Lactate as a metabolic marker for LDH-A activity, metastatic capacity and treatment response in tumors

Tumors with high tissue lactate concentrations and high lactate dehydrogenase A (LDH-A) expression have been linked to poor prognosis, and associated with greater metastatic potential. LDH-A is a bridge between several metabolic pathways and the product of LDH-A activity (lactate) can be assessed non-invasively and quantitatively using magnetic resonance spectroscopic (MRS) imaging (MRSI). While the protons in lactate have a very low concentration relative to water and they overlap with lipid in the proton spectrum requiring sophisticated editing techniques, these obstacles can be overcome by lactate MRS. Although lactate MRS is not a new technique, the correlation with LDH-A gene expression and LDH-A enzyme activity, its spatial association with LDH-A and non-invasive monitoring of LDH-A targeted therapy using lactate MRSI is novel.;In the first part of this study, I showed a substantial difference in LDH-A expression between isogenic murine breast tumor cells 67NR and 4T1 under normoxia and hypoxia that reflect LDH-A enzyme activity and lactate concentrations in tumors. I showed small orthotopic 4T1 tumors generate ten-fold more lactate than corresponding 67NR tumors. I also showed that lactate-MRSI has a greater dynamic range than [18F] FDG-PET and may be a more sensitive measure with which to evaluate the aggressive and metastatic potential of primary breast tumors.;Next, I went to transfect 4T1 cells were by shRNA plasmids that are designed to specifically knock down the expression of LDHA genes by RNA interference under stable transfection (for puromycin-resistance). I showed that the low tumor lactate in the 4T1 LDH-A shRNA transfected tumors is linked with slower tumor growth, lower tumor lactate accumulation and delayed metastases, compare to the control 4T1 cells transfected with scrambled shRNA. The LDH-A knockdown 4T1 tumors were also found to be accompanied by changes in metabolism including enhanced oxidative phosphorylation and oxygen consumption and decreased glycolysis.;In addition, I went to show that lactate has the potential to be a biomarker of treatment effect also. I have used human de-differentiated liposarcoma (DDLS) tumor xenograft implanted in mice in response to CPT-11 treatment. As early as 48 hours after treatment, a reduction in lactate was observed accompanied by growth arrest in CPT-11-treated DDLS tumors.;Collectively, these observations focused on LDH-A, as a critical point in glucose and glutamine metabolism, and showed that imaging tumor lactate levels will reflect LDH-A enzymatic activity, propensity for developing metastases and treatment efficacy.