Caspase activation

Apoptosis is compromised in cancers

The cell cycle is governed by various exogenous and endogenous stimuli and controls whether a cell engages in cell proliferation, enters growth arrest or initiates apoptosis, programmed cell death. In contrast to the traumatic cell death caused by external influences such as severe cellular injuries (necrosis), apoptosis is a self-digestive fragmentation of cells that prevents damage to surrounding tissue. It is triggered as a response to cellular stress caused by a variety of events including, e.g. damage to DNA and other subcellular structures, nutrient and oxygen deficiency and damage caused by heat or radiation. If cells lose their ability to precisely control the cell cycle and yet fail to initiate apoptosis, proliferation can occur at an enhanced and unregulated rate and might thereby cause the growth of tumours if not recognised by natural killer cells, cytotoxic lymphocytes that are part of the body’s immune system. Many anticancer therapies including cytotoxic drugs, irradiation or immunotherapy appear to induce cell death by activating downstream apoptosis effector mechanisms and thus use the cells’ own suicide mechanism to treat malignant tumours.

Caspases play a significant role in apoptosis

Caspases, a family of cysteine proteases, play an important part in the apoptotic process. These enzymes are inactive in normal cells and become activated through cleavage of the inactive precursor into fragments that constitute the active enzymes and engage in cell digestion. So far, 12 human caspases with different substrate specificities have been identified. Given the importance of caspases in a pathway leading to cell death, the activation of these enzymes is strictly regulated through a interconnecting network of effector and suppressor signalling. Thus, the ability of anticancer agents to initiate caspase activation might be a critical determinant of sensitivity or resistance to cytotoxic therapies.

Natural products that activate caspases

Betulinic acid is a pentacyclic triterpenoid of natural origin and was isolated from various plants such as Tryphyllum peltatum, Diospyros leucomelas or etracera boliviana. Several studies have shown that betulinic acid exhibits antitumour activity against a wide range of cancers including melanomas, lung and prostate carcinomas and colon cancer. The fact that betulinic acid showed enhanced cytotoxic activity under acidic conditions (pH < 6.8) might support the development of selective anticancer agents against solid tumours, as the pH value of these tissues are usually lower compared to normal tissue. Treatment of cancer cell lines with betulinic acid results in cell shrinkage, DNA fragmentation, membrane blebbing and caspase activation. Interestingly, this activity is independent of p53 tumour supressor gene status and the action of death inducing ligand/receptor pairs that play an important role in caspase activation.

The structurally related tricyclic diterpene ferruginol also showed caspase activation by affecting the apoptosis inducing factor and its translocalisation into the nucleus. In addition to this antitumour activity, it posesses antifungal, antibacterial and antiplasmodial properties. Preparation of betulinic acid analogues and related natural products like ferruginol to probe structure activity relationships has mainly focused on functional group interconversion while leaving the polycyclic terpene framework intact. New synthetic strategies giving rapid access to modified di- and triterpene frameworks might lead to new insights into the cellular mode of action and generate more potent analogues with improved pharmacokinetic properties.