Blockade of irradiation-induced autophagosome formation impairs proliferation but does not enhance cell death in HCT-116 human colorectal carcinoma cells

This work was undertaken to gain further information on the molecular mechanismsunderlying autophagosome formation and its relation with tumor cell survival inresponse to radiation in colon cancer. A human colon cancer cell line, HCT-116,was examined with respect to cell survival after blockade of irradiation-inducedautophagosome formation by pharmacological interference. Autophagosome formationwas confirmed using a kinetic study with incorporated bovine serum albumin gold-conjugate(BSA-Au) analyzed by electron microscopy and an autophagosome-associated LC3Bantibody measured by immunofluorescence and Western blotting. Annexin V/PI doublestaining was used to monitor cell death by apoptosis, and cell cycle profilesby flow cytometry. Ionizing radiation (IR) promoted autophagosome formation inthe HCT-116 IR-surviving cells. Pharmacological interference showed that PI3K/Aktand Src were involved in early stages of autophagosome formation. IR alone decreasedcell proliferation by arresting cells in the G2/M phase, and pharmacological interferenceof autophagosome formation decreased proliferation, but did not affect cell survival.Also, our data suggest that decreased proliferation caused by PI3K and Src inhibitorscould be through S phase cell cycle delay. Our results clearly indicate that blockadeof IR-induced autophagosome formation impairs proliferation but does not enhancecell death in colon cancer cells.