Fig 2: Overexpression of ALKBH5 suppresses human osteosarcoma growth in vivo. a. Overexpression of ALKBH5 effectively inhibited human osteosarcoma subcutaneous tumor growth in nude mice (n = 12 per group). b. The size of tumor was monitored every 5 days. c. Representative images of IHC staining, showing ALKBH5, SOCS3, and STAT3 in two subcutaneous tumor models. d. The tumor growth was effectively inhibited in YTHDF2 down-regulated group. However, ALKBH5 had no significant effect on tumor growth in YTHDF2 deficiency group (n = 4 per group). The data represent the mean± S.D. of three independent experiments. ANOVA test was used for statistical analysis. *P<0.05 or **P<0.01 indicates a significant difference between the indicated groups.

Fig 3: Proposed working model summarizing the mechanism of this study. m6A demethylase ALKBH5 decreases the m6A modification of SOCS3 mRNA, leading to inhibiting YTHDF2-dependent degradation of SOCS3, resulting in up-regulation of SOCS3 expression, thereby inactivates STAT3 pathway and suppresses the tumor proliferation and growth.

Fig 4: Overexpression of ALKBH5 enhances SOCS3 mRNA stability via an m6A-YTHDF2-dependent mechanism. a. Dual-luciferase reporter showed that knockdown of YTHDF2 increased luciferase activity of reporter carrying wild-type 3′ UTR fragment of SOCS3, while these changes were abolished when the m6A sites were mutated. b. Bioinformatics analysis of osteosarcoma showed a negative correlation between YTHDF2 and SOCS3 (http://hgserver1.amc.nl)(n = 88). c. inhibition of YTHDF2 in osteosarcoma cells improved the expression of SOCS3 to a similar degree to overexpression of ALKBH5. d. Streptavidin RNA pull-down assay demonstrated that YTHDF2 bound the SOCS3 full-length transcripts in osteosarcoma cells. e. The SOCS3 mRNA half-lives were significantly increased upon ALKBH5 overexpression and YTHDF2 inhibition in osteosarcoma cells. f. There was no significant difference in SOCS3 mRNA half-lives between ALKBH5 overexpression and control cells with YTHDF2 deficiency. g. Representative images showing high or low expression of SOCS3,ALKBH5 and YTHDF2 in osteosarcoma tissues. h. Correlation between SOCS3 and ALKBH5 or YTHDF2 in osteosarcoma microarray specimens. The data represent the mean± S.D. of three independent experiments. ANOVA or t-tests was used for statistical analysis. **P<0.01 indicates a significant difference between the indicated groups.

Fig 5: m6A methylation regulates the expression of regulators of STAT3 activation. a. There was no distinct effect on the STAT3 m6A modification after ALKBH5 overexpression. b. SOCS3 m6A methylation decreased after ALKBH5 overexpression. c. Mutations at the four putative m6A sites in SOCS3 (A to T) were generated. d. ALKBH5 decreased the m6A modifications of SOCS3-WTand SOCS3-MUTs (which contains only one potential m6A site) in osteosarcoma cells. And the decrease degree of m6A modification was suppressed in SOCS3-MUTs compared to SOCS3-WT.While the m6A modification of SOCS3-MUT1–4 (which contains all four potential m6A sites mutation) was not decreased in osteosarcoma cells with overexpression of ALKBH5. e. SOCS3 expression was elevated when ALKBH5 was overexpressed both in U2OS and KHOS cells. f. SOCS3 m6A modification level of osteosarcoma cells was decreased when treated with DAA. g. SOCS3 expression was augmented strongly when treated with DAA. h. The decrease of m6A modification caused by DAA was not detected in SOCS3-MUT1–4 cells. i. SOCS3 expression negatively correlated with m6A methylation in osteosarcoma tissues. The data represent the mean± S.D. of three independent experiments. ANOVA or t-tests was used for statistical analysis. *P<0.05 or **P<0.01 indicates a significant difference between the indicated groups.