title: A p21-activated kinase (PAK1) signaling cascade coordinately regulates F-actin remodeling and insulin granule exocytosis in pancreatic β cells.
abstract: Human islet studies implicate an important signaling role for the Cdc42 effector protein p21-activated kinase (PAK1) in the sustained/second-phase of insulin secretion. Because human islets from type 2 diabetic donors lack ∼80% of normal PAK1 protein levels, the mechanistic requirement for PAK1 signaling in islet function was interrogated. Similar to MIN6 β cells, human islets elicited glucose-stimulated PAK1 activation that was sensitive to the PAK1 inhibitor, IPA3. Given that sustained insulin secretion has been correlated with glucose-induced filamentous actin (F-actin) remodeling, we tested the hypothesis that a Cdc42-activated PAK1 signaling cascade is required to elicit F-actin remodeling to mobilize granules to the cell surface. Live-cell imaging captured the glucose-induced cortical F-actin remodeling in MIN6 β cells; IPA3-mediated inhibition of PAK1 abolished this remodeling. IPA3 also ablated glucose-stimulated insulin granule accumulation at the plasma membrane, consistent with its role in sustained/second-phase insulin release. Both IPA3 and a selective inhibitor of the Cdc42 GTPase, ML-141, blunted the glucose-stimulated activation of Raf-1, suggesting Raf-1 to be downstream of Cdc42→PAK1. IPA3 also inhibited MEK1/2 activation, implicating the MEK1/2→ERK1/2 cascade to occur downstream of PAK1. Importantly, PD0325901, a new selective inhibitor of MEK1/2→ERK1/2 activation, impaired F-actin remodeling and the sustained/amplification pathway of insulin release. Taken together, these data suggest that glucose-mediated activation of Cdc42 leads to activation of PAK1 and prompts activation of its downstream targets Raf-1, MEK1/2 and ERK1/2 to elicit F-actin remodeling and recruitment of insulin granules to the plasma membrane to support the sustained phase of insulin release.
authors.affiliation: Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, IN, USA.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Kalwat
authors.forename: Stephanie M
authors.initials: SM
authors.lastname: Yoder
authors.forename: Zhanxiang
authors.initials: Z
authors.lastname: Wang
authors.forename: Debbie C
authors.initials: DC
authors.lastname: Thurmond
mesh_terms: D000199:Actins / Q000378:metabolism*; D002460:Cell Line; D005089:Exocytosis; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D007515:Islets of Langerhans / Q000201:enzymology / Q000378:metabolism*; D018613:Microscopy, Confocal; D015398:Signal Transduction; D054462:p21-Activated Kinases / Q000378:metabolism*
chemical_list: D000199:Actins; D007328:Insulin; C515903:PAK1 protein, human; D054462:p21-Activated Kinases