Hedgehog pathway inhibition as a therapeutic target in acute myeloid leukemia

Introduction: The Hedgehog (HH) pathway constitutes a collection of signaling molecules which critically influence embryogenesis. In adults, however, the HH pathway remains integral to the proliferation, maintenance, and apoptosis of adult stem cells including hematopoietic stem cells.

Areas covered: We discuss the current understanding of the HH pathway as it relates to normal hematopoiesis, the pathology of acute myeloid leukemia (AML), the rationale for and data from combination therapies including HH pathway inhibitors, and ultimately the prospects that might offer promise in targeting this pathway in AML.

Expert opinion: Efforts to target the HH pathway have been focused on impeding this disposition and restoring chemosensitivity to conventional myeloid neoplasm therapies. The year 2018 saw the first approval of a HH pathway inhibitor (glasdegib) for AML, though for an older population and in combination with an uncommonly-used therapy. Several other clinical trials with agents targeting modulators of HH signaling in AML and MDS are underway. Further study and understanding of the interplay between the numerous aspects of HH signaling and how it relates to the augmented survival of AML will provide a more reliable substrate for therapeutic strategies in patients with this poor-risk disease.     

Signal transduction by guanine nucleotide binding proteins

High affinity binding of guanine nucleotides and the ability to hydrolyze bound GTP to GDP are characteristics of an extended family of intracellular proteins. Subsets of this family include cytosolic initiation and elongation factors involved in protein synthesis, and cytoskeletal proteins such as tubulin (Hughes, S.M. (1983) FEBS Lett. 164, 1-8). A distinct subset of guanine nucleotide binding proteins is membrane-associated; members of this subset include the ras gene products (Ellis, R.W. et al. (1981) Nature 292, 506-511) and the heterotrimeric G-proteins (also termed N-proteins) (Gilman, A.G. (1984) Cell 36, 577-579). Substantial evidence indicates that G-proteins act as signal transducers by coupling receptors (R) to effectors (E). A similar function has been suggested but not proven for the ras gene products. Known G-proteins include Gs and Gi, the G-proteins associated with stimulation and inhibition, respectively, of adenylate cyclase; transducin (TD), the G-protein coupling rhodopsin to cGMP phosphodiesterase in rod photoreceptors (Bitensky, M.W. et al. (1981) Curr. Top. Membr. Transp. 15, 237-271; Stryer, L. (1986) Annu. Rev. Neurosci. 9, 87-119), and Go, a G-protein of unknown function that is highly abundant in brain (Sternweis, P.C. and Robishaw, J.D. (1984) J. Biol. Chem. 259, 13806-13813; Neer, E.J. et al. (1984) J. Biol. Chem. 259, 14222-14229). G-proteins also participate in other signal transduction pathways, notably that involving phosphoinositide breakdown. In this review, I highlight recent progress in our understanding of the structure, function, and diversity of G-proteins.     

Bile-pancreatic juice exclusion promotes Akt/NF-kB activation and chemokine production in ligation-induced acute pancreatitis

Using a unique surgical model (the donor rat model), we showed previously that duodenal replacement of bile-pancreatic juice, obtained fresh from a donor rat, ameliorates ligation-induced acute pancreatitis. We hypothesize that bile-pancreatic juice exclusion from gut exacerbates Akt/nuclear factor-kB (NF-kB) pathway activation and induces chemokine production in ligation-induced acute pancreatitis. We compared rats with bile-pancreatic duct ligation to those with duodenal bile-pancreatic juice replacement fresh from a donor rat beginning immediately before duct ligation. Sham control rats had ducts dissected but not ligated. Rats were killed 1 or 3 hours after operation (n=7/group). Akt activation (immunoblotting, immune-complex kinase assay, and ELISA), inhibitory protein I-kB (I-kB) activation (immunoblotting), and production of chemokines MCP-1 and RANTES (ELISA) were measured in pancreatic homogenates. NF-kB was quantitated in nuclear fractions using electrophoretic mobility shift assay. Duct ligation produced significant increases in pancreatic Akt, IkB, and NF-kB activation and production of MCP-1 and RANTES. Activation of the Akt/NF-kB pathway and increased MCP-1 and RANTES production in response to duct ligation were significantly reduced by bile-pancreatic juice replacement (ANOVA, P<0.05). Bile-pancreatic juice exclusion stimulates Akt/NF-kB pathway activation and increases chemokine production in ligation-induced acute pancreatitis. Presented at the annual meeting of The Society for Surgery of the Alimentary Tract, Chicago, Illinois, May 16, 2005 (poster).