This following article is written for Prof. Abba Kastin’s Festschrift, to add to the tribute to his important role in the advancement of the role of peptides in physiological, as well as pathophysiological processes. There have been many advances during the 35 years of his prominent role in the Peptide field, not only as editor of the journal Peptides, but also as a scientific investigator and editor of two volumes of the Handbook of Biological Active Peptides [146,147]. Similar to the advances with many different peptides, during this 35 year period, there have been much progress made in the understanding of the pharmacology, cell biology and the role of (Bombesin) Bn receptors and their ligands in various disease states, since the original isolation of bombesin from skin of the European frog Bombina bombina in 1970 [76]. This paper will briefly review some of these advances over the time period of Prof Kastin 35 years in the peptide field concentrating on the advances since 2007 when many of the results from earlier studies were summarized [128,129]. It is appropriate to do this because there have been 280 articles published in Peptides during this time on Bombesin-related peptides and it accounts for almost 5% of all publications. Furthermore, 22 Bn publications we have been involved in have been published in either Peptides [14,39,55,58,81,92,93,119,152,216,225,226,231,280,302,309,355,361,362] or in the Prof Kastin’s Handbook of Biological Active Peptides [137,138,331].

Introduction Despite remarkable advances in tumor treatment, many patients still die from common tumors (breast, prostate, lung, CNS, colon, and pancreas), and thus, new approaches are needed. Many of these tumors synthesize bombesin (Bn)-related peptides and over-express their receptors (BnRs), hence functioning as autocrine-growth-factors. Recent studies support the conclusion that Bn-peptides/BnRs are well-positioned for numerous novel antitumor treatments, including interrupting autocrine-growth via the use of over-expressed receptors for imaging and targeting cytotoxic-compounds, either by direct-coupling or combined with nanoparticle-technology. Areas covered The unique ability of common neoplasms to synthesize, secrete, and show a growth/proliferative/differentiating response due to BnR over-expression, is reviewed, both in general and with regard to the most frequently investigated neoplasms (breast, prostate, lung, and CNS). Particular attention is paid to advances in the recent years. Also considered are the possible therapeutic approaches to the growth/differentiation effect of Bn-peptides, as well as the therapeutic implication of the frequent BnR over-expression for tumor-imaging and/or targeted-delivery. Expert opinion Given that Bn-related-peptides/BnRs are so frequently ectopically-expressed by common tumors, which are often malignant and become refractory to conventional treatments, therapeutic interventions using novel approaches to Bn-peptides and receptors are being explored. Of particular interest is the potential of reproducing BnRs in common tumors, such as the recent success of utilizing overexpression of somatostatin-receptors by neuroendocrine-tumors to provide the most sensitive imaging methods and targeted delivery of cytotoxic-compounds.

Human bombesin receptors, GRPR and NMBR, are two of the most frequently overexpressed G-protein-coupled-receptors by lung-cancers. Recently, GRPR/NMBR are receiving considerable attention because they act as growth factor receptors often in an autocrine manner in different lung-cancers, affect tumor angiogenesis, their inhibition increases the cytotoxic potency of tyrosine-kinase inhibitors reducing lung-cancer cellular resistance/survival and their overexpression can be used for sensitive tumor localization as well as to target cytotoxic agents to the cancer. The orphan BRS-3-receptor, because of homology is classified as a bombesin receptor but has received little attention, despite the fact that it is also reported in a number of studies in lung-cancer cells and has growth effects in these cells. To address its potential importance, in this study, we examined the frequency/relative quantitative expression of human BRS-3 compared to GRPR/NMBR and the effects of its activation on cell-signaling/growth in 13 different human lung-cancer cell-lines. Our results showed that BRS-3 receptor is expressed in 92% of the cell-lines and that it is functional in these cells, because its activation stimulates phospholipase-C with breakdown of phosphoinositides and changes in cytosolic calcium, stimulates ERK/MAPK and stimulates cell growth by EGFR transactivation in some, but not all, the lung-cancer cell-lines. These results suggest that human BRS-3, similar to GRPR/NMBR, is frequently ectopically-expressed by lung-cancer cells in which, it is functional, affecting cell signaling/growth. These results suggest that similar to GRPR/NMBR, BRS-3 should receive increased attention as possible approach for the development of novel treatments and/or diagnosis in lung-cancer.

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 CCK-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 CCK-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.

Pituitary adenylate-cyclase-activating polypeptide (PACAP) has widespread physiological/pathophysiological actions and there is increased interest for its use therapeutically, especially in the CNS (neuroprotection). Unfortunately, no selective PACAP-analogs exist for PACAP-preferring PAC1-receptors, primarily because of its high sequence identity to VIP and particularly, because of the inability of structure-function studies to separate the pharmacophore of PAC1-R from VPAC1-R, which has high affinity for PACAP and VIP. The present study attempted to develop PAC1-R-selective agonists primarily by making conformationally-restricted PACAP -analogs in positions important for receptor-selectivity/affinity. Forty-six PACAP-related-analogs were synthesized with substitutions in positions 1–4, 14–17, 20–22 ,28,34,38 and receptor-selectivity determined in PAC1-R,VPAC1-R,VPAC2-R-transfected or native cells from binding or cAMP-generation experiments. Fifteen PACAP-analogs had 6–78-fold higher affinities for PAC1-R than VPAC1-R and 13 were agonists. Although binding-affinities correlated significantly with agonist potency, the degree of receptor-spareness varied markedly for the different PACAP-analogs, resulting in selective potencies for activating the PAC1 receptor over the VPAC1 receptor from 0- to-103-fold. In addition, a number of PACAP-analogs were identified that had high selectivity for PAC1-R over VPAC2-R as well as PACAP-analogs that could prove more useful therapeutically because of substitutions known to extend their half-lives (substitutions at potential sites of proteolysis and attachment of long-chain fatty acids). This study provides for the first time a separation of the pharmacophores for PAC1-R and VPAC1-R, resulting in PACAP-related analogs that are PAC1-R-preferring. Some of these analogs, or their modifications, could prove useful as therapeutic agents for various diseases.

Neuropeptide G protein-coupled receptors (GPCRs) are overexpressed on numerous cancer cells. In a number of tumors, such as small cell lung cancer (SCLC), bombesin (BB) like peptides and neurotensin (NTS) function as autocrine growth factors whereby they are secreted from tumor cells, bind to cell surface receptors and stimulate growth. BB-drug conjugates and BB receptor antagonists inhibit the growth of a number of cancers. Vasoactive intestinal peptide (VIP) increases the secretion rate of BB-like peptide and NTS from SCLC leading to increased proliferation. In contrast, somatostatin (SST) inhibits the secretion of autocrine growth factors from neuroendocrine tumors (NETs) and decreases proliferation. SST analogs such as radiolabeled octreotide can be used to localize tumors, is therapeutic for certain cancer patients and has been approved for four different indications in the diagnosis/treatment of NETs. The review will focus on how BB, NTS, VIP, and SST receptors can facilitate the early detection and treatment of cancer.

In a recent study we explored Group-1-p21-activated kinases(GP.1-PAKs) in rat pancreatic acini. Only PAK2 was present; it was activated by gastrointestinal-hormones/neurotransmitters and growth factors in a PKC-,Src- and small-GTPases-mediated manner. PAK2 was required for enzyme-secretion and ERK/1-2-activation. In the present study we examined PAK2’s role in CCK and TPA-activation of important distal signaling cascades mediating their physiological/pathophysiological effects and analyzed its role in pathophysiological processes important in early pancreatitis. In rat pancreatic acini,PAK2-inhibition by the specific, GP.1.PAK-inhibitor, IPA-3-suppressed cholecystokinin(CCK)/TPA-stimulated activation of focal-adhesion kinases and mitogen-activated protein-kinases. PAK2-inhibition reversed the dual stimulatory/inhibitory effect of CCK/TPA on the PI3K/Akt/GSK-3β pathway. However, its inhibition did not affect PKC activation. PAK2-inhibition protected acini from CCK-induced ROS-generation; caspase/trypsin-activation, important in early pancreatitis; as well as from cell-necrosis. Furthermore, PAK2-inhibition reduced proteolytic-activation of PAK-2p34, which is involved in programmed-cell-death. To ensure that the study did not only rely in the specificity of IPA-3 as a PAK inhibitor, we used two other approaches for PAK inhibition, FRAX597 a ATP-competitive-GP.1-PAKs - inhibitor and by infection with an PAK2-dominat negative(DN)-Advirus. Those two approaches confirmed the results obtained with IPA-3. This study demonstrates that PAK2 is important in mediating CCK’s effect on the activation of signaling-pathways known to mediate its physiological/pathophysiological responses including several cellular processes linked to the onset of pancreatitis. Our results suggest that PAK2 could be a new, important therapeutic target to consider for the treatment of diseases involving deregulation of pancreatic acinar cells.

The use of proton pump inhibitors (PPIs) over the last 30 years has rapidly increased both in the United States and worldwide. PPIs are not only very widely used both for approved indications (peptic ulcer disease, gastroesophageal reflux disease (GERD), Helicobacter pylori eradication regimens, stress ulcer prevention), but are also one of the most frequently off-label used drugs (25–70% of total). An increasing number of patients with moderate to advanced gastroesophageal reflux disease are remaining on PPI indefinitely. Whereas numerous studies show PPIs remain effective and safe, most of these studies are <5 years of duration and little data exist for >10 years of treatment. Recently, based primarily on observational/epidemiological studies, there have been an increasing number of reports raising issues about safety and side-effects with very long-term chronic treatment. Some of these safety issues are related to the possible long-term effects of chronic hypergastrinemia, which occurs in all patients taking chronic PPIs, others are related to the hypo-/achlorhydria that frequently occurs with chronic PPI treatment, and in others the mechanisms are unclear. These issues have raised considerable controversy in large part because of lack of long-term PPI treatment data (>10–20 years). Zollinger–Ellison syndrome (ZES) is caused by ectopic secretion of gastrin from a neuroendocrine tumor resulting in severe acid hypersecretion requiring life-long antisecretory treatment with PPIs, which are the drugs of choice. Because in <30% of patients with ZES, a long-term cure is not possible, these patients have life-long hypergastrinemia and require life-long treatment with PPIs. Therefore, ZES patients have been proposed as a good model of the long-term effects of hypergastrinemia in man as well as the effects/side-effects of very long-term PPI treatment. In this article, the insights from studies on ZES into these controversial issues with pertinence to chronic PPI use in non-ZES patients is reviewed, primarily concentrating on data from the prospective long-term studies of ZES patients at NIH.