Sorbitol, ‘osmotic stress’, stimulates GLUT4 glucose transporter translocation to the plasma membrane and glucose transport by a phosphatidylinositol (PI) 3-kinase-independent mechanism that reportedly involves non-receptor proline-rich tyrosine kinase-2 (PYK2) but subsequent events are obscure. In the present study, we found that extracellular signal-regulated kinase (ERK) pathway components, growth-factor-receptor-bound-2 protein, son of sevenless (SOS), RAS, RAF and mitogen-activated protein (MAP) kinase/ERK kinase, MEK(−1), operating downstream of PYK2, were required for sorbitol-stimulated GLUT4 translocation/glucose transport in rat adipocytes, L6 myotubes and 3T3/L1 adipocytes. Furthermore, sorbitol activated atypical protein kinase C (aPKC) through a similar mechanism depending on the PYK2/ERK pathway, independent of PI 3-kinase and its downstream effector, 3-phosphoinositide-dependent protein kinase-1 (PDK-1). Like PYK2/ERK pathway components, aPKCs were required for sorbitol-stimulated GLUT4 translocation/glucose transport. Interestingly, sorbitol stimulated increases in phospholipase D (PLD) activity and generation of phosphatidic acid (PA), which directly activated aPKCs. As with aPKCs and glucose transport, sorbitol-stimulated PLD activity was dependent on the ERK pathway. Moreover, PLD-generated PA was required for sorbitol-induced activation of aPKCs and GLUT4 translocation/glucose transport. Our findings suggest that sorbitol sequentially activates PYK2, the ERK pathway and PLD, thereby increasing PA, which activates aPKCs and GLUT4 translocation. This mechanism contrasts with that of insulin, which primarily uses PI 3-kinase, D3-PO4 polyphosphoinositides and PDK-1 to activate aPKCs.

Abbreviations used: (a)PKC, (atypical) protein kinase C; ERK, extracellular signal-regulated kinase; PYK2, proline-rich tyrosine kinase-2; DOG, deoxyglucose; Grb2, growth-factor-receptor-bound-2 protein; SOS, son of sevenless; PI, phosphatidylinositol; PDK-1, 3-phosphoinositide-dependent protein kinase-1; PIP3, phosphatidylinositol-3,4,5-trisphosphate; DMEM, Dulbecco's modified Eagle's medium; KRP, Krebs Ringer phosphate; KI, kinase inactive; HA, haemagglutinin antigen; PLD, phospholipase D; MAP, mitogen-activated kinase; MEK(−1), mitogen-activated protein (MAP) kinase/ERK kinase; PA, phosphatidic acid.

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