Fight Aging!

Today I'll point out an open access paper on calcification of blood vessels in which the authors show that genetic engineering to alter levels of ATF4 in mice also alters the pace of the calcification process. Calcification of tissues, especially blood vessel walls, is an important component of the blood vessel stiffening that occurs with age. This stiffness causes raised blood pressure, or hypertension, as well as detrimental remodeling of heart tissue. Higher blood pressure in turn causes damage to important but comparatively delicate tissue structures such as those of the kidney and brain, as well as greater rates of structural failure in small blood vessels. It also sets the stage for larger and usually fatal ruptures that occur in blood vessels weakened by atherosclerosis. That the heart changes its structure over time in response to blood vessel stiffness paves the way towards the numerous varieties of heart failure. So, on the whole, we'd all be a lot better off without calcification, or more to the point with therapies that can get rid of it.

There is some debate over where blood vessel calcification sits in the chain of cause and effect that leads from the starting point of cell and tissue damage that occurs as a side-effect of the normal operation of cellular metabolism to the end point of aging, dysfunction, medical conditions, and death. Is it in and of itself a primary cause of aging that would occur even in absence of the others? Or is it secondary to one or more other known age-associated forms of damage, and if so which ones? Since at the present time the research community has only just found out how to safely remove one of the seven classes of fundamental damage that cause aging, which is to say the accumulation of senescent cells, it is still the case that it is very hard to produce good answers to the question of whether A causes B or B causes A (and how this is influenced by C, D, and E) once you are down at the detail level of molecular machinery inside a cell. Everything in living biochemistry is interconnected, and picking apart the connections is a slow and expensive job. That said, on the matter of calcification I'll point you to a great review paper on the topic, as well as results from another line of research that suggest a specific cellular dysfunction causes calcification, both of which imply that this is not a primary cause of aging. It still leaves open the question of which of the fundamental forms of damage are the significant causes of calcification, however. Good candidates include the cross-linking that is itself thought to be very important in blood vessel stiffening, and the accumulation of hardy waste compounds that clog cellular recycling mechanisms, and anything else that involves generation of chronic inflammation.

One interesting point to think about in connection with the research presented here is that it is a global reduction in ATF4 levels that produces benefits in the form of slower calcification. The authors are largely focused on the kidney, as that is one of the organs most damaged by the high blood pressure produced by calcification and stiffened blood vessels. Yet if you look back in the Fight Aging! archives, you'll find that many of the commonly studied methods of slowing aging in mice are characterized by increased levels of ATF4 in the liver - and it is certainly the case that of these methods, some are known to slow the process of vascular stiffening and calcification. Never let it be said that the molecular biology of living beings is anything other than very complicated. Every organ and tissue is its own special case, and most interventions will result in more or less of a specific protein produced in one organ versus another. This is yet another reason why the metabolic tinkering approach to aging, attempting to adjust levels of specific proteins to a more youthful configuration in order to produce benefits, is an enormous undertaking, far greater in scope and difficulty than the alternative approach of repairing the damage found in old tissues.

Activating transcription factor-4 promotes mineralization in vascular smooth muscle cells

Evidence is emerging that endoplasmic reticulum (ER) stress contributes to the pathogenesis of vascular calcification. The ER is a major site for the regulation of calcium and lipid homeostasis. ER stress is an integrated signal transduction pathway involved in the localization and folding of secreted and transmembrane proteins. Vascular calcification is an independent predictor for the mortality and morbidity of patients with chronic kidney disease (CKD). Vascular calcification is classified into two major types, atherosclerotic and medial, both of which are frequently and simultaneously observed in CKD patients. Vascular calcification is a highly regulated process that resembles skeletal bone formation. Many key transcriptional regulators involved in skeletal osteogenesis are expressed in both calcified medial arterial layers and atherosclerotic plaques. We recently reported that these factors induce ATF4 activation through the ER stress response, resulting in osteogenic differentiation and mineralization of vascular smooth muscle cells (VSMCs) in vitro. However, whether in vivo ATF4 activation causes vascular osteogenesis and the molecular mechanism by which ATF4 induces mineralization of VSMCs have not been determined. In this context, we determined the in vivo role of ATF4 in VSMCs in both atherosclerotic and medial calcification and its mechanisms by using several murine models with global ATF4 deficiency, smooth muscle cell-specific (SMC-specific) ATF4 deficiency, and SMC-specific ATF4 overexpression.

ATF4 is known to be a critical transcription factor that regulates skeletal osteogenesis and bone formation. We and other investigators previously reported that ER stress induces expression of aortic ATF4 in a number of in vitro and in vivo models of vascular calcification. In particular, CKD strongly activates the aortic ER stress response, resulting in a significant induction of aortic ATF4. In this study, we demonstrate that ATF4 expression in VSMCs plays a causative role in the pathogenesis of vascular calcification using a series of mouse models. As an initial model, we used global ATF4-haploinsufficient mice, which showed significantly smaller aortic medial calcified lesions under both CKD and normal kidney condition (NKD) conditions. We also used an SMC-specific ATF4-deficient model, in which both medial and atherosclerotic calcifications under NKD and CKD conditions were attenuated. Finally, we generated a mouse model that overexpresses ATF4 only in SMCs, in which severe medial and atherosclerotic calcification developed even under NKD. These findings strongly suggest that the induction of ATF4 in VSMCs through ER stress is a pivotal event in the development of vascular calcification and osteogenesis.

CHOP is a major target of ATF4, and it is a transcription factor that promotes apoptosis contributing to vascular calcification. We previously reported that global CHOP deficiency attenuated CKD-dependent vascular apoptosis and atherosclerotic calcification in ApoE-/- mice. In this study, SMC-ATF4 deficiency reduced aortic CHOP expression and CKD-dependent apoptosis accompanied by a marked attenuation of vascular calcification, whereas SMC-ATF4 overexpression induced CHOP expression and apoptosis. These results suggest that ATF4 mediates vascular calcification through the induction of phosphate uptake in VSMCs through CHOP-dependent and -independent mechanisms.