Hypoxia-inducible factors, mTOR, and astrin constitute an integrative regulatory network in colon cancer cells

Astrin/SPAG5 is a mitotic spindle protein found to be overexpressed in several human cancers, functioning as an oncogene. The expression of Astrin has not been reported so far in colon cancer, nor has it been related to HIFs expression or action. Since mTOR, Astrin, and hypoxia-inducible factors (HIFs) are involved in promoting the growth and survival of cancer cells, we investigated the possible interaction between them in cultured colon cancer cells. Both Astrin and HIF-1α and HIF-2α protein levels were found only expressed in colon cancer cells compared with nonmalignant cells. Our data indicate that mTOR stimulates both Astrin and HIFs expression, but notably, mTORC activity seems to be independent of Astrin expression levels. However, while HIF-1α or HIF-2α stable knockdown increased Astrin expression, mTOR activity was affected in an opposite way by HIF-1α or HIF-2α silencing, indicating that HIF-1α inhibits mTOR while HIF-2α stimulates its activity. These data suggest that mTOR, Astrin, and HIFs compose an integrative network interacting to activate positive or negative regulatory loops probably to coordinate cancer cell growth, metabolism, and survival under oncogenic stress.