Lack of vitamin D receptor causes stress-induced premature senescence in vascular smooth muscle cells through enhanced local angiotensin-II signals

OBJECTIVES

The inhibition of the renal renin-angiotensin system by the active form of vitamin D contributes to the cardiovascular health benefits of a normal vitamin D status. Local production of angiotensin-II in the vascular wall is a potent mediator of oxidative stress, prompting premature senescence. Herein, our objective was to examine the impact of defective vitamin D signalling on local angiotensin-II levels and arterial health.

METHODS

Primary cultures of aortic vascular smooth muscle cells (VSMC) from wild-type and vitamin D receptor-knockout (VDRKO) mice were used for the assessment of cell growth, angiotensin-II and superoxide anion production and expression levels of cathepsin D, angiotensin-II type 1 receptor and p57(Kip2). The in vitro findings were confirmed histologically in aortas from wild-type and VDRKO mice.

RESULTS

VSMC from VDRKO mice produced more angiotensin-II in culture, and elicited higher levels of cathepsin D, an enzyme with renin-like activity, and angiotensin-II type 1 receptor, than wild-type mice. Accordingly, VDRKO VSMC showed higher intracellular superoxide anion production, which could be suppressed by cathepsin D, angiotensin-II type 1 receptor or NADPH oxidase antagonists. VDRKO cells presented higher levels of p57(Kip2), impaired proliferation and premature senescence, all of them blunted upon inhibition of angiotensin-II signalling. In vivo studies confirmed higher levels of cathepsin D, angiotensin-II type 1 receptor and p57(Kip2) in aortas from VDRKO mice.

CONCLUSION

The beneficial effects of active vitamin D in vascular health could be a result of the attenuation of local production of angiotensin-II and downstream free radicals, thus preventing the premature senescence of VSMC.

Apoptotic volume decrease as a geometric determinant for cell dismantling into apoptotic bodies

Apoptosis is a mode of cell death through which cells are dismantled and cell remains are packed into small, membrane-bound, sealed vesicles called apoptotic bodies, which are easy to erase by phagocytosis by neighbouring and immune system cells. The end point of the process is to cleanly eliminate damaged or unnecessary cells without disrupting the surrounding tissue or eliciting an inflammatory response. The apoptotic process involves a series of specific events including deoxyribonucleic acid and nuclear fragmentation, protease-driven cleavage of specific substrates, which inhibits key survival functions and reorganizes the cell's structure, externalization of molecules involved in phagocytosis, membrane blebbing and cell shrinkage. Apoptotic volume decrease (AVD) leading to cell shrinkage is a core event in the course of apoptosis, the biological meaning of which has not been clearly ascertained. In this article we argue that volume loss is a geometrical requisite for cell dismantling into apoptotic bodies. This is derived from the cell's volume-to-surface ratio. Indeed, package of the original cell volume into smaller membrane-sealed vesicles requires that either cell membrane surface increase or cell volume decrease. In this sense, AVD provides a reservoir of membrane surface for apoptotic body formation. The strategic situation of AVD in the time course of apoptosis is also discussed in the context of apoptotic body formation.