A Novel Cytostatic Form Of Autophagy In Sensitization Of Non Small Cell Lung Cancer Cells To Radiation By Vitamin D And Vitamin D Analogue Eb 1089

The standard of care for unresectable lung cancer is chemoradiation. However, therapeutic options are limited and patients are rarely cured. While Radiation therapy is effective at killing tumor cells or inhibiting their growth initially, development of resistance to treatments and recurrence of tumors are major issues. One of the major goals of Dr. Gewirtz's laboratory has been to develop strategies to overcome the resistance and attenuate disease recurrence. One of these attempts involve employing vitamin D and its analogs in combination with radiation therapy. Our proposed studies were based on a previous finding where vitamin D and vitamin D analogs such as EB 1089, were shown to enhance the response to radiation in breast cancer through the promotion of autophagy. We extended these studies to non-small cell lung cancer (NSCLC) and were able to validate that 1,25-D3(the hormonally active form of vitamin D) and EB 1089 does in fact sensitize A549 and H460 cells and prolonged the growth arrest induced by radiation alone and suppressed proliferative recovery, which translated to a significant reduction in clonogenic survival. In H838 or H358 NSCLC cells, which lack the vitamin D receptor or functional p53, respectively, 1,25-D3 failed to modify the extent of radiation-induced growth arrest or suppress proliferative recovery post irradiation. Sensitization to radiation in H1299 NSCLC cells was evident only when p53 was induced in otherwise p53 null H1299 NSCLC cells. Sensitization by 1,25-D3/ EB 1089 was not associated with increased DNA damage, decreased DNA repair or an increase in apoptosis, necrosis or senescence. Instead sensitization appeared to be a consequence of the conversion of the cytoprotective autophagy induced by radiation alone to a novel cytostatic form of autophagy by the combination of 1,25-D3 or EB 1089 with radiation. While both pharmacological and genetic suppression of autophagy or inhibition of AMPK phosphorylation sensitized the NSCLC cells to radiation alone, inhibition of the cytostatic autophagy induced by the combination treatment reversed sensitization. Evidence for selectivity was provided by lack of radiosensitization in normal human bronchial cells and cardiomyocytes. Taken together, these studies have identified a unique cytostatic function of autophagy that appears to be mediated by the vitamin D receptor, p53 and possibly AMPK in the promotion of an enhanced response to radiation by 1,25-D3 and EB 1089 in NSCLC.