locus encodes three critical cell cycle inhibitors including p16Ink4a
and Arf (p14 in human and p19 in mouse). The two members of the Ink4 protein family inhibit the binding of D-type cyclins to cyclin dependent kinase 4 and 6 (Cdk4/6), inhibiting the phosphorylation of retinoblastoma (Rb) family proteins and preventing G1/S phase transition in cells. The Arf protein, which shares two common p16Ink4a
exons but contains a distinct open reading frame, is able to activate the p53 pathway through sequestration of the p53 negative regulator Mdm2 (reviewed in1,2
locus plays a critical role in determining cellular response to oncogenic signals. In normal cells, inappropriate oncogenic stimulation activates this locus and leads to cellular senescence. However, dysregulation of this locus can facilitate tumorigenesis through multiple oncogenic signaling pathways. The importance of this locus in cellular defense against tumorigenesis is further supported by evidence that Ink4a/Arf/Ink4b
is frequently deleted or mutated in a variety of human primary tumors3–6
. In addition, combined deletion of Ink4a/Arf
in mice results in a broader spectrum of tumors compared to mice with individual genetic deletions indicating that genes in this locus work synergistically to prevent tumor development and that p15Ink4b
is a critical tumor suppressor in the absence of p16Ink4a
. Since this locus controls both cellular senescence and tumorigenesis, tight regulation is crucial under physiological conditions. Although many oncoproteins and Polycomb group proteins have been shown to regulate Ink4a/Arf
, the mechanism that controls p15Ink4b
expression remains unclear.
The JmjC-domain containing histone demethylase 1b (Jhdm1b) is a paralogue of the first identified histone lysine demethylase, Jhdm1a, which targets H3K36 for demethylation10
. This fact, as well as their high homology within the catalytic JmjC domain (79%), led us to predict that the demethylase activity against the H3K36 methyl group is conserved between both paralogues (Supplementary Fig. 1
). However, in a recent report Jhdm1b was implicated in the demethylation of H3K4me3 in vivo11
. In addition, the reported biological functions of this protein are also controversial. Although two groups have identified Jhdm1b
as a hotspot for proviral insertion in murine tumors generated by random MMLV mutagenesis, the locus has paradoxically been identified as an oncogene and a tumor suppressor depending on the screen and functional studies used12,13
. In addition, subsequent studies have reported that Jhdm1b was a negative regulator of c-Jun14
or rRNA genes11
, further implicating Jhdm1b in tumor suppression.
In an effort to resolve these apparent discrepancies, we set out to use methods well established in our previous studies to characterize the biochemical properties as well as biological function of Jhdm1b using primary MEF cells, which maintain normal cell cycle regulatory pathways, to study protein function. We report here that Jhdm1b is an H3K36-specific histone demethylase that functions to promote cellular proliferation and inhibit cellular senescence through the silencing of the p15Ink4b tumor suppressor gene.