Matrix Metalloproteinases (MMPs)

Matrix metalloproteinases are a family of enzymes — more than twenty in human tissue — that cut the structural proteins holding the dermis together. They are named by number: MMP-1 is the collagenase that makes the first break in intact type I collagen, MMP-9 takes the resulting fragments apart further, MMP-3 works on proteoglycans and activates the others. Each carries a zinc ion at its active site, and each is held in check by a matching set of tissue inhibitors, the TIMPs.

They are not a defect. Wound remodelling, the ordinary turnover of the dermal matrix and the movement of cells through tissue all depend on controlled protein cutting. What matters is the balance. Ultraviolet exposure raises MMP activity within hours, through the AP-1 signalling pathway, and suppresses new procollagen production at the same time — so the scaffold is cut faster and rebuilt slower.

Repeated across years, that is a large part of what photoaging looks like in tissue: collagen fragmented rather than simply reduced, repaired imperfectly, and fibroblasts that, no longer held taut by an intact scaffold, make less collagen than they used to. The loop feeds itself, which is why this damage is far easier to prevent than to undo.

What follows for a routine is short. The trigger is ultraviolet, so daily sun protection is the intervention with the most behind it. Retinoids are documented to blunt the UV-driven rise in MMP-1 and to raise procollagen. Past that, a great many plant extracts are sold as MMP inhibitors on the strength of test-tube assays — inhibiting an enzyme in a dish is a long way from changing a dermis.