RESUMO
8-Cl-cAMP is a novel agent able to inhibit the growth of a wide variety of cancer cell types in vitro and in vivo by interfering with the protein kinase A type I (PKAI), a protein directly involved in mitogenic signalling and neoplastic transformation. In a recent phase I study conducted in cancer patients we have demonstrated that 8-Cl-cAMP, at doses devoid of toxicity, may achieve plasma concentrations in a range previously shown effective for cancer cell growth inhibition. In the present study we have investigated the effect of 8-Cl-cAMP in association with cytotoxic drugs acting by different mechanisms of action on the growth of LS174T and GEO human colon cancer cells. We here demonstrate that 8-Cl-cAMP administered after the cytotoxic drugs does not interfere with their growth inhibitory effect but rather is additive with most of them. Moreover, a synergistic effect was observed when 8-Cl-cAMP was administered after cisplatin or paclitaxel. The sequence of treatment seems to be important since pretreatment with 8-Cl-cAMP interferes with the effect of the cytoxic drugs. These results demonstrate that 8-Cl-cAMP is not only able to induce cell growth inhibition when used alone but also exhibit the capacity to enhance the efficacy of different cytotoxic drugs.
RESUMO
Protein kinase A type I (PKAI) and its regulatory subunit RI alpha are overexpressed in cancer cells and are induced by mitogenic hormones and growth factors in nontransformed cells. RI alpha/PKAI are directly involved in the G1>S transition and cell proliferation of non-transformed human breast MCF-10A cells. Retroviral vector-mediated overexpression of RI alpha in these cells (MCF-10A RI alpha) confers the ability to grow in serum-free medium. p53 controls a G1 check point before transition to the S phase, playing a key role in the regulation of cell proliferation and in the preservation of DNA integrity. In this study we evaluated the interaction of p53 and RI alpha on cell cycle progression and cell proliferation of MCF-10A cells. Retroviral vector-mediated overexpression of wild-type p53 in the MCF-10A neo and MCF-10A RI alpha cells determined a marked inhibition of RI alpha protein expression in MCF-10A-p53 cells and induced G0/G1 accumulation, cell gowth arrest and changes in cell morphology not due to apoptosis in both MCF-10A-p53 and MCF-10A RI alpha-p53 cells. On the other hand, in the MCF-10A RI alpha cells we observed an increased expression of the endogenous p53, although these cells were still able to proliferate. These results suggest that overexpression of wildtype p53 acts in a dominant fashion to abrogate the RI alpha induction of G1>S transition and cell proliferation. Moreover, overexpression of RI alpha leads to increased synthesis of endogenous p53 which, however, is unable to interfere with the RI alpha-dependent mitogenic signalling.
