Given the attention that descends upon any prospect of reversing skin aging, I should probably open by saying that much of the data here for extended low dose topical treatment with rapamycin over eight months, that regarding visible skin aging and collagen production, is no more exciting than that obtained by any number of other approaches, such as topical application of keratinocyte growth factor (KGF). Effect sizes are the only thing that matters, and also the one thing that all too many observers fail to consider. Looking at the paper, I would say that the primary point of interest is the 50% reduction in markers of cellular senescence in skin. Given what is known of rapamycin this seems unlikely to be a senolytic effect, so not destruction of existing senescent cells, but rather a reduction in the number of cells becoming senescent. This in turn suggests that there remains some meaningful level of ongoing natural clearance of lingering senescent cells at older ages.
This study demonstrates a clear impact of rapamycin treatment on both the molecular signature associated with senescence and the clinical signs of aging in the skin. These data support the idea that a reduction in the burden of senescent cells underlies these improvements. The results could reflect a modification of the senescent cells present in the skin or a reduction in the number of senescent cells. Although rapamycin has been shown to reduce pro-inflammatory secretions produced by senescent cells, the fact that p16INK4A is reduced suggests that the absolute number of senescent cells in the epidermis is reduced. This implies that rather than simply modifying senescent cells present in the tissue, rapamycin treatment is either reducing the number of cells entering senescence or increasing the clearance of senescent cells. Based on our studies demonstrating that rapamycin prevents the senescence transition and improves functionality in vitro, we favor the concept that rapamycin reduces entry into senescence, but we cannot rule out an additional role for clearance of senescent cells. Whether the reduction in senescent cells is due to reduced entry or increased clearance, a reduction in the burden of senescent cells would be expected to improve functionality.
Senescent cells produce pro-inflammatory cytokines, matrix metalloproteins, and reduced levels of anti-angiogenic factors, creating a secretory profile known as the Senescence-Associated Secretory Phenotype (SASP). Thus, we anticipate that rapamycin treatment reduces inflammatory cytokines in the skin, although the verification of this change represents a technical challenge due to the fact that such cytokines are present in picomolar amounts. One quantifiable aspect of skin biology that is improved by the rapamycin treatment is the incorporation of collagen VII into the basement membrane, which represents a functional measure of skin quality that is improved upon treatment with rapamycin. Collagen VII is essential for a functional skin barrier, and the levels of collagen VII decrease with age and specifically beneath wrinkles. Although the mechanism whereby rapamycin may increase collagen VII protein levels is not clear at this time, the known effects of rapamycin on autophagy and intracellular trafficking of vesicles may allow for intracellular processing of misfolded collagen and increase proper localization at the cell periphery and basement membrane.
A notable aspect of this study is the use of such a low dose of rapamycin (10 μM, or 0.001%) for topical application. Topical treatment with higher concentrations (0.1-1%) has been employed for the treatment of tuberous sclerosis complex (TSC) in adults and children and has shown efficacy. We chose to use rapamycin at a ten-fold lower dose because the concentrations used in TSC patients are intended to inhibit cell growth, while our aim was to improve cell function while maintaining proliferative potential and preventing senescence. The positive impact of our treatment regimen suggests that age-related therapy with rapamycin may be feasible at doses far below those associated with side effects; however, this possibility will require careful evaluation in each specific clinical setting.