STK38 at the crossroad between autophagy and apoptosis

Abstract

We describe the STK38 protein kinase as a conserved regulator of autophagy. We discovered STK38 as a novel binding partner of Beclin1, a key regulator of autophagy. By combining molecular, cell biological and genetic approaches, we show that STK38 promotes autophagosome formation in human cells and in Drosophila. Furthermore, we also provide evidence demonstrating that STK38 with the small GTPase RalB, assist the co-ordination between autophagic and apoptotic events upon autophagy induction, hence proposing a role for STK38 in determining cellular fate in response to autophagic conditions.

We recently identified the protein kinase STK38/NDR1, as a new partner of BECN1 that positively regulates autophagosome formation in human cells and fly larvae. We further showed that STK38 functions in the coordination of the autophagic and apoptotic pathways, being regulated by the small GTPase RALB. The specific action of STK38 on autophagy, connected to its ability to finely tune cell fate, identified this kinase as a very promising new therapeutic target.

The potentially “druggable” STK38 kinase could be a suitable target in several pathologies in which autophagy is critically involved. On the one hand, for the treatment of specific infectious and neurodegenerative disorders, it has been suggested that increased autophagy activity may be beneficial to remove pathogens and toxic protein aggregates, respectively. On the other hand, although the roles of autophagy in cancer progression are still under debate, data obtained in preclinical and clinical studies suggest that autophagy inhibition represents an interesting therapeutic option to reduce tumor burden and overcome therapy resistance. A prerequisite for the development of STK38-targeting compounds in these contexts, is to perform a careful evaluation of the potential additional effects that could result from the modulation of other STK38-dependent cellular functions. For example, studies of genetically engineered mice reveal that STK38 is required to limit inflammation by dampening cytokine secretion and to control T cell homeostasis. Moreover, aged Stk38 knockout mice are prone to develop T cell lymphoma and myeloproliferative diseases. Therefore, pharmacological inhibition or activation of STK38 could result in undesired alterations of the innate and/or adaptive immune response, which possibly could result in malignant proliferative disorders. The implication of STK38 in the regulation of cell cycle checkpoints, mitotic events and centrosome biology indicates that STK38 can play critical roles in cell cycle progression. Thus, since STK38 can contribute to several cellular functions, future studies are needed to address how modulating STK38 expression and/or activity affects essential physiological processes in vivo.

Our discovery that STK38 regulates autophagosome assembly proposes that phenotypes observed upon STK38 deficiency may be the result of alterations of the autophagic process at the molecular level. Specifically, independent studies have already established a causal link between autophagy inhibition in hematopoietic stem cells and the development of severe immune disorders, observations that echo the phenotypes observed in STK38-deficient mice. Therefore, it will be important to re-examine the development of immunopathologies and proliferative disorders that occur in STK38-deficient mice in the context of autophagy modulation. Possibly, the roles of STK38 in cell cycle regulation are also linked to autophagosome formation to some degree. This would be another interesting research avenue to follow since the impact of autophagy on the regulation of cell cycle processes has yet not been fully understood.

The involvement of STK38 in macroautophagy, a bulk degradation system here referred to as autophagy, suggests that it might be worth exploring whether NDR (nuclear dbf2-related) kinases also play a role in other types of autophagy. For example, in accordance with its role in immune competence, STK38 may have a role in xenophagy, the removal of pathogens through autophagy. Moreover, STK38 was recently implicated in mitochondria quality control. Therefore, the question of whether STK38 is involved in any form of selective autophagy (e.g. mitophagy, xenophagy) warrants more investigation.

Besides its cellular functions, STK38 is considered as a tumor suppressor because STK38 can act as a core component of Hippo signaling. However, as discussed for autophagy, STK38 could have a yin-yang role on tumor biology. Indeed, autophagic and apoptotic events both can depend on STK38. Under metabolic stress, STK38 activation is required for cell survival through autophagy stimulation, but further elevation of STK38 activation may lead to cell death through apoptosis. In this context, what are the upstream regulators that finely tune the regulatory phosphorylation of STK38? The kinases STK3/MST2-STK4/MST1, the mammalian orthologs of the fly hpo/Hippo kinase, can function upstream of STK38 in apoptosis, and fly hpo works upstream of trc/tricornered (fly STK38) in dendrite development and maintenance. In oxidative stress signaling the MAP4K4 kinase can signal through STK38. Another known upstream kinase, STK24/MST3, contributes to STK38-dependent cell cycle functions. All these kinases phosphorylate STK38 on Thr444 in the hydrophobic motif, which is required for the full activation of STK38. Interestingly, STK3/4 have been reported to have key roles in autophagy regulation. However, STK38 and STK3/4 are not acting at the same stages of the autophagy process, with STK38 functioning in autophagosome formation downstream of BECN1 while STK3/4 act in autophagosome maturation through the phosphorylation of LC3B. Moreover, it has been reported that STK4 can phosphorylate BECN1, but this inhibits autophagy. Therefore there is currently no clear link between the actions of STK3/4 and STK38 in autophagy. Any involvement of MAP4K4 or STK24 in autophagy has not been reported to date. Thus, in the context of the role of STK38 in autophagosome formation, another yet to be identified upstream kinase(s) is (are) likely to play a role. The serine-threonine kinase ULK1, could be a potential candidate that functions in the regulation of STK38 activity, since, upon autophagy, STK38 activation seems to depend on ULK1, which is needed at early steps of autophagosome biogenesis.

Another critical point is to enhance, in the future, our understanding of how STK38 regulates autophagy. So far, we know that STK38 binding to BECN1 does not depend on STK38 kinase activity, although STK38-BECN1 complex formation and STK38 kinase activity appear to be required for autophagy initiation. The identification of additional binding partners for the BECN1-STK38 complex and/or the substrate(s) of STK38 in autophagy and apoptosis signaling will be required in the future to fully unravel the complexity of STK38 signaling at the key junction for cellular fate and functions. Chemical genetics might be the method of choice to further identify and study the STK38-mediated phosphorylation of these molecules. Whatever the precise underlying molecular mechanisms may be, our discovery of STK38 as a regulator of autophagy has laid a foundation for basic and preclinical followup studies alike.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.