Molecular responses to hypoxia in tumor cells

Abstract

Highly aggressive, rapidly growing tumors are exposed to hypoxia or even anoxia which occurs as a consequence of inadequate blood supply. Both hypoxia and consecutive hypoxia/reoxygenation exert a variety of influences on tumor cell biology. Among these are activation of certain signal transduction pathways and gene regulatory mechanisms, induction of selection processes for gene mutations, tumor cell apoptosis and tumor angiogenesis. Most of these mechanisms contribute to tumor progression. Therefore, tissue hypoxia has been regarded as a central factor for tumor aggressiveness and metastasis. In this review, we summarize the current knowledge about the molecular mechanisms induced by tumor cell hypoxia with a special emphasis on intracellular signal transduction, gene regulation, angiogenesis and apoptosis. Interfering with these pathways might open perspectives for future innovative treatment of highly aggressive metastasizing tumors.

Figure 1

MAP kinase signalling pathways. Major pathways that transfer extracellular signals to the nucleus are the MAP kinase signalling pathways. The extracellular stimuli may be heterogeneous, deriving from exposure of cells to growth factors, phorbol esters, cytokines, or cellular stresses, such as osmotic shock and γ-irradiation. In principal, the Ras-Raf-MEK-ERK pathway transduces mitogenic signals involved in cellular proliferation or differentiation. The JNK/SAPK and p38 pathways regulate the cellular inflammatory or stress response. There are interactions between both pathways on MAPK kinase kinase (MAPKKK) levels immediately upstream of MEK (not indicated in the presented scheme). The downstream targets of the MAP kinase signalling pathways are the MAP kinases, ERK, JNK/SAPK and p38, which directly or indirectly interfere with transcription factors, such as Elk-1, ATF2 or cJun for activation of gene transcription. Upstream signalling components include the family of Rho GTPases such as Rho, Rac and Cdc42 which interfere with MAPKKK. Cellular stresses such as hypoxia may activate JNK/SAPK and p38 pathways which exert influence on cJun and ATF-2 activation.

Figure 2

Hypoxic activation of transcription factors HIF-1α, AP-1 and NF-κB. Cells exposed to hypoxia activate the cellular transcription factors HIF-1α, AP-1 and NF-κB. Under normoxic conditions the von Hippel Lindau tumor suppressor protein mediates ubiquitination and degradation of HIF-1α. This mechanism is inhibited under hypoxia. As a consequence, the HIF-1α protein is stabilized and shows enhanced expression. There are a series of factors that may interfere with HIF-1α under hypoxia, such as cJun/AP-1, heat shock protein (hsp) 90, and the transcriptional co-activator CBP/p300. Further mechanisms of HIF regulation include phosphorylation by extracellular signal regulated kinase (ERK), and phosphorylation of its interaction partner, cJun/AP-1 via stress activated protein kinase, JNK/SAPK. Recently, a factor inhibiting HIF-1α activation, FIH, has been described, representing a further level of HIF regulation. Upon reoxygenation, NF-κB, a well-known transcription factor involved in transcriptional regulation of immune response genes, is activated. However, evidence has been provided that NF-κB activation may also be induced by hypoxia.

Figure 3

Hypoxic activation of apoptosis pathways. Hypoxia activates intracellular signalling pathways involved in apoptosis and cell survival. One pathway of particular importance is hypoxia-induced mitochondrial membrane permability, leading to subsequent release of cytochrome C into the cytoplasm. Cytochrome C initiates the apoptosis cascade via activation of the apoptosis kinase Apaf-1, which in turn activates the caspase 9 apoptosis pathway. Hypoxia also activates JNK/SAPK signalling pathways which leads to apoptosis induction by an as yet unknown mechanism. The protein kinase Akt plays a central role in cell survival via induction of anti-apoptotic mechanisms, involving the anti-apoptotic function of Bcl-xL. Akt is also involved in hypoxia-induced HIF-dependent VEGF expression, a signalling cascade that can be inhibited by the tumor suppressor PTEN. PTEN exerts its negative regulatory effects via inhibition of phosphatidylinositol (3,4) and phosphatidylinositol (3,4,5) phosphorylation. Late-stage tumors often display mutated PTEN or show a complete loss of PTEN expression, which leads to a de-repression of the survival phosphatidylinositol (PtdIns) 3-kinase-Akt signalling pathway. PDK, PtdIns (3,4,5)P₃-dependent kinase.