Characterization of c-Kit expression and activation in KSHV-infected endothelial cells.

Kaposi's sarcoma (KS) herpesvirus (KSHV) is the etiological agent of several immunodeficiency-linked cancers, including KS. Our previous work showed that the proto-oncogene c-kit is upregulated in KSHV-infected endothelial cells (ECs), as well as in KS lesions. We show here that KSHV-dependent induction of both c-kit mRNA and protein requires the establishment of a latent infection and that this upregulation occurs in primary DMVECs as well as in immortalized DMVECs (eDMVECs). Interestingly, we find that while the lymphatic EC (LEC) subpopulation exhibits KSHV-induced c-Kit upregulation, the blood EC (BEC) subpopulation does not. Despite this upregulation of c-Kit, receptor activation and phosphorylation of downstream effectors such as MAP Kinase Erk 1/2 and GSK-3 still requires the addition of exogenous c-Kit ligand, stem cell factor (SCF). These data indicate that KSHV does not induce constitutive c-Kit signaling, but instead upregulates c-Kit receptor levels, thus allowing infected ECs to respond to endogenous and exogenous SCF. Nonetheless, inhibition of either c-Kit activation or its downstream effectors reverses the characteristic spindle phenotype of infected eDMVECs. Together, these results contribute to our overall understanding of the role that the c-kit proto-oncogene plays in KS pathogenesis.

Induction of tumorigenesis and metastasis by the murine orthologue of tumor protein D52.

Expression studies have consistently identified tumor protein D52 (TPD52) overexpression in tumor cells. Murine TPD52 (mD52) shares 86% identity with the human orthologue. To study a possible role for TPD52 in transformation, 3T3 fibroblasts were transfected with the full-length cDNA for mD52. Expression of mD52 was confirmed by reverse transcription-PCR (RT-PCR), real-time PCR, and Western blot analysis compared with 3T3 and vector-transfected 3T3 (3T3.V), and the resultant cell line was designated 3T3.mD52. At 4 weeks, 3T3.mD52 gained a 2-fold increase in growth rate, lost contact inhibition, and exhibited a marked phenotype change. Further characterization revealed an acquired ability for anchorage-independent cell growth. To determine whether 3T3.mD52 had become tumorigenic, naïve, healthy, immunocompetent syngeneic mice were inoculated subcutaneously with varying cell doses. Tumors measuring >1 cm(2) were detected 60 days postinoculation with 3T3.mD52, and a 50% subcutaneous tumor incidence was obtained with as few as 5 x 10(5) 3T3.mD52 cells. Remarkably, when lungs from 3T3.mD52 tumor-bearing mice were analyzed, numerous tumor nodules were observed, ranging from nodules less than 10 to nodules too numerous to count (inoculation with 1 x 10(5) and 5 x 10(6) cells, respectively). Further support for the metastatic capacity of 3T3.mD52 was the demonstration that transforming growth factor (TGF)-betaR1 (receptor) expression decreased and TGF-beta1 secretion increased in 3T3.mD52 compared with 3T3 controls. cDNA microarray analysis showed a gene expression pattern that further supported mD52-induced transformation and metastasis. Together, these data suggest that mD52 expression in 3T3 cells initiated cellular transformation, tumorigenesis, and progression to metastasis.