Exposure to cigarette smoke, air pollution and occupational hazards increases the risk of developing lung cancer. While the majority of these exposures are preventable, lung cancer causes the death of about 170,000 people a year in the United States, and overall survival statistics of patients with lung cancers have not been improved during the past several decades. Thus, there is a major need for the development of new therapeutic strategies against this disease. In our recent studies, ceramide, a bioactive sphingolipid, has been shown to exert anti-proliferative functions against lung cancers via degradation of the c-Myc proto-oncogene through directly binding to the inhibitor 2 of PP2A, (I2PP2A). However, mechanisms and biological roles by which ceramide-I2PP2A regulates c-Myc degradation remain unknown. This proposal is designed to examine the hypothesis that the direct interaction between ceramide and I2PP2A results in the restoration of nuclear PP2A activity leading to the degradation of c-Myc, and subsequently resulting in the suppression of lung tumor growth and/or progression. To test this hypothesis, two Specific Aims are proposed.
In Specific Aim 1, it will be determined how ceramide-I2PP2A binding regulates the association between I2PP2A-PP2A leading to activation of nuclear PP2A. More specifically, it will be determined if the association between nuclear ceramide and I2PP2A causes changes in the association between I2PP2A and PP2A. Additionally the conformational/structural changes of I2PP2A-PP2A complex via ceramide-I2PP2A binding will be explored. The results of this first Aim will establish how the association between ceramide-I2PP2A regulates the interaction between I2PP2A-PP2A, resulting in enhanced activity of PP2A in human lung cancer cells.
In Specific Aim 2, the mechanisms and biological roles of ceramide-I2PP2A binding on the regulation of c-Myc degradation via nuclear PP2A activation will be defined. More specifically, the contribution of endogenous ceramide generation in the nucleus by neutral sphingomyelinase 1 versus ceramide synthase 1 will be determined. Additionally, the signaling cascade causing c-Myc degradation via nuclear ceramide-I2PP2A binding, and subsequent activation of PP2A will be elucidated. Lastly, the biological roles of nuclear ceramide- I2PP2A binding in the suppression of lung tumor growth via regulation of PP2A dependent c-Myc degradation will be demonstrated. In summary, results of these Aims will define the structural, functional, and biological roles of ceramide-I2PP2A binding in the regulation of PP2A/c-Myc axis leading to the suppression of the growth and/or proliferation of human lung cancer cells. These studies will also lead to the development of mechanism-based therapeutic strategies, by specifically targeting oncoprotein I2PP2A and its downstream targets such as PP2A and c-Myc, through ceramide signaling for the treatment of human lung cancers.
PROJECT RELEVANCE Lung cancer is associated not only with smoking status, but also with exposure to environmental factors (smoke inhalation, combustion-generated carcinogens, air pollution) and/or occupational hazards (exposure to arsenic, asbestos, chromates, chloromethyl ethers, nickel, polycyclic aromatic hydrocarbons, radon). Regardless of the initiating factor, lung cancer remains to be the leading cause for cancer-related deaths in the United States, without much success for its therapeutic treatment in the clinic. Therefore, this proposal focuses on defining novel mechanisms by which bioactive sphingolipid ceramide mediates the suppression of lung cancer growth with the long-term objective to utilize these mechanistic data for the development of novel therapeutic strategies for the treatment of human lung cancers.
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