Biology Essay Sample: The Accumulation, Functions & Alteration of Macrophages

Macrophages and Hypoxic Tissues: The Accumulation, Functions and Alteration of Genetic Expression of Macrophages in Poorly Vascularized Tissues

Macrophages are cells of the immune system that are activated during the inflammatory response. As monocytes, they traverse the circulatory system before settling in the various tissues of the body as macrophages. Their primary role is to phagocytose foreign particles and microorganisms through four steps that begin with chemotaxis, attachment, ingestion and digestion. They are the most mobile of the cells in the immune system. In tissues, they are called histiocytes where they trap foreign pathogens. They are also present in the liver (kupffer cells), lungs (alveolar cells), brain (microglia), intestines, lymph nodes, spleen, bone marrow and blood vessels. (Burton, G. & Engelkirk, P, 2004; Johnson, G. & Losos, J., 2008).

Macrophages are also drawn towards areas with very poor vascularization and those with very low oxygen tissue or hypoxic areas. Examples of areas of hypoxia include a variety of inflamed tissues, tissues that are diseased such as “ malignant tumors, atherosclerotic plaques, myocardial infarcts, the synovial of joints with rheumatoid arthritis, healing wounds, and sites of bacterial infection” (Murdoch, C., et. al. 2005b; Stoeltzing, O., et. al., 2004). A number of studies have shown that hypoxia results in the alteration of the macrophage’s morphology including change in the expression of cell surface markers and excretion of cytokines. Hypoxia is also involved in the modification of the gene expression of macrophages (Lewis, J., et. al., 1999; Murdoch, C., et. al, 2005b; Murdoch, C., et. al., 2004a).

In view of the macrophage’s role in hypoxic tissues, this paper will explore in detail how macrophages accumulate in hypoxic tissues and its role in these types of tissues. The first part of this paper will describe the process involved in the gathering of macrophages in low-oxygen tissues. The second part will dwell on the functions of the macrophages in hypoxic tissues while the third part will focus on the alteration of gene expressions of macrophages that will enable them to perform their respective functions in hypoxic tissues. The last part will be a synthesis of this paper.

Accumulation of Macrophages in Hypoxic Tissues

How do Macrophages Accumulate in Hypoxic Tissues?

Macrophages are highly visible in hypoxic tissues, even composing up to 80% of the cells present in a breast carcinoma (Bingle, et. al., 2002). Numerous studies especially those led by Murdoch and his colleagues have implicated that CCL2 (monocyte chemoattractant protein-1); M-CSF or CSF-1 (macrophage-colony stimulating factor) and VEGF (vascular endothelial growth factor) play a vital role as tumor-derived chemoattractants of monocytes into tumors (Lewis, C. & Murdoch, C., 2005; Murdoch, C., et. al, 2004a; Murdoch, C., et. al., 2005b). Upon reaching hypoxic tissues such as malignant tumors, monocytes quickly differentiate into tumor-associated macrophages or TAMs. The following illustration will show how macrophages are recruited to hypoxic tissues:

Tumor Associated Macrophages or TAMs secrete low levels of TNF or tumor necrosis factor, carboxypeptidase M and CD51 (differentiation-associated macrophage antigens) but high levels of interleukin 1 and 6. These type of macrophages are said to be ‘immature macrophage phenotype’ (Lewis, C. & Murdoch, C., 2005). While macrophages from healthy tissues have the ability to lyse tumor cells, TAMS cannot entirely do this. Researchers have suggested that this may be due to their exposure to tumor-derived anti-inflammatory molecules (Bingle, L., et. al., 2002; Leek, R. & Harris, A., 2002). Instead of transforming themselves into M1 macrophages (the type of macrophages that engulf and disables microorganisms and tumor cells and produce high levels of immunostimulatory cytokines), TAMS are transformed into type 2 macrophages. The latter type is adapted to repairing lesions, engulfing debris and inducing angiogenesis in damaged tissues or wounds (Mantovani, A., et.al., 2002). The proliferation of TAMs in hypoxic tissues is directly correlated to the aggressiveness of the tumor. Tumors with high levels of hypoxia also have higher levels of TAMs. Studies have linked TAM activity with the promotion of tumor angiogenesis and metastasis (Leek, R. & Harris, A., 2002; Lewis, C. & Murdoch, C., 2005).

The accumulation of TAMs in hypoxic tissues may also be due to the release of more macrophage chemoattractants such as EMAP-II, Endothelin 3 and VEGF (vascular endothelial growth factor) (Murdoch, C., et. al., 2004a). Once they are in necrotic tissues, they are trapped and their mobility decreases dramatically. Researchers have suggest that while in necrotic tissues, the enzyme mitogen-activated protein kinase phosphatase (MKP-1), an enzyme responsible for dephosphorylating chemoattractant receptors in the surfaces of macrophages, terminates the response of TAMs to chemokines. This way, TAMs are securely immobilized inside the necrotic tissues. Furthermore, tumors result into formation of more areas that are hypoxic. This is brought about by the formation of new blood vessels in the tumor tissues that have collapsed (Lewis, C. & Murdoch, C., 2005).

Functions of Macrophages in Hypoxic Tissues

As previously mentioned in section 1, macrophages are densely located in hypoxic tissues such as myocardial infarcts, atherosclerotic plaques, malignant tumors and rheumatoid arthritis’ synovial joints and wounds. In tumors, studies have shown that the presence of TAMs has been directly implicated in the aggressiveness of tumors and correlated with invasion of lymph nodes with tumor cells that consequently led to metastasis. These studies suggest that in actuality, TAMs may play a role in the promotion of tumor angionesis and subsequently, its metastasis (see for example, Lewis, C. & Murdoch, C., 2005; Leek, R. & Harris, A., 2002; Lewis, J., et. al., 1999).

Once in the hypoxic sites, the functions of TAMs are affected by a number of factors. Hypoxia induces TAMs to secrete the following: mitogeneic factors, proangiogenic cytokines and enzymes and immunosuppressive agents. As a whole, these substances promote aggressive tumor growth and metastasis. Thus, a large number of TAMs in malignant tumor are often associated with poor prognosis. Vascular endothelial growth factor (VEGF) plays a vital role in the increase of the number of TAMs in tumor tissues. Through a positive feedback mechanism, hypoxia causes the release of VEGH by TAMs and tumor cells. In return, these VEGF stimulate more macrophages to migrate to these tumor tissues. As a result, the VEGF produced by macrophages exerts an ‘angiogenic and anti-apoptopic’ effect on the tissues (Knowles, H. & Harris, A., 2001).

In atherosclerosis, monocytes infiltrate the endothelium of arteries. Once inside, they then differentiate into macrophages and engulf huge amounts of LDL (low-density lipoproteins) forming ‘foam’ cells.’ This formation eventually will lead to the assembly of fatty streaks in the arterial walls which will later on progress to atherosclerotic plaques (Murdoch, C., et. al. 2005b). Studies have shown that once macrophages are exposed to hypoxia in vitro, lipooxygenase-2 secretion is up-regulated. This enzyme is found to play a major role in low-density lipoprotein oxidation. Aside from expression of lipooxygenase-2, hypoxia also increases the expression of VLDL (very low-density lipoprotein) receptors in the surfaces of plaque macrophages. Consequently, these studies show that hypoxia plays a role in increasing lipid metabolism in atherosclerotic lesions (Rydberg, et. al., 2003).

Meanwhile, another study showed the relationship of hypoxia with the presence of macrophages and the expression of HIF (hypoxia-inducible transcription factor) and VEGF in human carotid atherosclerosis. HIF is implicated in the progression of atherosclerosis and in the regulation of intraplaque angionesis (Sluimer, J., et. al., 2008). Another study shows the expression of macrophage migration inhibitory factor (MIF) by the myocardium. MIF is vital in controlling inflammatory responses and in the myocardium; it is released during redox stress. This implies that MIF has a role in the ‘pathogenesis of myocardial ischemia’ (Takahashi, M., et. al., 2001).

Rheumatoid arthritis is characterized by chronic inflammation of the affected joint’s synovium. The destruction of the cartilage covering the ends of the bones in the joint is preceded by an increase of synovial fibroblasts and invasion of leukocytes (Firestein, 2003). The pressure in the joints is increased and upon movement, the blood vessels collapse causing ‘hypoxic-reperfusion injury.’ This generates reactive oxygen species (ROS) that leads to a number of events including alteration of macrophage interaction (Mapp, I., et. al., 1995). Hypoxic tissues are present in synovial joints and are thought to play a role in its inflammation and destruction (Ahn, J., et. al. 2008).

In wounds, monocytes are recruited at days 2 to 5. These monocytes, now transformed into phagocytes, are responsible in engulfing cellular and foreign debris and in secreting factors that will attract other cells that are vital in the neovascularization of the wounded tissue. Macrophages also excrete HIF-1 in response to hypoxia in wound tissues (Murdoch, C., et. al., 2005).

Alteration of Gene Expression of Macrophages in Hypoxic Tissues

There are a number of macrophage genes that are shown to be up-regulated once they accumulate in hypoxic tissues. In atherosclerosis, foam cells respond to hypoxia through up-regulation of a molecule called oxygen-regulated protein 150 (ORP 150). Although the exact function of ORP 150 is not yet elucidated, it might be linked to the survival of macrophages during hypoxia (Ozawa, K. et. al, 1999). While in atherosclerotic tissues, macrophages also alter themselves by expressing more HIF and VEGF and increase expression of VLDL receptors that result in increased lipid metabolism and more deposition of atherosclerotic plaques (Sluimer, J., et. al., 2008; Takahashi, M., et. al., 2001).

In diseased tissues, macrophages show phenotypic heterogeneity. Hypoxia alters a number of macrophage activities such as release of cytokines or metabolic and phagocytic activities. Specifically, their low-density lipoprotein (LDL) uptake and tumor necrosis factor-α (TNF-α) secretions are altered. Since macrophages have adapted to conditions in hypoxic areas, they are believed to play a vital role in the progression of inflammatory vascular sites (Degrossoli, A. & Giorgio, S., 2007).

A study done by Bernard Burke and his colleagues (2003) demonstrated what genes are up-regulated by macrophages when they are present in hypoxic tissues. His study revealed that there was an up-regulation of glucose transporter 1 (GLUT-1) and matrix metalloproteinase-7 (MMP-7) mRNA in human breast tumors. Burkes group concluded that these genes could be vital for the functioning and survival of macrophages in necrotic lesions or areas of hypoxia. The promoters of these genes could be useful in ‘macrophage-delivered gene therapy (Burke, et. al., 2003).

There are also studies done that exemplify the uses of macrophages in delivering therapy to inflamed joints. Macrophages will be transduced with therapeutic genes and delivered to synovial joints where the transgene will be ‘switched on’ (Hollander, A., et. al, 2001). Another study has also shown the use of macrophage in delivering gene therapy to tumors (Griffiths, et. al., 2000).

Summary and Conclusion

Macrophages play an important role in the phagocytosis of foreign debris and microorganisms. While in the blood, these monocytes, as precursors of macrophages, travel to areas of infection and transform themselves to macrophages. Macrophages are present in many tissues. They are also attracted to areas with low oxygen tissues or hypoxic tissues and necrotic lesions such as tumors, myocardial infarcts, atherosclerosis and rheumatoid arthritis joints. Their metabolic and phagocytose activities are dramatically altered upon reaching these tissues. They are believed to promote progression of these lesions through a positive feedback manner. In tumors, hypoxic tissues secrete substances that will draw monocytes to the tumor area and transform them to TAMs. The latter is shown to enhance tumor angionesis and metastasis. In atherosclerosis, foam cells (macrophages laden with lipids) increase the deposition of atherosclerotic plaques while in synovial joints; macrophages enhance inflammation and deposition of synovial fibers and destruction of bones and cartilage in the joints. The alteration of the phenotype and up-regulation of certain genes of macrophages are believed to be important in fulfilling their functions in hypoxic tissues. At the same time, these same macrophages could be transduced with therapeutic genes and administered to lesions of tumors, myocardial infarcts, atherosclerosis and rheumatoid arthritis. Studies have shown that these transduced macrophages might be successful as a form of therapy for this type of disease.