Cell contact activates AMP-Activated Protein Kinase (AMPK) through CaMKKβ and the GTPase Rac1
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AMP-activated protein kinase (AMPK) has been identified as a target for cancer and metabolic diseases such as diabetes and obesity. Canonical activation of AMPK occurs as a result of energetic stress; however, within the past decade there is increasing evidence that AMPK can be activated by entirely different events, including some without energetic stresses. AMPK subunits have been implicated to interact with elements of the actin cytoskeleton and proteins involved in cell adhesion, migration, and invasion. Loss of AMPK activity in breast cancer progression correlates with loss of cell adhesion and increased cell migration. We hypothesized that AMPK activation is a result of cell-cell adhesion/contact. We verified that AMPK was indeed activated in cells that were plated at high density and confirmed that AMPK activation was resistant to traditional nutrient alterations when cells had neighbors. We found that AMPK activation occurs as a result of cell contact induced signaling through CaMMKβ, as using the inhibitor STO-609 completely abolished AMPK activation in response to cell contact when cells aggregated as well as plated on normal tissue culture treated dishes. Further, we saw that Rac1 activity was increased due to cell contact and that blocking this activation by transiently transfecting a dominant negative Rac1 T17N reduced AMPK activation. Rac1 knockdown by siRNA had no effect, but a pan-PAK inhibitor reduced AMPK activation as well, indicating that PAK1 activity, and not necessarily Rac1 activity is a closer signal that regulates AMPK activity in response to cell contact. Our results show a novel AMPK activation mechanism by cell cell contact that is dependent on CaMMKβ and the GTPase activity of Rac1. These findings illustrate how important cell density is in future in vitro experiments, as cell contact signaling can effect cell metabolism through AMPK. More studies need to be performed to determine how AMPK activation during cell contact is altered by pharmacologic activators as well as inhibitors to fully comprehend AMPK’s therapeutic potential as an anti-cancer agent.