Poly(ADP-ribose) polymerase, commonly known as PARP, is an enzyme that plays an important role in cell biology. The specific cutting of this enzyme, a process referred to as PARP cleavage, is an important event within cells. This enzymatic modification is involved in maintaining cell health and is linked to various diseases.
Understanding PARP
Poly(ADP-ribose) polymerase (PARP) is a family of enzymes that modify other proteins by adding chains of ADP-ribose units, a process known as poly(ADP-ribosyl)ation (PARylation). The most extensively studied member, PARP-1, is a nuclear enzyme that binds to DNA via zinc finger motifs. It becomes activated upon detecting breaks in DNA, such as single-strand and double-strand breaks.
PARP-1’s primary function in its un-cleaved state is in DNA repair and maintaining the integrity of the cell’s genetic material. When DNA damage occurs, PARP-1 is activated, synthesizing poly(ADP-ribose) (PAR) chains using nicotinamide-adenine-dinucleotide (NAD+) as a substrate. These PAR chains act as signals, recruiting other DNA repair proteins to the damaged sites.
This post-translational modification by PARP-1 helps assemble multiprotein complexes for various DNA repair pathways, including base excision repair (BER). By facilitating DNA repair, PARP-1 helps prevent the accumulation of DNA lesions, which could otherwise lead to genomic instability. However, excessive activation of PARP-1 can lead to the depletion of NAD+ and ATP, potentially resulting in cell death.
The Cleavage Event
PARP cleavage is the enzymatic cutting of the PARP protein into fragments. This cutting occurs during programmed cell death, known as apoptosis. Apoptosis is a regulated mechanism that removes damaged or unwanted cells, playing a part in development and tissue maintenance.
The enzymes responsible for PARP cleavage are caspases, a family of cysteine proteases. These caspases are synthesized as inactive forms and activate in a cascade during apoptosis. Caspase-3 is the main executioner caspase responsible for cleaving PARP.
Caspase-3 cleaves the 116 kDa form of PARP-1 at a specific site, resulting in two fragments: an N-terminal 24 kDa fragment and a C-terminal 89 kDa fragment. This cleavage inactivates PARP, preventing further PARylation and DNA repair. The 89 kDa fragment retains some basal enzymatic activity but loses the ability to recognize DNA damage, while the 24 kDa fragment remains in the nucleus, binding to damaged DNA and acting as an inhibitor of active PARP-1.
The purpose of PARP cleavage in apoptosis is to prevent the cell from attempting to repair its DNA when it is already committed to dying. By inactivating PARP, the cell ensures that energy resources, such as NAD+ and ATP, are not wasted on futile repair efforts. This inactivation facilitates the orderly dismantling of the cell, preventing a messy cell death that could trigger inflammation.
Implications of PARP Cleavage
PARP cleavage has implications for cellular fate and disease pathology. As a characteristic feature of apoptosis, it serves as a biochemical indicator for detecting programmed cell death in research and clinical settings. The 89 kDa cleaved PARP fragment is used as a marker to distinguish apoptotic cells from those undergoing other forms of cell death, such as necrosis.
Dysregulation of PARP cleavage is implicated in various diseases. In cancer, where apoptotic pathways are impaired, alterations in PARP cleavage contribute to tumor survival and chemotherapy resistance. Conversely, excessive PARP activation and its subsequent cleavage play a role in neurodegenerative disorders like Alzheimer’s, Parkinson’s, and multiple sclerosis.
Understanding PARP cleavage mechanisms also contributes to drug development. PARP inhibitors are a class of drugs used in cancer therapy. While these inhibitors target the un-cleaved, active form of PARP to prevent DNA repair and induce cell death, the subsequent cleavage of PARP remains a relevant event in the overall cell death pathway they influence. Beyond cancer, PARP inhibitors are investigated for their neuroprotective effects in neurodegenerative diseases by preventing harmful protein aggregation.