The pharmacological inhibitors of poly(ADP-ribose) polymerase (PARP) family of proteins have shown promising results in preclinical studies and clinical trials as a monotherapy or in combination therapy for some cancers. Thus, usage of PARP-inhibitors (PARPi) in cancer therapy is bound to increase with time, but resistance of cancer cells to PARPi is also beginning to be observed. Here we review different known and potential mechanisms by which: (i) PARPi kill cancer cells; and (ii) cancer cells develop resistance to PARPi. Understanding the lethality caused by PARPi and the countermeasures deployed by cancers cells to survive PARPi will help us rationalize the use of this new class of drugs in cancer therapy.

In the human airway epithelium, VIP/PACAP receptors are distributed in nerve fibers and in epithelial cells but their role in transepithelial ion transport have not been reported. Here, we show that human bronchial epithelial Calu‐3 cells expressed the VPAC1 receptor subtype which shares similar high affinity for VIP and PACAP‐27. The stoichiometric binding parameters characterizing the 125I‐VIP and 125I‐PACAP‐27 binding to these receptors were determined. We found that VIP (EC50∼7.6 nM) and PACAP‐27 (EC50∼10 nM) stimulated glibenclamide‐sensitive and DIDS‐insensitive iodide efflux in Calu‐3 cells. The protein kinase A (PKA) inhibitor, H‐89 and the protein kinase C (PKC) inhibitor, chelerythrine chloride prevented activation by both peptides demonstrating that PKA and PKC are part of the signaling pathway. This profile corresponds to the pharmacological signature of CFTR. In the cystic fibrosis airway epithelial IB3‐1 cell lacking functional CFTR but expressing VPAC1 receptors, neither VIP, PACAP‐27 nor forskolin stimulated chloride transport. Ussing chamber experiments demonstrated stimulation of CFTR‐dependent short‐circuit currents by VIP or PACAP‐27 applied to the basolateral but not to the apical side of Calu‐3 cells monolayers. This study shows the stimulation in human bronchial epithelial cells of CFTR‐dependent chloride secretion following activation by VIP and PACAP‐27 of basolateral VPAC1 receptors. British Journal of Pharmacology (2004) 141, 698–708. doi:

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An accurate and sensitive detection of catalytic activation of poly(ADP-ribose) polymerase-1 (PARP-1) is required to be performed in a wide variety of samples because this activity plays a role in various cellular responses to DNA damage ranging from DNA repair to cell death, as well as in housekeeping functions, such as transcription. Since PARP-1 gene is expressed constitutively, its activation cannot be surmised from increased expression of its mRNA or protein, but by demonstrating the consequences of its catalytic -reaction which results in consumption of the substrate nicotinamide adenine dinucleotide (NAD(+)) and formation of three products, namely, polymer of ADP-ribose (pADPr or PAR), nicotinamide, and protons. Here, we describe various approaches commonly used in our laboratory for detection of PARP-1 activation in vivo (cells, tissues, and tumors), in situ, and in vitro via assessment of formation of pADPr, depletion of the substrate NAD, or formation of protons resulting in rapid and reversible intracellular acidification. It is important to note that although some other members of the PARP family can carry out the same catalytic reaction, many of these assays largely reflect PARP-1 activation in a vast majority of the experimental circumstances and more specifically in DNA damage responses. However, if required, PARP-1-specific action should be confirmed by use of PARP-1 knockout or RNAi-mediated knockdown approaches.