Para-amino benzoic acid (PABA) modulates expression and stability of CDC25A in tumor cells and enhances radiosensitivity in vitro and in vivo

Abstract

PABA or p-Amino Benzoic Acid is a water-soluble natural vitamin-like compound thought to be important in the synthesis of folic acid. Moreover, PABA has also been implicated in amino acid synthesis, and the biosynthetic pathway of purines and pyrimidine. Interestingly, PABA has been used for a variety of purposes. It is a commonly included in sunscreen lotions do to its ability to absorb UV light. Importantly, PABA has been evaluated in human clinical trials for its efficacy on a number of connective tissue diseases including Dermatomyositis, Peyronie disease and Scleroderma and thus, much of the human pharmacokinetics and toxicities are known. Our previous studies have suggested that PABA can inhibit melanogenesis in melanoma cell and enhance the chemo and radiosensitivity of a variety of tumor cells (AACR-NCI-EORTC Meeting 2003). Compelling evidence suggests that modulation of cell cycle checkpoints can enhance radiosensitivity of certain tumor cells. Moreover, studies have suggested that modulation of CHK1 /CDC25A signaling pathways may increase radiosensitivity. Based on previous studies suggesting a role for CDC25A in regulating radiosensitivity, we sought to examine the effects of PABA on the expression and stability of CDC25A in tumor cells in vitro and in vivo. To examine the effects of PABA and ionizing radiation on tumor cell proliferation, mitochondrial dehydrogenase activity was assessed with a commercially available in vitro proliferation assay kit. To begin to assess the effects of PABA on expression of CDC25A, cDNA array analysis was performed on tumor cells incubated in the presence or absence of PABA. To confirm any changes in the relative expression of CDC25A, western blot analysis was carried out on whole cell lysates derived from tumor cells incubated in the presence or absence or PABA. Finally, the effects of PABA on tumor radiosensitivity was assessed in vivo, both in chick embryo and murine tumor models. Our previous studies suggested that PABA enhanced the sensitivity of malignant melanoma and breast cancer cells to chemotherapy and ionizing radiation. Here we extend these studies and show that pre-treatment of breast carcinoma cells with PABA enhanced the anti-proliferative effects of ionizing radiation in vitro and enhanced radiosensitivity of breast carcinoma tumors grown in vivo. Interestingly, analysis of cDNA array data suggested that CDC25A gene expression was enhanced by approximately 2 fold in both murine and human melanoma cells as compared to untreated cells. This increase in CDC25A expression was confirmed by Western Blot analysis of whole cell lysates derived from both melanoma and breast carcinoma cells incubated in the presence of PABA. Interestingly, pre-treatment of breast carcinoma cells with PABA resulted in a reduction of radiation-induced degradation of CDC25A in vitro. These findings extend our previous results and suggest that PABA may enhance the radiosensitivity of both melanoma and breast carcinoma in vitro and in vivo. Moreover, the findings are consistent with the possibility that the enhanced radiosensitivity observed after PABA may be associated with an increase in CDC25A protein expression and stability. These data, taken together with the relatively low toxicities associated with oral doses of PABA in humans, suggest that administration of PABA in combination with ionizing radiation may represent a promising new approach to enhance radiotherapy effects on tumors