Conserved Regulation of MAP Kinase Expression by PUF RNA-Binding Proteins

Abstract

Mitogen-activated protein kinase (MAPK) and PUF (for Pumilio and FBF [fem-3 binding factor]) RNA-binding proteins control many cellular processes critical for animal development and tissue homeostasis. In the present work, we report that PUF proteins act directly on MAPK/ERK-encoding mRNAs to downregulate their expression in both the Caenorhabditis elegans germline and human embryonic stem cells. In C. elegans, FBF/PUF binds regulatory elements in the mpk-1 3′ untranslated region (3′ UTR) and coprecipitates with mpk-1 mRNA; moreover, mpk-1 expression increases dramatically in FBF mutants. In human embryonic stem cells, PUM2/PUF binds 3′UTR elements in both Erk2 and p38α mRNAs, and PUM2 represses reporter constructs carrying either Erk2 or p38α 3′ UTRs. Therefore, the PUF control of MAPK expression is conserved. Its biological function was explored in nematodes, where FBF promotes the self-renewal of germline stem cells, and MPK-1 promotes oocyte maturation and germ cell apoptosis. We found that FBF acts redundantly with LIP-1, the C. elegans homolog of MAPK phosphatase (MKP), to restrict MAPK activity and prevent apoptosis. In mammals, activated MAPK can promote apoptosis of cancer cells and restrict stem cell self-renewal, and MKP is upregulated in cancer cells. We propose that the dual negative regulation of MAPK by both PUF repression and MKP inhibition may be a conserved mechanism that influences both stem cell maintenance and tumor progression. The mitogen-activated protein (MAP) kinase (MAPK) enzyme is crucial for regulation of both stem cell maintenance and tumorigenesis. Two conserved controls of MAPK include its activation by RAS signaling and a kinase cascade as well as its inactivation by MAPK phosphatases (MKPs). We identify a third mode of conserved MAPK regulation. We demonstrate that PUF (for Pumilio and FBF [fem-3 binding factor]) RNA-binding proteins repress mRNAs encoding MAPK enzymes in both the Caenorhabditis elegans germline and human embryonic stem cells. PUF proteins have emerged as conserved regulators of germline stem cells in C. elegans, Drosophila, and probably vertebrates. Their molecular mode of action relies on binding to sequence elements in the 3′ untranslated region of target mRNAs. We report that PUF proteins bind and repress mRNAs encoding C. elegans MPK-1 as well as human ERK2 and p38α. We also report that PUF repression and MKP inactivation function redundantly in the C. elegans germline to restrict MPK-1/MAPK activity and prevent germ cell apoptosis. We suggest that this dual regulation of MAPK activity by PUF and MKP proteins may be a conserved mechanism for the control of growth and differentiation during animal development and tissue homeostasis.