胞凋亡率47.4%）增加;转染后caspase-3蛋白表达下调。结论 microRNA-let-7a可促进黑素瘤细胞系A375的凋亡,同时下调caspase-3蛋白的表达。