Role of Mild Hypoxia- and UVB-Induced Signalling Pathways in the Development of Cutaneous

Mild hypoxia- and UVB-activated signalling cascades in the development of cutaneous malignant melanoma

*Kris Nys K (1),Joost Van den Oord (2), Carine Michiels (3), Léon Van Kempen (4), Marjan Garmyn (5) and Patrizia Agostinis (1)

1) Dept. Molecular & Cell Biology, Campus Gasthuisberg, K.U.Leuven,Leuven,Belgium

2)Dept. Pathology, K.U.Leuven,Leuven, Belgium

3) Laboratory of Biochemistry and Cellular Biology, University of Namur,Namur,Belgium

4) Dept. Pathology, UniversityHospitals, University of Nijmegen,The Netherlands

5) Laboratory of Dermatology, K.U.Leuven,Leuven, Belgium

Cutaneous malignant melanoma represent less than 10% of all dermatological cancers but are responsible for the overwhelming majority of skin cancer-related deaths. Recent studies suggest that the mild hypoxia (1-5 %O2) of the human skin represents a tumourpromoting environmental factor in melanomagenesis. In combination with other environmental stresses, such as UVB exposure, and mutations causing defects in cell death and/or survival pathways the hypoxic state of the skin could aid melanoma cell survival by increasing adaptation to cellular stress, induce a more aggressive phenotype andalso affect the notorious chemoresistance of melanoma.Since the molecular elements of thecellular response to mild hypoxia and UVB arenot completely understood, we set out to investigate thesignalling pathways underlying the response of human melanocytes and melanoma cells to these relevant cellular stresses. Because of their crucial involvement in hypoxia and UVB signalling, and their role in melanomagenesis, we focused on the role and cross-talk between Hif-1 (hypoxia inducible factor-1) and MAPKs(mitogen activated protein kinases).

Exposure of melanocytes and melanomacell lines to mild hypoxia or UVB results in the upregulation and nuclear translocation of Hif-1α, albeit with different intensity and kinetics. Hif-1α upregulation by both stress factors involves a redox-sensitive mechanism, but only for mild hypoxia are the mitochondria-generated ROS crucially important. The transient and biphasic Hif-1α upregulation following UVB is correlated with the rapid and sustained activation of the p38MAPK and JNK-pathways and with a transient activation of ERK-and Akt- pathways. The selective inhibition or knockdown of p38MAPKαsignificantly attenuates Hif-1α accumulation in normal and cancer cells, indicating that p38MAPKα is a key molecular element of the cascade linking UVB irradiation to Hif-1α stabilization.Moreover, Hif-1α modulation following UVB is associated with the induction of Noxa, concomitant downregulation of Bip/GRP78, and cell death, which occurs through both caspase-dependent and –independent pathways. Conversely, enhanced Hif-1α upregulationin melanoma cells and melanocytes following mild hypoxia does not involve p38MAPKα or JNK-signals, and correlates with Akt activation,sustained Bip/GRP78 levels and cell survival. Moreover, mild hypoxia stimulates autophagy, which is possibly coordinated through Hif-1-dependentBNIP3 upregulation. These results indicate that mild hypoxia and UVB modulate the activity of different redox pathways impinging on Hif-1upregulation, and which can eventually lead to cell survival (e.g. mild hypoxia) or cell death (e.g. UVB). We are currentlycharacterizing the functional role of Hif-1-dependent pathwaysin the response of normal melanocytes and melanoma cells to these skin microenvironmental stress factors, in order to further define their role in melanoma development.

1. Bedogni et al., 2005, Cancer Cell 8:443-54