Kaspareit-Rittinghausen described a rodent model of inherited polycystic kidney disease (PKD), the Han:SPRD rat [1, 2], in which heterozygotes develop renal cysts and renal failure (in males) over several months, whereas homozygous animals develop rapidly progressive renal enlargement that leads to death in a few weeks. In this study, we examined selected elements of the pathogenesis of this disease in heterozygotes and homozygotes from birth to advanced disease. Heterozygous male rats developed slowly progressive renal cystic disease with interstitial fibrosis and azotemia seen by six months of age. Female heterozygotes developed slowly progressive renal cystic disease, but did not develop interstitial fibrosis or azotemia. Epithelial cells lining cyst cavities showed various degrees of morphologic immaturity. Cyst walls also developed basement membrane thickening, especially in areas of cellular immaturity, suggesting an interrelationship between this basement membrane thickening and cellular dedifferentiation. Thickened basement membranes were associated with increased immunoreactivity for type IV collagen, laminin, and fibronectin. Homozygous rats developed massive renal enlargement, marked azotemia, and died near three weeks of age. Renal c-myc proto-oncogene expression was elevated in homozygous cystic infants and in adult heterozygotes. In situ hybridization showed high levels of c-myc mRNA in cyst epithelia, suggesting abnormal regulation of cellular proliferation in the cells lining cysts, as seen in other models of PKD. The Han:SPRD rat is the only well-documented animal model of inherited PKD with an autosomal-dominant inheritance pattern and appears to have several features which resemble human ADPKD.

Two G protein-coupled receptors (Edg-2) and (Edg-4) for the lysolipid phosphoric acid mediator lysophosphatidic acid have been described by molecular cloning. However, the calcium-mobilizing receptor Edg-4 is not expressed in some cell lines that exhibit robust calcium responses to this ligand, thus predicting the existence of additional receptor subtypes. We report here on the characterization of a third human lysophosphatidic acid receptor subtype, Edg-7, which mediates lysophosphatidic acid-evoked calcium mobilization. In a rat hepatoma Rh7777 cell line that lacks endogenous responses to lysophosphatidic acid, this lipid mediator, but not others, evokes calcium transients when the cells have been transfected with Edg-7 or Edg-4 DNAs. Furthermore, frog oocytes exhibit a calcium-mediated chloride conductance in response to mammalian-selective lysophosphatidic acid mimetics after injection of Edg-7 mRNA. Edg-7-expressing Rh7777 cells do not show inhibition of forskolin-driven rises in cAMP in response to lysophosphatidic acid. However, membranes from HEK293T cells cotransfected with Edg-7 and G(i2)alpha protein DNAs show lysophosphatidic acid dose-dependent increases in [gamma-(35)S]GTP binding with an EC(50) value of 195 nM. When we used this assay to compare various synthetic LPA analogs at Edg-2, Edg-4, and Edg-7 receptors, we found that ethanolamine-based compounds, which are full LPA mimetics at Edg-2 and Edg-4, exhibit little activity at the Edg-7 receptor. Edg-7 RNA was detected in extracts of several rat and human tissues including prostate. Together, our data indicate that Edg-7 is a third lysophosphatidic acid receptor that couples predominantly to G(q/11)alpha proteins.

Hypoxia drives malignant progression in part by promoting accumulation of the oncogenic transcription factor HIF-1α in tumor cells. Tumor aggressiveness also relates to elevation of the cancer stem cell-associated membrane protein CD24, which has been causally implicated in tumor formation and metastasis in experimental models. Here we link these two elements by showing that hypoxia induces CD24 expression through a functional hypoxia responsive element (HRE) in the CD24 promoter. HIF-1α overexpression induced CD24 mRNA and protein under normoxic conditions, with this effect traced to a recruitment of endogenous HIF-1α to the CD24 promoter. shRNA mediated-attenuation of HIF-1α or CD24 expression reduced cancer cell survival in vitro and in vivo at the levels of primary and metastatic tumor growth. CD24 overexpression in HIF-1α-depleted cancer cells rescued this decrease while HIF-1α overexpression in CD24-depleted cells did not. Analysis of clinical tumor specimens revealed a correlation between HIF-1α and CD24 levels and an association of their co-expression to decreased patient survival. Our results establish a mechanistic linkage between two critically important molecules in cancer, identifying CD24 as a critical HIF-1α transcriptional target and biological effector, strengthening the rationale to target CD24 for cancer therapy.