In order for the cancer cells to adapt with the hypoxic environment; they switches on hypoxia indubitable factor (HIF) which enhance new blood vessels to grow around the tumor and provides oxygen to the cancer cells. It also helps the tumor to adapt to hypoxic conditions by using alternative methods to produce energy.
I share with you the so called Warburg Effect ^_^ enjoy! http://www.cellsignal.com/contents/science-pathway-research-cellular-metabolism/warburg-effect-signaling-pathway/pathways-warburg