p21-activated kinases (PAKs) are highly conserved
serine/threonine protein kinases, which are divided into two groups: group-I (PAKs1-3) and group-II (PAKs4-6). In various tissues, Group-II PAKs play important roles in cytoskeletal dynamics and cell growth as well as neoplastic development/progression. However, little is known about Group-II PAK's role in a number of physiological events, including their ability to be activated by gastrointestinal (GI)
hormones/
neurotransmitters/
growth factors (GFs). We used rat pancreatic acini to explore the ability of GI
hormones/
neurotransmitters/GFs to activate Group-II-PAKs and the signaling cascades involved. Only PAK4 was detected in pancreatic acini. PAK4 was activated by
endothelin,
secretagogues-stimulating
phospholipase C (
bombesin, CCK-8, and
carbachol), by pancreatic GFs (
insulin,
insulin-like growth factor 1,
hepatocyte growth factor,
epidermal growth factor,
basic fibroblast growth factor, and
platelet-derived growth factor), and by postreceptor stimulants (12-O-tetradecanoylphobol-13-acetate and A23187 ).
CCK-8 activation of PAK4 required both high- and low-affinity CCK1-receptor state activation. It was reduced by PKC-, Src-, p44/42-, or p38-inhibition but not with
phosphatidylinositol 3-kinase-inhibitors and only minimally by
thapsigargin. A
protein kinase D (PKD)-inhibitor completely inhibited CCK-8-stimulated PKD-activation; however, stimulated PAK4 phosphorylation was only inhibited by 60%, demonstrating that it is both PKD-dependent and PKD-independent.
PF-3758309 and
LCH-7749944, inhibitors of PAK4, decreased CCK-8-stimulated PAK4 activation but not PAK2 activation. Each inhibited ERK1/2 activation and
amylase release induced by
CCK-8 or
bombesin. These results show that PAK4 has an important role in modulating signal cascades activated by a number of GI
hormones/
neurotransmitters/GFs that have been shown to mediate both physiological/pathological responses in acinar cells. Therefore, in addition to the extensive studies on PAK4 in
pancreatic cancer, PAK4 should also be considered an important signaling molecule for pancreatic acinar physiological responses and, in the future, should be investigated for a possible role in pancreatic acinar pathophysiological responses, such as in
pancreatitis. NEW & NOTEWORTHY This study demonstrates that the only Group-II
p21-activated kinase (PAK) in rat pancreatic acinar cells is PAK4, and thus differs from islets/pancreatic
cancer. Both
gastrointestinal hormones/
neurotransmitters stimulating PLC and pancreatic
growth factors activate PAK4. With
cholecystokinin (CCK), activation is PKC-dependent/-independent, requires both CCK1-R affinity states, Src, p42/44, and p38 activation. PAK4 activation is required for CCK-mediated p42/44 activation/
amylase release. These results show PAK4 plays an important role in mediating CCK physiological signal cascades and suggest it may be a target in pancreatic acinar diseases besides
cancer.