Eucalyptal A inhibits glioma by rectifying oncogenic splicing of MYO1B mRNA via suppressing SRSF1 expression

Abstract

Glioma is the most common primary intracranial tumor, in which glioblastoma (GBM) is the most malignant and lethal. However, the current chemotherapy drugs are still unsatisfactory for GBM therapy. As the natural products mainly extracted from Eucalyptus species, phloroglucinol-terpene adducts have the potential to be anti-cancer lead compounds that attracted increasing attention. In order to discover the new lead compounds with the anti-GBM ability, we isolated Eucalyptal A with a phloroglucinol-terpene skeleton from the fruit of E. globulus and investigated its anti-GBM activity in vitro and in vivo. Functionally, we verified that Eucalyptal A could inhibit the proliferation, growth and invasiveness of GBM cells in vitro. Moreover, Eucalyptal A had the same anti-GBM activity in tumor-bearing mice as in vitro and prolonged the overall survival time by maintaining mice body weight. Further mechanism research revealed that Eucalyptal A downregulated SRSF1 expression and rectified SRSF1-guided abnormal alternative splicing of MYO1B mRNA, which led to anti-GBM activity through the PDK1/AKT/c-Myc and PAK/Cofilin axes. Taken together, we identified Eucalyptal A as an important anti-GBM lead compound, which represents a novel direction for glioma therapy.

Introduction

Gliomas are the most common intracranial primary tumors, in which glioblastoma (GBM) is the most malignant and lethal (Tirosh and Suvà, 2018; Wang et al., 2018). Even with the standard therapies of surgery plus radiation and temozolomide (TMZ), the survival is only 15–18 months in GBM patients (Lefranc et al., 2009). To meet the urgent needs in clinic, natural products have become an indispensable source of new chemotherapy drugs and lead compounds (Gouveia et al., 2019). Eucalyptus globulus Labill. (E. globulus) belongs to Eucalyptus species in Myrtaceae family (Poke et al., 2005; www.theplantlist.org). E. globulus was called as “Yi-Kou-Zhong” in China that is used to treat inflammation, eczema, influenza and rheumatoid arthritis (Pham et al., 2019). Moreover, preliminary studies have found that the phloroglucinol-terpene adducts extracted from Eucalyptus species exhibit antitumor effects by inhibiting proliferation, migration and invasion of cancer cells, and inducing their apoptosis (Shang et al., 2016; Jian et al., 2017; Qin et al., 2018; Yang et al., 2012; Pham et al., 2019). Therefore, phloroglucinol-terpene adducts have become an important source to discover antitumor lead compounds, which has opened up a novel direction for the research in this field.

Alternative splicing (AS) of pre-mRNA is an indispensable step in mRNA processing, during which mature mRNA is formed by removing introns from the pre-mRNA and attaching the exons together (Donaldson and Beazley-Long, 2016; Gonçalves et al., 2017; Pohl et al., 2013). However, aberrant events in the AS of pre-mRNA process usually cause the production of detrimental subtypes, such as carcinogenic splicing variants, and often occur during tumor growth and development (Oltean and Bates, 2014; Srebrow and Kornblihtt, 2006; Dehm, 2013). Consequently, a plethora of small molecule inhibitors have been found to correct these aberrant splicing errors by inhibiting the carcinogenic variants or upstream splicing factors; this has become a potentially novel strategy for tumor treatment (Wang and Lee, 2018). In our latest research (Zhou et al., 2019a), we observed that serine/arginine-rich splicing factor 1 (SRSF1) is upregulated in gliomas, which switches the AS of myosin 1 B (MYO1B) mRNA to contribute to the proliferation, survival, and invasion of GBM cells.

In this research, we plan to discover a natural product with a phloroglucinol-terpene skeleton and anti-GBM activity from E. globulus, and conduct a preliminary study on its anti-GBM mechanism by reversing the above-mentioned abnormal AS of MYO1B.