G Sandeep, S Bhasker, Y Sri Ranganath
apoptosis, camp, chemotherapy, cytotoxicity, phosphodiesterase
G Sandeep, S Bhasker, Y Sri Ranganath. Phosphodiesterase as a novel target in Cancer Chemotherapy. The Internet Journal of Pharmacology. 2008 Volume 7 Number 1.
The Phosphodiesterase inhibitors have been used for the treatment of non-malignant conditions like asthma, pulmonary hypertension and erectile dysfunction etc, but the recent research suggests that they have great therapeutic value in the adjunctive therapy of Cancer. This article reviews the various substances elevating the intracellular cyclic AMP levels such as cAMP analogues and Phosphodiesterases inhibitors to induce apoptosis selectively in a variety of cancer cell lines with out affecting the normal cells. Their usefulness in the adjunctive therapy of cancer is explained.
Research aimed for treatment of cancer had been the focus of many industries and academic groups since last two decades. Besides radiation therapy and surgery, chemotherapy is the major modality for the treatment of cancer. But most of the chemotherapeutic agents exhibit different ranges of toxicity because they are administered at their maximum tolerated doses. But with the use of the drugs that elevate the intracellular cyclic AMP levels such as cAMP analogues and Phosphodiesterase inhibitors, we can achieve the therapeutic target just above the minimum effective concentration of the currently used anti-cancer drugs, thus avoiding the potential adverse effects. This improves the patient compliance too.
In this article, an attempt has been made in elucidating the role of raised levels of cAMP in inducing apoptosis by agents such as cAMP analogues and Phosphodiesterase inhibitors. The recent advances of research on the potential role of Phosphodiesterase inhibitors to selectively induce apoptosis in various experimental models have been explained.
Cyclic adenosine monophosphate (cAMP, cyclic AMP or 3'-5'-cyclic adenosine monophosphate) is a molecule that is important in many biological processes. cAMP is derived from adenosine triphosphate (ATP) and used for intracellular signal transduction in many different organisms. It is synthesised from ATP by adenylyl cyclase which is located at the cell membranes.
In humans, cAMP works by activating protein kinase A (PKA, also known as cAMP-dependent protein kinase). Cyclic adenosine monophosphate (cAMP) has a dual effect i.e., proliferation and differentiation processes in cells. The main target of cAMP action in the cell is cAMP-dependent protein kinase, which may exist as two different isozymes, designated as type I (PKA-I) and type II (PKA-II). PKA-I acts as a positive growth effector, whereas PKA-II inhibits cell division 1 .
Cyclic AMP as a tool to combat cancer
Cyclic AMP is a positive intracellular signal for cell proliferation in many differentiated cells. But in many tumor cells it is a negative messenger for proliferation, showing a much lower basal level than in normal cells 2 . The lowest concentration of cyclic AMP found to affect cell growth was o.1 mM & at a concentration of 0.5mM it arrested cell multiplication after 24 h in monolayer cultures of human liver cells 3 . Additions of high, nonphysiological concentrations (10 -5 M to 10 -3 M) of cyclic AMP suppress the induction of DNA synthesis after serum addition in quiescent mouse fibroblasts 4 .
Selective toxicity of cAMP towards Cancer cells
In malignant tissue, the concentration of cyclic AMP is lower in cells transformed in vitro by oncogenic viruses than in untransformed cells, and is lower in certain tumours grown in vivo than in the corresponding normal tissue 5 .The growth of four tumorigenic cell lines (Fl amnion, HEp-2, HeLa(line 229) and strain L (NCTC clone 929) was inhibited from 70 to 89% by adenosine 3' , 5'-cyclic monophosphate (0.3 mM) whereas a non-malignant cell line (WI-38) was affected only slightly (13%). Guanosine 3', 5'-cyclic monophosphate and quanosine 2', 3'-cyclic monophosphate) did not show selectivity; they inhibited the tumorigenic strains and the Wl-38 cell line. This shows that cAMP growth inhibition is selective towards tumorigenic cell lines 6 .
In malignant cells, exogenous cyclic AMP or agents that increase the intracellular concentration of this cyclic nucleotide like Phosphodiesterase Inhibitors decrease the rate of growth and induce morphological and biochemical differentiation 5 .
Cyclic nucleotide Phosphodiesterase, a potential target to combat cancer
A Phosphodiesterase is an enzyme that breaks a phosphodiester bond. The phosphodiesterase enzyme families are namely, Cyclic nucleotide phosphodiesterases, phospholipases C and D, autotaxin, sphingomyelin phosphodiesterase, DNAses, RNAses, and restriction endonucleases. But the cyclic nucleotide phosphodiesterases have great clinical significance.
The cyclic nucleotide phosphodiesterases are coded by 21 PDE genes in humans 7 , which are divided into 11 families based on protein sequences, regulatory considerations & sensitivity to inhibitors besides their activity towards cAMP and cGMP.
Why Phosphodiesterase as a target
High intracellular levels of cAMP can effectively kill the cancer cells in vitro but substances elevating cAMP such as forskolin, 8-bromo-cAMP, 8-chloro-cAMP, monobutyryl or dibutyryl cAMP are not recommended to be used as anticancer drugs as they are highly cytotoxic 9 . Phosphodiesterase inhibitor induced apoptosis is selective towards malignant cells with out effecting normal ones as evidenced by F Mentz
The selective toxicity of Phosphodiesterase Inhibitors towards malignant cells is further confirmed by Marika Sarfati
In studies of cAMP metabolism by Eun-Yi Moon
Phosphodiesterase (PDE) inhibitors and their anti cancer effects
The Phosphodiesterase inhibitors are classified as non-selective and selective types. The effect of various phosphodiesterase inhibitors and various experimental models has been explained.
Non-selective PDE inhibitors
B. Isobutyl Methyl Xanthine (IBX):
Sulindac sulfone (exisulind), although a nonsteroidal anti-inflammatory drug derivative, induces apoptosis in tumor cells by a mechanism that does not involve cyclooxygenase inhibition..Exisulind induced apoptosis by sustained increases in cGMP levels and cGMP-dependent protein kinase (PKG) induction which phosphorylates the accumulated beta –catenin and the phosphorylation of it leads to proteosomal degradation of b-catenin and apoptosis. The cytoplasmic and nuclear beta-catenin accumulations can be prevented by Exisulind in Familial Adenomatous Polyposis (which occurs due to Germ-line mutations in the APC tumor suppressor gene), so it can be considered as a potential therapy in FAP 20 .
Selective PDE inhibitors
In a study by Murata
DC-TA-46(7-benzylamino-6-chloro-2 piperazino-4-pyrrolidino-pteridine), PDE-4-specific inhibitor, as reported by Doris Marko et. al., that it arrested the cell cycle in G0/G1 phase after 24 hr treatment in the highly malignant Spindle Cell carcinoma cell line(CarB) with highest sensitivity at IC50=0.8±0.3 μM 24 .
In another study by Markus Drees
Doo Ho Kim
Sildenafil and Vardenafil (Selective PDE5 inhibitors)
PDE5 Inhibition & immune-mediated anti tumor Activity
Paolo Serafi ni
To confirm that the antitumor effect of PDE5 inhibitors was immune mediated, the experiments were repeated in immune-compromised BALB/c-Rag-2−/− mice. In these hosts, Sildenafil demonstrated no antitumor efficacy as these mice lacked T and B lymphocytes but have normal or enhanced NK and NKT activity.
PDE5 inhibition restores T cell proliferation
Paolo Serafi ni
PDE5 inhibition and apoptosis
The recent research suggests that phosphodiesterase inhibitors are selectively toxic towards the cancer cells and are synergistic with the current anticancer drugs, suggesting their potentials in the adjunctive cancer chemotherapy. There is also a need to look into the new compounds which inhibit angiogenesis, metastasis and invasion the major problem with malignant tumors. The characterisation of specific PDE isoenzymes that are over expressed in various tumor cells is to be explored for effective targeting.