a431 xenografts in nude mice revealed that knockdown of ythdf2 increased tumor growth, whereas forced overexpression of ythdf2 decreased tumor growth ( fig. 7d, e, and supplementary fig. s17b ). furthermore, skin - specific ythdf2 deletion accelerated skin tumorigenesis induced by chronic uvb irradiation in both male and female mice, demonstrating an inhibitory role of ythdf2 in tumor initiation ( fig. 7f – i ). histological analysis showed that uvb irradiation induces skin tumor formation in both wt and df2 cko mice, while squamous skin carcinoma was detected in the df2 cko mice ( supplementary fig. s17c ). this was supported by immunoflu orescence analysis that indicated expression of the basal keratinocyte marker krt14 ( k14 ) in both basal layer of mouse skin and mouse tumors, while the differentiation marker krt10 ( k10 ) was only detected in mouse skin, hair follicles, tumor from wt mice, but not in the tumor from df2 cko mice ( supplementary fig. s17c ). fig. 7ythdf2 suppresses skin tumorigenesis in mice through inhibiting tlr3. a immunoblot confirming ythdf2 knockdown and overexpression in a431 cells. b cell proliferation of cells as in a. c cell migration of cells as in a. d tumor growth of a431 xenografts in nude mice. e tumor weight for a431 xenografts. f - i number of tumor per mouse ( f and g ) and percentage of tumor - free mice ( h and i ) in wt and df2 cko mice following chronic uvb irradiation. j immunoblot of ythdf2 and cox - 2 in shnc and shdf2 a431 cells with or without u6 knockdown. k cell proliferation of shnc and shdf2 a431 cells with or without u6 knockdown. cell migration ( l ) and proliferation ( m ) in a431 cells with or without ythdf2 knockdown, treated with or without celecoxib ( 20 μm, cox - 2 inhibitor, 24 h ). tumor volume ( n ) and tumor weight ( o ) in nude mice injected with shnc or shdf2 a431 cells with or without tlr3 knockdown. p schematic of the role of ythdf2 in regulating u6 snrna decay and interaction with tlr3 to control uvb - induced inflammation and tumorigenesis. statistical analyses were conducted using a two - tailed unpaired student ’ s t - test ( b – f, h, k – o ) and log - rank test ( g, i ). data are shown as mean ± sd ( n = 3 for b – e, k, l – o; n = 9 for each group in f, g; n = 8 [ wt ] and n = 9 [ df2 cko ] in h, i ). all experiments used biologically independent samples. a immunoblot confirming ythdf2 knockdown and overexpression in a431 cells. b cell proliferation of cells as in a. c cell migration of cells as in a. d tumor growth of a431 xenografts in nude mice. e tumor weight for a431 xenografts. f - i number of tumor per mouse ( f and g ) and percentage of tumor - free mice ( h and i ) in wt and df2 cko mice following chronic uvb irradiation. j immunoblot of ythdf2 and cox - 2 in shnc and shdf2 a431 cells with or without u6 knockdown. k cell proliferation of shnc and shdf2 a431 cells with or without u6 knockdown. cell migration ( l ) and proliferation ( m ) in a431 cells with or without ythdf2 knockdown, treated with or without celecoxib ( 20 μm, cox - 2 inhibitor, 24 h ). tumor volume ( n ) and tumor weight ( o ) in nude mice injected with shnc or shdf2 a431 cells with or without tlr3 knockdown. p schematic of the role of ythdf2 in regulating u6 snrna decay and interaction with tlr3 to control uvb - induced inflammation and tumorigenesis. statistical analyses were conducted using a two - tailed unpaired student ’ s t - test ( b – f, h, k – o ) and log - rank test ( g, i ). data are shown as mean ± sd ( n = 3 for b – e, k, l – o; n = 9 for each group in f, g; n = 8 [ wt ] and n = 9 [ df2 cko ] in h, i ). all experiments used biologically independent samples. next, we assessed whether ythdf2 protein level is altered in human skin cancer. immunoflu orescence analysis showed that ythdf2 was lower in human skin squamous cell carcinoma samples ( both stage 1 and stage 2 ) than in normal human epidermis ( supplementary fig. s17d, e ). furthermore, ythdf2 was lower in stage 2 scc than stage 1 scc ( supplementary fig. s17d, e ), suggesting an active role for ythdf2 in skin tumorigenesis as well as tumor progression. to determine the role of u6 in ythdf2 ’ s functional role, we assessed whether u6 inhibition rescues the effect of ythdf2 knockdown on cell proliferation. u6 knockdown drastically inhibited the effect of ythdf2 knockdown on cox - 2 expression and cell proliferation in a431 cells ( fig. 7j, k and supplementary fig. s17f ). next, we assessed the importance of inflammatory genes in ythdf2 function. among these inflammatory mediators regulated by ythdf2 ( fig. 1 ), cox - 2 is a uvb - inducible enzyme that promotes inflammation and tumorigenesis. indeed, the cox - 2 inhibitor celecoxib drastically suppressed the proliferation and migration of ythdf2 knockdown and control cells; it also diminished the effect of ythdf2 knockdown in a431 cells ( fig. 7l, m, supplementary fig. s17g ). these findings indicate that ythdf2 loss promotes tumor cell proliferation by increasing u6 / cox - 2. in addition, inhibition of tlr3 also reversed the tumor - promoting effect of ythdf2 knockdown in mice from a431 cells ( fig. 7n, o and supplementary fig. s17h, i ). these findings demonstrate that ythdf2 suppresses tumorigenesis through inhibiting the tlr3 pathway. despite emerging investigations into the critical roles of ma mrna methylation in inflammation and cancer, the regulatory and functional role of ma methylation of self non - coding rna in inflammation and tumorigenesis remains largely unknown. here, we show that ythdf2 recognizes ma methylation of non - coding u6 snrna to control tlr3 activation in inflammation and tumorigenesis ( fig. 7p ). ma - methylated u6 snrna is recognized by ythdf2, leading to u6 decay. in addition, both ythdf2 and u6 localize to endosomes via sidt2 and the endocyt osis pathway. ythdf2 binds to ma u6 to compete with tlr3 binding to ma u6, thus inhibiting the expression of inflammatory modulators, cell proliferation, and tumor growth. in addition, uvb irradiation inhibits ythdf2 phosphorylation at s39, which is critical for ythdf2 interaction with cnot1 and localization in endosomes. ythdf2 is also downregulated by uvb irradiation at later time points through p62 - mediated selective autophagy. ythdf2 is reduced in human skin cancer, sle, and type i diabetes, as compared with normal controls. skin - specific ythdf2 deletion sensitizes mice to uvb - induced inflammation and tumorigenesis. our findings demonstrated a critical role of ythdf2 in regulating self non - coding rna in inflammation and tumorigenesis ( fig. 7p ). ythdf2 was initially shown to be an mrna - binding protein that recognizes ma methylation in mrna and promotes mrna decay by directly interacting with cnot1 and recruiting the ccr4 – not deadenylase complex. here, we show that ythdf2 binds to ma - modified self u6 snrna to mediate its decay. as a key component of the spliceosome, u6 binds to and interacts with many protein factors. upon ma methylation by mettl16, u6 structure may be altered such that a sequence on or around the ma site ( a43 ) becomes available to preferentially bind to ythdf2. previously it has been shown that ma rna methylation regulates the rna - structure - dependent accessibility of rna binding motifs to affect rna – protein interactions for mrnas. thus, it is possible that ma methylation of u6 induces u6 structural remodeling to permit ythdf2 binding. in addition, the interaction between ythdf2 and u6 snrnp proteins may also facilitate ythdf2 access to u6 snrna. moreover, we also show that cnot1, which is critical for the ccr4 - not mrna deadenylase activity in the decay of mrnas, including ma - methylated mrnas, binds to u6 snrna through interacting with ythdf2 in an ma - methylation dependent manner, as ythdf2 deletion or mettl16 knockdown inhibited the cnot1 - u6 snrna interaction. previously, the 3 ’ tail of a small fraction of u6 snrna has been shown to be adenylated.