P53 upregulated modulator of apoptosis

The p53 upregulated modulator of apoptosis (PUMA) also known as Bcl-2-binding component 3 (BBC3), is a pro-apoptotic protein, member of the is_associated_with::Bcl-2 protein family. In humans, the Bcl-2-binding component 3 is_associated_with::protein is encoded by the BBC3 is_associated_with::gene.

The expression of PUMA is regulated by the tumor suppressor is_associated_with::p53. PUMA is involved in is_associated_with::p53-dependent and -independent is_associated_with::apoptosis induced by a variety of signals, and is regulated by is_associated_with::transcription factors, not by post-translational modifications. After activation, PUMA interacts with antiapoptotic is_associated_with::Bcl-2 family members, thus freeing Bax and/or Bak which are then able to signal is_associated_with::apoptosis to the mitochondria. Following mitochondrial dysfunction, the caspase cascade is activated ultimately leading to is_associated_with::cell death.

Structure
The PUMA protein is part of the BH3-only subgroup of is_associated_with::Bcl-2 family proteins. This group of proteins only share sequence similarity in the BH3 domain, which is required for interactions with Bcl-2-like proteins, such as is_associated_with::Bcl-2 and is_associated_with::Bcl-xL. Structural analysis has shown that PUMA directly binds to antiapoptotic Bcl-2 family proteins via an amphiphatic α-helical structure which is formed by the BH3 domain. The mitochondrial localization of PUMA is dictated by a is_associated_with::hydrophobic domain on its is_associated_with::C-terminal portion. No posttranslational modification of PUMA has been discovered yet.

Mechanism of action
Biochemical studies have shown that PUMA interacts with antiapoptotic Bcl-2 family members such as is_associated_with::Bcl-xL, is_associated_with::Bcl-2, is_associated_with::Mcl-1, Bcl-w, and A1, inhibiting their interaction with the proapoptotic molecules, Bax and Bak. When the inhibition of these is lifted, they result in the translocation of Bax and activation of mitochondrial dysfunction resulting in release of mitochondrial apoptogenic proteins is_associated_with::cytochrome c, SMAC, and is_associated_with::apoptosis-inducing factor (AIF) leading to caspase activation and cell death.

Because PUMA has high affinity for binding to is_associated_with::Bcl-2 family members, another hypothesis is that PUMA directly activates Bax and/or Bak and through Bax multimerization triggers mitochondrial translocation and with it induces is_associated_with::apoptosis. Various studies have shown though, that PUMA does not rely on direct interaction with Bax/Bak to induce is_associated_with::apoptosis.

Induction
The majority of PUMA induced is_associated_with::apoptosis occurs through activation of the tumor suppressor protein is_associated_with::p53. p53 is activated by survival signals such as glucose deprivation and increases expression levels of PUMA. This increase in PUMA levels induces is_associated_with::apoptosis through mitochondrial dysfunction. p53, and with it PUMA, is activated due to is_associated_with::DNA damage caused by a variety of genotoxic agents. Other agents that induce p53 dependent apoptosis are is_associated_with::neurotoxins, is_associated_with::proteasome inhibitors, microtubule poisons, and transcription inhibitors. PUMA is_associated_with::apoptosis may also be induced independently of p53 activation by other stimuli, such as oncogenic stress is_associated_with::growth factor and/or is_associated_with::cytokine withdrawal and is_associated_with::kinase inhibition,  ER stress, altered is_associated_with::redox status,  is_associated_with::ischemia, is_associated_with::immune modulation,  and is_associated_with::infection.

Degradation
PUMA levels are downregulated through the activation of caspase-3 and a protease inhibited by the serpase inhibitor N-tosyl-L-phenylalanine chloromethyl ketone, in response to signals such as the cytokine TGFβ, the death effector TRAIL or chemical drugs such as anisomycin. PUMA protein is degraded in a proteasome dependent manner and its degradation is regulated by phosphorylation at a conserved serine residue at position 10.

Role in cancer
Several studies have shown that PUMA function is affected or absent in is_associated_with::cancer cells. Additionally, many human is_associated_with::tumors contain is_associated_with::p53 is_associated_with::mutations, which results in no induction of PUMA, even after is_associated_with::DNA damage induced through is_associated_with::irradiation or is_associated_with::chemotherapy drugs. Other is_associated_with::cancers, which exhibit overexpression of antiapotptic is_associated_with::Bcl-2 family proteins, counteract and overpower PUMA-induced is_associated_with::apoptosis. Even though PUMA function is compromised in most is_associated_with::cancer cells, it does not appear that genetic inactivation of PUMA is a direct target of cancer. Many cancers do exhibit p53 gene mutations, making gene therapies that target this gene impossible, but an alternate pathway may be to focus on therapeutic to target PUMA and induce apoptosis in cancer cells. Animal studies have shown that PUMA does play a role in is_associated_with::tumor suppression, but lack of PUMA activity alone does not translate to spontaneous formation of malignancies. Inhibiting PUMA induced apoptosis may be an interesting target for reducing the side effects of cancer treatments, such as chemotherapy, which induce apoptosis in rapidly dividing healthy cells in addition to rapidly dividing cancer cells.

PUMA can also function as an indicator of p53 mutations. Many cancers exhibit mutations in the p53 gene, but this mutation can only be detected through extensive DNA sequencing. Studies have shown that cells with p53 mutations have significantly lower levels of PUMA, making it a good candidate for a protein marker of p53 mutations, providing a simpler method for testing for p53 mutations.

Cancer therapeutics
Therapeutic agents targeting PUMA for is_associated_with::cancer patients are emerging. PUMA inducers target is_associated_with::cancer or is_associated_with::tumor cells, while PUMA inhibitors can be targeted to normal, healthy cells to help alleviate the undesired side effects of chemo and is_associated_with::radiation therapy.

Cancer treatments
Research has shown that increased PUMA expression with or without is_associated_with::chemotherapy or is_associated_with::irradiation is highly is_associated_with::toxic to cancer cells, specifically lung, head and neck, esophagus, melanoma, malignant glioma, gastric glands, breast and prostate. In addition, studies have shown that PUMA adenovirus seems to induce apoptosis more so than p53 adenovirus. This is beneficial in combating is_associated_with::cancers that inhibit is_associated_with::p53 activation and therefore indirectly decrease PUMA expression levels.

is_associated_with::Resveratrol, a plant-derived stilbenoid, is currently under investigation as a cancer treatment. Resveratrol acts to inhibit and decrease expression of antiapoptotic is_associated_with::Bcl-2 family members while also increasing is_associated_with::p53 expression. The combination of these two mechanisms leads to is_associated_with::apoptosis via activation of PUMA, Noxa and other proapoptotic proteins, resulting in mitochondrial dysfunction.

Other approaches focus on inhibiting antiapoptotic is_associated_with::Bcl-2 family members just as PUMA does, allowing cells to undergo is_associated_with::apoptosis in response to cancerous activity. Preclinical studies involving these inhibitors, also described as BH3 mimetics, have produced promising results.

Side-effect treatment
is_associated_with::Irradiation therapy is dose-limited by undesired side effects in healthy tissue. PUMA has been shown to be active in inducing is_associated_with::apoptosis in is_associated_with::hematopoietic and is_associated_with::intestinal tissue following γ-irradiation. Since inhibition of PUMA does not directly cause spontaneous malignancies, therapeutics to inhibit PUMA function in healthy tissue could lessen or eliminate the side effects of traditional cancer therapies.