Hypoxia‑mediated activation of autophagic flux inhibits apoptosis of keratinocytes via blocking tumor necrosis factor‑related apoptosis‑inducing ligand

The signaling involved in autophagy machinery in keratinocytes and therapeutic approaches for skin diseases

Autophagy is responsible for the lysosomal degradation of proteins, organelles, microorganisms and exogenous particles. Epidermis primarily consists of keratinocytes which functions as an extremely important barrier. Investigation on autophagy in keratinocytes has been continuously renewing, but is not so systematic due to the complexity of the autophagy machinery. Here we reviewed recent studies on the autophagy in keratinocyte with a focus on interplay between autophagy machinery and keratinocytes biology, and novel autophagy regulators identified in keratinocytes. In this review, we discussed the roles of autophagy in apoptosis, differentiation, immune response, survival and melanin metabolism, trying to reveal the possible involvement of autophagy in skin aging, skin disorders and skin color formation. Since autophagy routinely plays a double-edged sword role in various conditions, its functions in skin homeostasis and potential application as a therapeutic target for skin diseases remains to be clarified. Furthermore, more investigations are needed on optimizing designed strategies to inhibit or enhance autophagy for clinical efficacy.

Hypoxia regulates TRAIL sensitivity of colorectal cancer cells through mitochondrial autophagy

The capacity of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) to selectively induce cell death in malignant cells triggered numerous attempts for therapeutic exploitation. In clinical trials, however, TRAIL did not live up to the expectations, as tumors exhibit high rates of TRAIL resistance in vivo. Response to anti-cancer therapy is determined not only by cancer cell intrinsic factors (e.g. oncogenic mutations), but also modulated by extrinsic factors such as the hypoxic tumor microenvironment.Here, we address the effect of hypoxia on pro-apoptotic TRAIL signaling in colorectal cancer cells. We show that oxygen levels modulate susceptibility to TRAIL-induced cell death, which is severely impaired under hypoxia (0.5% O2). Mechanistically, this is attributable to hypoxia-induced mitochondrial autophagy. Loss of mitochondria under hypoxia restricts the availability of mitochondria-derived pro-apoptotic molecules such as second mitochondria-derived activator of caspase (SMAC), thereby disrupting amplification of the apoptotic signal emanating from the TRAIL death receptors and efficiently blocking cell death in type-II cells. Moreover, we identify strategies to overcome TRAIL resistance in low oxygen environments. Counteracting hypoxia-induced loss of endogenous SMAC by exogenous substitution of SMAC mimetics fully restores TRAIL sensitivity in colorectal cancer cells. Alternatively, enforcing a mitochondria-independent type-I mode of cell death by targeting the type-II phenotype gatekeeper X-linked inhibitor of apoptosis protein (XIAP) is equally effective.Together, our results indicate that tumor hypoxia impairs TRAIL efficacy but this limitation can be overcome by combining TRAIL with SMAC mimetics or XIAP-targeting drugs. Our findings may help to exploit the potential of TRAIL in cancer therapy.

The TRAIL to acne pathogenesis: let's focus on death pathways

This hypothesis presents acne as a PI3K-Akt-mTORC1-driven pro-survival disease of the sebaceous follicle with impaired TRAIL-mediated death signalling. It is predicted that anti-acne agents such as isotretinoin enhance death signalling and thereby readjust the disturbed balance of pro-survival and death signalling of the sebaceous follicle in acne vulgaris. For this purpose, immortalized sebocyte cultures are regarded as inapproproate models to study the key features of acne pathogenesis.