ABSTRACT: In the chronobiology field, a fundamental dichotomy exists to explain daily rhythmicity of biological processes: these can be elicited in response to cyclic extrinsic/environmental signals such as light, or driven endogenously by the circadian clock. In mammals, the circadian clock ticks in almost every cell of the body, and functions based on a network of transcription-translation feedback loops. The PI3K-AKT signaling pathway relays environmental information of nutritional/metabolic state to regulate cell size and proliferation. AKT, a Serine/Threonine protein kinase, is activated by phosphorylation, where phospho-serine 473 (pAKT) serves as a hallmark for its activation. Following activation, it proceeds to phosphorylate dozens of target proteins that convey the signal to regulate gene expression and other key cellular functions. Overall, this pathway is widely known to be activated in response to feeding related signals, and previous studies in mice found elevated pAKT levels in correspondence with food ingestion. However, it is still unknown whether this can (also) be driven through intrinsic mechanisms, such as the circadian clock. Here, we inspected daily activation of AKT both in cultured cells and animal models. Unexpectedly, we found, that neither environmental cues nor the circadian clock were necessary for pAKT rhythms, which exhibited ultradian, rather than circadian, cycles of phosphorylation. In addition, hepatic gene expression also exhibited short rhythms in clock disrupted mice, corresponding with AKT related genes/functions. Reciprocally, inhibition of AKT phosphorylation did not affect the rhythmicity of the circadian clock. Overall, our findings uncover temporal regulation of AKT activation and reveal ultradian molecular rhythmicity that cycles independently of the canonical circadian clock.
