Lung adenocarcinoma is among the deadliest cancer types world-wide. Solid tumors such as these almost universally acquire adaptive changes to survive hypoxic energy stress. This confers an increase in aggressiveness and resistance to chemotherapy and radiotherapy. However, the basic genetic and biochemical mechanisms of metabolic adaptation in cancer are relatively unknown. We have discovered that a gene essential for de novo adenosine 54-monophosphate (AMP) synthesis, Adenylosuccinate Synthetase 1 (ADSS1), is inactivated at a high frequency in lung adenocarcinomas, the major histotype of human lung cancer. Our results suggest that ADSS1 is a biochemical switch for a major energy sensitive pathway that controls the cellular response to hypoxia. By controlling cellular AMP levels, ADSS1 controls the activity of serine/threonine kinase 11 (STK11) (also LKB1), which is the apical kinase that regulates cell growth and survival during energy stress. LKB1 activates AMP-activated protein kinase (AMPK), which in turn inhibits tumor survival via signal transduction through numerous effector pathways. LKB1 activation of AMPK requires AMP, and the cellular ratio of AMP to ATP is increased under energy stress to a level sufficient for AMPK activation [11-13]. We found that ADSS1 inactivation circumvents this process by lowering AMP to levels insufficient for LKB/AMPK activation under hypoxia. Based on our evidence, it is hypothesized that ADSS1 inactivation promotes adaptation to hypoxia in LAC development by disabling LKB1/AMPK. This hypothesis will be tested in the following Specific Aims.
In Specific Aim 1, the biochemical mechanisms by which ADSS1 mediates resistance or adaptation to hypoxia will be characterized. The functional relationship between ADSS1, LKB1, AMPK and HIF1 will be the focus of this investigation.
In Specific Aim 2, the role of ADSS1 in the development of lung adenocarcinomas will be characterized. The effect of ADSS1 on tumor hypoxia and LKB1/AMPK1 activity will be examined. Metabolic fingerprinting by 1H-NMR will be used to identify markers of ADSS1 inactivation on tumor energy metabolism, particularly the metabolic pathways controlled by LKB1/AMPK. There has been no prior indication of ADSS1 or de novo nucleotide synthesis defects in cancer. With this innovation, this proposal has the potential to uncover a novel mechanism by which tumors survive energy stress and progress to malignancy. The significance of this proposal is that it may also lead to a better understanding of lung adenocarcinoma development, and foster new therapeutic strategies.
Changes in tumor metabolism occur as tumors outgrow their nutrient and oxygen supply marking a transition toward increased aggressiveness and treatment resistance. We have discovered a novel genetic defect in lung cancer that appears to promote this change. Its characterization, as proposed, may lead to new targets and strategies for the lung cancer treatment.
