Ms. Wiesener et al., Constitutive activation of hypoxia-inducible genes related to overexpression of hypoxia-inducible factor-1 alpha in clear cell renal carcinomas, CANCER RES, 61(13), 2001, pp. 5215-5222
The transcription factor hypoxia-inducible factor (HIF)-1 is an important m
ediator of hypoxic adaptation of tumor cells and controls several genes tha
t have been implicated in tumor growth. Oxygen-dependent degradation of HIF
-1 alpha, the regulatory subunit, requires binding to the von Hippel Lindau
(VHL) protein. Because functional inactivation of the VHL tumor suppressor
gene occurs in up to 70% of clear cell renal carcinomas, we investigated w
hether this results in overexpression of HIF-1 alpha and its target genes,
Immunoblotting revealed increased expression of HIF-1 alpha in 24 of 32 (75
%) clear cell renal carcinomas but only 3 of 8 non-clear cell renal tumors.
Somatic mutations of the VHL gene were detected only in clear cell renal c
arcinomas that overexpressed HIF-1 alpha, None of the HIF-1 alpha -negative
tumors displayed a VHL mutation. The level of HIF-1 alpha mRNA was not dif
ferent between tumors and adjacent kidney tissue. Immunohistochemistry reve
aled distinct patterns of nuclear staining for HIF-1 alpha, depending on hi
stological type and overall abundance of HIF-1 alpha, In those clear cell r
enal carcinomas that showed increased expression on immunoblots, HIF-1 alph
a was expressed in almost all cells. In the remaining clear cell and in non
-clear cell tumors, staining was focal; these different patterns thus were
compatible with genetic stabilization in contrast to microenvironmental sti
mulation of HIF-1 alpha as the primary mechanism. The mRNA expression of tw
o known target genes of HIF-1 alpha, vascular endothelial growth factor and
glucose transporter 1, increased progressively with increasing amounts of
HIF-1 alpha in tumor extracts. In addition, glucose transporter 1 protein l
evels correlated with HIF-1 alpha abundance, In conclusion, the data provid
e in vivo evidence for a constitutive up-regulation of HIF-1 alpha in the m
ajority of clear cell renal carcinomas, which leads to more widespread accu
mulation of this transcription factor than hypoxic stimulation. These obser
vations are most likely linked to functional inactivation of the VHL gene p
roduct. Increased expression of HIF-1 alpha is associated with alterations
in gene expression patterns that are likely to contribute to tumor phenotyp
e and progression.