Myo-Inositol modulates AKT signalling, mitochondrial protein expression and intracellular Ca²⁺dynamics in human dermal fibroblasts

Myo-Inositol (Myo-Ins) is increasingly recognised as a metabolic regulator, yet its cellular actions under physiological conditions remain incompletely understood. Here, we investigated the effects of Myo-Ins in primary human dermal fibroblasts. Myo-Ins selectively reduced AKT Ser473 phosphorylation without affecting Thr308, indicating site-specific modulation of AKT signalling. Myo-Ins also increased the abundance of several mitochondrial proteins without altering mitochondrial membrane potential, mtDNA copy number, or basal cellular redox state, consistent with a qualitative remodelling of mitochondrial protein content rather than true mitochondrial biogenesis. A modest enhancement of respiratory reserve capacity accompanied these changes. In addition, Myo-Ins potentiated agonist-evoked calcium (Ca²⁺) responses and increased intracellular Ca²⁺ store content, highlighting its role in modulating Ca²⁺ homeostasis and dynamics. To determine whether these effects require intact phosphoinositide (PIP) signalling, we examined fibroblasts carrying mutations in MTMR5/SBF1, a catalytically inactive member of the myotubularin family involved in regulating PIP metabolism. In these cells, Myo-Ins reduced AKT Ser473 phosphorylation but failed to increase mitochondrial protein expression, indicating that intact PIP-dependent pathways are required for mitochondrial remodelling. Overall, these findings identify Myo-Ins as a physiological modulator of AKT signalling, mitochondrial protein homeostasis, and intracellular Ca²⁺ dynamics, and define the molecular context that enables cellular responsiveness to Myo-Ins.