Laminin 111 triggers cell quiescence and long-term survival by inducing IQGAP1-mediated cytosolic scaffolding of ERK and BAD inactivation

In an adult human body, only a minority (∼1%) of cells are dividing; all others are either quiescent, senescent or terminally differentiated. Cellular quiescence, also called G0, is a reversible non-proliferative state in which cells, such as adult stem cells, exist until stimuli trigger their re-entry into the cell cycle. Quiescent cells are known to reside within microenvironment niches of specific extracellular matrix (ECM) composition, but the molecular mechanisms that control their entry and maintenance into G0 and their long-term survival are poorly understood. Here, using a reproducible and homogenous in vitro model of quiescence, ex vivo tissue histology, phosphoproteomics, and molecular cell biological assays, we revealed that Laminin 111 was sufficient to trigger i) reversible cell cycle exit into G0; ii) sustained and elevated MAPK/ERK signaling; and iii) long-term survival. We found that ERK was activated through the Rap1-BRAF-MEK arm underneath Laminin-binding Integrin α3β1. Activated pERK was scaffolded into the cytoplasm by IQGAP1, thereby blocking its translocation into the nucleus and the activation of proliferative transcription factors. Instead, cytoplasmic pERK inhibited pro-apoptotic protein BAD, which mediated the survival of quiescent cells even in absence of mitogen stimuli. Importantly, we confirmed that pERK was elevated and retained in the cytoplasm of Lgr5 ⁺ stem cells when they were located within Laminin α1-positive niches in porcine intestine. These findings uncovered a molecular mechanism that may explain how quiescent cell pools, such as dormant adult stem cells, can survive many years despite low mitogen stimuli and be resistant to apoptotic challenges, including chemotherapy.