Summary RAC1 P29 is the third most commonly mutated codon in human cutaneous melanoma, after BRAF V600 and NRAS Q61. Here, we study the role of RAC1 P29S in melanoma development and reveal that RAC1 P29S activates PAK, AKT, and a gene expression program initiated by the SRF/MRTF transcriptional pathway, which results in a melanocytic to mesenchymal phenotypic switch. Mice with ubiquitous expression of RAC1 P29S from the endogenous locus develop lymphoma. When expressed only in melanocytes, RAC1 P29S cooperates with oncogenic BRAF or with NF1 -loss to promote tumorigenesis. RAC1 P29S also drives resistance to BRAF inhibitors, which is reversed by SRF/MRTF inhibitors. These findings establish RAC1 P29S as a promoter of melanoma initiation and mediator of therapy resistance, while identifying SRF/MRTF as a potential therapeutic target.

The sea lamprey (Petromyzon marinus) is a genetically programmed animal model for biliary atresia as it loses its bile ducts and gallbladder during metamorphosis. However, in contrast to patients with biliary atresia or other forms of cholestasis who develop progressive disease, the post-metamorphosis lampreys grow normally to adult size. To understand how the adult lamprey thrives without the ability to secrete bile, we examined bile salt homeostasis in larval and adult lampreys. Adult livers were severely cholestatic with levels of bile salts >1 mM, but no evidence of necrosis, fibrosis, or inflammation. Interestingly, both larvae and adults had normal plasma levels (~10 μM) of bile salts. In larvae, petromyzonol sulfate (PZS) was the predominant bile salt, whereas the major bile salts in adult liver were sulfated C27 bile alcohols. Cytotoxicity assays revealed that PZS was highly toxic. Pharmacokinetic studies in free-swimming adults revealed that ~35% of intravenously injected bromosulfophthalein (BSP) was eliminated over a 72 hr period. Collection of urine and feces demonstrated that both endogenous and exogenous organic anions, including biliverdin, bile salts and BSP, were predominantly excreted via the kidney with minor amounts also detected in feces. Gene expression analysis detected marked up-regulation of orthologs of known organic anion and bile salt transporters in the kidney with lesser effects in the intestine and gills in adults compared to larvae. These findings indicate that adult lampreys tolerate cholestasis by altering hepatic bile salt composition, while maintaining normal plasma bile salt levels predominantly through renal excretion of bile products. Therefore, we conclude that strategies to accelerate renal excretion of bile salt and other toxins should be beneficial for patients with cholestasis.

Cholestasis impairs liver regeneration following partial liver resection (PHx). Bile acid receptor farnesoid X-receptor (FXR) is a key mediator of liver regeneration. The effects of FXR agonist obeticholic acid (OCA) on liver (re)growth were therefore studied in cholestatic rats. Animals underwent sham surgery or reversible bile duct ligation (rBDL). PHx with concurrent internal biliary drainage was performed 7 days after rBDL. Animals were untreated or received OCA (10 mg/kg/day) per oral gavage from rBDL until sacrifice. After 7 days of OCA treatment, dry liver weight increased in the rBDL + OCA group, indicating OCA-mediated liver growth. Enhanced proliferation in the rBDL + OCA group prior to PHx concurred with a rise in Ki67-positive hepatocytes, elevated hepatic Ccnd1 and Cdc25b expression, and an induction of intestinal fibroblast growth factor 15 expression. Liver regrowth after PHx was initially stagnant in the rBDL + OCA group, possibly due to hepatomegaly prior to PHx. OCA increased hepatobiliary injury markers during BDL, which was accompanied by upregulation of the bile salt export pump. There were no differences in histological liver injury. In conclusion, OCA induces liver growth in cholestatic rats prior to PHx but exacerbates biliary injury during cholestasis, likely by forced pumping of bile acids into an obstructed biliary tree.