Metastasizing cancer cells seem to know exactly where they are going. But cancer cells don't normally express the chemokine receptors that are required for homing to target organs, so how do they develop this sense of direction? According to a report in Nature by Wilhelm Krek and colleagues, expression of the CXCR4 chemokine receptor is negatively regulated by the von Hippel–Lindau (VHL) tumour suppressor protein under normoxic conditions, and is induced by hypoxia-inducible factor (HIF) during hypoxic conditions.

The authors were comparing gene-expression profiles of VHL-deficient A498 renal carcinoma cells (RCCs) with A498 cells that were engineered to express a haemagglutinin-tagged VHL, and noticed that CXCR4 mRNA expression was strongly suppressed in the VHL-expressing cells. However, they went on to show that a mutant form of VHL — which prevents VHL from degrading HIF1α — could not suppress CXCR4 expression, indicating that CXCR4 is directly regulated by HIF. CXCR4 mRNA accumulation in hypoxic renal epithelial cells occurs at a similar rate to GLUT3 — a known target of HIF — providing further evidence that CXCR4 is a hypoxia-inducible gene. So, there seems to be a link between loss of VHL function, hypoxia and overexpression of CXCR4.

They analysed the CXCR4 promoter and intron, and found four potential hypoxia-responsive elements. Luciferase reporter assays that analysed this region showed a twofold increase in luciferase expression in response to hypoxia in VHL-positive human embryonic kidney cells. Co-transfection with wild-type HIF1α enhanced reporter activity by tenfold and deletion analysis revealed an upsteam hypoxia-response element that was crucial for this HIF1α-inducible reporter activity. Electrophoretic mobility-shift assays showed that HIF2α binds to this element, confirming that the CXCR4 promoter is targeted by HIF.

so, does is expression of CXCR4 have an effect on cell migration? In transwell assays, increasing concentrations of stromal-derived factor 1α (SDF1α) — the chemokine that is specific for CXCR4 — caused directed migration of A498 cells, and restoration of wild-type VHL abrogated this response. SDF1α stimulation activates rapid accumulation of LIM kinase-1 and extracellular signal-related kinases (ERKs) that control cell movement and proliferation. But, is inactivation of VHL and upregulation of CXCR4 occurring in vivo?

They studied expression of CXCR4 and the HIF target genes carbonic anhydrase ( CA9 ) and GLUT1 in RCCs, and mRNA levels of all three were significantly higher in clear cell RCC than in papillary RCC or normal renal tissue. CXCR4 expression was investigated in a wide range of RCCs using renal cancer tissue microarrays. Although there was no correlation between high level CXCR4 expression and tumour stage and/or differentiation grade, there was a strong correlation between CXCR4 expression and poor survival.

These results provide a mechanism of how RCCs acquire the ability to metastasize. The absence of oxygen — characteristic of highly aggressive tumours — induces expression of CXCR4, allowing metastatic RCCs to find their way to distant sites. In addition, tumour cells could be primed to spread early in tumorigenesis by acquiring mutations in VHL.