HNRNPLL stabilizes mRNA for DNA replication proteins and promotes cell cycle progression in colorectal cancer cells

Heterogeneous nuclear ribonucleoprotein L-like (HNRNPLL), an RNA-binding protein that regulates alternative splicing of pre-mRNA, has been shown to regulate differentiation of lymphocytes, as well as metastasis of colorectal cancer cells. Here, we show that HNRNPLL promotes cell cycle progression and, hence, proliferation of colorectal cancer cells. Functional annotation analysis of those genes whose expression levels were changed threefold or more in RNA sequencing analysis between SW480 cells overexpressing HNRNPLL and those knocked down for HNRNPLL revealed enrichment of DNA replication-related genes by HNRNPLL overexpression. Among 13 genes detected in the DNA replication pathway, PCNA, RFC3 and FEN1 showed reproducible upregulation by HNRNPLL overexpression both at mRNA and at protein levels in SW480 and HT29 cells. Importantly, knockdown of any of these genes alone suppressed the proliferation-promoting effect induced by HNRNPLL overexpression. RNA-immunoprecipitation assay presented a binding of FLAG-tagged HNRNPLL to mRNA of these genes, and HNRNPLL overexpression significantly suppressed the downregulation of these genes during 12h of actinomycin D treatment, suggesting a role of HNRNPLL in mRNA stability. Finally, analysis of a public RNA sequencing dataset of clinical samples suggested a link between overexpression of HNRNPLL and that of PCNA, RFC3 and FEN1. This link was further supported by immunohistochemistry of colorectal cancer clinical samples, whereas expression of CDKN1A, which is known to inhibit the cooperative function of PCNA, RFC3 and FEN1, was negatively associated with HNRNPLL expression. These results indicate that HNRNPLL stabilizes mRNA encoding regulators of DNA replication and promotes colorectal cancer cell proliferation.