RNAseq reveals extensive metabolic disruptions in the sensitive SF-295 cell line treated with schweinfurthins

The schweinfurthin family of natural compounds exhibit a unique and potent differential cytotoxicity against a number of cancer cell lines and may reduce tumor growth in vivo. In some cell lines, such as SF-295 glioma cells, schweinfurthins elicit cytotoxicity at nanomolar concentrations. However, other cell lines, like A549 lung cancer cells, are resistant to schweinfurthin treatment up to micromolar concentrations. At this time, the precise mechanism of action and target for these compounds is unknown. Here, we employ RNA sequencing of cells treated with 50 nM schweinfurthin analog TTI-3066 for 6 and 24 h to elucidate potential mechanisms and pathways which may contribute to schweinfurthin sensitivity and resistance. The data was analyzed via an interaction model to observe differential behaviors between sensitive SF-295 and resistant A549 cell lines. We show that metabolic and stress-response pathways were differentially regulated in the sensitive SF-295 cell line as compared with the resistant A549 cell line. In contrast, A549 cell had significant alterations in response genes involved in translation and protein metabolism. Overall, there was a significant interaction effect for translational proteins, RNA metabolism, protein metabolism, and metabolic genes. Members of the Hedgehog pathway were differentially regulated in the resistant A549 cell line at both early and late time points, suggesting a potential mechanism of resistance. Indeed, when cotreated with the Smoothened inhibitor cyclopamine, A549 cells became more sensitive to schweinfurthin treatment. This study therefore identifies a key interplay with the Hedgehog pathway that modulates sensitivity to the schweinfurthin class of compounds.

ERK2 MAP kinase regulates SUFU binding by multisite phosphorylation of GLI1

There is considerable evidence that cross-talk between the Hedgehog pathway and MAPK signaling pathways occurs in several types of cancer, and contributes to the emergence of clinical resistance to Hedgehog pathway inhibitors. Here, we demonstrate that MAP kinase-mediated phosphorylation weakens the binding of the GLI1 transcription factor to its negative regulator SUFU. We show that ERK2 phosphorylates GLI1 on three evolutionarily-conserved target sites (S102, S116 and S130) located near the high-affinity binding site for the negative regulator SUFU; furthermore, these phosphorylation events cooperate to weaken the affinity of GLI1-SUFU binding by over 25 fold. Phosphorylation of any one, or even any two, of the three sites does not result in the level of SUFU release seen when all three sites are phosphorylated. Tumor-derived mutations in R100 and S105, residues bordering S102, also diminish SUFU binding, collectively defining a novel evolutionarily-conserved SUFU-affinity-modulating region. In cultured mammalian cells, mutant GLI1 variants containing phosphomimetic substitutions of S102, S116 and S130 displayed an increased ability to drive transcription. We conclude that of multisite phosphorylation of GLI1 by ERK2 or other MAP kinases weakens GLI1-SUFU binding, thereby facilitating GLI1 activation and contributing to both physiological and pathological crosstalk.

Plexins Promote Hedgehog Signaling Through Their Cytoplasmic GAP Activity

Hedgehog signaling controls tissue patterning during embryonic and postnatal development and continues to play important roles throughout life. Characterizing the full complement of Hedgehog pathway components is essential to understanding its wide-ranging functions. Previous work has identified Neuropilins, established Semaphorin receptors, as positive regulators of Hedgehog signaling. Neuropilins require Plexin co-receptors to mediate Semaphorin signaling, but a role for Plexins in Hedgehog signaling has not yet been explored. Here, we provide evidence that multiple Plexins promote Hedgehog signaling in NIH/3T3 fibroblasts and that Plexin loss-of-function in these cells results in significantly reduced Hedgehog pathway activity. Catalytic activity of the Plexin GTPase activating protein (GAP) domain is required for Hedgehog signal promotion, and constitutive activation of the GAP domain further amplifies Hedgehog signaling. Additionally, we demonstrate that Plexins promote Hedgehog signaling at the level of GLI transcription factors and that this promotion requires intact primary cilia. Finally, we find that Plexin loss-of-function significantly reduces the response to Hedgehoga pathway activation in the mouse dentate gyrus. Together, these data identify Plexins as novel components of the Hedgehog pathway and provide insight into their mechanism of action.

METTL3 facilitates hepatic fibrosis progression via m6A-YTHDF2 dependent silencing of GPR161

Hepatic fibrosis (HF) is a very common condition seen in millions of patients with various liver diseases. N6-methyladenosine (m6A) plays critical roles in various biological and pathological processes. However, the role of m6A and its main methyltransferase METTL3 in HF remains obscure. Here, we reported that METTL3 expression was elevated in HSCs from CCl4 induced fibrotic liver. METTL3 knockdown in HSCs mediated by recombinant adeno-associated-virus serotype 9 packed short hairpin RNA against METTL3 alleviated liver injury and fibrosis compared to empty carrier group. Mechanistically, the decreased liver fibrosis in CCl4-treated HSC-specific METTL3 knockdown mice was due to the increased GPR161 that is a suppressor of Hedgehog pathway, a well-known pathway to activate in liver injury and regeneration. As expect, GPR161 transferred into HSCs alleviated liver fibrosis and HSC activation. Forced GPR161 expression inhibited Gli3 activated form nuclear accumulation and subsequently suppressed fibrosis-associate gene expression. Conclusion, HSC-specific deletion of METTL3 inhibits liver fibrosis via elevated GPR161 expression, which subsequently suppressed Hedgehog pathway activation and fibrosis-associated genes expression, providing novel therapeutic targets for HF therapy.