The 58-kilodalton inhibitor of the interferon-induced double-stranded RNA-activated protein kinase is a tetratricopeptide repeat protein with oncogenic properties

The interferon-induced RNA-dependent protein kinase (PKR) is considered to play an important role in the cellular defense against viral infection and, in addition, has been suggested to be a tumor suppressor gene because of its growth-suppressive properties. Activation of PKR by double-stranded RNAs leads to the phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha) and a resultant block to protein synthesis initiation. To avoid the consequences of kinase activation, many viruses have developed strategies to down-regulate PKR. Recently, we reported on the purification and characterization of a cellular inhibitor of PKR (referred to as p58), which is activated during influenza virus infection. Subsequent cloning and sequencing has revealed that p58 is a member of the tetratricopeptide repeat (TPR) family of proteins. To further examine the physiological role of this PKR inhibitor, we stably transfected NIH 3T3 cells with a eukaryotic expression plasmid containing p58 cDNA under control of the cytomegalovirus early promoter. By taking advantage of a recently characterized p58 species-specific monoclonal antibody, we isolated cell lines that overexpressed p58. These cells exhibited a transformed phenotype, growing at faster rates and higher saturation densities and exhibiting anchorage-independent growth. Most importantly, inoculation of nude mice with p58-overexpressing cells gave rise to the production of tumors. Finally, murine PKR activity and endogenous levels of eIF-2 alpha phosphorylation were reduced in the p58-expressing cell lines compared with control cells. These data, taken together, suggest that p58 functions as an oncogene and that one mechanism by which the protein induces malignant transformation is through the down-regulation of PKR and subsequent deregulation of protein synthesis.     

Measurement of response to treatment in colorectal liver metastases

Assessment of tumour response to chemotherapy is important when assessing efficacy of treatment and comparing differing therapeutic regimens. Percentage hepatic replacement (PHR) is commonly used to assess response to treatment of colorectal hepatic metastases. PHR is dependent not only on tumour volume, but also on hepatic parenchymal volume. The effect of tumour growth on hepatic parenchymal volume is unclear but is of importance owing to its effect on PHR. We assessed tumour and hepatic parenchymal weights in an animal tumour model using dissection, and tumour and hepatic parenchymal volumes in patients with colorectal hepatic metastases using CT scanning, in order to establish how hepatic parenchyma varied with change in metastasis size. There was no significant correlation between tumour and liver parenchyma in either the animal model (r = -0.03, P > 0.05) or the patient study (r = 0.3, P < 0.05). This suggests that hepatic parenchymal volume was preserved in the presence of increasing tumour volume. In a further study of computerised tomographic (CT) scans before and after treatment in patients whose tumours either responded to chemotherapy or continued to grow, change in PHR (median proportion of PHR change = 0.40) significantly (P = 0.04) underestimated the change in tumour volume (median proportion of tumour volume change = 0.56), particularly at higher (> 400 ml) volumes. There was good correlation between change in tumour volume and WHO criteria in assigning patients to tumour growth, stable disease or tumour response categories. This study suggests that, in clinical trials comparing colorectal liver metastasis treatments, metastasis volume and not PHR should be used to assess extent of disease and the effect of treatment.