Intracellular factors that regulate
nitric oxide (NO) synthesis represent important targets in
tumor progression. Overexpression of
dimethylarginine dimethylaminohydrolase (DDAH), which metabolizes the endogenous inhibitors of NO synthesis
asymmetric dimethylarginine and N-monomethyl-
L-arginine, results in C6
gliomas with enhanced growth rate compared with wild type. To investigate the effects of DDAH on
tumor vascular morphogenesis in vivo, we have measured the transverse relaxation rates R(2)* and R(2) in clone D27
gliomas overexpressing DDAH and C6 wild-type
gliomas using intrinsic susceptibility magnetic resonance imaging (MRI), sensitive to changes in endogenous [
deoxyhemoglobin], and susceptibility contrast-enhanced MRI using the intravascular blood pool
contrast agent NC100150, and we compared the results with fluorescence microscopy of the
tumor uptake of the perfusion marker
Hoechst 33342. The baseline R(2)* was significantly faster in the D27
tumors, consistent with a greater vascular development (P < 0.02, ANOVA). There was no significant difference between the response of the two
tumor types to
hypercapnia (5% CO(2)/95% air), used as a probe for vascular maturation, or
hyperoxia (5% CO(2)/95% O(2)), used as a probe for vascular function.
NC100150 increased the R(2)* and R(2) rates of both
tumor types and demonstrated a significantly larger blood volume in the D27
tumors (P < 0.02, ANOVA). This correlated with a significantly greater uptake of
Hoechst 33342 in the D27
tumors compared with C6 wild-type
tumors (P < 0.02, ANOVA). Despite the increased
tumor blood volume, the Delta R(2)*/Delta R(2) ratio, an index of microvessel size, showed that the capillaries in the two
tumor types were of a similar caliber. The data highlight the potential of susceptibility MRI-derived quantitative end points to noninvasively assess
tumor angiogenesis, and in this regard, the use of intravascular blood pool
contrast agents such as
NC100150 appears very promising. Overexpression of DDAH results in increased neovascularization of C6
gliomas in vivo. The lack of significant difference in hypercapnic/hyperoxic response between the C6 and D27
tumors and the similar vessel caliber are also consistent with a role for DDAH in the initial stages of vasculogenesis.