**1. Introduction**

Aldehyde dehydrogenases (ALDH) belong to the oxidoreductase family, which catalyze the conversion of aldehydes to their corresponding acids. As a group of NAD(P)+-dependent enzymes, aldehyde dehydrogenases (ALDHs) are involved in oxidation of a large number of aldehydes into their weak carboxylic acids (Moreb*, et al.*, 2012). ALDH is found in every subcellular region such as cytosol, endoplasmic reticulum, mitochondria, and the nucleus, with some even found in more than one location (Marchitti*, et al.*, 2008).

ALDH is also found in stem cells. During early life and growth, stem cells **(**SCs) have a spectacular potential to develop into several cell types in the body. In many tissues, SCs behave as a kind of internal repair system, dividing essentially without limit to replenish other cells (Fuchs & Segre, 2000). Stem cells are distinguished from other cell types by two important characteristics: (1) Their unspecialized properties and renewal potencies; and (2) differentiation into other cell types under certain physiologic or experimental conditions (Discher*, et al.*, 2009, Solis*, et al.*, 2012). These cells are identified by their expression of a particular panel of surface molecules, with the presence of CD73, CD90, CD105, and the absence of CD14, CD34, CD45, and HLA-DR. They show no proliferative response from alloreactive lymphocytes because of the negligible levels of extracellular MHC class I and II determinants and they also have important immunomodulatory functions in all the cells involved in both the innate and adaptive immune responses (Nauta & Fibbe, 2007). On the other hand, cancer stem cell theory is supported by biological reason for aging. The theory postulates that cancer SCs, a small subset of tumor cells also have stem cell-like properties (epithelial-to-mesenchymal progression, differentiation and self-renewing capacity). ALDH expression has demonstrated itself to be a possibly relevant prognostic marker. For this

© 2012 Allahverdiyev et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

reason, the subpopulation of cancer SCs (CSCs) can present a therapeutic target for poorprognostic, treatment-resistant and recurrent breast cancer. Through its role in oxidizing retinol to RA, which is a modulator of cell proliferation, ALDH1 might have a role in early differentiation of SCs and stem cell proliferation (Mieog*, et al.*, 2012).

Aldehyde Dehydrogenase: Cancer and Stem Cells 5

through lipid, and aldehydic intermediates through carbohydrate metabolism. Along with these endogenous aldehydes, there are also exogenously present aldehydes in a variety of industrial processes, including the production of polyester plastics (formaldehyde, acetaldehyde, acrolein, etc.), polyurethane, smog, cigarette smoke or motor vehicle exhaust. With their malodorous properties, some dietary and aromatic aldehydes are accepted as additives in food and cosmetics (e.g., citral, cinnamaldehyde, benzaldehyde, and retinal), though many others are cytotoxic (Chen*, et al.*, 2010). Aldehydes could interact with thiol compounds of some proteins, leading to structural and functional alterations of these molecules (Weiner*, et al.*, 2008). In order to protect the human body from the deleterious effects of aldehydes in general, and myocardium and the brain in particular, a fast aldehyde detoxification mechanism is essential. Aldehydes are significantly reactive and possess high diffusion capacities in cells, thus they can easily form complexes with DNA, proteins and lipids, of which they can alter the function and cause their inactivation. As a result of DNA damage induced by these complexes, many aldehydes are classified as mutagenic or carcinogenic, including acetaldehyde, which is derived from ethanol consumption. Over– consumption of ethanol has been related to liver disease and several gastrointestinal and upper aerodigestive cancers. Numerous other cytotoxic and reactive aldehydes have been shown to be linked with other types of diseases (Hofseth & Wargovich, 2007, Perluigi*, et al.*,

Aldehyde dehydrogenases [EC 1.2.1.3; *systematic name*: aldehyde: NAD(P)+oxidoreductase] catalyze aldehyde conversion into their matching acids by NAD(P)+-dependent nearly irreversible reaction. In 1949, mammalian ALDH was first discovered in ox liver. After that, many types of ALDH were distinguished according to their physico-chemical characteristics, enzymological properties, subcellular localization, and tissue distribution (Oraldi*, et al.*, 2011). They are involved in several cell functions such as proliferation, differentiation, survival as well as cellular response to oxidative stress (Jackson*, et al.*, 2011). ALDHs are commonly delivered from bacteria and humans (Moreb, 2008). Based on their physico-chemical characteristics, subcellular localization, tissue distribution and enzymological properties, a number of types of ALDH have been distinguished since the 1960s, around the time when mammalian ALDH activity was observed in ox for the first time. In 1985, 2 ALDH genes were cloned and characterized. Genes or cDNAs for more than 50 animals, fungi and bacterial ALDHs in addition to protein sequences have been

The human genome contains 19 ALDH functional genes and 3 pseudogenes (Black*, et al.*, 2009). At least 5 ALDH isozymes function in the mitochondria, and all the ALDH genes are

All of the ALDH gene superfamily plays an important role in the enzymic detoxification of endogenous and exogenous aldehydes. They are also involved in the formation of molecules that are important in cellular processes like RA, betaine and gamma aminobutyric acid

2009, Chen*, et al.*, 2010).

**1.2. Aldehyde dehydrogenases** 

discovered (Yoshida*, et al.*, 1998).

encoded in the nucleus (Chen*, et al.*, 2010).

There are several isoforms of ALDH (ALDH1A1, ALDH1A2, ALDH1A3 and ALDH8A1) that play a role in RA formation by oxidation of all-trans-retinal and 9-cis-retinal in RA cell signaling, which has been related to the "stemness" characteristics of SCs (Marcato*, et al.*, 2011). ALDH1 is better as a marker of breast cancer SCs than CD44+/CD24- (Tanei*, et al.*, 2009). While cellular markers including CD133 have been used to identify tumor SCs, especially for glioblastomas (GBMs) ALDH1 was desrcribed as a marker for the identification of non-neoplastic SCs and tumor stem cells (TSCs) (Corti*, et al.*, 2006, Ginestier*, et al.*, 2007, Huang*, et al.*, 2009).

After CD133- GBMs are characterized to behave as brain TSCs(Beier*, et al.*, 2007). ALDH1 has also been described as a stem cell marker in various solid neoplasms including lung cancer (Jiang*, et al.*, 2009), breast carcinoma (Ginestier*, et al.*, 2007), colorectal cancer (Huang*, et al.*, 2009), and GBM. ALDH1B1 and ALDH1A1 are differentially expressed in normal human tissues. ALDH1B1 is expressed at higher levels than ALDH1A1 in human epithelial cancers. ALDH1B1 was abundantly expressed in adenocarcinomas originating from the tissue and particularly in colonic adenocarcinoma (Chen*, et al.*, 2011).

ALDHbr cells can be detected with ALDEFLUOR reagent by using flow cytometry or fluorescent microscopy. Aldefluor assay is based on the conversion of fluorescent non-toxic substrate for ALDH substrate to the fluorescent reaction product. Non-toxic substrate for ALDH can freely diffuse into intact, viable cells. The BODIPY aminoacetaldehyde is converted to the fluorescent product BODIPY aminoacetate by ALDH activity. These cell populations, which are known as ALDH bright (ALDHbr) cells are isolated from adult tissues by flow sorting. ALDHbr cells were also found in various cancer tissues including breast, liver, colon, pancreas, prostate, lung, ovarian and acute myelogenous leukemia and are related to cancer chemo resistance (Siclari & Qin, 2010).

ALDHbr population may play an important role in regenerative medicine. The regenerative potential of ALDHbr cells obtained from different tissues was investigated in various disease models such as ischemic tissue damage, hind limb model, brain damage (spinal motor atrophy, etc.) and pancreatitis.

Thus, as mentioned above, ALDH is an important enzyme for cancer and stem cells. This chapter aims to represent the important role of aldehyde dehydrogenases in stem cells, cancer stem cells, therapy and regenerative medicine.
