The protein histidine phosphatase LHPP is a tumour suppressor.

Histidine phosphorylation, the so-called hidden phosphoproteome, is a poorly characterized post-translational modification of proteins. Here we describe a role of histidine phosphorylation in tumorigenesis. Proteomic analysis of 12 tumours from an mTOR-driven hepatocellular carcinoma mouse model revealed that NME1 and NME2, the only known mammalian histidine kinases, were upregulated. Conversely, expression of the putative histidine phosphatase LHPP was downregulated specifically in the tumours. We demonstrate that LHPP is indeed a protein histidine phosphatase. Consistent with these observations, global histidine phosphorylation was significantly upregulated in the liver tumours. Sustained, hepatic expression of LHPP in the hepatocellular carcinoma mouse model reduced tumour burden and prevented the loss of liver function. Finally, in patients with hepatocellular carcinoma, low expression of LHPP correlated with increased tumour severity and reduced overall survival. Thus, LHPP is a protein histidine phosphatase and tumour suppressor, suggesting that deregulated histidine phosphorylation is oncogenic.

Plasma PPi Deficiency Is the Major, but Not the Exclusive, Cause of Ectopic Mineralization in an Abcc6-/- Mouse Model of PXE.

Pseudoxanthoma elasticum (PXE), a prototype of heritable ectopic mineralization disorders, is caused in most cases by inactivating mutations in the ABCC6 gene. It was recently discovered that absence of ABCC6-mediated adenosine triphosphate release from the liver and consequently reduced plasma inorganic pyrophosphate (PPi) levels underlie PXE. This study examined whether reduced levels of circulating PPi, an antimineralization factor, is the sole mechanism of PXE. The Abcc6-/- and Enpp1asj mice were crossed with transgenic mice expressing human ENPP1, an ectonucleotidase that generates PPi from adenosine triphosphate. We generated Abcc6-/- and Enpp1asj mice, either wild-type or hemizygous for human ENPP1. Plasma levels of PPi and the degree of ectopic mineralization were determined. Overexpression of human ENPP1 in Enpp1asj mice normalized plasma PPi levels to that of wild-type mice and, consequently, completely prevented ectopic mineralization. These changes were accompanied by restoration of their bone microarchitecture. In contrast, although significantly reduced mineralization was noted in Abcc6-/- mice expressing human ENPP1, small mineralization foci were still evident despite increased plasma PPi levels. These results suggest that PPi is the major mediator of ectopic mineralization in PXE, but there might be an alternative, as yet unknown mechanism, independent of PPi, by which ABCC6 prevents ectopic mineralization under physiologic conditions.