A Bidirectional Relationship Between AMPK and α-Klotho Expression
Klotho is one of the few longevity-associated proteins that works in both directions: in animal models less of it than usual means worse health and a shorter live, more of it than usual means better health and a longer life. In humans researchers have observed correlations between klotho expression and late-life health. Klotho has a number of forms, the important one of which is α-klotho. α-klotho localizes to the cell membrane, where part is cut off to form the circulating α-klotho protein. This circulating protein appears to produce broad benefits via its interaction with cell receptors, including protection of kidney function and enhanced cognitive function.
Klotho is a great example of just how long it takes to figure out the biochemistry for one protein and its immediate interactions. The work is far from done, nearly 20 years since the effects on longevity were first discovered and published. How α-klotho works to produce improved tissue function is only understood in a very sketchy form, and there is every possibility that important aspects remain unknown. This is the case despite the fact that α-klotho is a topic of great interest, with many researchers working on it. For proteins that don't have this level of interest, little progress is made on similar time frames. So very much of cellular metabolism remains unknown at the detail level.
Today's open access paper on the relationship between AMPK and α-klotho is interesting because it has already been demonstrated that klotho expression increases AMPK expression. Thus there exists a bidirectional regulation between the two proteins and their effects on all of the processes that they subsequently influence. AMPK is one of the all-too-many proteins that appears to play a role in regulating all of the interesting core cellular processes relating to growth, regeneration, and maintenance. AMPK has long been a target of interest for researchers because it appears critical in the machinery responsible for the beneficial reactions to exercise and calorie restriction.
AMP-dependent kinase stimulates the expression of αKlotho
Renal transmembrane protein αKlotho has several important functions. On the one hand, it serves as a co-receptor for phosphaturic hormone fibroblast growth factor 23 (FGF23), which is mainly produced in bone. On the other hand, enzymatic cleavage of transmembrane αKlotho results in an extracellular form, called soluble Klotho (sKL), which exerts endocrine and paracrine effects in several tissues and organs. FGF23 exerts further effects in other organs including heart and is correlated with outcomes in kidney and cardiovascular disease.
The joint action of FGF23 and αKlotho in the kidney is pivotal for phosphate and vitamin D metabolism. Lack of either FGF23 or αKlotho results in massive phosphate and active vitamin D excess in mice, causing a phenotype of rapid aging with a plethora of aging-associated diseases that are reminiscent of human aging and affect almost all tissues and organs. Conversely, overexpression of αKlotho has powerful antiaging effects, expanding the life span by about 30% in mice. αKlotho has also been demonstrated to be highly beneficial in several acute and chronic disorders. Further putatively health-promoting effects of αKlotho may include the reduction in oxidative stress or anti-inflammatory effects to name a few.
AMP-dependent kinase (AMPK) is basically expressed in all cell types and consists of three subunits, α, β, γ. Physiologically, it is activated by increase in cellular AMP concentration, indicating lack of ATP and hence energy deficiency. In rough summary, AMPK reduces cellular processes consuming energy and induces pathways providing energy. Higher AMPK activity is associated with some remarkable health benefits. These may include the protection of the heart during ischemia, the reduction of microvascular disease in diabetes, or nephroprotection in insulin resistance. These beneficial effects are largely attributed to improvements in cell metabolism or stimulation of autophagy.
Given that both αKlotho and AMPK have beneficial effects in similar organs, we studied whether AMPK regulates αKlotho gene expression in Madin-Darby canine kidney cells, normal rat kidney 52E cells, and human kidney 2 cells. We measured αKlotho expression upon pharmacological manipulation or siRNA-mediated knockdown of AMPKα. AMPK activator 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) enhanced αKlotho expression, an effect reduced in the presence of AMPK inhibitor compound C or siRNA targeting AMPK catalytic subunits. Similarly, AMPK activators metformin and phenformin upregulated αKlotho transcripts. Taken together, our results suggest that AMPK is a powerful inducer of αKlotho and could thereby contribute to the development of future therapeutic interventions.