Inverse relationship between Alzheimer's disease and cancer, and other factors contributing to Alzheimer's disease: a systematic review.

Background: The AD etiology is yet not properly known. Interactions among environmental factors, multiple susceptibility genes and aging, contribute to AD. This study investigates the factors that play role in causing AD and how changes in cellular pathways contribute to AD.

Methods: PUBMED database, MEDLINE database and Google Scholar were searched with no date restrictions for published articles involving cellular pathways with roles in cancers, cell survival, growth, proliferation, development, aging, and also contributing to Alzheimer's disease. This research explores inverse relationship between AD and cancer, also investigates other factors behind AD using several already published research literature to find the etiology of AD.

Results: Cancer and Alzheimer's disease have inverse relationship in many aspects such as P53, estrogen, neurotrophins and growth factors, growth and proliferation, cAMP, EGFR, Bcl-2, apoptosis pathways, IGF-1, HSV, TDP-43, APOE variants, notch signals and presenilins, NCAM, TNF alpha, PI3K/AKT/MTOR pathway, telomerase, ROS, ACE levels. AD occurs when brain neurons have weakened growth, cell survival responses, maintenance mechanisms, weakened anti-stress responses such as Vimentin, Carbonic anhydrases, HSPs, SAPK. In cancer, these responses are upregulated and maintained. Evolutionarily conserved responses and maintenance mechanisms such as FOXO are impaired in AD. Countermeasures or compensatory mechanisms by AD affected neurons such as Tau, Beta Amyloid, S100, are last attempts for survival which may be protective for certain time, or can speed up AD in Alzheimer's microenvironment via C-ABL activation, GSK3, neuro-inflammation.

Conclusions: Alzheimer's disease and Cancer have inverse relationship; many factors that are upregulated in any cancer to sustain growth and survival are downregulated in Alzheimer's disease contributing to neuro-degeneration. When aged neurons or genetically susceptible neurons have weakened growth, cell survival and anti-stress responses, age related gene expression changes, altered regulation of cell death and maintenance mechanisms, they contribute to Alzheimer's disease. Countermeasures by AD neurons such as Beta Amyloid Plaques, NFTs, S100, are last attempts for survival and this provides neuroprotection for certain time and ultimately may become pathological and speed up AD. This study may contribute in developing new potential diagnostic tests, interventions and treatments.