Báo cáo khoa học: Energy metabolism in tumor cells

Energy Metabolism in Tumor Cells

Tác giả: Rafael Moreno-Sánchez, Sara Rodríguez-Enríquez, Alvaro Marín-Hernández and Emma Saavedra

Lĩnh vực: Biochemistry, Biochemistry/Metabolism

Nội dung tài liệu: Bài viết này đánh giá các cơ chế di truyền và sinh hóa mà qua đó tế bào khối u đạt được tốc độ đường phân tăng cường. Đồng thời, bài viết xem xét các cơ chế được đề xuất dẫn đến giảm phosphoryl hóa oxy hóa trong tế bào khối u. Đặc biệt, bài viết tái đánh giá con đường này liên quan đến việc sử dụng cơ chất oxy hóa và đóng góp của nó vào việc cung cấp ATP so với đường phân. Tác giả cũng mô tả và thảo luận về ảnh hưởng của các loại thuốc tác động lên quá trình chuyển hóa năng lượng của khối u và sự tăng sinh tế bào. Cuối cùng, bài viết đề xuất rằng quá trình chuyển hóa năng lượng có thể là một mục tiêu điều trị thay thế cho cả khối u thiếu oxy (đường phân) và khối u oxy hóa.

Mục lục chi tiết:

• Introduction
• Keywords
• Correspondence
• (Received 31 October 2006, revised 2 January 2007, accepted 10 January 2007)
• doi:10.1111/j.1742-4658.2007.05686.x
• In early studies on energy metabolism of tumor cells, it was proposed that the enhanced glycolysis was induced by a decreased oxidative phosphorylation.
• Since then it has been indiscriminately applied to all types of tumor cells that the ATP supply is mainly or only provided by glycolysis, without an appropriate experimental evaluation.
• In this review, the different genetic and biochemical mechanisms by which tumor cells achieve an enhanced glycolytic flux are analyzed.
• Furthermore, the proposed mechanisms that arguably lead to a decreased oxidative phosphorylation in tumor cells are discussed.
• As the O2 concentration in hypoxic regions of tumors seems not to be limiting for the functioning of oxidative phosphorylation, this pathway is re-evaluated regarding oxidizable substrate utilization and its contribution to ATP supply versus glycolysis.
• In the tumor cell lines where the oxidative metabolism prevails over the glycolytic metabolism for ATP supply, the flux control distribution of both pathways is described.
• The effect of glycolytic and mitochondrial drugs on tumor energy metabolism and cellular proliferation is described and discussed.
• Similarly, the energy metabolic changes associated with inherent and acquired resistance to radiotherapy and chemotherapy of tumor cells, and those determined by positron emission tomography, are revised.
• It is proposed that energy metabolism may be an alternative therapeutic target for both hypoxic (glycolytic) and oxidative tumors.
• In biochemical and physiological studies, tumor cells are usually classified according to their rate of growth: low; intermediate; or fast [1].
• For tumors in experimental animals, the growth rate is determined by size and volume, mitotic count, degree of differentiation and thymidine incorporation [2].
• Examples of fast-growth tumors in mice include several experimental cancers, such as Ehrlich ascites tumor, fibrosarcoma 1929 and lymphocytic leukemia L1210; and in rats, fast-growth tumors include the hepatomas of Morris (3924A, 7793, 7795, 7800, 7288C, 7316B, 3683), Reuber H-35, Novikoff, AH130 and AS-30D, breast carcinosarcoma Walker 256, hepatocellular carcinoma HC-252, hepatoma induced by dimethylazobenzene and DS-carcinosarcoma [1].
• In human tumors, classification is based on their histological characteristics and stage of clinical progression.
• By their advanced developmental stage and metastatic properties, some human tumors considered to be of fast growth are breast carcinoma, ovarian carcinoma, melanoma, thyroid carcinoma, uterine carcinoma and lung carcinoma [1,3].
• Human primary brain tumors, such as gliomas, glioblastomas and meduloblastomas, are also considered as fast-growth tumors because of their high rate of proliferation (average transfer in days or weeks) and their conversion to a poorly differentiated status [4,5].
• Tumor cells exhibit profound genetic, biochemical and histological differences with respect to the original, nontransformed cellular types.
• The vast majority of fast-growth tumor cell types display a markedly modified energy metabolism in comparison to the tissue of origin (Figs 1 and 2), which has been widely documented for human cervix (HeLa), pharynx and mammary gland (MCF-7, MDA-MB-453) tumors, as well as for astoblastomas, gliomas (U-251MG, D-54MG, U-87 and U118MG) and oligodendrogliomas.
• The same applies for tumors experimentally developed in rodents (hepatomas of Ehrlich, Ehrlich-Lettré, Morris and AS-30D; Walker 256 carcinoma; C6 glioma) [1,5-9].
• The most notorious and well-known energy metabolism alteration in tumor cells is an increased glycolytic capacity, even in the presence of a high O2 concentration [1,6-11].
• For instance, the glycolytic flux is 2-17 times higher in rat hepatomas than in normal hepatocytes [3,11].
• It has been proposed that this increase in the glycolytic flux is a metabolic strategy of tumor cells to ensure survival and growth in environments with low O2 concentrations [10].
• Several mechanisms for the enhanced glycolysis in tumor cells have been advanced and documented (Table 1).
• It has to be emphasized that there is no reason to apply the mechanisms, described below, to all cancer cells automatically; each particular tumor cell line has its own combination of mechanisms and degree of expression for increasing glycolysis.
• Glycolytic enzymes and transporters in tumor cells
• Transcriptional regulation of the glycolytic genes
• A great body of evidence suggests that the main mechanism by which glycolysis is substantially higher in tumor cells than in nontumorigenic cells is the enhanced transcription of genes of several or all pathway enzymes and transporters, which is accompanied by an enhanced protein synthesis [12-15]; activity has, however, rarely been determined.
• For instance, in comparison to normal rat hepatocytes (Fig. 1), all glycolytic enzymes are over-expressed by two- to fourfold in rat AS-30D hepatoma [hexose-6-phosphate isomerase, aldolase (ALD), triose-phosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoglycerate kinase, phosphoglycerate mutase, enolase and lactate dehydrogenase (LDH)], pyruvate kinase is over-expressed by eight- to 10-fold, and hexokinase (HK) and phosphofructokinase type 1 (PFK-1) are over-expressed by up to 17- to 300-fold (Fig. 1) [11,16].
• For human cervix HeLa cells, all enzymes, including HK and PFK-1, are over-expressed by two- to sevenfold, with the exception of phosphoglycerate mutase and LDH, which are expressed at a level two- to seven-fold lower than in rat hepatocytes [11].
• However, for this last case a more rigorous comparison should be made with normal uterine cervix epithelial cells (i.e. the original source) when data become available.
• In Morris hepatomas, the activity of HK, PFK and pyruvate kinase is 5- to 500-fold higher than in liver [17], whereas the activity of HK, ALD, pyruvate kinase and LDH is 3.7- to 7-times higher in human breast cancer than in normal tissue [18].
• Perhaps the prime driving mechanism for the enhanced glycolysis is activation, via the hypoxia inducible factor 1 (HIF-1), of the transcription and translation of glycolytic genes in tumor cells.
• HIF-1 is a transcription factor constituted by two subunits, HIF-1α and HIF-1β.
• Factor stability mostly depends on HIF-1α.
• Under aerobiosis, an active process of HIF-1α degradation is promoted, whereas in anaerobiosis, HIF-1α becomes highly stable [19,20].
• In addition to hypoxia, HIF-1α may be induced, under aerobiosis, by cytokines, growth factors, reactive oxygen species and nitric oxide; or by the energy-metabolism intermediates pyruvate (Pyr), lactate and oxaloacetate [20-22].
• The von Hippel-Lindau protein, a tumor suppressor, binds to HIF-1α and induces its degradation by the proteasome; in some aggressive tumors, the von Hippel-Lindau protein is mutated, thus becoming ineffective in promoting HIF-1α degradation.
• This might be the reason why HIF-1α is only detected in malignant tumors, but not in normal, healthy tissues or benign tumors [20,23].
• In turn, HIF-1 enhancement promotes the expression of HK, PFK-1, phosphofructokinase type 2 (PFK-2), ALD, GAPDH, phosphoglycerate kinase, enolase, pyruvate kinase and LDH [24,25], which leads to a stimulation of the glycolytic flux.
• Notwithstanding the O2 level, metastatic tumor cell lines (breast MDA, U87 glioblastoma, DU145 prostate, renal RCC4 and CaSKi) show high levels of HIF-1α, over-expression of glycolytic enzymes and high glycolysis, whereas non-metastatic tumor cells (breast MCF-7, HT-29 colon, MiaPaCa pancreatic, A549 lung, BX-PC3 prostate) increase HIF-1α, enzyme over-expression and glycoly- sis only under hypoxia [23].
• HIF-1α also favors the glycolytic flux by increasing the expression of pyruvate dehydrogenase complex (PDH) kinase 1, which inhibits, by phosphorylation, the PDH complex activity, thus decreasing Pyr oxida- tion in the Krebs cycle and increasing the generation of lactate from Pyr [26].
• Further association of HIF-1α with the expression of other mitochondrial proteins has yet to be found.
• Table 1. Mechanisms explaining the accelerated glycolytic rate in fast-growing tumor cells.
• GLUT, glucose transporter; HK, hexokinase; PFK-1, phosphofructokinase type 1; PFK-2, phosphofructokinase type 2.
• Tumor cell type
• Rodent
• Human
• 1. Increase in the isoform expression of the glycolytic enzymes and glucose transporters
• GLUT
• AS-30D, Novikoff, Ehrlich, and Morris 3924A hepatomas; ependymoblastoma; thyroid and Lewis lung carcinomas [34]
• HepG2 carcinomas; brain tumors (A-172, H4) [34]; breast cancer (MCF-7 and T47D); leukemias (Jurkat, HL60, U937,U1); pancreatic, lung, renal (HEK-293), cutaneous, gastric and esophageal tumors [35]
• HK
• AS-30D hepatoma [11,16]; Morris 7800,5123-D, 7288-C, 3924-A; H19 cells [31]; 3683 and Novikoff hepatomas [44]
• HeLa carcinoma [11], ependymoma, astrocytoma, glioma [45]
• PFK-1
• AS-30D hepatoma [11,16]; mouse ascites carcinoma [33]; thyroid carcinoma [51]; Morris (7800,5123-D,7288-C, 3924-A, 3683); Ehrlich Lettré [53]
• HL-60, KG-1, K-562 myeloid leukemia, MOLT-4 leukemia, lymphoma [32], HeLa and KB carcinoma [32], glioma [45]
• PFK-2
• Ehrlich hepatoma [25]
• HeLa, HepG2 [55,57], Hek-293, Lewis lung carcinoma, K562 leukemia, MCF-7 breast carcinoma, TD47 cells [15]
• All enzymes
• AS-30D hepatoma [11]
• HeLa [11] and CaSKi carcinoma, U87 glioblastoma, DU145 prostate tumor, renal RCC4 tumor [24]
• 2. Decreased expression of mitochondrial oxidative enzymes and transporters
• Ehrlich [59,60], Morris (16, 44, 777, 3924A, 7794A, 7800) [1,61,66,72], Novikoff, Yoshida, Reuber H-35, and BW7756 hepatomas [1,69,75]; L1210 leukemia; leukemic B82T tumor; SV40-transformed fibroblast [1]
• HeLa carcinoma; mammary tumors (Cf7, C3H) [1]; meningioma; ependymoma; pituitary adenoma [74]; human kidney carcinoma [77]
• 3. Lowering in the amount of mitochondria per cell
• C-57, HC-252 carcinomas [1], mammary adenocarcinoma [73]
• 4. Inhibition of oxidative phosphorylation by glycolysis activation (Crabtree effect)
• Ehrlich-Lettré [80], AS-30D [64] hepatomas; EL-4 thymoma [83]; sarcoma 180 [81]; tumor pancreatic islet cells; insulinoma RINm5F [82]
• HeLa [84], HT29 [85]
• 5. Increased amount in the natural inhibitor protein (IF1) of the mitochondrial ATP synthase
• Zadjela and Yoshida sarcomas [90], AS-30D hepatoma [91]
• 6. Higher sensitivity of mitochondrial DNA to oxidative stress
• Breast, colon, stomach, liver, kidney, bladder, head/neck and lung tumors; leukemia; lymphoma [93]
• The oncogene, c-myc, encodes the transcription factor, c-Myc, which in transformed cells may also activate glycolytic genes, such as those for glucose transporter 1 (GLUT1), hexose-6-phosphate isomerase, PFK-1, GAPDH, phosphoglycerate kinase, enolase and LDH, thus increasing glycolysis under aerobiosis [13,14].
• Isoform expression and activity
• HK and PFK-1 are among the main controlling steps of the glycolytic flux in erythrocytes, hepatocytes, and cardiac and skeletal muscle cells [27-30].
• Changes in the isoform pattern of HK and PFK-1 expression occur in several tumor cells in comparison to normal cells (Fig. 1) [1,2,31-33].
• As described below, it seems that such modifications in these and other glycolytic steps are also part of the mechanisms involved in the increased glycolytic flux of tumor cells.
• Glucose transporter
• It is well documented that GLUT levels of mRNA and protein are higher in tumor cells than in normal, healthy tissues [34-36].
• This increase in the protein levels of GLUT might be part of the mechanisms promoting the increased glycolysis in tumor cells as long as the GLUT activity also increases and significant control of the pathway resides in this step (discussed in more detail in the section entitled ‘Metabolic control analysis of glycolysis and oxidative phosphorylation in intact tumor cells’).
• There are several isoforms of GLUT expressed in mammalian cells.
• The GLUT1 isoform is present in all tissues; GLUT2 is abundant in liver, pancreas, intestine and kidney; GLUT3 prevails in brain; GLUT4 is present in skeletal muscle, heart, brain and adipose tissue; GLUT5 is present in small intestine, testis, skeletal muscle, adipose tissue and kidney; GLUT6 is present in spleen, leukocytes and brain; GLUT7 is the less-well known member of the family and the sites of expression are unknown; GLUT8 is present in testis and brain; GLUT9 is present in liver and kidney; GLUT10 is present in liver and pancreas; GLUT11 is present in heart and skeletal muscle; and GLUT12 is present in heart, small intestine and prostate [34].
• In several tumor cells, the predominant over-expressed isoform is GLUT1 (Table 1) [34,35].
• However, other isoforms, which are not usually found in the tissue of origin, may also be over-expressed.
• For instance, in some human leukemias (U937, HL60 y U1), GLUT5, which is an isoform not found in normal leukocytes, is over-expressed [34].
• GLUT3 is detected in lung, ovarian and gastric cancers, but not in the corresponding normal tissues [35].
• In most studies on GLUT expression in tumor cells, an enhanced mRNA or protein content has certainly been detected, but unfortunately these results have not been accompanied by an effort to determine whether indeed an increased GLUT activity is also achieved, perhaps because it is not an easy assay.
• Nevertheless, some kinetic parameters of GLUT in tumor cells have been reported [37,38].
• However, these last experiments were not carried out with glucose, but with glucose analogues (some of which are indeed nonmetabolizable, although 2-deoxyglucose can be phosphorylated by HK and dehydrogenated by glucose-6-phosphate dehydro